Download Disorders of Acid

CHAPTER
34
Disorders of Acid-Base
Balance
MECHANISMS OF ACID-BASE BALANCE
Acid-Base Chemistry
Metabolic Acid and Bicarbonate Production
Carbon Dioxide and Bicarbonate Production
Production of Noncarbonic Acids and Bases
Calculation of pH
Regulation of pH
Intracellular and Extracellular Buffer Systems
Respiratory Control Mechanisms
Renal Control Mechanisms
Laboratory Tests
Carbon Dioxide and Bicarbonate Levels
Base Excess or Deficit
Anion Gap
DISORDERS OF ACID-BASE BALANCE
Metabolic Versus Respiratory Acid-Base Disorders
Primary Versus Compensatory Mechanisms
Metabolic Acidosis
Causes
Manifestations
Treatment
Metabolic Alkalosis
Causes
Manifestations
Treatment
Respiratory Acidosis
Causes
Manifestations
Treatment
Respiratory Alkalosis
Causes
Manifestations
Treatment
M
etabolic activities of the body require the precise
regulation of acid-base balance, which is reflected
in the pH of extracellular fluids. Membrane excitability, enzyme systems, and chemical reactions depend on
acid-base balance being regulated within a narrow physiologic range to function in an optimal way. Many conditions,
pathologic or otherwise, can alter body pH. This chapter
has been organized into two sections: mechanisms of acidbase balance and disorders of acid-base balance.
Mechanisms of Acid-Base Balance
After you have completed this section of the chapter, you should be
able to meet the following objectives:
✦ Characterize an acid and a base
✦ Cite the source of metabolic acids
✦ Describe the three forms of carbon dioxide transport and
their contribution to acid-base balance
✦ Define pH and use the Henderson-Hasselbalch equation
to calculate the pH and to compare compensatory
mechanisms for regulating pH
✦ Describe the intracellular and extracellular mechanisms
for buffering changes in body pH
✦ Compare the role of the kidneys and respiratory system
in regulation of acid-base balance
✦ Explain how potassium and hydrogen ions and how
bicarbonate and chloride ions interact in pH regulation
Normally, the concentration of body acids and bases
is regulated so that the pH of extracellular body fluids is
maintained within a very narrow range of 7.35 to 7.45.
This balance is maintained through mechanisms that generate, buffer, and eliminate acids and bases. This section of
the chapter focuses on acid-base chemistry, the production
and regulation of metabolic acids and bicarbonate, calculation of pH, and laboratory tests of acid-base balance.
ACID-BASE CHEMISTRY
An acid is a molecule that can release a hydrogen ion (H+),
and a base is a molecule that can accept or combine with
an hydrogen ion.1,2 When an acid (HA) is added to water,
it dissociates reversibly to form H+ and anions (A−); for example, HA
H+ + A−. The degree to which an acid dissociates and acts as a H+ ion donor determines whether
it is a strong or weak acid. Strong acids, such as sulfuric
acid, dissociate completely; weak acids, such as acetic acid,
dissociate only to a limited extent. The same is true of a
base and its ability to dissociate and accept an H+ ion. Most
៞
៝
1
2
UNIT VIII
Renal Function and Fluids and Electrolytes
MECHANISMS OF ACID-BASE BALANCE
៞
៝
➤ The pH is determined by the ratio of the bicarbonate (HCO 3−)
base to the volatile carbonic acid (H2CO3 H+ + HCO3−). At a
normal pH of 7.4, the ratio is 20⬊1.
➤ The pH is regulated by extracellular (carbonic acid [H2CO3]/
bicarbonate [HCO3−]) and intracellular (proteins) systems that
buffer changes in pH that would otherwise occur because of
the metabolic production of volatile (CO2) and nonvolatile
(i.e., sulfuric and phosphoric) acids.
៞
៝
៞
៝
➤ The respiratory system regulates the concentration of the
volatile carbonic acid (CO2 + H2O H2CO3 H+ + HCO3−) by
changing the rate and depth of respiration.
➤ The kidneys regulate the plasma concentration of HCO 3− by
two processes: reabsorption of the filtered HCO3− and generation of new HCO3− or the elimination of H+ ions that have
been buffered by tubular systems (phosphate and ammonia)
to maintain a luminal pH of at least 4.5.
of the body’s acids and bases are weak acids and bases; the
most important are carbonic acid (H2CO3), which is a weak
acid derived from carbon dioxide (CO2), and bicarbonate
(HCO3−), which is a weak base.
The concentration of the H+ ion in body fluids is low
compared with other ions. For example, the sodium ion
(Na+) is present at a concentration approximately 1 million times that of the H+ ion. Because of its low concentration in body fluids, the H+ ion is commonly expressed
in terms of pH. Specifically, pH represents the negative logarithm (p) of the H+ ion concentration in milliequivalents
per liter; a pH value of 7.0 implies an H+ ion concentration
of 10−7 (0.0000001) equivalents per liter (mEq/L). The pH is
inversely related to the H+ ion concentration; a low pH indicates a high concentration of H+ ions, and a high pH
indicates a low concentration of H+ ions.
The dissociation constant (K) is used to describe the degree to which an acid or base dissociates. The symbol pKa
refers to the negative logarithm of the dissociation constant for an acid and represents the pH at which the acid is
50% dissociated.1 Use of a negative logarithm for the dissociation constant allows pH to be expressed as a positive
value. Each acid in an aqueous solution has a characteristic pKa that varies slightly with temperature and pH. At normal body temperature, the pKa for the HCO3− buffer system
of the extracellular fluid compartment is 6.1.
METABOLIC ACID AND
BICARBONATE PRODUCTION
Acids are continuously generated as byproducts of metabolic processes. Physiologically, these acids fall into two
groups: the volatile acid H2CO3 and all other nonvolatile or
fixed acids.
The difference between the two types of acids arises
because H2CO3 is in equilibrium with the volatile gas CO2,
which leaves the body by way of the lungs (Fig. 34-1). The
concentration of H2CO3 is therefore determined by the lungs
and their respiratory capacity. The fixed acids (e.g., sulfuric,
hydrochloric, phosphoric) are nonvolatile and are not eliminated by the lungs. Instead, they are buffered by body
proteins or extracellular buffers, such as HCO3−, and then
excreted by the kidney.
Carbon Dioxide and Bicarbonate Production
Body metabolism results in the production of approximately 15,000 mmol of CO2 each day.1 Carbon dioxide is
transported in the circulation in three forms: attached to
hemoglobin, as dissolved CO2 in the plasma, and as HCO3−
(Fig. 34-2). Collectively, dissolved CO2 and HCO3− constitute approximately 77% of the CO2 that is transported in the
extracellular fluid; the remaining CO2 travels attached to
hemoglobin.2 Although CO2 is not an acid, a small percentage of the gas combines with water in the bloodstream to
form H2CO3.
The reaction that generates H2CO3 from CO2 and water
is catalyzed by an enzyme called carbonic anhydrase, which
is present in large quantities in red blood cells, renal tubular cells, and other tissues in the body. The rate of the reaction between CO2 and water is increased approximately
5000 times by the presence of carbonic anhydrase. Were it
not for this enzyme, the reaction would occur too slowly
to be of any significance.
Because it is almost impossible to measure H2CO3, dissolved CO2 measurements are commonly substituted when
calculating pH. The H2CO3 content of the blood can be calculated by multiplying the partial pressure of CO2 (PCO2) by
its solubility coefficient, which is 0.03. This means that the
concentration of H2CO3 in arterial blood, which normally
has a PCO2 of approximately 40 mm Hg, is 1.2 mEq/L (40 ×
0.03 = 1.2).
Production of Noncarbonic Acids and Bases
The metabolism of dietary proteins and other substances
results in the generation of noncarbonic acids and bases.1
Oxidation of the sulfur-containing amino acids (e.g., methionine, cysteine, cystine) results in the production of
sulfuric acid. Oxidation of arginine and lysine produces
hydrochloric acid, and oxidation of phosphorus-containing
nucleic acids yields phosphoric acid. Incomplete oxidation of glucose results in the formation of lactic acid, and
incomplete oxidation of fats results in the production of
ketoacids. The major source of base is the metabolism of
amino acids such as aspartate and glutamate and the metabolism of certain organic anions (e.g., citrate, lactate, acetate). Acid production normally exceeds base production,
with the net effect being the addition of approximately
1 mmol/kg body weight of nonvolatile or fixed acid to the
body each day.1 A vegetarian diet, which contains large
amounts of organic anions, results in the net production
of base.
CHAPTER 34
Disorders of Acid-Base Balance
3
Food intake
Digestion
Absorption
Cell metabolism
of foodstuffs
Intracellular
and extracellular
buffering
Renal
buffering
Respiratory
buffering
CO2
CO2
H2CO3
Nonvolatile
acids
Proteins and
bicarbonate
Excreted
(combined with
urinary
buffer bases)
Nonvolatile
acids
FIGURE 34-1 The role of intracellular and extracellular buffer, respiratory, and renal mechanisms in
maintaining normal blood pH. (Rhoades R. A., Tanner G. A. [1996]. Medical physiology [p. 468]. Boston:
Little, Brown)
CALCULATION OF pH
Tissue cell
Red blood cell
H2O
Hgb CO2
H2CO3
H2O
CA
Hgb
+
CO2
CO2
CO2
HCO3- H+ + Hgb
HHgb
HCO3-
Cl-
CO2
Carbon dioxide (CO2)
is transported as:
dissolved CO2
hemoglobin (Hgb)
bicarbonate (HCO -)
FIGURE 34-2 Mechanisms of carbon dioxide transport.
CA, carbonic anhydrase
3
The plasma pH can be calculated using an equation called
the Henderson-Hasselbalch equation. This equation uses the
negative logarithm of the dissociation constant and the
logarithm of the HCO3− to CO2 (HCO3−/CO2) ratio to calculate pH: pH = pKa (6.1) + log10 HCO3−/(0.03 × CO2). It
should be noted that it is the ratio rather than the absolute
values for bicarbonate and dissolved CO2 that determines
pH (e.g., when the ratio is 20⬊1, pH = 7.4). Plasma pH decreases when the ratio is less than 20:1, and it increases
when the ratio is greater than 20⬊1 (Fig. 34-3).
Because it is the ratio rather than the absolute values
of HCO3− or CO2 that determines pH, the pH can remain
within relatively normal values as long as changes in
HCO3− are accompanied by similar changes in CO2, or vice
versa. For example, the pH will remain at 7.4 when plasma
HCO3− has increased from 24 to 48 mEq/L as long as CO2
4
UNIT VIII
Renal Function and Fluids and Electrolytes
FIGURE 34-3 Normal and compensated states of pH and acid-base balance represented as a balance
scale. (A) When the ratio of bicarbonate (HCO3−) to carbonic acid (H2CO3, arterial CO2 × .03) = 20⬊1, the
pH = 7.4. (B) Metabolic acidosis with an HCO3−⬊H3CO3 ratio of 10⬊1 and a pH of 7.1. (C) Respiratory compensation lowers the H3CO3 to 0.6 mEq/L and returns the HCO3−⬊H3CO3 ratio to 20⬊1 and the pH to 7.4.
