THE EFFECTS OF EPIDURAL-ANESTHESIA ON UTERINE VASCULAR-RESISTANCE, PLASMA ARGININE-VASOPRESSIN CONCENTRATIONS, AND PLASMA-RENIN ACTIVITY DURING HEMORRHAGE IN GRAVID EWES

In an earlier study, epidural anesthesia increased uterine vascular resistance and fetal acidosis during hemorrhage in gravid ewes. But, it is unclear whether epidural anesthesia modifies the uterine vascular resistance response during hemorrhage, independent of changes in arterial blood pressure. The purpose of this study was to determine the effects of epidural an esthesia on: 1) the mean arterial pressure/uterine vascular resistance relationship; and 2) arginine vasopressin concentrations and plasma renin activity during hemorrhage in gravid ewes. Twenty-four experiments were performed in 12 chronically instrumented animals between 0.8 and 0.9 of timed gestation. The experimental sequence included: 1)T = 0 min: epidural administration of 0.5% bupivacaine (epidural group) or normal saline (control group); 2)T = 30 min: maternal hemorrhage 0.5 mL.kg(-1).min(-1) until maternal mean arterial pressure was 60% of baseline measurements (time H); 3) T = H to H+60 min: adjust hemorrhage to maintain maternal mean arterial pressure at 60% of baseline. At 30 min, epidural bupivacaine resulted in a median sensory level of T-8 in the epidural group. At that time, uterine vascular resistance was similar in both groups despite lower (P = 0.0001) mean arterial pressure in the epidural group. Between H and H+60 min, uterine vascular resistance was lower (P = 0.045) in the epidural group than in the control group. Also, fetal PCO2 was lower (P = 0.020) in the epidural group than in the control group, but fetal pH and PO2 did not differ significantly between groups. Plasma arginine vasopressin concentrations and plasma renin activity were not significantly lower (P < 0.10) in the epidural group versus the control group during hemorrhage. We conclude that epidural anesthesia attenuated the increase in uterine vascular resistance during hemorrhage. However, the clinical benefit of epidural anesthesia before hemorrhage seems minimal because there were no significant differences in fetal PO2 or pH between groups.