Synopsis
Identify primary and mixed metabolic acidosis systematically, distinguish high-gap from bicarbonate-loss mechanisms and treat the time-critical cause.
- Confirm the primary process from pH, bicarbonate and carbon dioxide: metabolic acidosis lowers bicarbonate, while appropriate respiratory compensation lowers carbon dioxide rather than normalising pH completely.
- Calculate anion gap as sodium minus chloride plus bicarbonate using values from the same sample; a raised gap signals unmeasured anions but a normal gap does not mean mild disease.
- Low albumin lowers the expected anion gap, so correct or interpret the gap for albumin in critical illness before dismissing lactate, ketones, renal acids or toxins.
Key red flags
Deep rapid breathing reflects respiratory carbon dioxide clearance; slowing respiration, exhaustion or reduced consciousness during severe acidosis signals impending decompensation.
Investigation priorities
Confirm acidemia, quantify bicarbonate and carbon dioxide and identify respiratory compensation or a mixed disorder.
Management branches
A gas shows low pH, low bicarbonate or an unexpected carbon dioxide concentration.
- Confirm sampling context, pH and primary bicarbonate reduction, then compare carbon dioxide with expected compensation to identify respiratory failure or additional respiratory alkalosis.
- Calculate an albumin-aware anion gap from the matching chemistry sample and measure lactate, ketones and renal indices directly rather than relying on a mnemonic alone.
Key medicines
Intravenous sodium bicarbonateFor a defined severe metabolic acidosis indication, calculate and titrate the amount to the clinical base deficit using the current BNF, product and cause-specific protocol under senior critical-care, renal or toxicology supervision.