(D) Respiratory alkalosis with an HCO3−⬊H3CO3 ratio of 40⬊1 and a pH of 7.7. (E) Renal compensation
eliminates HCO3−, reducing serum levels to 12 mEq/L, returning the HCO3−⬊H3CO3 ratio to 20⬊1 and the
pH to 7.4. Normally, these compensatory mechanisms are capable of buffering large changes in pH, but
do not return the pH completely to normal, as illustrated here.
levels are also doubled (see Fig. 34-3). Likewise, the pH will
remain at 7.4 when plasma HCO3− has decreased from 24
to 12 mEq/L as long as CO2 levels are reduced by one half.
REGULATION OF pH
The pH of body fluids is regulated by intracellular and
extracellular buffering systems that prevent large changes
in the extracellular pH from occurring through respiratory mechanisms that eliminate CO2 and by renal mech-
anisms that conserve HCO3− ions and eliminate H+ ions
(see Fig. 34-1). The pH is further influenced by the electrolyte composition of the intracellular and extracellular
compartments.
Intracellular and Extracellular Buffer Systems
The moment-by-moment regulation of pH depends on intracellular and extracellular buffer systems. A buffer system
consists of a weak acid and the base salt of that acid or of
a weak base and its acid salt. In the process of preventing
CHAPTER 34
large changes in pH, the system trades a strong acid for a
weak acid or a strong base for a weak base.
The two major buffer systems that protect the pH of
body fluids are proteins and the bicarbonate buffer system.3,4
These buffer systems are immediately available to combine
with excess acids or bases and prevent large changes in pH
from occurring during the time it takes for respiratory and
renal mechanisms to become effective. Bone also represents an important site for buffering of acids and bases.3
Although it is difficult to measure, it has been estimated
that 40% of acute acid-base buffering occurs in bone.1 The
role of bone buffers is even higher in chronic acid-base disorders. One consequence of bone buffering is the release
of calcium from bone and increased renal excretion of calcium. In addition to causing demineralization of bone, it
also predisposes to kidney stones.
Protein Buffer Systems. Proteins are the largest buffer system in the body. Proteins are amphoteric, meaning that
they can function as acids or bases. They contain many
ionizable groups that can release or bind H+. The protein
buffers are largely located in cells, and H+ ions and CO2 diffuse across cell membranes for buffering by intracellular
proteins. Albumin and plasma globulins are the major protein buffers in the vascular compartment.
Bicarbonate Buffer System. The bicarbonate buffer system
uses H2CO3 as its weak acid and a bicarbonate salt such as
sodium bicarbonate (NaHCO3) as its weak base. It substitutes the weak H2CO3 for a strong acid such as hydrochloric acid (HCl + NaHCO3
H2CO3 + NaCl) or the weak
bicarbonate base for a strong base such as sodium hydroxide (NaOH + H2CO3 NaHCO3 + H2O). The HCO3−/CO2
buffer system is a particularly efficient system because the
buffer components can be readily added or removed from
the body.2,5 Metabolism provides an ample supply of CO2,
which can replace any H2CO3 that is lost when excess base
is added, and CO2 can be readily eliminated when excess
acid is added. Likewise, the kidney can conserve or form
new HCO3− when excess acid is added, and it can excrete
HCO3− when excess base is added.
៞
៞ ៝
៝
Disorders of Acid-Base Balance
5
Respiratory Control Mechanisms
The second line of defense against acid-base disturbances is
the control of CO2 by the respiratory system. Excess CO2 or
excess H+ ions in the blood mainly act directly on the respiratory center in the brain to control ventilation. Although
H+ ions do not easily cross the blood–brain barrier, CO2
crosses with ease and in the process reacts with water to
form carbonic acid, which dissociates into H+ and HCO3−
ions. It is the H+ ion that stimulates the respiratory center,
causing an increase or decrease in ventilation.
The respiratory control of pH is rapid, occurring within
minutes, and is maximal within 12 to 24 hours.1 Although
the respiratory response is rapid, it does not completely return the pH to normal. It is only about 50% to 75% effective as a buffer system. This means that if the pH falls from
7.4 to 7.0, the respiratory system can return the pH to a
value of about 7.2 to 7.3.2 In acting rapidly, however, it
prevents large changes in pH from occurring while waiting for the much more slowly reacting kidneys to respond.
Although CO2 readily crosses the blood–brain barrier,
there is a lag for entry of the HCO3− ion. Thus, blood pH
and HCO3− levels drop more rapidly than cerebrospinal
fluid (CSF) levels. In metabolic acidosis, for example, in
which there is a primary decrease in HCO3− ions, there is
often a 12- to 24-hour delay in maximal respiratory response.1 Likewise, when metabolic acid-base disorders are
corrected rapidly, the respiratory response may persist because of a delay in CSF adjustments.
Renal Control Mechanisms
The kidneys regulate acid-base balance by excreting either
an acidic or an alkaline urine. Excreting an acidic urine reduces the amount of acid in the extracellular fluid, and excreting an alkaline urine removes base from the extracellular
fluid. The renal mechanisms for regulating acid-base balance cannot adjust the pH within minutes, as respiratory
mechanisms can, but they continue to function for days
until the pH has returned to normal or near-normal range.
Plasma Potassium–Hydrogen Exchange. Potassium ions
Hydrogen Ion Elimination and Bicarbonate Conservation. The kidney regulates pH by excreting excess H+ ions
(K+) and H+ ions interact in important ways in the regulation of acid-base balance. Both ions are positively charged,
and both ions move freely between the intracellular and
extracellular compartments. In situations of acidosis, excess H+ ions move into the intracellular compartment for
buffering. When this happens, another cation, in this case
the K+ ion, must leave the cell and move into the extracellular fluid. When extracellular potassium levels fall, K+
ions move out of the cell and are replaced by H+ ions. Thus,
alterations in extracellular potassium levels can affect acidbase balance, and changes in acid-base balance can influence extracellular potassium levels. Potassium shifts tend
to be more pronounced in metabolic acidosis than respiratory acidosis.5,6 Also, metabolic acidosis caused by an accumulation of nonorganic acids (e.g., HCl that occurs in
diarrhea, phosphoric acid that occurs in renal failure) produces a greater increase in extracellular potassium than
does acidosis caused by an accumulation of organic acids
(e.g., lactic acid, ketoacids).
and reabsorbing or regenerating HCO3− ions. Bicarbonate is
freely filtered in the glomerulus (approximately 4500 mEq/
day) and reabsorbed in the tubules.5 Loss of even small
amounts of HCO3− impairs the body’s ability to buffer its
daily load of metabolic acids. Because H+ ions are not filtered in adequate amounts to maintain acid-base balance,
they are secreted from blood in the peritubular capillaries
into the urine filtrate in the renal tubules.
Most of the H+ ion secretion and reabsorption of HCO3−
ions takes place in the proximal tubule. The process begins
with a coupled Na+/H+ transport system in which a H+ ion
is secreted into the tubular fluid and a Na+ ion is reabsorbed
into the tubular cell (Fig. 34-4). The secreted H+ ion combines with a filtered HCO3− ion to yield CO2 and H2O. The
water is eliminated in the urine, and the CO2 diffuses into
the tubular cell, where it combines with water, in a carbonic anhydrase–mediated reaction to form a HCO3− ion
and a H+ ion. The HCO3− ion is then reabsorbed into the
blood along with the Na+ ion, and the newly generated H+
6
UNIT VIII
Tubular lumen
(Urine filtrate)
Renal Function and Fluids and Electrolytes
Tubular cell
Peritubular capillary
(Extracellular fluid)
Tubular lumen
(Urine filtrate)
Tubular cell
Peritubular capillary
(Extracellular fluid)
HCO3- + Na +
Na+
H+
H2CO 3
CA
H2 O + CO2
Na+
H ++ HCO 3H2CO3
CA
H2O
+
CO 2
Na+
ATP
K+
NaHPO4- + Na +
H+
Na+
H + HCO 3-
H2CO 3
CO2
NaH 2PO4
H2O + CO 2
CA
ATP
K+
HCO 3CO2
FIGURE 34-4 Hydrogen ion (H+) secretion and bicarbonate ion
(HCO3−) reabsorption in a renal tubular cell. Carbon dioxide (CO2) diffuses from the blood or urine filtrate into the tubular cell, where it
combines with water in a carbonic anhydrase-catalyzed reaction
that yields carbonic acid (H2CO3). The H2CO3 dissociates to form H+
and HCO3−. The H+ is secreted into the tubular fluid in exchange for
Na+. The Na+ and HCO3− enter the extracellular fluid.
FIGURE 34-5 The renal phosphate buffer system. The monohydrogen phosphate ion (HPO42−) enters the renal tubular fluid in the
glomerulus. A H+ combines with the HPO42− to form H2PO4− and is
then excreted into the urine in combination with Na+. The HCO3−
moves into the extracellular fluid along with the Na+ that was exchanged during secretion of the H+.
ion is secreted into the tubular fluid to begin another cycle.
Normally, only a few of the secreted H+ ions remain in the
tubular fluid because the secretion of H+ ions is roughly
equivalent to the number of HCO3− ions that are filtered in
the glomerulus.
H+ ions by NH3 in the collecting tubules.1,2 The metabolism of glutamate in the proximal tubule results in the
formation of two NH4+ and two HCO3− ions (Fig. 34-6A).
The two NH4+ ions are secreted into the tubular fluid by
a countertransport mechanism in exchange for a Na+ ion.
The two HCO3− ions move out of the tubular cell along
with the reabsorbed Na+ ion to enter the peritubular capillaries system. Thus, for each molecule of glutamine metabolized in the proximal tubule, two NH4+ ions are
secreted into the tubular filtrate, and two HCO3− ions are
reabsorbed into the blood. The HCO3− generated by this
process constitutes new HCO3−. A second buffering mechanism involves the recycling of NH4+ by tubular cells in
the medullary portion of the kidney. Here, NH4+ is converted to NH3 and secreted into the tubular lumen. In the
collecting tubules, H+ ions that are secreted into the tubular lumen combine with NH3 to form NH4+ ions (see Fig.
34-6B). However, this part of the tubule is relatively impermeable to NH4+; therefore, once the H+ has reacted
with NH3 to NH4+, it becomes trapped in the tubular
lumen and is eliminated in the urine. In the process of
being converted to NH3, the H+ from the recycled NH4+
promotes the reabsorption of HCO3− by combining with
HCO3− delivered from the proximal tubule. Thus, an additional new HCO3− is generated and added to the blood
for each NH4+ that is recycled.
