information about the various parameters measured in ABG

1. PaO2 - partial pressure of oxygen dissolved in blood
plasma.
PaO2 reflects only 3% of total oxygen in blood.
PaO2 : 80 – 100mm of hg.
SaO2 - represents oxygen combined with available
haemoglobin – approximately 97% of total oxygen in
blood
Hence the oxygenation status of the patient can be
properly assessed based on 3 elements
PaO2, SaO2 and Haemoglobin

2. pH
 pH is the hydrogen ion concentration in plasma. Ideal
value for pH is 7.40
 Condition were pH lower than 7.35 is called acidosis and
above 7.45 is called alkalosis
You don’t have to remember the equation given below as the pH is already calculated for you by the blood gas
machine.
Henderson – Hasselbalch equation for blood pH
pH = pK + log HCO3
CO2

3. PaCO2
 Partial pressure of carbon dioxide dissolved in blood
plasma. PaCO2 value indicates whether the patient can
ventilate well enough to rid the body of carbon dioxide
produced as a consequence of metabolism.
 Normal range : 35 – 45 mm of Hg
 PaCO2 less than 35 defines respiratory alkalosis
 PaCO2 greater than 45 defines respiratory acidosis

4. HCO3
-
 HCO3
- bicarbonate level is the acid base component
that reflects kidney function.
 Normal range: 22 – 26mEq/L
 Bicarbonate is increased or decreased in the blood
plasma by renal mechanism.
 Bicarbonate level less than 22 defines metabolic
acidosis and greater than 26 defines metabolic
alkalosis.

5. Base Excess and Base deficit
 Base excess and base deficit reflect the non respiratory
contribution to acid-base balance
 Normal range: -2mmol/L to +2mmol/L.
 Negative base level is reported as base deficit and is
associated with metabolic acidosis
 Positive base level is reported as base excess and is
associated with metabolic alkalosis.
 Bicarbonate level and base level goes in the same
direction

6. Anion Gap
 Anion gap is computed by subtracting the major plasma anions
(chloride and bicarbonate) from the major plasma cations
(sodium).
 Normal range: 8 to 16 mEq/L. This value represents the
amount of unmeasured anions such as ketones or lactate in
plasma.
 Plasma anion gap = (Na+ + K+) – (Cl- + HCO3-)
A non-anion gap metabolic acidosis can occur through the loss of
bicarbonate and the retention of chloride ion (hyperchloremic
metabolic acidosis) . Clinically a non-anion gap acidosis is
associated with diarrhoea, renal failure, hyperalimentation etc.

7. Oxyhaemoglobin dissociation curve
The relationship between the two axes of this curve assumes normal values for
haemoglobin, pH, temperature and PCO2 and
In the tissues, the oxygen-haemoglobin curve dissociation curve shifts to the right. As
pH
decreases, PCO2 increases, or temperature rises, the curve shifts to the right, resulting
in an increased release of oxygen.
In the lungs, the oxygen-haemoglobin curve dissociation curve shifts to the left. As pH
increases, PCO2 decreases, or temperature falls, the curve shifts to the left, resulting
in an increased ability of haemoglobin to pick up oxygen.