In a 75-year-old patient undergoing craniotomy for resection of a large astrocytoma, with a PaCO2 of 30 mmHg while on isoflurane, the global cerebral blood flow would be approximately which value?

In a 75-year-old patient undergoing craniotomy for resection of a large astrocytoma, with a PaCO2 of 30 mmHg while on isoflurane, the global cerebral blood flow would be approximately which value?

• A. 10 mL × 100 g brain weight-1 × min-1
• B. 20 mL × 100 g brain weight-1 × min-1
• C. 30 mL × 100 g brain weight-1 × min-1
• D. 40 mL × 100 g brain weight-1 × min-1

Answer: (D)

The key idea is how CO2 levels and volatile anesthetics shape cerebral blood flow. Global CBF in adults is roughly 50–60 mL/100 g/min at normal CO2. Lowering the PaCO2 to 30 mmHg causes cerebral vessels to constrict, reducing CBF by about 1–2 mL/100 g/min for each mmHg drop below 40, so a 10 mmHg drop would cut CBF by roughly 10–20 mL/100 g/min. Isoflurane, a volatile anesthetic, tends to dilate cerebral vessels and can raise CBF, but in the setting of hypocapnia the constrictive effect dominates enough to keep the overall value in the vicinity of 40 mL/100 g/min. Given the patient’s age and intracranial pathology, this estimate aligns with an approximate global CBF of about 40 mL/100 g/min.

The key idea is how CO2 levels and volatile anesthetics shape cerebral blood flow. Global CBF in adults is roughly 50–60 mL/100 g/min at normal CO2. Lowering the PaCO2 to 30 mmHg causes cerebral vessels to constrict, reducing CBF by about 1–2 mL/100 g/min for each mmHg drop below 40, so a 10 mmHg drop would cut CBF by roughly 10–20 mL/100 g/min. Isoflurane, a volatile anesthetic, tends to dilate cerebral vessels and can raise CBF, but in the setting of hypocapnia the constrictive effect dominates enough to keep the overall value in the vicinity of 40 mL/100 g/min. Given the patient’s age and intracranial pathology, this estimate aligns with an approximate global CBF of about 40 mL/100 g/min.