One of the most important features of the ammonia
buffer system is that it is subject to physiologic control.
Under normal conditions, the amount of H+ ion eliminated by the ammonia buffer system is about 50% of the
acid excreted and new HCO3− regenerated. However, with
chronic acidosis, it can become the dominant mechanism
for H+ excretion and new HCO3− generation.
Tubular Buffer Systems. Because an extremely acidic
urine would be damaging to structures in the urinary tract,
the pH of the urine is maintained within a range from 4.5
to 8.0. This limits the number of unbuffered H+ ions that
can be excreted by the kidney. When the number of free
H+ ions secreted into the tubular fluid threatens to cause
the pH of the urine to become too acidic, they must be carried in some other form. This is accomplished by combining H+ ions with intratubular buffers before they are
excreted in the urine. There are two important intratubular buffer systems: the phosphate buffer system and the
ammonia buffer system.
The phosphate buffer system uses HPO42− and H2PO4−
that are present in the tubular filtrate. Both become concentrated in the tubular fluid because of their relatively
poor absorption and because of reabsorption of water from
the tubular fluid. The combination of H+ with HPO42− to
form H2PO4− allows the kidneys to increase their secretion
of H+ ions (Fig. 34-5).
Another important but more complex buffer system
is the ammonia buffer system. The excretion of H+ and
generation of HCO3− by the ammonia buffer system occurs in three major steps: (1) the synthesis of ammonium
(NH4+) from the amino acid glutamine in the proximal
tubule, thick ascending loop of Henle, and distal tubules;
(2) the reabsorption and recycling of NH3 within the
medullary portion of the kidney; and (3) the buffering of
CHAPTER 34
Disorders of Acid-Base Balance
7
FIGURE 34-6 The ammonia buffer systems. (A) In the proximal, ascending loop of
Henle, and distal tubule, two ammonium (NH4+) ions and two bicarbonate (HCO3−)
ions are synthesized from the amino acid glutamine. The two NH4+ are secreted into
the tubular fluid in exchange for a two sodium (Na+) ions and are either eliminated in
the urine as ammonium chloride (NH4+Cl) or recycled in the medullary nephrons. The
two HCO3− ions, which represent new HCO3− are returned to the circulation. (B) A second NH4+ exchange system occurs in the collecting tubules where secreted hydrogen
(H+) ions combine with ammonia (NH3) from recycled NH4+ and are then eliminated
in the urine as NH4+Cl.
Hydrogen and Potassium Ions Compete for Elimination in
the Urine. Plasma K+ levels influence renal elimination of
H+ ions, and vice versa. When plasma K+ levels fall, there
is movement of K+ ions from body cells into the extracellular fluid and a reciprocal movement of H+ ions from
the extracellular fluid into body cells. In the kidney, these
movements lower the intracellular pH of tubular cells,
causing an increase in H+ ion secretion. Potassium depletion also stimulates ammonia synthesis by the kidney as a
means of buffering the excess H+ ions. The net result is an
increased reabsorption of the filtered HCO3− ions and the
development of metabolic alkalosis. An elevation in plasma
K+ levels has the opposite effect. Plasma K+ levels are similarly altered by acid-base balance. Acidosis tends to increase H+ ion elimination and decrease K+ ion elimination,
with a resultant increase in plasma potassium levels. Alkalosis has the opposite effect.
Aldosterone also influences H+ ion elimination by the
kidney. It acts in the collecting duct to stimulate H+ ion
secretion indirectly, while increasing Na+ ion reabsorption and K+ ion secretion (see Chapter 32). Hyperaldosteronism tends to lead to a decrease in plasma K+ levels
and increased pH and alkalosis because of increased H+
ion secretion. Hypoaldosteronism has the opposite effect.
It leads to increased K+ levels, decreased H+ ion secretion,
and acidosis.
Influence of Sodium Chloride–Bicarbonate Exchange on
pH. Body sodium levels can indirectly influence acid-base
balance by way of the Cl−/HCO3− exchange system. Sodium
reabsorption in the kidneys requires the reabsorption of an
accompanying anion. The two major anions in the extracellular fluid are Cl− and HCO3−.
Renal Function and Fluids and Electrolytes
Base excess or deficit measures the level of all the buffer systems of the blood—hemoglobin, protein, phosphate, and
HCO3−. The base excess or deficit describes the amount of a
fixed acid or base that must be added to a blood sample to
achieve a pH of 7.4 (normal ± 3.0 mEq/L).1 For practical
purposes, base excess or deficit is a measurement of bicarbonate excess or deficit. Base excess indicates metabolic alkalosis, and base deficit indicates metabolic acidosis.
regulation of acid-base balance. The pH of the extracellular
fluid is normally maintained within the narrow physiologic
range of 7.35 to 7.45. Metabolic processes produce volatile
and fixed or nonvolatile metabolic acids that must be buffered
and eliminated from the body. The volatile acid, H2CO3, is in
equilibrium with dissolved CO2, which is eliminated through
mEq
Normal
140
130
120
110
100
Acidosis
due to excess
organic acids
Acidosis
due to excess
chloride levels
Anion gap
12 mEq/L
150
Sodium 142 mEq/L
Base Excess or Deficit
In summary, normal body function depends on the precise
Chloride 116 mEq/L
Bicarbonate 14 mEq/L
The PCO2 of the arterial blood gases provides a means of
assessing the respiratory component of acid-base balance. Arterial blood gases are used because venous blood
gases are highly variable, depending on metabolic demands of the various tissues that empty into the vein
from where the sample is being drawn. The dissolved CO2
levels can be determined from arterial blood gas measurements using the PCO2 and the solubility coefficient
for CO2 (normal arterial PCO2 is 38 to 42 mm Hg). Arterial blood gases also provide a measure of blood oxygen
(PO2) levels. This can be important in assessing respiratory function.
Laboratory tests include measurements of the CO2
content and HCO3− in the blood. The CO2 content that is
included in these measurements is different from the PCO2
that is measured in blood gases. Instead, it refers to the
total CO2 content of blood, including that contained in
HCO3−. More than 70% of the CO2 in the blood is in the
form of bicarbonate. The CO2 content is determined by
adding a strong acid to a plasma sample and measuring
the amount of CO2 generated. The plasma HCO3− concentration is then determined from the total CO2 content of
the blood. The normal range of values for venous HCO3−
concentration is 24 to 29 mEq/L (24 to 29 mmol/L).
Anion gap 25 mEq/L
Carbon Dioxide and Bicarbonate Levels
Chloride 103 mEq/L
Bicarbonate 14 mEq/L
Laboratory tests that are used in assessing acid-base balance
include those for arterial blood gases and pH, CO2 content
and HCO3− levels, base excess or deficit, and the anion gap.
Although useful in determining whether acidosis or alkalosis is present, the pH of the blood as measured by a pH
meter or electrode provides little information about the
cause of an acid-base disorder.
The anion gap describes the difference between the plasma
concentration of the major measured cation (Na+) and the
sum of the measured anions (Cl− and HCO3−). This difference represents the concentration of unmeasured anions,
such as phosphates, sulfates, organic acids, and proteins
(Fig. 34-7). Normally, the anion gap ranges between 8 and
12 mEq/L (a value of 16 mEq/L is normal if both sodium
and potassium concentrations are used in the calculation).
The anion gap is increased in conditions such as lactic acidosis and ketoacidosis that result from elevated levels of
metabolic acids. A low anion gap is found in conditions
that produce a fall in unmeasured anions (primarily albumin) or rise in unmeasured cations. The latter can occur in
hyperkalemia, hypercalcemia, hypermagnesemia, lithium
intoxication, or multiple myeloma, in which an abnormal
immunoglobulin is produced.1
The anion gap of urine can also be measured. It uses
values for the measurable cations (Na+ and K+) and measurable anion (Cl−) to provide an estimate of ammonium
(NH4+) excretion. Because ammonium is a cation, the
value of the anion gap becomes more negative as the ammonium level increases. In normal persons secreting 20 to
40 mmol of ammonium per liter, the urine anion gap is
close to zero. In metabolic acidosis, the amount of unmeasurable NH4+ should increase if renal excretion of H+ is
intact; as a result, the urine anion gap should become more
negative.
Sodium 142 mEq/L
LABORATORY TESTS
Anion Gap
Anion gap
12 mEq/L
One of the mechanisms that the kidneys use in regulating the pH of the extracellular fluids is to conserve or
eliminate HCO3− ions; in the process, it often is necessary
to shuffle anions. Chloride is the most abundant anion in
the extracellular fluid and can substitute for HCO3− when
an anion shift is needed. As an example, plasma HCO3−
levels normally increase as hydrochloric acid is secreted
into the stomach after a heavy meal, causing what is called
the postprandial alkaline tide. Later, as the Cl− is reabsorbed in
the small intestine, the pH returns to normal. Hypochloremic alkalosis refers to an increase in pH that is induced by
a decrease in plasma Cl− levels. Hyperchloremic acidosis occurs when excess levels of Cl− are present.
Sodium 142 mEq/L
UNIT VIII
Chloride 103 mEq/L
Bicarbonate 27 mEq/L
8
FIGURE 34-7 The anion gap in acidosis due to excess metabolic acids
and excess plasma chloride levels. Unmeasured anions such as phosphates, sulfates, and organic acids increase the anion gap because
they replace bicarbonate. This assumes there is no change in sodium
content.
CHAPTER 34
the lungs. The nonvolatile metabolic acids, most of which are
excreted by the kidneys, are derived mainly from protein metabolism and incomplete carbohydrate and fat metabolism. It
is the ratio of the HCO3− ion concentration to dissolved CO2
(H2CO3 concentration) that determines body pH. When this
ratio is 20⬊1, the pH is 7.4.
The ability of the body to maintain pH within the normal
physiologic range depends on intracellular and extracellular
mechanisms for buffering excess H+ and HCO3− ions and on the
respiratory and renal mechanisms for eliminating or conserving excess acids and bases. Proteins are the most important intracellular buffers, and the HCO3− buffer system includes the
most important extracellular buffers. The respiratory system
contributes to the regulation of pH by controlling the elimination of CO2. The kidney aids in regulation of pH by eliminating
H+ ions or conserving HCO3− ions. In the process of eliminating
H+ ions, it uses the phosphate and ammonia buffer systems.
Body pH is also affected by the distribution of exchangeable
cations (K+ and H+) and anions (Cl− and HCO3−).
Laboratory tests that are used in assessing acid-base balance include arterial blood gas measurements, CO2 content
and HCO3− levels, base excess or deficit, and the anion gap.
