Download Quality Assurance of PH and Blood Gas Determination

Blood Gases, pH, and
Buffer system
Introduction
• Acid: substances that yields H ions in H2O.
• Base: yields a hydroxyl ion (OH).
• A buffer: the combination of a weak acid or weak base and its
salt, is a system that resists changes in pH.
Acid-Base balance:
Maintenance of hydrogen ions:
• Body produces 40-80 mmol of H/day,
• Normal concentration of H in ECF ranges from 36-44 nmol of
hydrogen ion.
• Any deviation from the values. the body will try to compensate.
• however, through metabolism, the body produces
much greater quantities of H+. Through exquisite
mechanisms that involve the lungs and kidneys, the
body controls and excretes H in order to maintain pH
homeostasis.
• Any H+ value outside this range will cause alterations
in the rates of chemical reactions within the cell and
affect the many metabolic processes of the body, e.g.
• >44 nmol/L: altered awareness, coma- death
• <36 nmol/L: neuromuscular irritability, tetany, loss of
awareness, death.
Concentration of H ions and pH
•
•
•
•
•
Reciprocal relationship in the concentration of H ions
and pH
Increase pH: decrease in H ion
Decrease pH: increase H ions
The reference value for arterial blood pH is 7.40 and
is equivalent to an H concentration of 40 nmol/L.
Arterial blood pH is controlled by:
1. Buffers
2. Respiratory System
3. Kidneys
Buffer System: Regulation of H+
• First line of defense to changes in H+ concentration is the buffer
systems present in all body fluids. All buffers consist of a weak
acid carbonic acid (H2CO3) & its salt (HCO3) bicarbonate.
• Add acid to the bicarbonate-carbonic acid system- the HCO3
combines with H from the acid to form H2CO3.
• Add a base to the system, H2CO3 combines with OH to form
H2O and HCO3
• Keeps the body at the correct pH (7.35-7.45).
• Bicarbonate: carbonic acid system has low buffering capacity
but still an important buffer system for 3 reasons
1. H2CO3 dissociates into CO2 & H2O allowing H+ to be
eliminated as CO2 by lungs
2. Changes in PCO2 modify the ventilation rate
3. HCO3 conc. can be altered by kidney
Other systems
• HPO4, H2PO4system: plays a role in plasma and red
blood cells and is involved in the exchange of sodium
ion in the urine H+ filtrate.
• Proteins are capable of binding H+: Most circulating
proteins have a net negative charge and are capable of
binding H.
• The lungs regulate pH through retention or elimination
of CO2 by changing the rate and volume of ventilation
and the kidneys regulate pH by excreting acid, primarily
in the ammonium ion,
Regulation of Acid-Base Balance: Lungs
• Carbon dioxide, the end product of most aerobic
metabolic processes, easily diffuses out of the tissue
where it is produced and into the plasma and red cells
in the surrounding capillaries.
• In plasma, a small amount of CO2 is physically
dissolved or combined with proteins to form
carbamino compounds.
• Most of the CO2 combines with H2O to form H2CO3,
which quickly dissociates into H+ and HCO3
Regulation of Acid-Base Balance: Lungs
 RBC regulation
• CO2 and O2 exchange, some CO2 remains in the RBC in
combination to HB (carboxyhemoglobin)
• CO2 combines to water to form carbonic acid and is transported
in the blood.
• Carbonic anhydrase enzymes in the RBC accelerate this process
• (CO2 + H2O→H2CO3(.
• The dissociation of H2CO3 causes the HCO3 concentration to
increase in the red cells and diffuse into the plasma.
• To maintain electroneutrality (the same number of positively
and negatively charged ions on each side of the red cell
membrane), chloride diffuses into the cell. This is known as the
chloride shift. Plasma proteins and plasma buffers combine with
the freed H to maintain a stable pH.
• Plasma proteins and plasma buffers combine with the
freed H+ to maintain a stable pH.
• In the lungs, the process is reversed. Inspired O2
diffuses from the alveoli into the blood and is bound
to hemoglobin, forming oxyhemoglobin (O2Hb).
