The effect of low-flow cardiopulmonary bypass on cerebral function: an experimental and clinical study

Systemic flow rates (Q) during nonpulsatile hypothermic cardiopulmonary bypass (CPB) that are consistent with preservation of cerebral function have not to our knowledge been objectively defined. The effect of a sequential reduction in flow rates on cerebral cortical metabolism and function was evaluated in 6 mongrel dogs during hypothermic (25 degrees C) CPB. Cerebral function was assessed using somatosensory cortical evoked potentials (SSEP); cerebral metabolism was assessed by adenosine triphosphate (ATP) and lactate content of snap-frozen gray matter biopsies taken from the hemisphere contralateral to that monitored for SSEP. A progressive decline in ATP levels was observed during flow reduction with virtually complete depletion of ATP at 0.25 L min-1 m-2(p = .0003). The significant (p = .028) dependence of cortical ATP levels on perfusion pressure was no longer evident after adjusting for the effects of flow rate. Lactate levels increased during flow reduction (p = .028), especially at flow rates less than 0.5 L min-1 m-2. Somatosensory neural transmission remained intact until flow was reduced to 0.25 L min-1 m-2 in 5 animals and until total circulatory arrest in 1, at which time loss of the signal occurred. In addition, 5 patients were subjected to brief periods of low-flow CPB (Q = 1.0 L min-1 m-2) at 21 degrees to 25 degrees C. SSEPs remained intact during flow reduction, and postoperative neurologic evaluation was normal in all patients. We conclude that, in the absence of cerebral vascular disease, the flow rate threshold for incurring functional cerebral injury during hypothermic (25 degrees C) nonpulsatile CPB is less than 1.0 L min-1 m-2.(ABSTRACT TRUNCATED AT 250 WORDS)