The base excess or deficit describes the amount of a fixed acid
or base that must be added to a blood sample to achieve a pH
of 7.4. The anion gap describes the difference between the
plasma concentration of the major measured cation (Na+) and
the sum of the measured anions (Cl− and HCO3−). This difference represents the concentration of unmeasured anions,
such as phosphates, sulfates, organic acids, and proteins, that
are present.
Disorders of Acid-Base Balance
After you have completed this section of the chapter, you should be
able to meet the following objectives:
✦ Differentiate the terms acidemia, alkalemia, acidosis, and
alkalosis
✦ Describe a clinical situation involving an acid-base
9
✦ Explain the use of the plasma anion gap in
differentiating types of metabolic acidosis
✦ List common causes of metabolic and respiratory
acidosis and metabolic and respiratory alkalosis
✦ Contrast and compare the clinical manifestations and
treatment of metabolic and respiratory acidosis and of
metabolic and respiratory alkalosis
The terms acidosis and alkalosis describe the clinical
conditions that arise as a result of changes in dissolved CO2
and HCO3− concentrations. An alkali represents a combination of one or more alkali metals such as sodium or
potassium with a highly basic ion such as a hydroxyl ion
(OH−). Sodium bicarbonate is the main alkali in the extracellular fluid. Although the definitions differ somewhat,
the terms alkali and base are often used interchangeably.
Hence, the term alkalosis has come to mean the opposite
of acidosis.
METABOLIC VERSUS RESPIRATORY
ACID-BASE DISORDERS
There are two types of acid-base disorders: metabolic and
respiratory (Table 34-1). Metabolic disorders produce an
alteration in bicarbonate concentration and result from
the addition or loss of nonvolatile acid or alkali to or
from the extracellular fluid. A reduction in pH due to a
decrease in HCO3− is called metabolic acidosis, and an elevated pH due to increased HCO3− levels is called metabolic
alkalosis. Respiratory disorders involve an alteration in the
PCO2, reflecting an increase or decrease in alveolar ventilation. Respiratory acidosis is characterized by a decrease
in pH, reflecting a decrease in ventilation and an increase
in PCO2. Respiratory alkalosis involves an increase in pH,
resulting from an increase in alveolar ventilation and a
decrease in PCO2.
PRIMARY VERSUS
COMPENSATORY MECHANISMS
Acidosis and alkalosis typically involve a primary or initiating
event and a compensatory or adaptive state that results from
homeostatic mechanisms that attempt to correct or prevent
large changes in pH. For example, a person may have a primary metabolic acidosis as a result of overproduction of
disorder in which primary and compensatory
mechanisms might be active
✦ Define metabolic acidosis, metabolic alkalosis,
respiratory acidosis, and respiratory alkalosis
TABLE 34-1
Disorders of Acid-Base Balance
Summary of Acid-Base Imbalances
Acid-Base Imbalance
Primary Disturbance
Respiratory Compensation
Renal Compensation
Metabolic acidosis
Decrease in bicarbonate
Metabolic alkalosis
Increase in bicarbonate
Respiratory acidosis
Increase in PCO2
Hyperventilation to
decrease PCO2
Hypoventilation to
increase PCO2
None
Respiratory alkalosis
Decrease in PCO2
None
If no renal disease, increased H+ excretion
and increased HCO3− reabsorption
If no renal disease, decreased H+ excretion
and decreased HCO3− reabsorption
Increased H+ excretion and increased
HCO3− reabsorption
Decreased H+ excretion and decreased
HCO3− reabsorption
10
UNIT VIII
Renal Function and Fluids and Electrolytes
ketoacids and respiratory alkalosis because of a compensatory increase in ventilation (see Table 34-1). Compensatory mechanisms adjust the pH toward a more normal
level without correcting the underlying cause of the disorder (see Fig. 34-3). A mixed acid-base disorder is one in
which there is both a primary and a compensatory change
in acid-base balance. The respiratory mechanisms, which
compensate by increasing or decreasing ventilation, are
rapid but seldom able to return the pH to normal because
as the pH returns toward normal, the respiratory stimulus
is lost. The kidneys compensate by conserving HCO3− or H+
ions. It normally takes longer to recruit renal compensatory mechanisms than it does respiratory compensatory
mechanisms. Renal mechanisms are more efficient, however, because they continue to operate until the pH has
returned to a normal or near-normal value.
Compensatory mechanisms provide a means to control pH when correction is impossible or cannot be immediately achieved. Often, compensatory mechanisms are
interim measures that permit survival while the body attempts to correct the primary disorder. Compensation requires the use of mechanisms that are different from those
that caused the primary disorder. In other words, the lungs
cannot compensate for respiratory acidosis that is caused
by lung disease, nor can the kidneys compensate for metabolic acidosis that occurs because of renal failure. The body
can, however, use renal mechanisms to compensate for
respiratory-induced changes in pH, and it can use respiratory mechanisms to compensate for metabolically induced
changes in acid-base balance. Because compensatory mechanisms become more effective with time, there are often
differences between the level of pH change that is present
in acute and chronic acid-base disorders.
METABOLIC ACIDOSIS
Metabolic acidosis involves a primary deficit in base
HCO3− along with a decrease in plasma pH. In metabolic
acidosis, the body compensates for the decrease in pH by
increasing the respiratory rate in an effort to decrease CO2
and H2CO3 levels. The PCO2 can be expected to fall by 1 to
1.5 mm Hg for each 1 mEq/L fall in HCO3−.3,4
Causes
Metabolic acidosis can be caused by one of four mechanisms: increased production of nonvolatile metabolic acids,
decreased acid secretion by the kidney, excessive loss of
bicarbonate, or an increase in Cl−.7–9 The anion gap is often
useful in determining the cause of the metabolic acidosis
(Chart 34-1). The presence of excess metabolic acids produces an increase in the anion gap as sodium bicarbonate
is replaced by the sodium salt of the offending acid (e.g., sodium lactate). When acidosis results from increased chloride levels (e.g., hyperchloremic acidosis), the anion gap
remains within normal levels. The causes of metabolic acidosis are summarized in Table 34-2.
Increased Production of Metabolic Acids. Metabolic acids
increase when there is an accumulation of lactic acid,
overproduction of ketoacids, or drug or chemical anion
ingestion.
METABOLIC ACID-BASE IMBALANCE
➤ Metabolic acid-base disorders represent a primary change
in the plasma HCO3− ion concentration.
➤ Metabolic acidosis can be defined as a decrease in plasma
HCO3− and a pH that is caused by an excess of production
or accumulation of fixed acids or loss of HCO3− ion. Compensatory responses include an increase in ventilation and
elimination of CO2 and the reabsorption and generation of
bicarbonate by the kidney.
➤ Metabolic alkalosis can be defined as an increase in
plasma HCO3− and pH that is initiated by excess H+ ion loss
or HCO3− ion gain and maintained by conditions that
impair the ability of the kidney to excrete the excess HCO3−
ion. Compensatory responses include a decreased respiratory rate with retention of PCO2 and increased elimination
of HCO3− by the kidney.
Lactic Acidosis. Acute lactic acidosis is one of the most com-
mon types of metabolic acidosis.8 Lactic acidosis develops
when there is excess production of lactic acid or diminished lactic acid removal from the blood. Lactic acid is produced by the anaerobic metabolism of glucose. Virtually
all tissues can produce lactic acid under appropriate cir-
CHART 34-1
The Anion Gap in Differential Diagnosis
of Metabolic Acidosis
Decreased Anion Gap (<8 mEq/L)
Hypoalbuminemia (decrease in unmeasured anions)
Multiple myeloma (increase in unmeasured cationic IgG
paraproteins)
Increased unmeasured cations (hyperkalemia, hypercalcemia, hypermagnesemia, lithium intoxication)
Increased Anion Gap (>12 mEq/L)
Presence of unmeasured metabolic anion
Diabetic ketoacidosis
Alcoholic ketoacidosis
Lactic acidosis
Starvation
Renal insufficiency
Presence of drug or chemical anion
Salicylate poisoning
Methanol poisoning
Ethylene glycol poisoning
Normal Anion Gap (8–12 mEq/L)
Loss of bicarbonate
Diarrhea
Pancreatic fluid loss
Ileostomy (unadapted)
Chloride retention
Renal tubular acidosis
Ileal loop bladder
Parenteral nutrition (arginine and lysine)
CHAPTER 34
TABLE 34-2
Disorders of Acid-Base Balance
11
Metabolic Acidosis
Causes
Manifestations
Excess Metabolic Acids (Increased Anion Gap)
Excessive production of metabolic acids
Lactic acidosis
Diabetic ketoacidosis
Alcoholic ketoacidosis
Fasting and starvation
Poisoning (e.g., salicylate, methanol,
ethylene glycol)
Impaired elimination of metabolic acids
Kidney failure or dysfunction
Blood pH, HCO3−, CO2
pH decreased
HCO3− (primary) decreased
PCO2 (compensatory) decreased
Excessive Bicarbonate Loss (Normal Anion Gap)
Loss of intestinal secretions
Diarrhea
Intestinal suction
Intestinal or biliary fistula
Increased renal losses
Renal tubular acidosis
Treatment with carbonic anhydrase
inhibitors
Hypoaldosteronism
Increased Chloride Levels (Normal Anion Gap)
Excessive reabsorption of chloride by
the kidney
Sodium chloride infusions
Treatment with ammonium chloride
Parenteral hyperalimentation
Gastrointestinal Function
Anorexia
Nausea and vomiting
Abdominal pain
Neural Function
Weakness
Lethargy
General malaise
Confusion
Stupor
Coma
Depression of vital functions
Cardiovascular Function
Peripheral vasodilation
Decreased heart rate
Cardiac dysrhythmias
Skin
Warm and flushed
Skeletal System
Bone disease (e.g., chronic acidosis)
Signs of Compensation
Increased rate and depth of respiration
(i.e., Kussmaul breathing)
Hyperkalemia
Acid urine
Increased ammonia in urine
cumstances. Tissues such as red blood cells, intestine, and
skeletal muscle do so under normal conditions. Excess lactate is produced with vigorous exercise, during which there
is a local disproportion between oxygen supply and demand in the contracting muscles. The liver and, to a lesser
extent, the kidney normally remove lactic acid from the
blood and use it for energy or convert it back to glucose.
Most cases of lactic acidosis are caused by inadequate
oxygen delivery, as in shock or cardiac arrest.8 Such conditions increase lactic acid production, and they impair
lactic acid clearance because of poor liver perfusion. Mortality rates are high for persons with lactic acidosis because
of shock and tissue hypoxia.8
Lactic acidosis is also associated with disorders in
which tissue hypoxia does not appear to be present. It has
been reported in patients with leukemia, lymphomas, and
other cancers; those with poorly controlled diabetes; and
patients with severe liver failure. Mechanisms causing lactic acidosis in these conditions are poorly understood. Some
conditions, such as neoplasms, may produce local increases
in tissue metabolism and lactate production or interfere
with blood flow to noncancerous cells. Ethanol produces a
slight elevation in lactic acid, but clinically significant lac-
tic acidosis does not occur in alcohol intoxication unless
other problems, such as liver failure, are present.