• The H+ that was carried on the (reduced) hemoglobin
in the venous blood is released to recombine with
HCO3 to form H2CO3, which dissociates into H2O
and CO2.
• The CO2 diffuses into the alveoli and is eliminated
through ventilation.
Interrelationship of the bicarbonate and hemoglobin buffering systems.
• When the lungs do not remove CO2 at the rate of its
production (as a result of decreased ventilation or
disease), it accumulates in the blood, causing an
increase in H+ concentration.
• If, however, CO2 removal is faster than production
(hyperventilation), the H+ concentration will be
decreased
Acid-Base Disorders
• Acidosis (decrease pH) → acidemia
• vs. Alkalosis (increased pH) → alkalemia
• Metabolic (kidneys) or respiratory (Lung)
• Inadequate elimination and excess production of CO2
in the body.
• Body compensates by respiration rate and kidney.
Acidosis
1. Metabolic (non-respiratory) Acidosis
 Metabolic acidosis is defined as a bicarbonate level of less
than 22 mEq/L with a pH of less than 7.35.
 Reduce excretion of acids (Diarrhea and intestinal fistulas
may cause decreased levels of base).
 the treatment of metabolic acidosis is dependent upon the
cause.
 Causes of increased acids include:
•
•
•
•
Renal failure
Diabetic ketoacidosis
Anaerobic metabolism
Starvation
Acidosis
2. Respiratory Acidosis:
 Is defined as a pH less than 7.35 with a PaCO2 greater than
45 mm Hg.
 Caused by hypoventilation (decrease the elimination of CO2
in the lungs, it builds up in the blood)
 In plasma → increase in CO2, decrease in pH, increase in H
and HCO3
 Diseases: emphysema, drugs , congestive heart failure,
bronchopneumonia.
• Respiratory compensation
Hyperventilation
• Renal compensation
Increase H excretion & increase reabsorption of
HCO3-
Signs and Symptoms of Respiratory Acidosis
Pulmonary
• dyspnea
• respiratory distress
Neurological
• headache
• Restlessness (‫)االرق‬
• confusion
Cardiovascular
• tachycardia
Alkalosis
1. Metabolic alkalosis:
 Metabolic alkalosis is defined as a bicarbonate
level greater than 26 mEq/liter with a pH greater
than 7.45.
 An excess of base or a loss of acid within the
body can cause metabolic alkalosis.
 Metabolic alkalosis is one of the most difficult
acid-base imbalances to treat. Bicarbonate
excretion through the kidneys can be stimulated
with drugs such as acetazolamide (Diamox®), but
resolution of the imbalance will be slow.
Signs and Symptoms of Metabolic Alkalosis
Pulmonary
• Respiratory depression
Neurological
• Seizures
• coma
Musculoskeletal
• Weakness
• muscle cramps
• tetany
Gastrointestinal
• Nausea
• vomiting
Alkalosis
2. Respiratory alkalosis:
 Respiratory alkalosis is defined as a pH greater
than 7.45 with a PaCO2 less than 35 mm Hg.
 Any condition that causes hyperventilation can
result in respiratory alkalosis.
These conditions include:
•
•
•
Increased metabolic demands, such as fever, sepsis,
pregnancy.
Medications, such as respiratory stimulants
Central nervous system lesions
Signs and Symptoms of Respiratory Alkalosis
Neurological
•
•
•
•
light-headedness
confusion
inability to concentrate
blurred vision
• Cardiovascular
• palpitations
• diaphoresis (heavy sweating)
• Miscellaneous
• dry mouth
• tetanic spasms of the arms and legs
• Blood gases: is a measurement of how much oxygen
and carbon dioxide is in your blood. It also
determines the acidity (pH) of your blood.
• Blood gas measurements are used to evaluate a
person's lung function and acid/base balance.
• arterial blood gas (ABG) test: measures the acidity
(pH) and the levels of oxygen and carbon dioxide in
the blood from an artery. This test is used to check
how well your lungs are able to move oxygen into the
blood and remove carbon dioxide from the blood.
Blood Gas measures
• Partial pressure of oxygen (PaO2): This measures the
pressure of oxygen dissolved in the blood.
