THE MAINTENANCE OF CEREBRAL PERFUSION PRESSURE (CPP)
Until recently, too much emphasis was placed on the treatment of raised intracranial pressure (ICP) rather than on the maintenance of the cerebral perfusion pressure (CPP = mean BP - ICP). Raised ICP in the absence of any easily treatable condition (e.g. intracranial haematoma or raised PC02) requires careful management. The various techniques used to lower ICP have already been described (pages 79-80) but these must not be applied indiscriminately.
Studies have failed to confirm that active treatment of raised ICP following head injury improves outcome. Failure to show benefit from ICP treatment in the past may have resulted from a 'blind' use of hyperventilation. The vasoconstriction produced by lowering the PC02 reduces ICP by reducing intracranial blood volume. But vasoconstriction may reduce cerebral blood flow to ischaemic levels. Only prior measurement of the amount of 02 taken up by the brain (arteriovenous oxygen difference (AVD02)) indicates whether or not neurones can withstand a further reduction in 02 supply (caused by the vasoconstriction). Provided arterial 02 saturation remains constant, then the jugular bulb 02 saturation (i.e. 02 saturation of the venous outflow of the brain (SJv02)) can provide this guide. Low SJv02 is a contraindication to hyperventilation (see page 80).
Recent studies show that the number of 'insults' (high ICP, low BP or CPP), sustained by patients in the first few days after head injury, adversely affect outcome and suggest that the CPP is the most important factor, this should be maintained at more than 70 mmHg.
Patient selection for ICP monitoring: Monitoring ICP and CPP is most relevant in patients with a flexion response or worse to painful stimuli (a response of 'localising to pain' signifies a milder degree of injury and spontaneous recovery is likely). Such patients may have already undergone removal of an intracranial haematoma or may have had no mass lesion on CT scan (i.e. diffuse injury or contusional damage). Each neurosurgical unit is likely to have its own policy for ICP monitoring but the following outline may serve as a guide.
& BP low (e.g. mean <100 mm Hg) -» measure central venous pressure (CVP) or insert Swan Ganz catheter.
- if CVP low —► give plasma volume expanders e.g. haemacel
(do not use mannitol)
- if CVP normal -> give inotropic agents (e.g. dobutamine)
& BP normal insert jugular bulb monitor and measure SJv02
- if SJv02 low (< 60%) give hypnotics e.g.Propofol,
Etomidate. (see page 80) Do not hyperventilate.
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