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Severinghaus Electrode — FRCA Primary MCQ

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ModeratePhysics & Clinical MeasurementSeveringhaus ElectrodeFRCA Primary

A blood-gas analyser contains a glass pH electrode separated from the blood sample by a carbon-dioxide-permeable membrane, with a thin bicarbonate solution between the membrane and the glass. Which arterial variable is this Severinghaus electrode assembly designed to determine?

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Correct answer: C — PaCO2

The correct answer is C, PaCO2. Carbon dioxide diffuses through the gas-permeable membrane into the bicarbonate solution. It forms carbonic acid, increasing hydrogen-ion concentration and changing the potential across the internal glass pH electrode. By the Henderson–Hasselbalch relationship, this potential varies approximately with the logarithm of PCO2, allowing the analyser to report PaCO2 after calibration. Although the sensing element detects a local pH change, the assembly is a PCO2 electrode rather than the analyser's blood pH electrode. PaO2 is measured polarographically by a Clark electrode. Bicarbonate is normally calculated from measured pH and PaCO2, while oxygen saturation is measured by co-oximetry or estimated from PaO2 rather than by the Severinghaus electrode.

Reference: Association for Respiratory Technology and Physiology. ARTP statement on pulmonary function testing 2020, section on blood-gas analysis. BMJ Open Respiratory Research 2020;7:e000575. https://bmjopenrespres.bmj.com/content/7/1/e000575