Hemodynamic effects of surface cooling-induced hypothermia on immature pigs with ventricular septal defects

Clinical interventions for repair of congenital heart defects, such as hypothermia and cardiopulmonary bypass, may cause changes in pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR) leading to deleterious alterations in blood flow. Since the relationship between the pulmonary and systemic circulations in patients with ventricular septal defects (VSDs) is dynamic and susceptible to stimuli, we directly measured hemodynamic variables and blood flow distribution in infant pigs with VSDs during surface cooling. In 12 piglets (aged 4 weeks and weighing 4 kg), VSDs were created by caval occlusion, right ventriculotomy, and septal perforation, resulting in a 2.2±0,4:1 shunt. SVR and PVR were derived by both the flow probe (FP) and microsphere (μS) techniques. Systemic and pulmonary blood flow distribution were measured (μS). Qp/Qs ratios were derived by oximetry, FP, μS techniques. Pigs were surface cooled in 28° C while measurements were made at 37°, 32°, and 28° C. SVR increased by FP (4034±55.4 to 10450±2132 dynes-sec/cm⁵ [p<0.05]) and by μS (3097±497 to 1022±2583 dynes-sec/cm⁵ [p<0.05]), while PVR remained unchanged. Qp/Qs ratios increased during hypothermia by FP (2.4±0.4:1 to 6.3±1.4:1 [p<0.05]), by μS (2.2±0.4:1 to 3.5±0.8:1 [p<0.05]), and by oximetry (1.4±0.1:1 to 2.6±0.3:1 [p<0.05]). Visceral flow (34.8±5 to 17.5±4 ml/100 gm/min [p<0.05]) and renal flow (127±21 to 53±11 ml/100 gm/min) [p<0.05]) both decreased during hypothermia. Systemic surface cooling-induced hypothermia in pigs with VSD causes an increase in SVR but no change in PVR resulting in an increased left to right intracardiac shunt confirmed by three measurement techniques. Redistribution of blood flow favors the lungs while lowering renal and visceral flow. Surface cooling-induced hypothermia and circulatory arrest in infants with VSDs may cause circulatory alterations leading to increased left to right shunt and decreased renal and visceral flow even before cardiopulmonary bypass and intracardiac repair.