Endogenous vasopressin and renin-angiotensin systems support blood pressure after epidural block in humans

Background: Studies in experimental animals show that endogenous Arg- vasopressin (AVP) and the renin-angiotensin system support blood pressure when the sympathetic system is impaired pharmacologically or after epidural anesthesia. However, extrapolation to humans is uncertain. Therefore, we administered an AVP type V₁ receptor antagonist and an angiotensin- converting enzyme inhibitor to volunteers and measured the effect on blood pressure after epidural anesthesia. Methods: Healthy volunteers in whom epidural catheters were placed were randomly assigned to receive 1.25 mg intravenous enalapril or saline placebo followed by 0.5 mg AVP type V₁ antagonist β-mercapto-β,β-cyclopentamethylene-proprionyl-o-Met-Tyr-Arg- vasopressin (AVPA) or saline placebo. Finally, 2% lidocaine was injected to obtain a T2 level. Controls received intramuscular lidocaine. Results: Blood pressure did not significantly change after T-₂ epidural anesthesia in subjects treated with saline placebo, AVPA or enalapril alone (n = 10, for each treatment). In contrast, combined treatment with enalapril and AVPA resulted in a 36 ± 11% decrease in blood pressure after epidural dosing (n = 6). Controls given intramuscular lidocaine, in place of the epidural did not develop hypotension after AVPA and enalapril treatment (n = 10). Conclusions: Endogenous AVP and the renin-angiotensin system play important roles in maintaining blood pressure after epidural anesthesia in healthy subjects.