In the apnoea test used to confirm brainstem death, the ventilator is disconnected after preoxygenation and observation continues. Absence of any respiratory movement, together with arterial carbon dioxide tension rising above which value, confirms loss of the medullary respiratory centre?

• A PaCO2 greater than 30 mmHg
• B PaCO2 greater than 45 mmHg
• C PaCO2 greater than 100 mmHg
• D PaCO2 greater than 60 mmHg (or 20 mmHg above baseline) ✓

Explanation

The apnoea test demonstrates that even a maximal chemical stimulus to breathing fails to provoke respiratory effort. Preoxygenation allows safe disconnection while PaCO2 climbs; a rise to 60 mmHg, or at least 20 mmHg above an elevated baseline in chronic CO2 retainers, provides the strongest physiological drive to breathe, so its failure excludes brainstem respiratory function. A threshold below this would not exclude a depressed but potentially responsive centre, and waiting for extreme hypercapnia risks arrhythmia and cerebral injury before declaration.

Reference: Ganong's Review of Medical Physiology, 27th ed.

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