Induced hypotension and brain ischaemia.

Autoregulation is the term used to describe the maintenance of constant perfusion over a range of arterial pressures. Autoregulation occurs in all vascular beds, but other mechanisms also act on vascular control, for example autonomic activity and plasma concentrations of hormones. The influence of these other mechanisms varies widely between different circuits, so that the effect of induced hypotension is different on the vascular systems of the brain, lung and kidney. For example, sudden blood loss causes more marked vasoconstriction in kidney than in myocardium. In brain, the influence of mechanisms other than autoregulation is relatively small. Thus, maximal sympathetic activity reduces CBF by only 20% (Harper et al., 1972) which is approximately the same as the change produced by a decrease of 1 kPa in arterial PCO2. Autoregulation is therefore predominant in the cerebral circulation and acts to maintain CBF constant over a range of mean arterial pressures (MAP) which extend from 60 to 130 mm Hg in man (Lassen and Christensen, 1976). If, during induced hypotension, MAP is reduced beyond the lower limit of autoregulation, CBF starts to decrease in parallel with MAP until the ischaemic thresholds discussed by Symon (1985) and Heuser and Guggenberger (198 5) are reached. It is important to note that the lower limit of autoregu-