Expansion
pH is determined by the strong ion difference, weak acids and carbon dioxide
Expansion
Stewart argued that hydrogen ion concentration is a dependent variable, determined by three independent ones:
- Strong ion difference: mainly (sodium + potassium) minus chloride, normally about 40 mmol/l. A narrower difference means acidosis
- Total weak acid concentration, chiefly albumin and phosphate. Less albumin means alkalosis
- PaCO2
Practical insights this gives that the traditional approach does not:
- 0.9 per cent saline contains 154 mmol/l each of sodium and chloride, a strong ion difference of zero. Infusing it narrows the plasma difference and causes a hyperchloraemic metabolic acidosis, without any acid having been given
- Hypoalbuminaemia in critical illness causes a metabolic alkalosis that can mask a coexisting acidosis, which is why the anion gap must be corrected for albumin
- Balanced crystalloids such as Hartmann’s solution have a strong ion difference closer to physiological and avoid this
The approaches are complementary rather than contradictory: the traditional method is quicker at the bedside, while Stewart explains mechanisms the traditional method cannot.