Lactic acidosis may also occur in mitochondrial disorders that impair oxidative metabolism. Lactic acidosis occurs in genetic mitochondrial disorders that impair lactate
metabolism.10,11 One of these disorders, referred to by the
acronym MELAS, involves mitochondrial encephalopathy
(ME), lactic acidosis (LA), and strokelike episodes (S). Children with the disorder function normally for the first few
years of their lives and then begin to display impaired
motor and cognitive development. The mitochondrial defect also leads to short stature, seizure disorders, and multiple strokes. Lowering the plasma lactate level of children
with severe lactic acidosis may result in marked clinical improvement. A variety of drugs can also produce life-threatening lactic acidosis by inhibiting mitochondrial function.
These drugs include the biguanide antidiabetic drugs and
the antiretroviral nucleoside analogs that are used to treat
acquired immunodeficiency syndrome (AIDS).8
A unique form of lactic acidosis, called D-lactic acidosis, can occur in persons with intestinal disorders that
involve the generation and absorption of D-lactic acid
(L-lactic acid is the usual cause of lactic acidosis). D-Lactic
12
UNIT VIII
Renal Function and Fluids and Electrolytes
acidosis can occur in persons with jejunoileal bypass, small
bowel resection, or short bowel syndrome, in which there
is impaired reabsorption of carbohydrate in the small intestine.8,12 In these cases, the unabsorbed carbohydrate is
delivered to the colon, where it is converted to D-lactic
acid by an overgrowth of gram-positive anaerobes. Persons
with D-lactic acidosis experience episodic periods of metabolic acidosis often brought on by eating a meal high in
carbohydrates. Manifestations include confusion, cerebellar ataxia, slurred speech, and loss of memory. They may
complain of feeling (or may appear) intoxicated. Treatment
includes use of antimicrobial agents to decrease the number of D-lactic acid–producing microorganisms in the bowel
along with a low-carbohydrate diet.
Ketoacidosis. Ketoacids (i.e., acetoacetic and β-hydroxybu-
tyric acid), produced in the liver from fatty acids, are the
source of fuel for many body tissues. An overproduction of
ketoacids occurs when carbohydrate stores are inadequate
or when the body cannot use available carbohydrates as a
fuel. Under these conditions, fatty acids are mobilized from
adipose tissue and delivered to the liver, where they are
converted to ketones. Ketoacidosis develops when ketone
production exceeds tissue use.
The most common cause of ketoacidosis is uncontrolled diabetes mellitus, in which an insulin deficiency
leads to the release of fatty acids from adipose cells with
subsequent production of excess ketoacids (see Chapter 43).
Ketoacidosis may also develop as the result of fasting or
food deprivation, during which the lack of carbohydrates
produces a self-limited state of ketoacidosis. The selflimited nature of ketoacidosis results from a decrease in insulin, which further suppresses the release of fatty acids
from fat cells.
Ketones are also formed during the oxidation of alcohol, a process that occurs in the liver. A condition called
alcoholic ketoacidosis can develop in persons who engage in
excess alcohol consumption.13 It usually follows prolonged
alcohol ingestion, particularly if accompanied by decreased
food intake and vomiting—conditions that result in using
fatty acids as an energy source. The ketoacids responsible
for alcoholic ketoacidosis are formed in part as a result of
alcohol metabolism. Ketone formation may be further
enhanced by the hypoglycemia that results from alcoholinduced inhibition of glucose synthesis (i.e., gluconeogenesis) by the liver and impaired ketone elimination by the
kidneys because of dehydration. An extracellular fluid volume deficit caused by vomiting and decreased fluid intake often contributes to the acidosis. Numerous other
factors, such as elevations in cortisol, growth hormone,
glucagon, and catecholamines, mediate free fatty acid release and thereby contribute to the development of alcoholic ketoacidosis.
Salicylate Toxicity. Salicylates are another potential source of
metabolic acids. Aspirin (acetylsalicylic acid) is rapidly
converted to salicylic acid in the body. Although aspirin is
the most common cause of salicylate toxicity, other salicylate preparations such as methyl salicylate, sodium salicylate, and salicylic acid may be involved. Salicylate overdose
produces serious toxic effects, including death. A fatal overdose can occur with as little as 10 to 30 g in adults and 3 g
in children.1
A variety of acid-base disturbances occur with salicylate
toxicity. The salicylates cross the blood–brain barrier and
directly stimulate the respiratory center, causing hyperventilation and respiratory alkalosis. The kidneys compensate by secreting increased amounts of HCO3−, K+, and
Na+, thereby contributing to the development of metabolic
acidosis. Salicylates also interfere with carbohydrate metabolism, which results in increased production of metabolic
acids.
One of the treatments for salicylate toxicity is alkalinization of the plasma. Salicylic acid, which is a weak
acid, exists in equilibrium with the alkaline salicylate
anion. It is the salicylic acid that is toxic because of its
ability to cross cell membranes and enter brain cells. The
salicylate anion crosses membranes poorly and is less toxic.
With alkalinization of the extracellular fluids, the ratio of
salicylic acid to salicylate is greatly reduced. This allows
salicylic acid to move out of cells into the extracellular
fluid along a concentration gradient. The renal elimination of salicylates follows a similar pattern when the urine
is alkalinized.
Methanol and Ethylene Glycol Toxicity. Ingestion of methanol
and ethylene glycol results in the production of metabolic
acids and causes metabolic acidosis. Both produce an osmolar gap because of their small size and osmotic properties. Methanol (wood alcohol) is a component of shellac,
varnish, deicing solutions, sterno, and other commercial
products. Methanol can be absorbed through the skin or
gastrointestinal tract or inhaled through the lungs. A dose
as small as 30 mL can be fatal.14 In addition to metabolic
acidosis, methanol produces severe optic nerve and central nervous system toxicity. Organ system damage occurs
after a 24-hour period in which methanol is converted to
formaldehyde and formic acid.
Ethylene glycol is a solvent found in products ranging
from antifreeze and deicing solutions to carpet and fabric
cleaners. It tastes sweet and is intoxicating, factors that
contribute to its abuse potential. A lethal dose is approximately 100 mL.15,16 It is rapidly absorbed from the intestine, making treatment with gastric lavage and syrup of
ipecac ineffective. Acidosis occurs as ethylene glycol is
converted to oxalic and lactic acid. Manifestations of ethylene glycol toxicity occur in three stages: neurologic symptoms ranging from drunkenness to coma, which appear
during the first 12 hours; cardiorespiratory disorders such
as tachycardia and pulmonary edema; and flank pain and
renal failure caused by plugging of the tubules with oxalate
crystals (from excess oxalic acid production).16
The enzyme alcohol dehydrogenase metabolizes methanol and ethylene glycol into toxic metabolites. This is the
same enzyme that is used in the metabolism of ethanol.
Because alcohol dehydrogenase has an affinity for ethanol
10 times its affinity for methanol or ethylene glycol, intravenous or oral ethanol is used as an antidote for methanol
and ethylene glycol poisoning.1 Extracellular volume expansion and hemodialysis are also used. Fomepizole, with
CHAPTER 34
specific indications for ethylene glycol poisoning, was
recently approved by the U.S. Food and Drug Administration.15,17 In a manner similar to ethanol, it is thought
to act as an inhibitor of alcohol dehydrogenase, thereby
preventing the formation of the toxic ethylene glycol
metabolites.
Decreased Renal Function. Kidney disease is the most
common cause of chronic metabolic acidosis. The kidneys
normally conserve HCO3− and secrete H+ ions into the
urine as a means of regulating acid-base balance. In renal
failure, there is loss of glomerular and tubular function,
with retention of nitrogenous wastes and metabolic acids.
In a condition called renal tubular acidosis, glomerular
function is normal, but the tubular secretion of H+ or
reabsorption of HCO3− is abnormal (see Chapter 35).
Increased Bicarbonate Losses. Increased HCO3− losses
occur with the loss of bicarbonate-rich body fluids or with
impaired conservation of HCO3− by the kidney. Intestinal
secretions have a high HCO3− concentration. Consequently,
excessive loss of HCO3− ions occur with severe diarrhea;
small bowel, pancreatic, or biliary fistula drainage; ileostomy drainage; and intestinal suction. In diarrhea of microbial origin, HCO3− is also secreted into the bowel as a
means of neutralizing the metabolic acids produced by the
microorganisms causing the diarrhea. Creation of an ileal
bladder, which is done for conditions such as neurogenic
bladder or surgical removal of the bladder because of cancer, involves the implantation of the ureters into a short,
isolated loop of ileum that serves as a conduit for urine collection. With this procedure, contact time between the
urine and ileal bladder is normally too short for significant
anion exchange, and HCO3− is lost in the urine.1
Hyperchloremic Acidosis. Hyperchloremic acidosis occurs
when Cl− ion levels are increased. Because Cl− and HCO3−
are anions, the HCO3− ion concentration decreases when
there is an increase in Cl− ions. Hyperchloremic acidosis
can occur as the result of abnormal absorption of chloride
by the kidneys or as a result of treatment with chloridecontaining medications (i.e., sodium chloride, amino acid–
chloride hyperalimentation solutions, and ammonium
chloride). Ammonium chloride is broken down into NH4+
and Cl−. The ammonium ion is converted to urea in the
liver, leaving the Cl− ion free to react with H+ to form HCl.
The administration of intravenous sodium chloride or parenteral hyperalimentation solutions that contain an amino
acid–chloride combination can cause acidosis in a similar
manner.18 With hyperchloremic acidosis, the anion gap is
within the normal range, whereas the chloride levels are
increased, and bicarbonate levels are decreased.
Manifestations
Metabolic acidosis is characterized by an increased extracellular H+ ion concentration with a decrease in pH (<7.35) and
a decrease in HCO3− levels (<24 mEq/L). Acidosis typically
produces a compensatory increase in respiratory rate with a
decrease in PCO2.
The manifestations of metabolic acidosis fall into three
categories: signs and symptoms of the disorder causing the
Disorders of Acid-Base Balance
13
acidosis, changes in body function related to recruitment
of compensatory mechanisms, and alterations in cardiovascular, neurologic, and musculoskeletal function resulting from the decreased pH (see Table 34-2). The signs and
symptoms of metabolic acidosis usually begin to appear
when the plasma HCO3− concentration falls to 20 mEq/L or
less. A fall in pH to less than 7.0 to 7.10 can reduce cardiac
contractility and predispose to potentially fatal cardiac
dysrhythmias.1
Metabolic acidosis is seldom a primary disorder; it
usually develops during the course of another disease. The
manifestations of metabolic acidosis frequently are superimposed on the symptoms of the contributing health problem. With diabetic ketoacidosis, which is a common cause
of metabolic acidosis, there is an increase in blood and
urine glucose and a characteristic smell of ketones to the
breath. In metabolic acidosis that accompanies renal failure, blood urea nitrogen levels are elevated, and other tests
of renal function yield abnormal results.
