Fig. 3 Cerebral autoregulation curve: CBF, cerebral blood flow; CPP, cerebral perfusion pressure.
The concept of autoregulation can be extended to discussions of cerebral perfusion pressure in the uninjured brain. Autoregulation in terms of perfusion pressure has a lower limit of 50 mmHg and an upper limit of approximately 140 mmHg. With a perfusion pressure below 50 mmHg, cerebral arterioles are maximally dilated and perfusion cannot match the brain's metabolic needs. If this state persists, brain ischemia follows. As previously noted, the brain tolerates poorly ischemic conditions which aggravate secondary injury. If the perfusion pressure is above 140 mmHg, cerebral arterioles are maximally constricted and blood flow increases passively with perfusion pressure.
Discussions of cerebral autoregulation are generally predicated on the brain's physiological responses to changes in systemic arterial pressure. However, in these discussions it is assumed that the physiological responses occurring in an uninjured brain are also in place after brain injury. This has been a topic of debate and study. Up to half of severely head-injured patients will have some autoregulatory impairment. The autoregulatory curve appears to be shifted to the right with an increase in the lower limit at which autoregulation is effective. Patients with severe head injury probably require a cerebral perfusion pressure of 60 to 70 mmHg to maintain autoregulation of cerebral blood flow and prevent ischemic complications. Although no randomized trials assessing outcome based on cerebral perfusion pressure have been completed, a number of observations can be made from retrospective and historically controlled studies in severely head-injured individuals.
1. Outcome is significantly worse in patients with a cerebral perfusion pressure below 60 mmHg.
2. A significantly better outcome and survival rate are achieved if the cerebral perfusion pressure is above 80 mmHg in the first 48 h after head injury.
3. A more favorable outcome has also been anticipated when cerebral perfusion pressure is maintained above 70 mmHg.
Despite the absence of a prospective randomized study, the available data suggest that it is best to maintain cerebral perfusion pressure at 60 to 70 mmHg.
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