Nitroprusside abolishes the deleterious effects of surface cooling-induced hypothermia on immature pigs with ventricular septal defects

The efficacy of systemic hypothermia in combination with cardiopulmonary bypass for the repair of congenital cardiac malformations is established. Surface cooling in infants with ventricular septal defects as a prebypass adjunct has been associated with visceral ischemic complications. Surface cooling in infant pigs with ventricular septal defects results in increased systemic vascular resistance and unchanged pulmonary vascular resistance with increased left-to-right shunting and a maldistribution of blood flow away from the viscera and kidneys. This study was to determine whether nitroprusside ameliorates the deleterious effects of increased systemic vascular resistance during hypothermia in infant pigs with ventricular septal defects. Nine neonatal pigs (4 weeks, 4 kg) had experimental ventricular septal defects resulting in a 2.3 ± 0.3 : 1 shunt. Systemic and pulmonary vascular resistances were determined by the flow probe and microsphere techniques. Systemic and pulmonary blood flow distributions were measured by the microsphere technique. Left-to-right shunt ratios were determined by oximetry, flow probe, and microsphere. The pigs were cooled to 28°C and measurements were made at 37°, 32°, and 28° C. We found that nitroprusside abolishes the deleterious effects of surface cooling by blocking the rise of systemic vascular resistance without significant changes in aortic pressure. As a result, the animals maintained control pulmonary-to-systemic flow levels and maintained control levels of oxygen delivery while showing a decline in oxygen consumption. Additionally, the vasodilatory effect allowed the animals to cool twice as fast as animals without nitroprusside. Regional blood flow distribution as percent cardiac output and absolute tissue flow were protected during surface cooling. This technique may have a role in cardiac operations on infants with left-to-right shunts.