The effects of morphine on the mechanical properties of the systemic circulation in the dog.

We studied the effects of morphine on the mechanical properties of the systemic circulation in nine mongrel dogs. A right heart bypass preparation allowed the separation of venous return into splanchnic and extrasplanchnic (peripheral) flows. Each channel was drained by gravity into an external reservoir. Blood then was pumped into the pulmonary artery at a constant but adjustable rate. Venous resistances and compliances of both channels were calculated from transient and steady state volume shifts which occurred after rapid drops in splanchnic and peripheral venous pressures. Arterial pressure and changes in reservoir volume were measured. The following data, (mean SE) were obtained with the pump flow set at 80 ml -min ' kg" 1 . The administration of morphine did not produce a significant change from control splanchnic compliance (0.0177 0.0023 liter/mm Hg), peripheral compliance (0.0259 0.0017 liter/mm Hg), or peripheral venous resistance (5.6 0.6 mm Hg-liter' 1 min). Morphine increased splanchnic venous resistance from 13.2 1.4 to 20.6 2.4 mm Hg liter" 1 min (P < 0.05); it decreased the percentage of cardiac output perfusing the splanchnic channel from 33 2 to 24 2 (P < 0.01) and increased the percentage perfusing the peripheral channel from 67 2 to 76 2 (P < 0.001). Arterial pressure fell from 92 7 to 59 5 mm Hg (P < 0.001). Splanchnic arterial resistance decreased from 140 19 to 111 17 mm Hg liter ' min (P < 0.01) and peripheral arterial resistance fell from 68 8 to 36 6 mm Hg liter ' min (P < 0.001). Total peripheral resistance similarly declined from 51 5 to 33 4 mm Hg liter ' min (P < 0.001). Following the administration of morphine, the volume in the extracorporeal reservoir decreased by 0.508 0.108 liter (P < 0.001). In four dogs before morphine and in all dogs after morphine, volume-flow relationships were determined by setting the pump to deliver five different flows (48, 64, 80, 96, and 112 ml -min' 1 -kg' 1 ) and observing the steady state change in reservoir volume. On the average, morphine shifted the volume-flow curve by 0.423 liter to the right along the volume axis (P < 0.02) and produced a slight statistically not significant increase in the slope.