Download respiratory compensation - King Edward Medical University

RESPIRATORY MODULE
RESPIRATORY MODULE
FAWAD AHMAD RANDHAWA
MBBS ( King Edward Medical College)
M.C.P.S; F.C.P.S. ( Medicine)
F.C.P.S. ( Endocrinology)
Assistant Professor of Endocrinology
King Edward Medical University
Objectives
At the end of this session the participants should be able to:
• Grasp a clear concept governing acid base homeostasis and
Clinical implementation of acid base concepts
• Identify basic pathophysiology of respiratory failure
Learning outcomes
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Basic terminologies
Normal buffers of the body
Respiratory component of acid base homeostasis
Compensatory mechanisms in acid base disorders
Definition of respiratory failure with its types
The Body and pH
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Homeostasis of pH is tightly controlled
Extracellular fluid = 7.4
Blood = 7.35 – 7.45
< 6.8 or > 8.0 death occurs
Acidosis (acidemia) below 7.35
Alkalosis (alkalemia) above 7.45
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Terms
• Acid
– Any substance that can yield a hydrogen ion (H+) or hydronium ion when
dissolved in water
– Release of proton or H+
• Base
– Substance that can yield hydroxyl ions (OH-)
– Accept protons or H+
Terms
• Buffer
– Combination of a weak acid and /or a weak base and its salt
– What does it do?
• Resists changes in pH
Terms
• Acidosis
– pH less than 7.35
• Alkalosis
– pH greater than 7.45
• Note: Normal pH is 7.35-7.45
Acid-Base Balance
• Function
– Maintains pH homeostasis
– Maintenance of H+ concentration
• Potential Problems of Acid-Base balance
– Increased H+ concentration yields decreased pH
– Decreased H+ concentration yields increased pH
Blood Buffer Systems
• Why do we need them?
– If the acids produced in the body from the catabolism of food and
other cellular processes are not removed or buffered, the body’s
pH would drop
– Significant drops in pH interferes with cell enzyme systems.
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Small changes in pH can produce major
disturbances
• Most enzymes function only with narrow pH ranges
• Acid-base balance can also affect electrolytes (Na+, K+, Cl-)
• Can also affect hormones
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The body produces more acids than bases
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Acids take in with foods
Acids produced by metabolism of lipids and proteins
Cellular metabolism produces CO2.
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-
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Control of Acids
1. Buffer systems
Take up H+ or release H+ as conditions change
Buffer pairs – weak acid and a base
Exchange a strong acid or base for a weak one
Results in a much smaller pH change
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Regulation of pH
• Weak acids good buffers since they can tilt a
reaction in the other direction
• Strong acids are poor buffers because they make
the system more acid
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Respiratory mechanisms
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Exhalation of carbon dioxide
Powerful, but only works with volatile acids
Doesn’t affect fixed acids like lactic acid
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3Body pH can be adjusted by changing rate and depth of
breathing
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Kidney excretion
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Can eliminate large amounts of acid
Can also excrete base
Can conserve and produce bicarb ions
Most effective regulator of pH
If kidneys fail, pH balance fails
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Rates of correction
• Buffers function almost instantaneously
• Respiratory mechanisms take several minutes to hours
• Renal mechanisms may take several hours to days
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Acid base abnormalities- Causes
• Respiratory
– abnormal processes which tend to alter pH because of a primary
change in pCO2 levels
• acidosis
• alkalosis
• Metabolic
– abnormal processes which tend to alter pH because of a primary
change in [HCO3-]
• acidosis
• alkalosis
Acid-Base Imbalances
• pH< 7.35 acidosis
• pH > 7.45 alkalosis
• The body response to acid-base imbalance is called
compensation
• May be complete if brought back within normal limits
• Partial compensation if range is still outside norms.
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Compensation
• If underlying problem is metabolic, hyperventilation or
hypoventilation can help : respiratory compensation.
• If problem is respiratory, renal mechanisms can bring about
metabolic compensation.
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Respiratory Acidosis
• Carbonic acid excess caused by blood levels of CO2 above
45 mm Hg.
• Hypercapnia – high levels of CO2 in blood
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Compensation for Respiratory Acidosis
• Kidneys eliminate hydrogen ion and retain bicarbonate ion
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Respiratory Alkalosis
• Carbonic acid deficit
• pCO2 less than 35 mm Hg (hypocapnea)
• Most common acid-base imbalance
• Primary cause is hyperventilation
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Compensation of Respiratory Alkalosis
• Kidneys conserve hydrogen ion
• Excrete bicarbonate ion
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Diagnosis of Acid-Base Imbalances
1. Note whether the pH is low (acidosis) or high (alkalosis)
2. Decide which value, pCO2 or HCO3- , is outside the normal range
and could be the cause of the problem. If the cause is a change in
pCO2, the problem is respiratory. If the cause is HCO3- the problem
is metabolic.
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3. Look at the value that doesn’t correspond to the
observed pH change. If it is inside the normal range,
there is no compensation occurring. If it is outside the
normal range, the body is partially compensating for
the problem.
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Example
• A patient is in intensive care because he suffered a severe myocardial
infarction 3 days ago. The lab reports the following values from an
arterial blood sample:
• pH 7.3
• HCO3- = 20 mEq / L ( 22 - 26)
• pCO2 = 32 mm Hg (35 - 45)
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Diagnosis
• Metabolic acidosis
• With compensation
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RESPIRATORY FAILURE
Definition
• Inability of the lung to meet the metabolic
demands of the body.
• This can be from failure of tissue oxygenation
and/or failure of CO2 homeostasis.
Diagnosis
• PaO2 <60 mmHg while breathing air,
• or a PaCO2 >50 mmHg.
Respiratory system includes:
CNS (medulla)
Peripheral nervous system (phrenic nerve)
Respiratory muscles
Chest wall
Lung
Upper airway
Bronchial tree
Alveoli
Pulmonary vasculature
Potential causes of Respiratory Failure
RESPIRATORY FAILURE- TYPE 1
• PaO2 <60mmHg with normal or low PaCO2 
normal or high pH
• Most common form of respiratory failure
• Lung disease is severe to interfere with
pulmonary O2 exchange, but over all ventilation is
maintained
• Physiologic causes: V/Q mismatch and shunt
Causes of Hypoxemic Respiratory
failure
• Caused by a disorder of heart, lung or blood.
Respiratory Failure -Type II
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PaCO2 >50 mmHg
Hypoxemia is always present
pH depends on level of HCO3
HCO3 depends on duration of hypercapnia
Renal response occurs over days to weeks
Take Home Message
• Body is naturally tuned up to maintain pH of the
blood between 7.35-7.45
• This maintenance is done with the help of buffers
• Buffers are of two types:
– Chemical and physiological
• The process of correcting any acid base disorder is
called compensation
Take Home Message
• Compensation can be either respiratory or
metabolic(kidneys)
• Respiratory failure are of two types
• pCO2 helps to classify the type of respiratory failure