Arterial blood gases

information about the various parameters measured in ABG

1. PaO 2 PaO 2 - partial pressure of oxygen dissolved in blood plasma. PaO 2 reflects only 3% of total oxygen in blood. PaO 2 : 80 – 100mm of hg. SaO 2 - 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 PaO 2, SaO 2 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 HCO 3 CO 2

3. PaCO 2 Partial pressure of carbon dioxide dissolved in blood plasma. PaCO 2 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 PaCO 2 less than 35 defines respiratory alkalosis PaCO 2 greater than 45 defines respiratory acidosis

4. HCO 3 - HCO 3 - 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.