Working out the expected compensatory response to an acid base disturbance often reveals a second acid-base problem that was otherwise hidden. I regularly use Winter’s formula when I see a metabolic acidosis, but I have trouble remembering the others, so here they are, from Harwood-Nuss’ Clinical Practice of Emergency Medicine (apologies if you ‘think’ in kilopascals):
Formulas Describing Expected Compensatory Response to Primary Acid–Base Disturbances
Simple Metabolic Acidosis

• Predicted decreased PCO2 mm Hg = 1.2 × Δ(HCO3-) mEq/L
• Predicted PCO2 mm Hg = 1.5(HCO3-) mEq/L + 8 ± 2
• Anticipated PCO2 approximates last two digits of arterial pH

Simple Metabolic Alkalosis

• Predicated increased Δ PCO2 mm Hg = 0.67 × Δ(HCO3-) mEq/L

Simple Acute Respiratory Acidosis

• Predicted decreased ΔpH units = 0.8 × Δ PCO2 mm Hg
• Predicted increased Δ(HCO3-) mEq/L = 0.1 × Δ PCO2 mm Hg

Simple Chronic Respiratory Acidosis

• Predicted decreased ΔpH units = 0.3 × Δ PCO2 mm Hg
• Predicted increased Δ(HCO3-) mEq/L = 0.35 × Δ PCO2 mm Hg

Simple Acute Respiratory Alkalosis

• Predicted increased ΔpH units = 0.8 × Δ PCO2 mm Hg
• Predicted decreased Δ(HCO3-) mEq/L = 0.2 × Δ PCO2 mm Hg

Simple Chronic Respiratory Alkalosis

• Predicted increased ΔpH units = 0.17 × Δ PCO2 mm Hg
• Predicted decreased Δ(HCO3-) mEq/L = 0.5 × Δ PCO2 mm Hg