Introduction 25. Autoregulation may also be impaired in neonatal brain asphyxia and infec- tions of the central nervous system, but appears to be intact in spreading depres- sion and migraine, despite impairment of chemical and metabolic control of cerebral blood flow. In chronic hypertension, the limits of autoregulation are shifted toward high blood pressure. Acute hypertensive encephalopathy, on the other hand, is thought to be due to autoregulatory failure at very high pressure. In long-term diabetes mellitus there may be chronic impairment of cerebral autoregulation, probably due to diabetic microangiopathy. The exact function of cerebral autoregulation still remains unclear. Three differ- ent mechanisms are thought to play a role contributing to cerebral autoregula- tion 1: metabolic, myogenic and neurogenic regulation. Metabolic regulation adjusts blood flow to both metabolic demand and oxygen supply. Myogenic regulation is the effect of transmural blood pressure changes on vascular smooth muscle tension to keep blood flow constant. Neurogenic regulation is the control of the vascular smooth muscle tension through the autonomic nervous system. Techniques used for investigating CBF regulation In the past, measurements performed to investigate cerebral blood flow regula- tion involved arterial blood pressure changes that were achieved by pharmacol- ogical means and cerebral blood flow measuring mainly depended on indicator- dilution techniques, such as133Xenon or nitrous oxide. In this situation blood pressure and cerebral blood flow data could only be recorded under steady-state conditions where data were collected over several minutes. This resulted in the well-known classical cerebral autoregulation curve (Figure 1), showing a plateau in cerebral blood flow in the case of normal autoregulation, also known as the “static” autoregulation curve 27. If the blood flow changes significantly with either an increase or a decrease in arterial blood pressure, cerebral autoregula- tion is said to be impaired. If blood flow is maintained at or near the baseline level, despite a change in arterial blood pressure, cerebral autoregulation is said to be intact. 9
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