Cerebral Blood Flow and Metabolism

Quick Answer

Cerebral blood flow (CBF) is tightly regulated to maintain constant oxygen and glucose delivery to the brain, which has high metabolic demand (20% of resting oxygen consumption, 2% of body weight). Normal CBF: 50 mL/100g/min (gray matter 80 mL/100g/min, white matter 20 mL/100g/min), total 750-1000 mL/min (15% cardiac output). Cerebral metabolic rate for oxygen (CMRO₂): 3.0-3.5 mL O₂/100g/min. Autoregulation: Maintains constant CBF across MAP 60-160 mmHg (lower limit 50 mmHg in chronic hypertension), mediated by myogenic response (vascular smooth muscle contraction/relaxation) and metabolic factors (CO₂, H⁺, adenosine). CO₂ reactivity: CBF changes 2-4% per mmHg PaCO₂ (hypercapnia vasodilates, hypocapnia vasoconstricts; extreme hypocapnia <25 mmHg causes ischaemia). O₂ reactivity: Minimal until PaO₂ <50 mmHg (below this, marked vasodilation). Intracranial pressure (ICP): Normal <15 mmHg; Monro-Kellie doctrine states intracranial volume (brain 80%, CSF 10%, blood 10%) is fixed, so compensatory mechanisms (CSF displacement, venous compression) maintain pressure until exhausted, then small volume increases cause exponential ICP rise. Cerebral perfusion pressure (CPP): MAP - ICP (or CVP, whichever higher), target 60-70 mmHg in TBI. Anaesthetic effects: Volatiles cause dose-dependent cerebral vasodilation (increase CBF, CMRO₂ uncoupling at >1 MAC), propofol decreases CBF and CMRO₂ (coupled reduction), opioids minimal direct effect. Indigenous considerations: Higher rates of hypertension and stroke may impair autoregulation; careful BP management essential. [1-10]