Henderson-Hasselbalch Blood pH — FRCA Primary MCQ

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ModeratePhysiology - RespiratoryHenderson-Hasselbalch Blood pHFRCA Primary

Which equation correctly expresses the Henderson–Hasselbalch relationship for arterial blood at 37°C when plasma bicarbonate concentration is measured in mmol/L and PaCO2 is measured in kPa?

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Correct answer: C — pH = 6.1 + log10([HCO3−]/(0.23 × PaCO2))

Explanation lettering: E = shown as B · B = shown as D · D = shown as E

C is correct. For the bicarbonate buffer system, pH equals the apparent pKa (approximately 6.1 at 37°C) plus the base-10 logarithm of bicarbonate concentration divided by dissolved CO2 concentration. Dissolved CO2 equals its solubility coefficient multiplied by PaCO2; the coefficient is approximately 0.23 mmol/L/kPa, or 0.03 mmol/L/mmHg. A reverses the base-to-acid ratio, predicting lower pH as bicarbonate rises. B incorrectly uses the natural logarithm rather than log10. D substitutes normal arterial pH for the apparent pKa. E uses the coefficient appropriate to mmHg despite PaCO2 being specified in kPa. With [HCO3−] 24 mmol/L and PaCO2 5.3 kPa, the equation gives a pH of approximately 7.4.

Reference: Weber M, Cave G. Trending peripheral venous PCO2 in patients with respiratory failure using mathematically arterialised venous blood gas samples. BMJ Open Respiratory Research. 2021;8:e000896. https://pubmed.ncbi.nlm.nih.gov/3365906/