Cerebral Perfusion Pressure
Maintenance of an adequate Cerebral Perfusion Pressure is a cornerstone of modern brain injury therapy.
After brain injury, and especially in the multiply injured patient, cerebral blood flow may be lowered to the ischaemic threshold. To prevent further neuronal death (the secondary brain injury), this flow of well oxygenated blood must be restored. There is no class I evidence for the optimum level of CPP, but 70-80mmHg is probably the critical threshold. Mortality increases approximately 20% for each 10mmHg loss of CPP. In those studies where CPP is maintained above 70mmHg, the reduction in mortality is as much as 35% for those with severe head injury.
Cerebral Perfusion Pressure may be maintained by raising the Mean Arterial Pressure or by lowering the Intracranial Pressure. In practice ICP is usually controlled to within normal limits (<20mmHg) and MAP is raised therapeutically. It is unknown whether ICP control is necessary providing CPP is maintained above the critical threshold.
Control of intracranial hypertension is discussed on the pages on intracranial pressure.
There is substantial evidence now that early hypotension (BP < 90mmHg) is associated with increased morbidity and mortality following severe brain injury. Even patients with one episode of hypotension during their ICU stay have a significantly reduced prognosis. Maintenance of an adequate MAP requires primarily a normovolaemic patient. Control of other sites of haemorrhage has the highest priority (with oxygenation). These patients should NOT be kept 'dry' with fluid restriction, but maintained in zero balance. Further elevation of MAP, once normovolaemia is achieved, is usually accomplished with norepinephrine, though dopamine may be used. There is little evidence to recommend any one agent over another.
trauma.org 5:1 2000
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