Cerebral autoregulation

Three different mechanisms are thought to contribute to the process of cerebral autoregulation. These are metabolic, myogenic and neurogenic. Metabolic regulation Metabolic regulation is driven by the difference between cerebral metabolism (demand) and oxygen delivery through cerebral blood flow (supply) and acts by means of a vasoactive substance. In principle, this is a negative feedback control system that seeks to balance blood flow to its demand. Myogenic regulation The effect of transmural blood pressure changes is directly detected by the vascular smooth muscle in arterioles, probably via a stress sensing mechanism. Then, the calibers are adjusted accordingly to keep blood flow constant. Neurogenic regulation The vascular smooth muscle actuators in the resistance arterioles are controlled via sympathetic innervation, receiving the input from the appropriate brainstem autonomous control center. Nitric oxide released by parasympathetic fibers may also play a role. ==Assessment of cerebral autoregulation==

In order to assess cerebral autoregulation one must at least continuously measure arterial blood pressure and cerebral blood flow. Because CO2 levels are of great influence to cerebral autoregulation it is recommended to also continuously measure CO2. Measuring arterial blood pressure Arterial blood pressure can be measured invasively using an arterial line. However, noninvasive finger arterial pressure can also be measured using a volume clamp technique. This technique uses a combination of an inflatable finger cuff and an infrared plethysmograph. Measuring cerebral blood flow Cerebral blood flow can be quantified in various ways of which three noninvasive means are currently much used. These are Transcranial Doppler sonography, Magnetic Resonance Imaging and Near Infrared Spectroscopy. Quantification of cerebral autoregulation The quantification of cerebral autoregulation always involves variation seen in cerebral blood flow in relation to changes in blood pressure. This blood pressure variation can either be evoked or spontaneous. Evoked blood pressure changes can be the result of: • releasing leg cuffs that were inflated above systolic pressure • breathing at a fixed rate • performing a Valsalva maneuver • performing squat-stand or sit-stand maneuvers • lower body negative pressure • pharmaceutical methods to raise or lower blood pressure The quantification depends on the experimental setup and can involve methods such as regression, cross-correlation, transfer function analysis or fitting mathematical models. Measuring and understanding cerebral autoregulation remains a big challenge. Despite great clinical interest and much research effort, benefit to patients has so far been limited. ==See also==