Effects of isoflurane on cerebral autoregulation in dogs

Abstract

The effect of decreased cerebral perfusion pressure (CPP) on regional cerebral blood flow (CBF) with radiolabeled microspheres in dogs receiving either 1.4% or 2.8% isoflurane following anesthesia induction with thiopental (12 mg/kg, iv bolus) was studied. Mean arterial blood pressure (MABP), cerebrospinal fluid pressure (Pcsf), and sagittal sinus pressure (Pss) were measured. CPP of 83, 63, 53, 43, 33, and 23 mmHg were studied in three groups of animals. In group 1 (n = 6, isoflurane 1.4%) CPP was decreased by hemorrhage, in group 2 (n = 6, isoflurane 1.4%) CPP was decreased by increasing Pcsf while MABP was maintained constant, and in group 3 (n = 6, isoflurane 2.8%) CPP was decreased by hemorrhage. Control total CBF in groups 1, 2, and 3 was 69 ± 8, 72 ± 7, and 150 ± 25 ml·min-1·100 g-1, respectively, at CPP of 84 ± 1 mmHg. Flow to both cerebral hemispheres and brain stem in animals receiving 2.8% isoflurane was approximately twice that found with 1.4% isoflurane while CMRO2 was similar (about 3.5 ml·min-1·100 g-1). In groups 1 and 2 flow in all brain areas was maintained as CPP decreased to 43 mmHg and then flow decreased as CPP decreased further (P < 0.05). In group 3 flow to all brain areas decreased progressively as CPP decreased from 83 to 23 mmHg. At CPP of 43 mmHg and below, flow to cerebrum was similar in the three groups; however, flow to brain stem in group 3 animals remained elevated above groups 1 and 2 until CPP was decreased to 23 mmHg. Control cerebrovascular resistance (CVR) was 1.4 ± 0.2, 1.6 ± 0.2, and 0.8 ± 0.1 mmHg ml-1·min·100 g in groups 1, 2, and 3, respectively. In groups 1 and 2 CVR continuously declined as CPP decreased. In group 3 control CVR was less than groups 1 and 2 (P < 0.05) and was unchanged from control as CPP decreased from 83 to 23 mmHg. CMRO2 was 3.5, 3.8, and 3.1 ml·min-1·100 g-1 in groups 1, 2, and 3, respectively and was unchanged as CPP decreased. Cerebral autoregulation to decreased CPP is preserved with 1.4% isoflurane, whereas autoregulation is eliminated by 2.8% isoflurane. The increased CBF produced by 2.8% isoflurane maintains cerebral O2 delivery at or greater than that with 1.4% isoflurane as CPP is decreased.