College Anaesthesiologists Exam Prep | Hong Kong SAR China

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A patient with a traumatic brain injury is undergoing an emergency craniotomy. The surgeon requests hyperventilation to reduce brain swelling. What is the mechanism of this effect, and what is the recommended lower limit of PaCO2 to avoid cerebral ischemia?

Hyperventilation causes hypocapnia, leading to cerebral vasodilation; lower limit of PaCO2 is 25 mmHg (3.3 kPa). Hyperventilation causes hypocapnia, leading to cerebral vasoconstriction; lower limit of PaCO2 is 30 mmHg (4.0 kPa). Hyperventilation causes hypercapnia, leading to cerebral vasoconstriction; lower limit of PaCO2 is 35 mmHg (4.7 kPa). Hyperventilation increases intrathoracic pressure, improving venous drainage; lower limit of PaCO2 is 20 mmHg (2.7 kPa).

Correct Answer:

Hyperventilation causes hypocapnia, leading to cerebral vasoconstriction; lower limit of PaCO2 is 30 mmHg (4.0 kPa).

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