Afterload reduction and cardiac output in infants early after intracardiac surgery

Sixteen infants less than 18 months old were studied within 3 hours of intracardiac surgery. Initial mean arterial pressure and systemic vascular resistance levels were higher than normal in each infant (99 ± 17.2 mm Hg and 48 ± 18.1 units·m², respectively [mean ± standard deviation]); cardiac index was low (1.9 ± 0.48 liters·min^-1·m^-2) and mean left and right atrial pressures were 11.4 ± 2.39 and 12.5 ± 3.10 mm Hg, respectively. Mean pulmonary arterial pressure was 29 mm Hg; pulmonary vascular resistance was 8.6 units·m². When nitroprusside was infused to reduce mean arterial pressure to about normal, cardiac index increased 17 percent and mean left and right atrial pressures decreased 25 and 22 percent, respectively. Mean pulmonary arterial pressure decreased 31 percent. When atrial pressures returned to initial values after infusion of blood with continued infusion of nitroprusside, cardiac index increased another 24 percent. When administration of nitroprusside was discontinued, cardiac index decreased to 116 percent of the initial value and mean atrial pressure increased to 90 percent of the initial value. The study demonstrated the favorable effect on cardiac output of vasodilator therapy in combination with blood volume expansion in hypertensive infants early after intracardiac surgery. An empirical equation interrelating atrial and arterial pressures and cardiac index was derived from the data.