Bradycardia and hypotension associated with severe hemorrhage are reversed by morphine given centrally or peripherally in anesthetized rats

Background: Severe simple hemorrhage (blood loss in the absence of tissue damage and nociception) leads to a reflex bradycardia and hypotension. Earlier studies showed that this reflex can be attenuated by prior administration of morphine, However, some patients may receive morphine, e.g., for analgesia after they have suffered severe hemorrhage. The aim of this study was to determine whether an established bradycardia and hypotension could be reversed by morphine. Methods: Four groups of male Wistar rats (236-258 g) were anesthetized with alphadolone/alphaxalone (16-19 mg.hg.h(-1) intravenously), All groups received a hemorrhage of 40% total blood volume (BV) at 2% BV.min(-1). After the loss of 27% BV, bradycardia and hypotension were established equally in groups I and II and III and IV. Groups I(n = 8) and III (n = 10) received 0.9% saline (20 mu L intracerebroventricularly or 1 mL.kg(-1) intravenously, respectively), whereas groups II (n = 10) and IV (n = 10) received morphine (10 mu g intracerebroventricularly or 0.5 mg.kg(-1) intravenously, respectively). Results: In groups I and III, heart rate and mean arterial blood pressure continued to fall, whereas the bradycardia was completely reversed and the hypotension partly reversed in groups II and IV after treatment with morphine. Conclusion: Morphine, administered centrally or peripherally, can reverse the bradycardia and markedly can attenuate the hypotension associated with severe hemorrhage. However, any benefit may be more apparent than real because other studies suggest that mortality may be increased.