Normothermic cardiopulmonary bypass.

H YPOTHERMIA and cardiac surgery have been closely linked since Dr F. John Lewis performed the first successful "open heart" surgery under direct vision using vena caval inflow occlusion and generalized hypothermia accomplished by surface cooling in 1952. ~ The advantages of hypothermia in prolonging tolerance to periods of reduced blood flow were already known. 2 Although systemic hypothermia with inflow stasis safely allowed surgeons to perform cardiotomies and repair simple congenital defects, the universal failure encountered with more complex intracardiac lesions confirmed the need for a perfusion method if cardiac surgery was to progress. Anatomic practicalities convinced pioneering surgeons that it was almost mandatory to circumvent both the heart and the lungs so that any extracorporeal circuit must be both a pump and an oxygenator. The first attempts to employ a pump-oxygenator for total cardiopulmonary bypass (CPB) were compromised by the high perfusion rates that were considered necessary during normothermia) Early clinicians maintained systemic blood flow rates of 100 to 120 mL/kg/min on CPB, equivalent to basal cardiac output. Utilization of perfusion rates of this magnitude resulted in unexpected and substantial blood return out of the cardiotomy from well-developed systemic-topulmonary collaterals and hindered surgical exposure. This substantial blood loss made early perfusions physiologically precarious. 4 Difficulties in the procurement of large volumes of fresh heparinized blood necessary to prime early extracorporeal circuits and complications from the transfusion of multiple units of homologous blood 5 fueled the search for alternate techniques and strategies in CPB. It was not until Lillehei et al recognized, described, and pioneered into clinical practice the "azygous flow" concept that lowered systemic blood flow rates were accepted as safe and advantageous during CPB. They identified that the small cardiac output resulting from azygous vein blood return to a canine heart with occluded cavae was able to sustain vital organ fnnction for a substantial time at normothermia. 6 Zuhdi et al combined the use of low-volume perfusion rates, with intentional hemodilution and moderate hypothermia, 7 which liberated cardiac surgery from the massive blood banking requirements. Perfusion rates of 30 mL/kg/min were found adequate during periods of total bypass. Systemic hypothermia was used in conjunction with CPB in support of lowered pump flows and for its contribution to myocardial cooling initially via coronary perfusion with hypothennic blood, a The reductions in tissue metabolism and oxygen consumption associated with hypothermia appealed to surgeons performing total bypass at lowered perfusion rates as a strategy of providing better organ protection. Lowered perfusion flow rates also allowed better visualization by the surgeon during cardiac surgical procedures. Although in retrospect early oxygenators provided relatively inefficient gas exchange in comparison with modern-day oxygenators, systemic hypothermia was not introduced because of inability to satisfy normothermic oxygen consumption. In his Shattuck Lecture of 1959, pioneering cardiac surgeon Dr Robert Gross noted that in his experience with early oxygenators "there has been no difficulty in supplying fully oxygenated blood at excellent flow rates (2,300 to 2,500 mL/min/m2), even for large adults. 9 During the period 1963 to 1969, asanguinous priming of plastic disposable oxygenators became commonplace except in the smallest children. Reduced perfusion rates and the use of moderate systemic hypothermia took hold, although some still operated under normothermic or nearnormothermic conditions. During the 1970s, with the introduction of myocardial revascularization surgery as a treatment for coronary arteriosclerosis, techniques for myocardial preservation included an emphasis on myocardial hypothermia. The application of hypothermia to the problem of myocardial preservation during iatrogenic cardiac arrest, particularly in combination with chemical cardioplegia, further strengthened its association with cardiac surgery. Most cardiac surgical procedures are now performed using hemodilution, moderate systemic hypothermia, topical myocardial hypothermia, and chemical cardioplegic arrest. Hypothermic CPB has been the standard technique for cardiac surgery for the past 25 years.