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

Background: Severe simple hemorrhage (blood loss in the absence of tissue d amage and nociception) leads to a reflex bradycardia and hypotension. Earli er studies showed that this reflex can be attenuated by prior administratio n of morphine, However, some patients may receive morphine, e.g., for analg esia after they have suffered severe hemorrhage. The aim of this study was to determine whether an established bradycardia and hypotension could be re versed 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 rece ived a hemorrhage of 40% total blood volume (BV) at 2% BV.min(-1). After th e loss of 27% BV, bradycardia and hypotension were established equally in g roups 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, respect ively). Results: In groups I and III, heart rate and mean arterial blood pressure c ontinued to fall, whereas the bradycardia was completely reversed and the h ypotension partly reversed in groups II and IV after treatment with morphin e. Conclusion: Morphine, administered centrally or peripherally, can reverse t he bradycardia and markedly can attenuate the hypotension associated with s evere hemorrhage. However, any benefit may be more apparent than real becau se other studies suggest that mortality may be increased.