LOCAL VARIATIONS IN THE CEREBRAL MICROCIRCULATORY RESPONSE TO HYPERCAPNIA AND HEMORRHAGE

This study evaluates local variations of the cerebral vasomotor respon ses to hypercapnia and haemorrhagic hypotension in a pig model. Four l aser Doppler flow probes were used in each pig. There was considerable variation in laser Doppler signals between the four probes in baselin e recordings. The increases in flow after CO2 administration in 7 pigs had a mean coefficient of variation of 0.43 +/- 0.31, and the flow ch anges after blood loss in another 7 pigs had a mean coefficient of var iation of 0.45 +/- 0.34. The range of flow changes within each animal was large; the probe with the highest CO2 response showed on the avera ge a 273% +/- 157% larger CO2 response than the probe with the lowest CO2 response. Correspondingly, the probe with the best preserved blood flow after blood loss had on the average a flow value of 93% +/- 12% of the baseline value, while the probe that changed most with haemorrh age had a flow value of 44% +/- 24% of the baseline value. Single lase r Doppler recordings have been used for the monitoring of cerebral blo od flow in neurosurgical critical care, but our results suggest that a single laser Doppler flow probe is not an adequate method to monitor vasoreactivity in neurosurgical patients because flow signals from one probe may be unrepresentative for other sites in the brain.