Calculator for the Henderson–Hasselbalch relationship in arterial blood gas analysis: \( \mathrm{pH} = 6.1 + \log_{10}\!\left(\frac{HCO_3^-}{0.03 \times PaCO_2}\right) \), with stepwise computation, explanation, and clinical context.
The Henderson–Hasselbalch equation relates pH to the ratio of bicarbonate concentration to dissolved carbon dioxide. In arterial blood gas interpretation, a commonly used clinical form is pH = 6.1 + log10(HCO₃⁻ / (0.03 × PaCO₂)), where 6.1 approximates the apparent pK′ of the bicarbonate buffer system at physiologic temperature and 0.03 is the solubility coefficient for carbon dioxide in plasma. This equation links the respiratory component (PaCO₂) and the metabolic component (HCO₃⁻) to observed blood pH.
Clinically, the equation is useful for understanding why pH falls when carbon dioxide rises and why pH rises when bicarbonate increases. It also explains why the bicarbonate value reported on many arterial blood gas panels is calculated from measured pH and PaCO₂ rather than directly measured on the blood gas instrument. In bedside interpretation, the equation reinforces that acid–base disorders reflect a disturbance in the ratio between metabolic base and respiratory acid.
Despite its central role, the Henderson–Hasselbalch approach has limitations. It is a model of equilibrium and does not by itself identify the cause of an acid–base disorder, quantify compensation, or capture all contributors to nonbicarbonate buffering. Contemporary acid–base analysis may also incorporate anion gap, strong ion approaches, lactate, albumin, and clinical context. Accordingly, the equation should be integrated with the full ABG, serum chemistry, and the patient’s presentation rather than used in isolation.