Cerebral autoregulation may be defined as the maintenance of constant cerebral blood flow despite changes in cerebral perfusion pressure, where CPP is equivalent to MAP-ICP (or CVP, whichever is greater). Given that normal ICP is generally low (5-12 mmHg), CPP is mainly dependent upon MAP. The normal range of cerebral autoregulation is often quoted between MAPs of approximately 60-160 mmHg, keeping in mind some sources will define the range of cerebral autoregulation in terms of CPP.
In patients with chronic systemic hypertension, this range is “right-shifted”, or in other words, the normal range of MAP in which CBF remains constant due to cerebral autoregulation is higher. Both the lower and upper limits are shifted. The limits of cerebral autoregulation may return close to normal in patients on long-term antihypertensive therapy. It should be noted that the blood pressure limits of cerebral autoregulation do not represent an “all-or-none” physiologic response, but rather the ends of the range simply indicate the points at which the flow/pressure relationship maintained by autoregulation begin to change. Regardless, CBF becomes pressure-dependent once MAP is outside the range of cerebral autoregulation. Although cerebral autoregulation is rapid, it is not instantaneous. If MAP changes abruptly, CBF may transiently drop before autoregulation takes effect (usually within 1-2 minutes) even if cerebral autoregulation is intact.
There are several factors influencing cerebral autoregulation during the administration of anesthesia. For example, hyper- or hypocapnia can cause significant changes cerebral vascular resistance. Hypercapnia especially may impair cerebral autoregulation by causing cerebral vasodilatation. Hemodilution with associated anemia may cause a decrease in vascular tone, ultimately resulting in a decrease in cerebral autoregulation capacity. Body temperature can also affect cerebral autoregulation, such that mild hypothermia impairs and mild hyperthermia enhances cerebral autoregulation. Finally, several anesthetic drugs can affect cerebral autoregulation. In healthy individuals, Propofol and Remifentanil tend to maintain cerebral autoregulation, whereas inhalational anesthetics (excluding sevoflurane at clinically relevant doses) tend to impair cerebral autoregulation. In other words, the degree to which the cerebral vasculature can respond to a given MAP is influenced by the degree of background cerebral vascular tone.
- Armagan Dagal, Arthur M Lam Cerebral autoregulation and anesthesia. Curr Opin Anaesthesiol: 2009, 22(5);547-52