Autoregulation

Cerebral autoregulation is the intrinsic property of the blood vessels to maintain relatively constant cerebral blood flow by rapidly adjusting cerebrovascular resistance and compensating for fluctuations in cerebral perfusion pressure.

Changes in cerebrovascular resistance usually take place at the arteriole level, although larger vessels may also contribute. Cerebral autoregulation maintains mean cerebral blood flow between 50 and 170 millimeters mercury (mm Hg) of arterial pressure. Beyond the autoregulatory range, cerebral blood flow is pressure passive: at low blood pressures, the brain is at risk of ischemic injury; at high blood pressures, cerebral edema and breakdown of the blood-brain barrier may occur.

Two types of autoregulation exist:

dynamic autoregulation, which responds to immediate changes (within seconds).

static autoregulation, which responds to long-term (from minutes to hours) changes in blood pressures.

Earlier investigations of cerebral blood flow regulation relied on steady-state blood pressures for valid measures of cerebral autoregulation. This method was time consuming and required invasive procedures such as the Kety-Schmidt technique, using Xenon Xe 133 as a tracer. Moreover, the traditional steady-state techniques lacked the temporal resolution to identify the dynamic vascular changes that occur within seconds.

Assessment of dynamic cerebral autoregulation is based on transient changes in cerebral blood flow in response to sudden changes in arterial pressure. The sudden changes in arterial pressure can be induced by a variety of techniques such as deflation of bilateral thigh cuffs, postural alteration, Valsalva maneuver, lower-body negative pressure, isometric hand-grip exercise, and pharmacologic interventions.

Transcranial Doppler (TCD) ultrasound is a powerful noninvasive tool with high temporal resolution for assessing dynamic cerebral blood flow responses to various stimuli, including changes in arterial pressure. As TCD provides continuous measurements of cerebral blood flow velocity in the basal cerebral arteries, it has become the most commonly utilized tool to study cerebral blood flow regulation in humans.

Several analyses using time and frequency domains are utilized to examine dynamic cerebral autoregulation with TCD.
Abnormalities in cerebral autoregulation are thought to occur in a number of clinical disorders such as stroke, subarachnoid hemorrhage, postpartum angiopathy, eclampsia, syncope, and traumatic brain injury.