Manifestations related to respiratory and renal compensatory mechanisms usually occur early in the course of
metabolic acidosis. In situations of acute metabolic acidosis, the respiratory system compensates for a decrease in pH
by increasing ventilation to reduce PCO2; this is accomplished through deep and rapid respirations. In diabetic
ketoacidosis, this breathing pattern is referred to as Kussmaul breathing. For descriptive purposes, it can be said that
Kussmaul breathing resembles the hyperpnea of exercise—
the person breathes as though he or she had been running.
There may be complaints of difficult breathing or dyspnea with exertion; with severe acidosis, dyspnea may be
present even at rest. Respiratory compensation for acute acidosis tends to be somewhat greater than for chronic acidosis.
When kidney function is normal, H+ ion excretion increases
promptly in response to acidosis, and the urine becomes
more acid.
Changes in pH have a direct effect on body function
that can produce signs and symptoms common to most
types of metabolic acidosis, regardless of cause. A person
with metabolic acidosis often complains of weakness, fatigue, general malaise, and a dull headache. The person
also may have anorexia, nausea, vomiting, and abdominal
pain. Tissue turgor is impaired, and the skin is dry when
fluid deficit accompanies acidosis. In persons with undiagnosed diabetes mellitus, the nausea, vomiting, and abdominal symptoms may be misinterpreted as being caused
by gastrointestinal flu or other abdominal disease, such as
appendicitis. Acidosis depresses neuronal excitability and
decreases binding of calcium to plasma proteins so that
more free calcium is available to decrease neural activity.
As acidosis progresses, the level of consciousness declines,
and stupor and coma develop. The skin is often warm and
flushed because skin vessels become less responsive to
sympathetic stimulation and lose tone.
When the pH falls to 7.0 to 7.1, cardiac contractility and
cardiac output decrease, the heart becomes less responsive
to catecholamines (i.e., epinephrine and norepinephrine),
and dysrhythmias, including fatal ventricular dysrhythmias, can develop. A decrease in ventricular function may
be particularly important in perpetuating shock-induced
14
UNIT VIII
Renal Function and Fluids and Electrolytes
lactic acidosis, and partial correction of the acidemia may
be necessary before tissue perfusion can be restored.1
Chronic acidemia, as in renal failure, can lead to a
variety of musculoskeletal problems, some of which result
from the release of calcium and phosphate during bone
buffering of excess H+ ions.19 Of particular importance is
impaired growth in children. In infants and children, acidemia may be associated with a variety of nonspecific symptoms such as anorexia, weight loss, muscle weakness, and
listlessness.1,14 Muscle weakness and listlessness may result
from alterations in muscle metabolism.
Treatment
The treatment of metabolic acidosis focuses on correcting the condition that caused the disorder and restoring
the fluids and electrolytes that have been lost from the
body. The treatment of diabetic ketoacidosis is discussed
in Chapter 43.
The use of supplemental sodium bicarbonate (NaHCO3)
may be indicated in the treatment of some forms of normal anion gap acidosis. However, its use in treatment of
metabolic acidosis with an increased anion gap is controversial, particularly in cases of impaired tissue perfusion.4,20
In most patients with cardiac arrest, shock, or sepsis, impaired oxygen delivery is the primary cause of lactic acidosis. In these situations, the administration of large amounts
of NaHCO3 does not improve oxygen delivery and may
produce hypernatremia, hyperosmolality, and decreased
oxygen release by hemoglobin because of a shift in the oxygen dissociation curve.20
TABLE 34-3
METABOLIC ALKALOSIS
Metabolic alkalosis is a systemic disorder caused by an increase in pH due to a primary excess of plasma HCO3−
ions.21,22 It can be caused by a loss of H+ ions, net gain in
HCO3− ions, or loss of Cl− ions in excess of HCO3− ions.
Metabolic alkalosis is reported to be the second most common acid-base disorder in hospitalized adults, accounting
for about 32% of all acid-base disorders.22
Causes
Most of the body’s plasma HCO3− is obtained from three
sources: from CO2 that is produced during metabolic processes, from reabsorption of filtered HCO3−, or from
generation of new HCO3− by the kidney. Usually, HCO3−
production and renal reabsorption are balanced in a manner that prevents alkalosis from occurring. The proximal
tubule reabsorbs 99.9% of the filtered HCO3−. When the
plasma levels of HCO3− rise above the threshold for tubular reabsorption, the excess is excreted in the urine. Many
of the conditions that increase plasma HCO3− also raise the
level for HCO3− reabsorption; thus, an increase in HCO3−
contributes not only to the generation of metabolic alkalosis but also to its maintenance.
Metabolic alkalosis involves both the factors that generate the loss of H+ or gain of HCO3− ions and those that
maintain it by interfering with excretion of the excess
HCO3− (Table 34-3). Factors that serve to maintain metabolic alkalosis include extracellular fluid volume contraction accompanied by hypokalemia and hypochloremia
and mineralocorticoid (aldosterone) excess.
Metabolic Alkalosis
Causes
Manifestations
Excessive Gain of Bicarbonate or Alkali
Ingestion or administration of
sodium bicarbonate
Administration of hyperalimentation
solutions containing acetate
Administration of parenteral solutions
containing lactate
Administration of citrate-containing
blood transfusions
Blood pH, HCO3−, CO2
pH increased
HCO3 − (primary) increased
PCO2 (compensatory) increased
Excessive Loss of Hydrogen Ions
Vomiting
Gastric suction
Binge-purge syndrome
Potassium deficit
Diuretic therapy
Hyperaldosteronism
Milk-alkali syndrome
Increased Bicarbonate Retention
Loss of chloride with bicarbonate retention
Volume Contraction
Loss of body fluids
Diuretic therapy
Neural Function
Confusion
Hyperactive reflexes
Tetany
Convulsions
Cardiovascular Function
Hypotension
Dysrhythmias
Respiratory Function
Respiratory acidosis due to decreased
respiratory rate
Signs of Compensation
Decreased rate and depth of respiration
Increased urine pH
CHAPTER 34
Excess Alkali Intake. Excessive alkali ingestion, as in the
use of bicarbonate-containing antacids (e.g., AlkaSeltzer) or NaHCO3 administration during cardiopulmonary resuscitation, can cause metabolic alkalosis.
Other sources of alkali intake are acetate in hyperalimentation solutions, lactate in parenteral solutions such
as Ringer’s lactate, and citrate used in blood transfusions. A condition called the milk-alkali syndrome may
develop in persons who consume excessive amounts of
milk along with an antacid such as calcium carbonate. In
this case, the carbonate raises the plasma HCO3− while
the hypercalcemia prevents the excretion of excess
HCO3− . The most common cause at present is the use of
calcium carbonate as a phosphate buffer in persons with
renal failure.
Hydrogen, Chloride, and Potassium Ion Loss Associated
With Bicarbonate Ion. Vomiting, removal of gastric secretions through use of nasogastric suction, and low potassium levels resulting from diuretic therapy are the most
common causes of metabolic alkalosis in hospitalized patients. The binge and purge syndrome, or self-induced
vomiting, also is associated with metabolic alkalosis.22
Gastric secretions contain high concentrations of HCl
and lesser concentrations of potassium chloride (KCL). As
Cl− is taken from the blood and secreted into the stomach
with the H+ ion, it is replaced by HCO3−. Under normal
conditions, each 1 mEq of H+ ion that is secreted into the
stomach generates 1 mEq of plasma HCO3−.22 Because the
entry of acid into the duodenum stimulates an equal
amount of pancreatic HCO3− secretion, the increase in
plasma HCO3− concentration is usually transient, and pH
returns to normal within a matter of hours. However, loss
of H+ and Cl− ions from the stomach due to vomiting or
gastric suction stimulates continued production of gastric
acid and thus the addition of more bicarbonate into the
blood.
Maintenance of Metabolic Alkalasis by Volume Contraction, Hypokalemia, and Hypochloremia. Vomiting also results in the loss of water, sodium, and potassium (Fig. 34-8).
The resultant volume depletion and hypokalemia maintain the generated metabolic alkalosis by increasing the
renal reabsorption of HCO3− ion. Administration of diuretics (e.g., loop and thiazide diuretics) is often associated
with metabolic alkalosis. Loop diuretics decrease Na+, K+,
and Cl− reabsorption, leading to volume contraction and
hypokalemia. The thiazide diuretics increase renal K+ loss,
leading to increased HCO3− reabsorption.
Extracellular volume depletion is one of the most
important factors affecting HCO3− reabsorption in the
proximal tubule. A decrease in extracellular fluid volume
activates the renin-angiotensin-aldosterone system, which
increases Na+ reabsorption as a means of maintaining extracellular fluid volume. The reabsorption of Na+ requires
concomitant anion reabsorption; because there is a Cl−
deficit, HCO3− is reabsorbed along with Na+.
Hypokalemia is a potent stimulus for H+ secretion and
HCO3− reabsorption. The mechanisms by which hypokale-
Disorders of Acid-Base Balance
15
mia increases HCO3− reabsorption are likely the effects of
tubular K+/H+ ion exchange. In hypokalemia, K+ moves out
of the tubular cell into the blood and is replaced by the H+
ion. This results in intracellular acidosis and increased
HCO3− reabsorption. For some unknown reason, severe
hypokalemia may cause a reduction in Cl− reabsorption by
the distal tubule.1 As a result, Na+ reabsorption at this site
is associated with greater tubular electronegativity and
greater tendency to H+ secretion.
It has been proposed recently that hypochloremia and
the reduced delivery of Cl− to the distal tubule of the kidney
are responsible for maintaining metabolic alkalosis, rather
than volume depletion per se. A low intraluminal concentration of Cl− ion is interpreted by the kidney as a sign of
low extracellular fluid volume, thereby serving as a stimulus
for activation of the renin-angiotensin-aldosterone system.
Low intraluminal concentrations of Cl− ion also reduce the
driving force for bicarbonate reabsorption.22
Metabolic alkalosis can also result from excessive
adrenocorticosteroid hormones (e.g., hyperaldosteronism,
Cushing’s syndrome). The hormone aldosterone increases
H+ ion secretion as it increases Na+ and HCO3− ion reabsorption. In hyperaldosteronism, the concurrent loss of K+
in the urine serves to perpetuate the alkalosis.
