CBF: Factors influencing
Basic, Organ-Based and Clinical Sciences, Physiology
Normal cerebral blood flow (CBF) is 50 mL / min / 100g of brain tissue. Grey matter receives 80% of cerebral blood flow whereas white matter receives the remaining 20%. Factors that affect CBF include cerebral metabolic rate, autoregulation, reactivity to CO2 and O2, temperature, vasoactive drugs, and anesthetic agents.
Cerebral metabolic rate:
Normal cerebral metabolic rate for O2 (CMRO2) is 3.0 - 3.5/min/100g of brain tissue. Flow-metabolism coupling describes the process where increases in brain activity lead to increases in CMRO2 which in turn increases CBF. CMR is lower during coma, sleep, brain injury, sedation, and is higher with mental activity or seizures.
Autoregulation refers to changes in cerebral vascular resistance in order to maintain relatively constant CBF. Between a MAP of about ~60 mmHg to 160 mmHg (variable between individuals), CBF is relatively constant. Below this range, CBF decreases while above this range, the CBF increases (see figure below). Chronic hypertension shifts the autoregulation curve to the right. The equation relating CBF and blood pressure is as follows:
CBF = MAP - ICP (or CVP, whichever is higher)
CO2 and O2 reactivity:
PaCO2 and CBF have a roughly linear relationship, as seen in the figure below. For each 1 mmHg increase or decrease in PaCO2, CBF increases or decreases 1-2 mL/min/100 g brain tissue, respectively. PaO2 has little effect on CBF except when PaO2 becomes less than 60 mmHg, where CBF increases dramatically.
Hypothermia decreases both CBF and CMRO2. For each 1 °Celsius decrease in temperature, CMRO2 decreases 6-7%. Conversely, increases in temperature increase CMRO2 and CBF.
Systemic vasodilators such as sodium nitroprusside, nitroglycerin, hydralazine, and calcium channel blockers can cause cerebral vasodilation as well, likely increasing CBF.
Most intravenous agents, such as propofol, thiopental, etomidate, barbiturates, benzodiazepines, opioids, and dexmedetomidine decrease CBF and CMRO2 in a parallel manner, albeit to different degrees. The exception is ketamine, which increase both CBF and CMRO2. Inhalational anesthetic agents impair autoregulation in a dose-dependent manner, as shown in the figure below. Thus, at higher doses of inhalational anesthetics, CBF begins to have more of a linear relationship with MAP. At the same time, they decrease CMRO2 and cause cerebral vasodilation, which increases cerebral blood volume. Nitrous oxide is unique amongst inhalational anesthetic agents in that solo administration results in increases in CBF, CMRO2, and ICP.