Cerebral blood vol: Vasodilators
Basic, Organ-Based and Clinical Sciences
Under normal circumstances, cerebral blood flow (CBF) is approximately 50 mL/100 g/min. Gray matter receives 80% and white matter receives 20% of this blood flow. This is secondary to the brain having a high metabolic rate and receiving approximately 15% of cardiac output. Many factors impact cerebral blood volume and flow. These include metabolic rate, arterial carbon dioxide tension, intrinsic autoregulation, oxygen partial pressure, and commonly used medications during anesthesia. Focusing specifically on systemic vasodilators such as nitroglycerin, nitroprusside, hydralazine, adenosine and calcium channel blockers (i.e. nicardipine), these drugs vasodilate the cerebral circulation and can potentially, depending on the MAP, increase CBF and volume. In contrast, angiotensin-converting enzyme (ACE) inhibitors and Beta-Adrenergic blockers have no effect on CBF or volume. Inhaled anesthetics such as sevoflurane can simultaneously vasodilate the cerebral circulation and cause an increase in cerebral blood volume. Finally, vasopressors such as phenylephrine, norepinephrine, ephedrine, and dopamine do not have direct effects on the cerebral circulation; however, when the mean arterial pressure is less than the lower limit of autoregulation, these agents can increase the MAP and thereby increase CBF. If the MAP is within the limits of autoregulation, then vasopressor-induced increases in systemic pressure have little effect on CBF.