Chronic respiratory acidosis produces a compensatory
loss of H+ and Cl− ions in the urine along with HCO3− retention. When respiratory acidosis is corrected abruptly, as
with mechanical ventilation, a “posthypercapneic” metabolic alkalosis may develop because of a rapid drop in
PCO2; however, the concentration of HCO3− ions, which
are eliminated renally, remains elevated.
Manifestations
Metabolic alkalosis is characterized by a plasma pH above
7.45, plasma HCO3− level above 29 mEq/L (29 mmol/L), and
base excess above 3.0 mEq/L (3 mmol/L) (see Table 34-3).
Persons with metabolic alkalosis often are asymptomatic or
have signs related to volume depletion or hypokalemia.
Neurologic signs and symptoms (e.g., hyperexcitability)
occur less frequently with metabolic alkalosis than with
other acid-base disorders because the HCO3− ion enters the
CSF more slowly than CO2. When neurologic manifestations do occur, as in acute and severe metabolic alkalosis,
they include mental confusion, hyperactive reflexes, tetany,
and carpopedal spasm. Metabolic alkalosis also leads to a
compensatory hypoventilation with development of various degrees of hypoxemia and respiratory acidosis. Significant morbidity occurs with severe metabolic alkalosis
(pH >7.55), including respiratory failure, dysrhythmias,
seizures, and coma.
Treatment
The treatment of metabolic alkalosis usually is directed toward correcting the cause of the condition. A chloride
deficit requires correction. Potassium chloride usually is
the treatment of choice for metabolic alkalosis when there
is an accompanying K+ deficit. When KCl is used as a therapy, the Cl− anion replaces the HCO3− anion, and correcting the potassium deficit allows the kidney to conserve the
16
UNIT VIII
Renal Function and Fluids and Electrolytes
a sustained increase in arterial PCO2, resulting in renal adaptation and a more marked increase in plasma HCO3−.23–26
Causes
Respiratory acidosis occurs in acute or chronic conditions
that impair effective alveolar ventilation and cause an accumulation of PCO2 (Table 34-4). Impaired ventilation can
occur as the result of decreased respiratory drive, lung disease, or disorders of chest wall and respiratory muscle. Less
commonly, it results from excess CO2 production.
Acute Disorders of Ventilation. Acute respiratory acidosis can be caused by impaired function of the respiratory
center in the medulla (as in narcotic overdose), lung disease, chest injury, weakness of the respiratory muscles, or
airway obstruction. Almost all persons with acute respiratory acidosis are hypoxemic if they are breathing room
air. In many cases, signs of hypoxemia develop before
those of respiratory acidosis because CO2 diffuses across the
alveolar capillary membrane 20 times more rapidly than
oxygen.1,2
Chronic Disorders of Ventilation. Chronic respiratory aci-
FIGURE 34-8 Renal mechanisms for generating and maintaining metabolic alkalosis following depletion of extracellular fluid volume, chloride (Cl−), and potassium (K+) due to vomiting. HCO3−, bicarbonate; GFR,
glomerular filtration rate.
H+ ion while eliminating the K+ ion. Fluid replacement
with normal saline or one half normal saline often is used
in the treatment of patients with volume contraction alkalosis.
RESPIRATORY ACIDOSIS
Respiratory acidosis occurs in conditions that impair alveolar ventilation and cause an increase in plasma PCO2, also
known as hypercapnia, along with a decrease in pH. Respiratory acidosis can occur as an acute or chronic disorder. Acute
respiratory failure is associated with a rapid rise in arterial
PCO2 with a minimal increase in plasma HCO3− and large decrease in pH. Chronic respiratory acidosis is characterized by
dosis is a relatively common disturbance in patients with
chronic obstructive lung disease (see Chapter 31). In these
persons, the persistent elevation of PCO2 stimulates renal
H+ ion secretion and HCO3− reabsorption. The effectiveness of these compensatory mechanisms can often return
the pH to near-normal values as long as oxygen levels are
maintained within a range that does not unduly suppress
chemoreceptor control of respirations.
An acute episode of respiratory acidosis can develop
in patients with chronic lung disease who have chronically elevated PCO2 levels. This is sometimes called carbon
dioxide narcosis. In these persons, the medullary respiratory center has become adapted to the elevated levels of
CO2 and no longer responds to increases in PCO2. Instead,
the oxygen content of their blood becomes the major
stimulus for respiration. When oxygen is administered at
a flow rate that suppresses this stimulus, the rate and
depth of respiration decrease, and the CO2 content of the
blood increases.
Increased Carbon Dioxide Production. Carbon dioxide is
a product of the body’s metabolic processes, generating a
substantial amount of acid that must be excreted by the
lungs or kidney to prevent acidosis. An increase in CO2
production can result from numerous processes, including
exercise, fever, sepsis, and burns. For example, CO2 production increases by approximately 13% for each 1°C rise
in temperature above normal.27 Nutrition also affects the
production of carbon dioxide. A carbohydrate-rich diet
produces larger amounts of CO2 than one containing reasonable amounts of protein and fat. Although excess carbon dioxide production can lead to an increase in PCO2, it
seldom does. In cases of healthy persons, an increase in
CO2 is usually matched by an increase in CO2 elimination
by the lungs. In contrast, persons with respiratory diseases
may be unable to eliminate the excess CO2.
CHAPTER 34
RESPIRATORY ACID-BASE IMBALANCE
➤ Respiratory acid-base imbalances are due to a primary
disturbance in PCO2, reflecting an increase or decrease in
alveolar ventilation.
➤ Respiratory acidosis, or hypercapnia, represents an
increase in PCO2 and a decrease in plasma pH, resulting
from a decrease in effective alveolar ventilation. Compensatory mechanisms include increased conservation and
generation of HCO3− and elimination of H+ by the kidney.
➤ Respiratory alkalosis, or hypocapnia, represents a decrease in PCO2 and an increase in plasma pH, resulting
from increased alveolar ventilation. Compensatory
mechanisms include increased elimination of HCO3−
and conservation of H+ by the kidney.
Disorders of Acid-Base Balance
17
ache, blurred vision, irritability, muscle twitching, and psychological disturbances can occur with acute respiratory
acidosis. If the condition is severe and prolonged, it can
cause an increase in CSF pressure and papilledema. Impaired
consciousness, ranging from lethargy to coma, develops as
the PCO2 rises. Paralysis of extremities may occur, and there
may be respiratory depression. Less severe forms of acidosis
often are accompanied by warm and flushed skin, weakness,
and tachycardia.
Treatment
The treatment of acute and chronic respiratory acidosis is
directed toward improving ventilation. In severe cases, mechanical ventilation may be necessary. The treatment of
respiratory acidosis due to respiratory failure is discussed in
Chapter 31.
RESPIRATORY ALKALOSIS
Manifestations
Respiratory acidosis is associated with a plasma pH below
7.35 and an arterial PCO2 above 50 mm Hg (see Table 34-4).
The signs and symptoms of respiratory acidosis depend on
the rapidity of onset and whether the condition is acute or
chronic. Because respiratory acidosis often is accompanied
by hypoxemia, the manifestations of respiratory acidosis
often are intermixed with those of oxygen deficit. Carbon
dioxide readily crosses the blood–brain barrier, exerting its
effects by changing the pH of brain fluids. Elevated levels of
CO2 produce vasodilation of cerebral blood vessels. Head-
TABLE 34-4
Respiratory alkalosis is a systemic acid-base disorder characterized by a primary decrease in plasma PCO2, also referred to as hypocapnia, which produces an elevation in pH
and a subsequent decrease in HCO3−.25,27,28 Because respiratory alkalosis can occur suddenly, a compensatory decrease
in bicarbonate level may not occur before respiratory correction has taken place.
Causes
Respiratory alkalosis is caused by hyperventilation or a respiratory rate in excess of that needed to maintain normal
Causes and Manifestations of Respiratory Acidosis
Causes
Manifestations
Depression of Respiratory Center
Drug overdose
Head injury
Blood pH, CO2 , HCO3−
pH decreased
PCO2 (primary) increased
HCO3− (compensatory) increased
Lung Disease
Bronchial asthma
Emphysema
Chronic bronchitis
Pneumonia
Pulmonary edema
Respiratory distress syndrome
Airway Obstruction, Disorders of Chest Wall
and Respiratory Muscles
Paralysis of respiratory muscles
Chest injuries
Kyphoscoliosis
Extreme obesity
Treatment with paralytic drugs
Breathing Air With High CO2 Content
Neural Function
Dilation of cerebral vessels and depression
of neural function
Headache
Weakness
Behavior changes
Confusion
Depression
Paranoia
Hallucinations
Tremors
Paralysis
Stupor and coma
Skin
Skin warm and flushed
Signs of Compensation
Acid urine
18
UNIT VIII
Renal Function and Fluids and Electrolytes
plasma PCO2 levels (Table 34-5). It may occur as the result
of central stimulation of the medullary respiratory center
or stimulation of peripheral (e.g., carotid chemoreceptor)
pathways to the medullary respiratory center.25
Mechanical ventilation may produce respiratory alkalosis if the rate and tidal volume are set so that CO2 elimination exceeds CO2 production. Carbon dioxide crosses
the alveolar capillary membrane 20 times more rapidly
than oxygen. Therefore, the increased minute ventilation
may be necessary to maintain adequate oxygen levels while
producing a concomitant decrease in CO2 levels. In some
cases, respiratory alkalosis may be induced medically as a
means of controlling disorders such as severe intracranial
hypertension.27
Central stimulation of the medullary respiratory center
occurs with anxiety, pain, pregnancy, febrile states, sepsis,
encephalitis, and salicylate toxicity. Respiratory alkalosis
has long been recognized as a common acid-base disorder
in critically ill patients and is a consistent finding in both
septic shock and the systemic inflammatory response syndrome27 (see Chapter 28). Progesterone increases ventilation in women; during the progesterone phase of the
menstrual cycle, normal women increase their PCO2 values by 2 to 4 mm Hg and their pH by 0.01 to .02.3 Women
also develop substantial hypocapnia during pregnancy,
most notably during the last trimester, with PCO2 values
of 29 to 32 mm Hg.3,27
One of the most common causes of respiratory alkalosis is the hyperventilation syndrome, which is characterized
by recurring episodes of overbreathing often associated
with anxiety. Persons experiencing panic attacks frequently present in the emergency room with manifestations of acute respiratory alkalosis.
Hypoxemia exerts its effect on pH through the peripheral chemoreceptors in the carotid bodies. Stimulation
of peripheral chemoreceptors occurs in conditions that
cause hypoxemia with relatively unimpaired CO2 trans-
TABLE 34-5
port such as exposure to high altitudes, asthma, and respiratory disorders that decrease lung compliance.
