Cardiopulmonary Bypass affects most of the body's physiologicprocesses. Some of these effects are controlled completely or at leastpartially during or after CPB, but some are out of direct control. Anexample of the latter group is the systemic inflammatory responseinduced by the contact of blood with non-endothelial foreign surfaces.Partially controllable parameters are systemic vascular resistance, totalbodyoxygen consumption, regional and organ blood circulation,metabolic rate, and lactic acidosis. The effects of completely controllableparameters, such as blood flow rate and pattern, perfusion temperature,perfusate characteristics, and systemic arterial, venous, and pulmonarypressures, have been widely investigated and clarified. Nevertheless, it isevident that ideal CPB management can only be obtained by investigatingthese uncontrollable or partially controllable effects. These effects areamong the principal causes of increased morbidity, especially in infantsand children undergoing cardiac surgery.Patients undergoing cardiac surgery are often cooled to 25 to 28°Ccore temperatures during cardiopulmonary bypass (CPB) and rewarmedbefore termination of CPB. During the rewarming period, the aim of theanesthesiologist is to achieve a uniform rewarming of the whole body.Despite rewarming core temperatures to 37°C, decreases up to severaldegrees in core temperatures after discontinuation of CPB have beendocumented by many authors. This drop in core temperatures aftertermination of CPB is referred to as "afterdrop" and is said to be a sign ofinadequate total body rewarming on CPB. The surgeon’s decision about when to rewarm is critical; adequaterewarming requires time, but rewarming too soon removes the protectiveeffects of hypothermia. Rapid rewarming often results in largetemperature gradients between well-perfused organs and peripheralvasoconstricted tissues; subsequent equilibration following separationfrom CBP decreases core temperature again. Infusion of a vasodilatordrug by allowing higher pump flows often speeds the rewarming processand decreases large temperature gradients.The aim of this study is to investigate whether, the negative effectsof CPB on pediatric patients' arterial tone and microcirculation can beovercome by pharmacologic vasodilation. Therefore, the effects ofintravenous Na nitroprusside and inhaled sevoflurane on body heatdistribution, oxygen consumption, and blood lactate values will beassessed during and after CPB.