Effect of cerebral ischaemia on the cerebrovascular and cardiovascular response to haemorrhage.

Reports studying the combination of low blood pressure and cerebral ischaemia are few, and it remains to be determined how cerebral circulatory insufficiency modifies the cerebral perfusion and the central haemodynamic response to blood loss. We hypothesised that occlusion of arteries to the brain modifies the cerebrovascular and cardiovascular responses to blood loss. Continuous measurements of the cerebral microcirculation with laser Doppler microprobes in the cerebral cortex were performed in anaesthetised pigs during cerebral ischaemia and haemorrhagic hypotension. The response to rapid bleeding (25% of the blood volume) was recorded during normal conditions and during cerebral ischaemia induced by bilateral occlusion of the common carotid arteries. During normal conditions haemorrhage caused insignificant decreases in cerebral microcirculation. Haemorrhage during bilateral carotid artery occlusion, however, caused significantly greater changes in cerebral microcirculation and a greater posthaemorrhagic increase in cerebrovascular resistance shortly after the blood loss. Haemorrhage during bilateral carotid artery occlusion also caused greater reductions in cardiac output and arterial pressure than similar blood loss caused during normal conditions. This study showed a disproportionate decrease in cerebral blood flow with haemorrhage during bilateral carotid occlusion, caused by an immediate increase in cerebrovascular resistance. The results suggest that even a moderate blood loss in patients with impaired cerebral circulation could be dangerous, because normal compensatory mechanisms to haemorrhage are impaired.