Drugs Used for Controlled Hypotension
The aims of controlled hypotension in neurosurgical practice are to decrease blood loss during surgery, to provide a dry surgical field and to diminish the risk of intraoperative aneurysm rupture. The benefits of controlled hypotension must be discussed in connection with the risks of development of cerebral ischaemia. For review, see McDowall (1985). Generally, induced hypotension to MABP levels of 50–60 mm Hg, which indicate the lower level of cerebral autoregulation in normotensive subjects is advocated (Strandgaard and Paulson 1984). In chronic arterial hypertension in baboons (Strandgaard et al 1975, Jones et al 1976), and rats (Barry et al 1982) the autoregulation curve is shifted to a higher level and similar findings have been observed in man (Strandgaard 1976). Other studies in baboons have shown that sympathetic vasoconstriction shifts the lower level of autoregulation towards higher pressures (Fitch et al 1975). Accordingly, the hypotensive level under these circumstances must be at a higher level. The lower level of CBF at which cerebral ischaemia threatens is supposed to be about 20 ml/100 g/min. Below this level the content of brain water increases. The increase in brain water is supposed to be a result of an increase in osmotic active molecules produced by anaerobic metabolism; it is a cytotoxic edema and is usually reversible on restoration of MABP (Symon et al 1979). If the degree of ischaemia is more severe so that CBF decreases to the threshold for failure of cell membrane ion haemostasis at about 10 ml/100 g/min, further cytotoxic edema occurs as a result of sodium passage into the cells. This is a cytotoxic oedema as well and it is reversible unless the cell has been severely damaged (Hossmann 1976, Iannotti and Hoff 1983).
KeywordsPlasma Renin Activity Cerebral Autoregulation Control Hypotension Intracranial Compliance Rebound Hypertension
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