Arterial Blood pH Calculator — ABG Interpretation

Formula

How this is calculated

Arterial blood gas interpretation centres on three measured values: pH, PaCO₂ (partial pressure of CO₂) and HCO₃⁻ (bicarbonate). This calculator uses them in two ways. First, it verifies internal consistency: the Henderson-Hasselbalch equation pH = 6.1 + log₁₀([HCO₃⁻] / (0.0307 × PaCO₂)) predicts what pH should be given the other two values — a large discrepancy signals a laboratory error, sample handling problem or unmeasured variable. The dissolved-CO₂ factor 0.0307 mmol/L per mmHg is the solubility constant at 37 °C body temperature.

Second, the calculator classifies the primary acid-base disorder from pH, PaCO₂ and HCO₃⁻ direction, then computes expected compensation. For metabolic acidosis, Winters formula gives the expected respiratory compensation: PaCO₂ = 1.5 × HCO₃⁻ + 8 (± 2 mmHg). For metabolic alkalosis: PaCO₂ = 0.7 × HCO₃⁻ + 21 (± 2). For respiratory acidosis, expected bicarbonate rises by ~1 mmol/L per 10 mmHg CO₂ acutely, and ~3.5 mmol/L per 10 mmHg chronically. For respiratory alkalosis: HCO₃⁻ falls by ~2 mmol/L per 10 mmHg acutely and ~5 per 10 mmHg chronically. A measured value outside the expected range suggests a second concurrent disorder.

This tool is a clinical reference aid only. Full ABG interpretation requires clinical context, oxygenation parameters (PaO₂, SaO₂), electrolytes (for anion gap), and evaluation by a qualified clinician. All decisions must be reviewed by appropriate medical personnel.