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 re sistance and fetal acidosis during hemorrhage in gravid ewes. But, it is unclear whether epidural anesthesia modifies the uterine vascular r esistance response during hemorrhage, independent of changes in arteri al blood pressure. The purpose of this study was to determine the effe cts of epidural an esthesia on: 1) the mean arterial pressure/uterine vascular resistance relationship; and 2) arginine vasopressin concentr ations and plasma renin activity during hemorrhage in gravid ewes. Twe nty-four experiments were performed in 12 chronically instrumented ani mals between 0.8 and 0.9 of timed gestation. The experimental sequence included: 1)T = 0 min: epidural administration of 0.5% bupivacaine (e pidural group) or normal saline (control group); 2)T = 30 min: materna l hemorrhage 0.5 mL.kg(-1).min(-1) until maternal mean arterial pressu re was 60% of baseline measurements (time H); 3) T = H to H+60 min: ad just 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 re sistance was similar in both groups despite lower (P = 0.0001) mean ar terial 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 ep idural group than in the control group, but fetal pH and PO2 did not d iffer significantly between groups. Plasma arginine vasopressin concen trations 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 uteri ne vascular resistance during hemorrhage. However, the clinical benefi t of epidural anesthesia before hemorrhage seems minimal because there were no significant differences in fetal PO2 or pH between groups.