Emergency Medicine

124 Acid–Base, Electrolyte and Renal Emergencies

Arterial blood gas interpretation

Blood gas analysis provides information regarding potential primary and

compensatory processes that affect the body’s acid–base buffering system.

Acidosis is an abnormal process that increases the serum hydrogen ion concen-

tration, lowers the pH, and results in acidaemia.

Alkalosis is an abnormal process with decrease in the hydrogen ion concentra-

tion, resulting in alkalaemia.

1 Blood gas analysis is used to:

(i) Determine the adequacy of oxygenation and ventilation.

(ii) Assess the respiratory function.

(iii) Determine the acid–base balance.

2 Interpret the arterial blood gas result in a stepwise manner as follows (see

Table 3.1):

(i) Determine the adequacy of oxygenation (PaO 2 ):

(a) normal range 80–100 mmHg (10.6–13.3 kPa)

(b) provides direct evidence of hypoxaemia

(c) determine if there is a raised A-a gradient due to VQ

mismatch/shunting, if there is a lower than expected PaO 2

ACID–BASE DISTURBANCES

Table 3.1 Determining the likely acid–base disorder from the pH, PaCO2 and HCO3

pH PaCO 2 HCO 3 Acid–base disorder

‚ N ‚ Primary metabolic acidosis

‚‚ ‚Metabolic acidosis with respiratory compensation

‚·N Primary respiratory acidosis

‚· ·Respiratory acidosis with renal compensation

‚· ‚Mixed metabolic and respiratory acidosis

·‚N Primary respiratory alkalosis

·‚ ‚Respiratory alkalosis with renal compensation

· N · Primary metabolic alkalosis

·· ·Metabolic alkalosis with respiratory compensation

·‚ ·Mixed metabolic and respiratory alkalosis

Note: respiratory compensation occurs rapidly by changes in PaCO 2. Renal

compensation occurs more slowly by changes in HCO 3.

N, normal.