Regional Cerebral Blood Flow following Hemorrhage during Isoflurane Anesthesia in the Rabbit

Abstract

The authors investigated whether the increase in cerebral blood flow (CBF) when pressure passive following hemorrhage was dependent on the technique used to increase cerebral perfusion pressure (CPP). Twenty-one New Zealand white rabbits anesthetized with isoflurane in oxygen were studied with four regional blood flow determinations in each. Blood flow was determined by injection of radioactive microspheres during the following experimental conditions: Injection 1:1 h after 1.0% inspired isoflurane; injection 2: 25 min after hypotension (MAP 50 mmHg) was induced by isoflurane; injection 3: 10-15 min after superimposed hemorrhagic hypotension to a MAP of 30 mmHg; injection 4: 25 min after restoring MAP to 50 mmHg by one of three techniques: decreasing inspired isoflurane concentration (group 1); restoration of blood volume (group 2); and phenylephrine infusion (group 3). Normocapnia was maintained throughout (35.8-38.4 mmHg). There was no group .times. time interaction (treatment effect) for total CBF (tCBF) (P = 0.309) or for any supratentorial region (frontal, parietal, occipital cortex P = 0.821, 0.413 and 0.342 respectively) for the three techniques used to increase CPP. A significant group .times. time interaction was seen for infratentorial structures (P = 0.002 for cerebellum and P = 0.035 for brain stem). When MAP was increased by decreasing the inspired isoflurane concentration significantly lower rCBF to infratentorial structures was seen compared with that following the other two techniques. A significant group .times. time interaction was seen for cardiac output following hemorrhage (P = 0.052) being significantly higher with blood volume expansion versus phenylephrine infusion. A group .times. time interaction was not seen for renal blood flow (P = 0.306). Thus, in the presence of isoflurane anesthesia, CBF increases for whole brain and supratentorial structures were independent of the three techniques used to increase CPP. In contrast, rCBF to posterior fossa structures was dependent on the technique with rCBF lowest when CPP was increased by decreasing the inspired isoflurane concentration. This finding is compatible with a shift downward and to the right in the regional CBF autoregulatory curves for infratentorial structures.