Expansion
Check pH, then carbon dioxide, then bicarbonate, then compensation, then the anion gap
Expansion
- Oxygenation: is the patient hypoxaemic, and what is the A-a gradient
- pH: acidaemia below 7.35, alkalaemia above 7.45
- PaCO2: high with acidaemia means respiratory acidosis; low with alkalaemia means respiratory alkalosis
- Bicarbonate or base excess: low with acidaemia means metabolic acidosis; high with alkalaemia means metabolic alkalosis
- Compensation: is it appropriate for the primary disorder, and is it acute or chronic
- Anion gap if there is a metabolic acidosis
- Delta ratio if the anion gap is raised, to detect a coexisting disorder
The key discipline is identifying the primary disorder from the pH: whichever abnormality would move the pH in the observed direction is primary, and the other is compensation.
Compensation is never complete, so a pH that has returned exactly to 7.40, or crossed to the opposite side, indicates a mixed disorder. Likewise, compensation outside the expected range signals a second process.