• Partial pressure of carbon dioxide (PaCO2): This
measures how much carbon dioxide is dissolved in the
blood.
• pH: The pH measures hydrogen ions (H+) in blood.
• Bicarbonate (HCO3): Bicarbonate is a chemical
(buffer) that keeps the pH of blood from becoming too
acidic or too basic.
• Oxygen saturation (O2Sat) values: Oxygen saturation
measures how much of the hemoglobin in the red blood
cells is carrying oxygen (O2).
Why the Test is Performed
• The test is used to evaluate respiratory diseases and
conditions that affect the lungs.
• It helps determine the effectiveness of oxygen therapy.
• The test also provides information about the body's
acid/base balance, which can reveal important clues
about lung and kidney function and the body's general
metabolic state.
• Check for severe breathing problems and lung
diseases, such as asthma, cystic fibrosis.
• Find out if you need extra oxygen or help with
breathing (mechanical ventilation).
Sampling
• Usually, Blood is most commonly drawn from
the radial artery because it is easily accessible.
• The femoral artery is also used, especially during
emergency situations or with children.
• The health care provider will insert a small needle
through the skin into the artery. You can choose to
have numbing medicine (anesthesia) applied to the site
before the test begins.
• In some cases, blood from a vein may be used, the
sample must be quickly sent to a laboratory for
analysis to ensure accurate results.
• There are plastic and glass syringes used for blood gas
samples. Most syringes come pre-packaged and contain
a small amount of heparin, to prevent coagulation.
• Once the sample is obtained, care is taken to eliminate
visible gas bubbles, as these bubbles can dissolve into
the sample and cause inaccurate results.
• If a plastic blood gas syringe is used, the sample should
be transported and kept at room temperature and
analyzed within 30 min.
• Heparin is the only anticoagulant that is suitable for pH
and blood gas determination.
• The specimen should delivered immediately to the
laboratory and analyzed.
• Normal specimen stable for to 2 hours
How It Feels
• Collecting blood from an artery is more painful than
collecting it from a vein because the arteries are
deeper and are protected by nerves.
• Most people feel a brief, sharp pain as the needle to
collect the blood sample enters the artery. If you are
given a local anesthetic, you may feel nothing at all
from the needle puncture, or you may feel a brief
sting or pinch as the needle goes through the skin.
• You may feel more pain if the person drawing your
blood has a hard time finding your artery, your artery
is narrowed, or if you are very sensitive to pain.
Risks
• There is very little risk when the procedure is done correctly.
• Taking blood from some people may be more difficult than
from others.
• Other risks associated with this test may include:
•
•
•
•
•
•
•
Bleeding at the puncture site
Blood flow problems at puncture site (rare)
Bruising at the puncture site
Delayed bleeding at the puncture site
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
Error in the collection of blood gas specimen:
Syringes
• Most of the plastic syringes available today are acceptable
Inexpensive, disposable, non- breakable syringe are
preferable to glass syringes for cost and safety reasons.
• Sample should not be sent to the lab with the needle still on
the syringe.
Vacuum tube
• Vacuum collection tubes are to be avoided.
• There will be dead air space after the tube filled and this will
result of decreasing of pO2 and pCO2.
• The tube must be opened.
31
Over dilution of specimen with Heparin
• An excessive amount of heparin will dilute the
specimen
• An insufficient amount of heparin will result of small
fibrin clot, which are capable of disabling pH\blood
gas analyzer.
• Kits with syringes containing premeasured amount of
powdered heparin are available.
• CO2 will travel to plasma to air space and resulting in
decrease of pCO2 and increase of pH
Blood gases (Result)
• Partial pressure of oxygen (PaO2) :Greater than 80 mm
Hg .
• Partial pressure of carbon dioxide (PaCO2): 35-45 mm
Hg .
• pH: 7.35-7.45.
• Bicarbonate (HCO3): 23-30 mEq/L, (23-30 mmol/L).
• Oxygen saturation (O2Sat): 95%-100% .
• Remember: pCO2 >45 = acidosis, pCO2 <35 = alkalosis.
