The predominance of metabolic regulation of cerebral blood flow and the lack of "Classic" autoregulation curves in the viable brain

BACKGROUND: The influence of cerebral perfusion pressure (CPP) on real-time focal cerebral blood flow (fCBF) is not fully understood, in either intact or injured brain. We wanted to evaluate that relationship, and by implication investigate the relative importance of perfusion pressure versus metabolism in the regulation and control of cerebral blood flow. Our hypothesis was that metabolic needs dominated over a physiologic range of blood pressure. METHODS: This was an observational study of 23 patients, most of them with closed head injury, three with subarachnoid hemorrhage, one with a gunshot wound to the brain, and one monitored after craniotomy for unruptured aneurysm. Arterial lines, ventriculostomies, and fCBF monitors were placed. CPP (mean arterial pressure - intracranial pressure) and fCBF were measured and recorded to a computer database every minute. The relationship between CPP and fCBF was graphed and correlation coefficients were compared between survivors and non-survivors. RESULTS: Graphs of CPP versus fCBF did not show any linearity over a range of 50-150 mm Hg in patients who survived. In those who died, four of seven showed some indication of linearity. The difference in the correlation coefficients between survivors and non-survivors was statistically significant (P < 0.05), with survivors having essentially no correlation, as expected with autoregulation intact, and non-survivors having a mean correlation of 0.311. CONCLUSIONS: In the functioning and viable brain, metabolic regulation of cerebral blood flow (CBF) predominates, leading to the lack of an obvious relationship between perfusion pressure and flow. This predominance of metabolic regulation is robust and preserved over a wide range of brain injury, with pressure autoregulation necessary but not clinically apparent in the metabolically active brain. This robust and constantly varying relationship of pressure and flow shown by our real-time measurements of fCBF has important implications for interpreting clinical measurements of autoregulation. Perhaps most importantly, the development of a correlation between pressure and flow may indicate and be an early warning of deterioration.