Blood Pressure and the Brain: How Low Can You Go?

There are occasionally intraoperative circumstances in which reduction of mean arterial pressure (MAP) to levels well below those that occur in nonanesthetized adults is necessary or unavoidable. In these situations, clinicians are inevitably concerned about the limits of the tolerance of the brain for hypotension. Reference to the phenomenon of cerebral blood flow autoregulation is frequently made in discussions of safe MAP limits. However, in several respects, prevalent conceptions about cerebral blood flow autoregulation may be incomplete or inaccurate. The principal theses offered by this review are: (1) that the average lower limit of cerebral blood flow autoregulation in normotensive adult humans is not less than a MAP of 70 mm Hg; (2) that there is considerable intersubject variability in both the lower limit of cerebral blood flow autoregulation and the efficiency of cerebral blood flow autoregulation; (3) that there is a substantial blood flow reserve that buffers the normal central nervous system against critical blood flow reduction in the face of hypotension; (4) that there are several common clinical phenomena that have the potential to compromise that buffer, and that should be taken into account in decision making about minimum acceptable MAPs; and (5) that the average threshold for the onset of central nervous system ischemic symptoms is probably a MAP of 40-50 mm Hg at the level of the circle of Willis in a normotensive adult in a vertical posture and 45-55 mm Hg in a supine subject. However, these MAPs should probably only be approached deliberately when the exigencies of the surgical situation absolutely require it.