During cardiopulmonary bypass (CPB), the patient becomes severely hypothermic (28°C core temperature). Which acid-base management strategy maintains physiologically normal ionised pH at the actual body temperature during hypothermia?

• A Alpha-stat strategy: maintain PaCO2 at 40 mmHg and pH 7.40 when measured at 37°C ✓
• B pH-stat strategy: maintain PaCO2 at 40 mmHg and pH 7.40 when corrected to 28°C
• C Alpha-stat strategy: maintain PaCO2 at 40 mmHg corrected to actual body temperature
• D Bicarbonate supplementation to raise pH regardless of temperature correction

Explanation

Alpha-stat management maintains blood gas values (pH 7.40, PaCO2 40 mmHg) as measured at 37°C without temperature correction. Since at hypothermic temperatures blood becomes more alkaline (as gas solubility increases with cooling), the true in vivo pH at 28°C may be 7.50–7.55, which is the natural physiological state for poikilotherms and matches the pH of neutral water at lower temperatures. This preserves autoregulation and enzyme function. pH-stat management instead corrects values to actual body temperature and adds CO2 to normalise measured pH — this increases cerebral blood flow and may improve brain cooling but risks loss of autoregulation. Alpha-stat is preferred for most adult cardiac procedures.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.