The Systematic Approach
- Check the pH — acidemia <7.35, alkalemia >7.45
- Check PaCO2 — this is the respiratory component
- Check HCO3- — this is the metabolic component
- Find which component moved in the same direction as the pH abnormality — that's your primary disorder
- Check whether the other system is compensating
- If metabolic acidosis is present, calculate the anion gap
- Check PaO2 for hypoxemia
Acidosis vs. Alkalosis at a Glance
| Disorder | pH | Primary change | Compensation |
|---|---|---|---|
| Respiratory acidosis | Low | High PaCO2 | High HCO3- (renal, slow) |
| Respiratory alkalosis | High | Low PaCO2 | Low HCO3- (renal, slow) |
| Metabolic acidosis | Low | Low HCO3- | Low PaCO2 (resp, fast) |
| Metabolic alkalosis | High | High HCO3- | High PaCO2 (resp, fast) |
Worked Examples
Example 1: pH 7.25, PaCO2 60 mmHg, HCO3- 26 mEq/L. pH is low. PaCO2 is high, moving the same direction: primary respiratory acidosis. HCO3- only slightly up, so compensation is incomplete. Think COPD exacerbation or opioid overdose.
Example 2: pH 7.30, PaCO2 25 mmHg, HCO3- 12 mEq/L. pH is low. HCO3- is low, moving the same direction: primary metabolic acidosis. PaCO2 is low, showing respiratory compensation. Next step: anion gap. Think DKA or lactic acidosis.