Effects of afterload and baroreceptors on cardiac function in fetal sheep

The fetal cardiac function curve, the relationship between cardiac output and right atrial pressure, was studied in 17 chronically catheterized fetal sheep. Combined biventricular cardiac output was measured by thermodilution while right atrial pressure was either lowered by haemorrhage, or raised by infusion of 5% (w/v) glucose. Cardiac output rose as right atrial pressure increased from 2 to 6 mmHg, but at higher right atrial pressures (up to 15 mmHg) cardiac output reached a plateau and remained constant at approximately 500 ml/min per kg. The normal right atrial pressure of 4.6 +/- 0.6 mmHg (SEM) placed these fetuses near the plateau of their cardiac function curves. When aortic pressure was increased from 11 to 16 mmHg by a methoxamine infusion, the cardiac function curve was depressed, maximum cardiac output on the plateau averaging only about 340 ml/min per kg. Ganglionic blockade with hexamethonium did not alter the position of the cardiac function curve, suggesting baroreceptors did not affect cardiac function. The cardiac function curve was shifted upward significantly during an isoproterenol infusion, which kept descending aortic pressure in the normal range by dilating the peripheral vasculature. If cardiac function was represented by the relationship between stroke work and right atrial pressure, no physiologically significant differences could be detected among the four conditions. Therefore, fetal cardiac function is normally near maximum, very sensitive to afterload and uninfluenced by baroreceptors. Cardiac output, however, with volume loading during beta-receptor stimulation, can be increased provided arterial pressure does not increase.