Respiratory Acid–Base Disorders

• With acute respiratory acidosis, pH falls 0.08 for every 10 mmHg rise in PaCO₂ (chronic: 0.03).
• Hypercapnia plus hypoxemia signals alveolar hypoventilation ± V/Q mismatch; address the driver (obstruction, NM weakness, CNS depression).
• ICU respiratory alkalosis comes from early sepsis, PE, pain, over-ventilation, or pregnancy; never just sedate the sepsis hyperventilator.
• Permissive hypercapnia is a lung-protective strategy, not a therapeutic target.
• Winter's formula for metabolic acidosis: expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ±2.

Evaluating Respiratory Acid–Base Disturbances

• Step 1 — Obtain an ABG plus basic metabolic panel and confirm internal validity: draw the ABG and verify serum bicarbonate [HCO₃⁻] via the Henderson–Hasselbach equation: [H⁺] = 24 × PaCO₂/[HCO₃⁻]. The [HCO₃⁻] on the ABG and basic metabolic panel must match within 2 mmol/L
• Step 2 — Identify the primary acid–base disturbance. A primary respiratory disorder exists when PaCO₂ is abnormal and pH shifts the opposite way: respiratory acidosis = PaCO₂ >45 mm Hg with pH <7.35 · respiratory alkalosis = PaCO₂ <35 mm Hg with pH >7.45
• Step 3 — Classify the respiratory disorder as uncompensated (acute) versus compensated (chronic). pH moves toward, but never fully reaches, normal. Serum HCO₃⁻ or pH well outside the expected range points to a superimposed metabolic acid–base disorder
• Step 4 — Look for a mixed respiratory and metabolic disorder: pH change exceeds what the PaCO₂ predicts: ΔpH = (ΔPaCO₂/10) × 0.08
• Worked example — pH 7.1 and PaCO₂ 60 mm Hg: ΔpH = (20/10) × 0.08 = 0.16; 7.4 − 0.16 gives expected pH 7.24 → a superimposed metabolic acidosis is present · Worked example — pH 7.55 and PaCO₂ 30 mm Hg: ΔpH = (10/10) × 0.08 = 0.08; 7.4 + 0.08 gives expected pH 7.48 → a superimposed metabolic alkalosis is present
• Normal pH despite abnormal PaCO₂ means a mixed acid–base disorder: pH 7.38 with PaCO₂ 60 → respiratory acidosis plus metabolic alkalosis · pH 7.42 with PaCO₂ 30 → respiratory alkalosis plus metabolic acidosis
• Step 5 — Match these findings to the clinical history and physical exam to reach the correct diagnosis
• Step 6 — Treat as indicated