• Remember: HCO3 > 26 = alkalosis, HCO3 < 22 =
acidosis.
Gas content in lungs and pulmonary and systemic circulation.
What Affects the Test
• Reasons you may not be able to have the test or why the
results may not be helpful include the following:
• You have a fever or an abnormally low body
temperature (hypothermia).
• You have a disease that affects how much oxygen is
carried in your blood, such as severe anemia.
• You smoke just before the test or breathe secondhand
smoke, carbon monoxide, or certain paint or varnish
removers in closed or poorly ventilated areas.
Factors effecting the affinity of Hb for O2
Measurements
Spectrophotometric ( co-oximeter)
• Determination of Oxygen Saturation.
• Designed to directly measure the various hemoglobin
species.
• Each species of hemoglobin has a characteristic
absorbance curve.
• The number of hemoglobin species measured will
depend on the number and specific wavelengths
incorporated into the instrumentation.
• For example, two-wavelength instrument systems can
measure only two hemoglobin species (i.e., O2Hb and
HHb).
Optical absorption of hemoglobin fractions.
• Because the primary purpose of determining O2Hb is
to assess oxygen transport from the lungs, it is best to
stabilize the patient’s ventilation status before blood
sample collection.
• An appropriate waiting period before the sample is
drawn should follow changes in supplemental O2 or
mechanical ventilation.
• All blood samples should be collected under
anaerobic conditions and mixed immediately with
heparin or other appropriate anticoagulant.
Blood Gas Analyzer
• Use electrode method for sensing and measuring:
PO2
PCO2
pH
• The pO2 measurement is amperometric, meaning that
the amount of current flow is an indication of the
oxygen present.
• The pCO2 and pH measurements are potentiometric,
in which a change in voltage indicates the activity of
each analyte.
Errors with the Instrument
• Incorrect calculation of gas % partial pressures for standard
gases used for calibration
• Improperly equilibrated gas mixture
• Bubble in measuring chamber
• Bacterial growth in measuring chamber on electrode
membranes :
• Protein coating membrane
• Instrument temperature not maintained at a consistent or
correct value
• Contaminated buffers and calibrating gases
• Defective, aging or improperly maintained electrodes
• Improperly grounded instrument (causes electrode drift)
Steps to an Arterial Blood Gas Interpretation
• Step One: Identify whether the pH, pCO2 and HCO3
are abnormal. For each component, label it as
“normal”, “acid” or “alkaline”.
The two matching values determine what the problem is. In this case,
an ACIDOSIS.
Step Two
• If the ABG results are abnormal, determine if the
abnormality is due to the kidneys (metabolic) or the
lungs (respiratory).
Match the two abnormalities: Respiratory (lung problem) +
Acidosis = Respiratory Acidosis.
Example One: John Doe is a 55 year-old male admitted
to your nursing unit with recurring bowel obstruction.
He has been experiencing intractable vomiting for the
last several hours despite the use of antiemetics. His
arterial blood gas result is as follows:
Alkaline
Normal
Alkaline
(MetabolicAlkalosis)
Lungs
kidney
Example 2: Jane Doe is a 55-year-old female admitted
to your nursing unit with sepsis. Here is her arterial
blood gas result: pH 7.31, pCO2 39, HCO3 17
Acidosis
Normal
Acidosis
(Metabolic Acidosis.)
Lung
kidneys
Example 3: Jane Doe is a 19 year-old female admitted
to your nursing unit with head injury. Her blood gas
• for the first time, that both the pCO2 and the HCO3 are
abnormal.
Step Two
• If both the pCO2 and the HCO3 are abnormal, but the
pH is in the normal range, we are going to use the
single value of 7.40 as our only “normal”.
• Any pH of <7.40 is now going to be considered
acidosis.
• Any pH > 7.40 is now going to be considered
alkalosis.
• Look at our pH in this example. The pH is <7.40.
• The two matching values determine what the problem is. In
this case, an ACIDOSIS.
• Match the two abnormalities: Respiratory (lungs) + Acidosis
= Respiratory Acidosis