A combined respiratory acidosis / metabolic alkalosis will result in elevated PaCO2 and serum bicarbonate. Which process is the primary disorder (e.g. primary respiratory acidosis with metabolic compensation versus primary metabolic alkalosis with respiratory compensation) is dependent on the pH – in an acidotic patient, the acidosis is primary (and the alkalosis is compensatory) and vice versa.
Compensation behaves in accordance with the following rules:
Metabolic Acidosis: As bicarbonate goes from 10 to 5, pCO2 will bottom out at 15.
pCO2 = 1.5 x [HCO3-] + 8 (or ↓ pCO2 = 1.25 x Δ[HCO3-])
Metabolic Alkalosis: compensation here is less because CO2 is driving force for respiration.
pCO2 = 0.7 x [HCO3-] + 21 (or ↑ pCO2 = 0.75 x Δ[HCO3-])
Acutely: ↑ [HCO3-] = 0.1 x Δ pCO2 or ↓ pH = 0.008 x Δ pCO2 Chronically: ↑ [HCO3-] = 0.4 x Δ pCO2 or ↓ pH = 0.003 x Δ pCO2
Respiratory Alkalosis: Metabolic compensation will automatically be retention of chloride (i.e., hyperchloremic, usually referred to as “loss of bicarb” although it is the strong ion difference that matters). If you have an anion gap, then you’ve automatically got a little bit of an acidosis on top of the compensation (because the compensation should be a NON-gap acidotic process.
Acutely: ↓ [HCO3-] = 0.2 x Δ pCO2 (or ↑ pH = 0.008 x Δ pCO2) Chronically: ↓ [HCO3-] = 0.4 x Δ pCO2 (or ↑ pH = 0.017 x Δ pCO2)