The effect of surface cooling on blood flow distribution in infant pigs with mature left to right shunts

Surface cooling as an adjunct to cardiopulmonary bypass, core cooling, and circulatory arrest has been effectively used to produce more homogeneous cooling and better tissue preservation. A previous study, using pigs with newly created aortopulmonary shunts, revealed a redistribution of blood flow away from the kidneys and viscera during surface cooling that did not occur in normal pigs. The present study tests the hypothesis that pigs with mature aortopulmonary shunts behave in a similar manner. Group I (N = 7, unshunted pigs) and Group II (N = 7, shunted pigs) underwent surface cooling and blood flow distribution measurements by microspheres at 37, 32, 28, and 25 °C. Both Groups I and II experienced a decrease in cardiac output with hypothermia. Group II had decreased absolute tissue flow to the viscera, kidneys, muscle, and skin at 37 °C compared with Group I, even before the onset of hypothermia. During hypothermia. Group I experienced a decrease in all tissue flows, but Group II had a decrease only in visceral and renal flows at 25 °C. Blood flow distribution, as a percentage of cardiac output, showed little change (decrease only in skin) in Group I with hypothermia. In Group II, however, a maldistribution of cardiac output developed resulting in decreased percentage of cardiac output to the kidneys and viscera and an increased percentage of cardiac output to the lungs that was confirmed by an increase in the Q_p Q_s ratio. Surface cooling in immature pigs with mature left-to-right shunts causes a maldistribution of cardiac output resulting in a decreased percentage of cardiac output to the viscera and kidneys and an increased percentage of cardiac output to the lungs. Systemic vascular resistance seems to rise to a greater degree than pulmonary vascular resistance during hypothermia and results in an increased left-to-right shunt (when present) and a maldistribution of blood flow.