Graphic analysis of serotonin effects in dog heart-lung preparation

Graphical analysis of the effects of serotonin on cardiac function and pulmonary circulation was performed, using the dog heart-lung preparation. The equilibrium points, at which the cardiac output (CO)-curve and venous return (VR)-curve cross in the right atrial pressure (RAP) or left atrial pressure (LAP)-CO relations, were directly recorded on two X-Y recorders. CO- and VR-curves were directly depicted by changing the blood level in the reservoir, and by inducing ventricular fibrillation and simultaneously occluding pulmonary arterial trunk, respectively. Single injections of serotonin, 300 micrograms, into the right or left atrium, induced a negative inotropic response. Low rate (less than 30 micrograms/min) of infusion of serotonin had no effect on the CO-curve or on the slope-gradient of VR-curve in the LAP-CO relation. At a rate of 60 or 120 micrograms/min, however, the CO-curve was moved downwards to the right, indicating a negative inotropic effect. Pulmonary mean filling pressure increased and the slope-gradient of pulmonary VR-curve decreased, indicating an increased resistance to venous return from the pulmonary circulation. Pulmonary arterial pressure was markedly elevated. In order to obtain the capacitance ratio between the extracorporeal circuit and the pulmonary circulation, a shift of blood volume to the pulmonary circulation was induced by elevating the aortic pressure, which also decreased the slope-gradient. The calculated capacitance ratio became greater during the infusion of serotonin, indicating that the capacitance in the pulmonary circulation was lowered. It is likely that serotonin has contractile effects on the pulmonary arterial and venous vascular beds, elevating the pulmonary filling pressure and resistance to venous return.