Manifestations
Respiratory alkalosis manifests with a decrease in PCO2
and a deficit in H2CO3 (see Table 34-5). In respiratory alkalosis, the pH is above 7.45, arterial PCO2 is below 35 mm
Hg, and plasma HCO3− levels usually are below 24 mEq/L
(24 mmol/L).
The signs and symptoms of respiratory alkalosis are
associated with hyperexcitability of the nervous system
and a decrease in cerebral blood flow. Alkalosis increases
protein binding of extracellular calcium. This reduces ionized calcium levels, causing an increase in neuromuscular
excitability. A decrease in the CO2 content of the blood
causes constriction of cerebral blood vessels. Because CO2
crosses the blood–brain barrier rather quickly, the manifestations of acute respiratory alkalosis are usually of sudden onset. The person often experiences light-headedness,
dizziness, tingling, and numbness of the fingers and toes.
These manifestations may be accompanied by sweating,
palpitations, panic, air hunger, and dyspnea. Chvostek’s
and Trousseau’s signs may be positive (see Chapter 33),
and tetany and convulsions may occur. Because CO2 provides the stimulus for short-term regulation of respiration,
short periods of apnea may occur in persons with acute
episodes of hyperventilation.
Treatment
The treatment of respiratory alkalosis focuses on measures
to correct the underlying cause. Hypoxia may be corrected
by administration of supplemental oxygen. Changing ventilator settings may be used to prevent or treat respiratory
alkalosis in persons who are being mechanically ventilated.
Persons with hyperventilation syndrome may benefit from
reassurance, rebreathing from a paper bag during symptomatic attacks, and attention to the psychological stress.
Causes and Manifestations of Respiratory Alkalosis
Causes
Manifestations
Excessive Ventilation
Anxiety and psychogenic
hyperventilation
Hypoxia and reflex stimulation of
ventilation
Lung disease that reflexly stimulates
ventilation
Stimulation of respiratory center
Elevated blood ammonia level
Salicylate toxicity
Encephalitis
Fever
Mechanical ventilation
Blood pH, CO2 , HCO3−
pH increased
PCO2 (primary) decreased
HCO3− (compensatory) decreased
Neural Function
Constriction of cerebral vessels and
increased neuronal excitability
Dizziness, panic, light-headedness
Tetany
Numbness and tingling of fingers and toes
Positive Chvostek’s and Trousseau’s signs
Seizures
Cardiovascular Function
Cardiac dysrhythmias
CHAPTER 34
In summary, acidosis describes a decrease in pH, and alkalosis describes an increase in pH. Acid-base disorders may be
caused by alterations in the body’s volatile acids (i.e., respiratory
acidosis or respiratory alkalosis) or nonvolatile or fixed acids
(i.e., metabolic acidosis or metabolic alkalosis). Acidosis and
alkalosis typically involve a primary or initiating event and a
compensatory or adaptive state that results from homeostatic
mechanisms that attempt to prevent or correct large changes in
pH. A mixed acid-base disorder is one in which there is both a
primary and a compensatory change in acid-base balance.
Metabolic acidosis is defined as a decrease in pH due to a
decrease in the HCO3− ion, and metabolic alkalosis as an increase in pH due to an increase in the HCO3− ion. It is caused
by an increased production of nonvolatile metabolic acids
such as lactic acids or ketoacids, decreased acid excretion by
the kidney, excessive loss of HCO3− as in diarrhea, or an increase in Cl− ion. Metabolic acidosis may present with an increase in anion gap in which sodium bicarbonate is replaced
by the sodium salt of the defending anion or with a normal
anion gap as when HCO3− is replaced by Cl−. Metabolic alkalosis involves factors related to the generation of a loss of H+ ion
or gain of HCO3− ion and those that serve to maintain it by interfering with the excretion of excess HCO3−. Factors that serve
to maintain the alkalosis include decreased glomerular filtration rate, extracellular volume contraction, hypokalemia,
hypochloremia, and aldosterone excess.
Respiratory acidosis reflects an increase in PCO2 levels and
is caused by conditions that impair alveolar ventilation. It can
occur as an acute disorder in which there is a rapid rise in PCO2,
a minimal increase in plasma HCO3−, and a large decrease in
pH. Respiratory alkalosis is caused by conditions that cause
hyperventilation and a reduction in PCO2 levels. Because respiratory alkalosis often occurs suddenly, a compensatory decrease in HCO3− levels may not occur before correction has
been accomplished.
The signs and symptoms of acidosis and alkalosis reflect
alterations in body function associated with the disorder causing the acid-base disturbance, the effect of the change of pH
on body function, and the body’s attempt to correct and maintain the pH within a normal physiologic range. In general, neuromuscular excitability is decreased in acidosis and increased in
alkalosis.
REVIEW EXERCISES
A 34-year-old woman with diabetes is admitted to the
emergency in a stuporous state. Her skin is flushed and
warm, her breath has a sweet odor, her pulse is rapid
and weak, and her respirations are rapid and deep.
Her initial laboratory tests indicate a blood sugar of
320 mg/dL, serum bicarbonate of 12 mEq/L (normal,
24 to 27 mEq/L), and a pH of 7.1.
A. What is the most likely cause of her lowered pH and
bicarbonate levels?
B. How would you account for her rapid and deep respirations?
Disorders of Acid-Base Balance
19
C. Using the Henderson-Hasselbalch equation and the
solubility coefficient for carbon dioxide that is given
in this chapter, what would you expect her PCO2
to be?
D. How would you explain her warm, flushed skin and
stuporous mental state?
A 65-year-old man with chronic obstructive lung disease has been using low-flow oxygen therapy because of
difficulty in maintaining adequate oxygenation of his
blood. He has recently had a severe respiratory tract infection and has had increasing difficulty breathing. He
is admitted to the emergency room because he has become increasingly lethargic and his wife has had trouble arousing him. She relates that he had “turned his
oxygen way up” because of difficulty breathing.
A. What is the most likely cause of this man’s problem?
B. How would you explain the lethargy and difficulty
in arousal?
C. Blood gases, drawn on admission to the emergency
room, indicated a PO2 of 85 mm Hg and a PCO2
of 90 mm Hg. His serum bicarbonate levels were
34 mEq/L. What is his pH?
D. What would be the main goal of his treatment in
terms of acid-base balance?
References
1. Rose B. D. (2001). Clinical physiology of acid-base and electrolyte
disorders (5th ed., pp. 302, 325–364, 578–647, 669). New York:
McGraw-Hill.
2. Guyton A., Hall J. E. (2000). Textbook of medical physiology
(10th ed., pp. 346–363). Philadelphia: W. B. Saunders.
3. Adrogu′e H. E., Adrogu′e H. J. (2001). Acid-base physiology.
Respiratory Care 46, 328–341.
4. Shapiro J. I., Kaehny W. D. (2003). Pathogenesis and management of metabolic acidosis and alkalosis. In Schrier R. W.
(ed). Renal and electrolyte disorders (pp.115–153). Philadelphia:
Lippincott Williams & Wilkins.
5. Rhoades R. A., Tanner G. A. (1996). Medical physiology
(pp. 465–483). Boston: Little, Brown.
6. Metheny N. M. (2000). Fluid and electrolyte balance (4th ed.,
p. 162). Philadelphia: J. B. Lippincott.
7. Kraut J. A., Madias N. E. (2001). Approach to patients with
acid-base disorders. Respiratory Care 46, 392–402.
8. Adrogue H. J., Madias N. E. (1998). Management of lifethreatening acid-base disorders: First of two parts. New England
Journal of Medicine 338, 26–34.
9. Gauthier P. M., Szerlip H. M. (2002). Metabolic acidosis in the
intensive care unit. Critical Care Medicine 18, 289–308.
10. Swenson E. R. (2001). Metabolic acidosis. Respiratory Care 46,
342–353.
11. Rothman S. M. (1999). Mutations of the mitochondrial
genome: Clinical overview and possible pathophysiology of
cell damage. Biochemical Society Symposia 66, 111–122.
12. Howell N. (1999). Human mitochondrial diseases: Answering
questions and questioning answers. International Review of
Cytology 186, 49–116.
13. Uribarri J., Oh M. S., Carroll H. J. (1998). D-Lactic acidosis: A review of clinical presentation, biochemical features and pathophysiological mechanisms. Medicine (Baltimore) 77(2), 73–82.
20
UNIT VIII
Renal Function and Fluids and Electrolytes
14. Umpierrez G. E., DiGirolamo M., Tuvlin J. A., et al. (2000).
Differences in metabolic and hormonal milieu in diabetic and
alcohol-induced ketoacidosis. Journal of Critical Care 15(2),
52–59.
15. Meyer R. J., Beard M. E., Ardagh M. W., Henderson S. (2000).
Methanol poisoning. New Zealand Medical Journal 113(1102),
3–11.
16. Scalley RD., Smart M. L., Archie T. E. (2002). Treatment of
ethylene glycol poisoning. American Family Physician 66,
807–812.
17. Egbert P. A., Abraham K. (1999). Ethylene glycol intoxication:
Pathophysiology, diagnosis, and emergency management.
ANNA Journal 26, 295–300.
18. Brent J., McMartin K., Phillips S. (2001). Fomepizole for treatment of methanol poisoning. New England Journal of Medicine
344, 424–429.
19. Powers F. (1999). The role of chloride in acid-base balance.
Journal of Intravenous Nursing 22, 286–291.
20. Alpern R. J., Sakhaee K. (1997). The clinical spectrum of chronic metabolic acidosis: Homeostatic mechanisms produce
21.
22.
23.
24.
25.
26.
27.
28.
significant morbidity. American Journal of Kidney Diseases 29,
291–302.
Forsythe S. M., Schmidt G. A. (2000). Sodium bicarbonate for
the treatment of lactic acidosis. Chest 117, 260–267.
Khanna A., Kurtzman N. A. (2001). Metabolic alkalosis. Respiratory Care 46, 354–365.
Galla J. H. (2000). Metabolic alkalosis. Journal of the American
Society of Nephrology 11, 369–375.
Adrogue H. J., Madias N. E. (1998). Management of lifethreatening acid-base disorders. New England Journal of Medicine 338, 107–111.
Kaehny W. D. (2003). Pathogenesis and management of respiratory and mixed acid-base disorders. In Schrier R. W. (ed).
Renal and electrolyte disorders (pp. 154–170). Philadelphia:
Lippincott Williams & Wilkins.
Epstein S. K., Singh N. (2001). Respiratory acidosis. Respiratory
Care 46, 366–383.
Laffey J. H., Kavenaugh B. P. (2002). Hypocapnia. New England
Journal of Medicine 347, 43–53.
Foste G. T., Vaziri N. D., Sassoon C. S. H. (2001). Respiratory
alkalosis. Respiratory Care 46, 384–391.