Arterial blood gas interpretation - acid-base disorder classification
Core concept
- Henderson-Hasselbalch: pH determined by HCO3- (metabolic, renal) over pCO2 (respiratory, ventilatory)
- Five steps, in order
- 1. pH - acidaemia (<7.35) or alkalaemia (>7.45)? A normal pH does not exclude a mixed disorder
- 2. Primary disorder - which of HCO3- or pCO2 moves in the direction that explains the pH?
- 3. Compensation - calculate the expected value. *Compensation never overshoots and rarely fully normalises pH; if it has, there is a second disorder*
- 4. Anion gap = Na - (Cl + HCO3); normal 8-12 (add ~2.5 per 10 g/L fall in albumin - hypoalbuminaemia hides a gap)
- 5. Delta-delta if the gap is raised - (rise in AG) / (fall in HCO3)
- <0.4 = concurrent NAGMA; ~1-2 = pure HAGMA; >2 = concurrent metabolic alkalosis
3 more sections, plus exam facts
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