The effects of epidural morphine on cardiac and renal sympathetic nerve activity in α-chloralose-anesthetized cats

Background Epidural morphine yields postoperative pain relief and hemodynamic stability. However, the effects of epidural morphine on sympathetic tone are unclear. This study was designed to elucidate the effects of epidural morphine on cardiac (CSNA) and renal (RSNA) sympathetic nerve activity by direct measurement in anesthetized cats. Methods: Thirty mongrel cats anesthetized with alpha-chloralose were randomly assigned to one of the following five groups: control(0.2 ml/kg thoracic epidural normal saline; n = 5); thoracic epidural morphine (n = 9); lumbar epidural morphine (n = 6); vagotomized, sinoaortic denervated, thoracic epidural morphine (n = 5); or intravenous morphine (n = 5), Mean arterial pressure (MAP), heart rate (HR), CSNA, and RSNA were measured 0, 15, 30, 60, 90, and 120 min after saline or morphine (200 mu g/hg) administration and 15 min after reversal with 200 mu g naloxone given intravenously. Results: In the control group, no changes in measured variables were found after either thoracic epidural saline or intravenous naloxone. Thoracic and lumbar epidural morphine both significantly reduced MAP, HR, CSNA, and RSNA 30 through 120 min after morphine administration (P < 0.05). These changes were reversed by intravenous naloxone. Changes after thoracic epidural morphine administration in vagotomized, baroreceptor-denervated cats were similar to those in intact cats. Intravenous morphine produced no significant changes except for a decrease in MAP, which was reversed by intravenous naloxone. Conclusion: In contrast to intravenous morphine, thoracic and lumbar epidural morphine both inhibited cardiac and renal sympathetic nerve activity and consequently reduced MAP and HR in alpha-chloralose anesthetized cats.