The critical closing pressure of the cerebral circulation.

The critical closing pressure (CrCP) of the cerebral circulation indicates the value of arterial blood pressure (ABP) at which cerebral blood flow (CBF) approaches zero. Measurements in animals and in humans, have shown that the CrCP is significantly greater than zero. A simple mathematical model, incorporating the effects of arterial elasticity and active wall tension, shows that CrCP can be influenced by several structural and physiological parameters, notably intracranial pressure (ICP) and active wall tension. Due to the non-linear shape of the complete ABP-CBF curve, most methods proposed for estimation of CrCP can only represent the linear range of the pressure-flow (or velocity) relationship. As a consequence, only estimates of apparent CrCP can be obtained, and these tend to be significantly higher than the true CrCP. Estimates of apparent CrCP have been shown to be influenced by arterial PCO2, ICP, cerebral autoregulation, intra-thoracic pressure, and mean ABP. There is a lack of investigation, under well-controlled conditions, to assess whether CrCP is altered in disease states. Studies of the cerebral circulation need to take CrCP into account, to obtain more accurate estimates of cerebrovascular resistance changes, and to reflect the correct dynamic relationship between instantaneous ABP and CBF.