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pH-Stat vs Alpha-Stat — FRCA Final MCQ

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HardCardiothoracic AnaesthesiapH-Stat vs Alpha-StatFRCA Final

During cardiopulmonary bypass, a patient has been cooled to 28°C. The blood-gas analyser measures samples at 37°C and currently reports an uncorrected pH of 7.42 and PaCO2 of 5.1 kPa. The perfusionist is instructed to change from alpha-stat to pH-stat management. Which instruction best implements this change?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: E — Use the temperature-corrected 28°C values and retain or add CO2 to maintain pH 7.40 and PaCO2 5.3 kPa

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

pH-stat management targets pH 7.40 and PaCO2 approximately 5.3 kPa after correction to the patient's actual temperature. Cooling increases CO2 solubility, so its partial pressure falls and pH rises at the lower temperature. Additional CO2 is therefore retained by reducing oxygenator sweep-gas flow or supplied to the oxygenator; the uncorrected 37°C analyser values consequently appear hypercapnic and acidotic. This CO2 produces cerebral vasodilatation and increases cerebral blood flow. Option A describes alpha-stat, which targets the uncorrected 37°C values and permits temperature-corrected pH to rise during cooling. Options C and D increase CO2 elimination and therefore move away from pH-stat targets. Bicarbonate in option E does not correct the defining relative hypocapnia and is not the mechanism used to institute pH-stat management.

Reference: Diop S, Thes J, Kassab MC, Mounier R. Hydrogen ion (pH) and gas behavior during deep hypothermic cardiopulmonary bypass: Physiology and clinical implications. Perfusion. 2026. https://pubmed.ncbi.nlm.nih.gov/15255667/