Morphine effects on cardiac output and regional blood flow distribution in conscious dogs

This study documents the changes in selected regional hemodynamics occurring in response to large doses of intravenous morphine. It was performed in normal conscious dogs. Their cardiovascular systems was unaltered by the presence of other drugs or recent surgery. The animals had been surgically prepared previously by implantation of chronic indwelling electromagnetic and Doppler ultrasonic flow probes on the aorta and the cranial mesenteric, renal and iliac arteries and placement of aortic catheters. Morphine, 1 mg/kg, produced mesenteric vascular dilation (30 per cent maximum decrease in resistance), renal vascular dilation (11 per cent maximum decrease in resistance) and nonsignificant changes in iliac vascular resistance. Morphine, 3 mg/kg, resulted in a similar amount of renal dilation (maximum 12 per cent decrease in resistance) but constricted the mesenteric vasculature (maximum 120 per cent increase in resistance) and the iliac vasculature (68 per cent maximum increase in resistance). These changes were observed despite the lack of sustained major changes in cardiac output, blood pressure, total peripheral resistance, or heart rate. It is therefore concluded that morphine has only minimal effects on systemic hemodynamics, but has significant effects when the alterations it produces in regional hemodynamics are considered. Such alterations may be physiologically significant when perfusion to these individual organs is a concern. The minimal systemic effects of high dose morphine is a phenomenon, for the most part, common to both dogs and humans. Whether similar regional hemodynamics changes as these occur in humans cannot be determined at this time since continuous flow monitoring techniques have, as yet, not been applied to humans.