Cardiopulmonary bypass strategies and neurological injury in children

The mortality for congenital heart surgery is now so low that the focus has shifted to improving the quality of life for children after heart surgery. Neurodevelopmental deficit is the most important surgical morbidity in congenital heart disease. This review article describes both clinical and laboratory studies in this area, most of which were performed at Children's Hospital Boston over the last 15 years. An epidemic of choreoathetoses at Children's Hospital in the late 1980[s] was the initial stimulus for studies of neurological injury after congenital heart surgery. A high index of suspicion associated the epidemic with a change in pH strategy from the more acidosis pH stat strategy to the alpha stat strategy during cardiopulmonary bypass. A more alkaline pH like alpha stat in a shift in the oxyhemoglobin dissociation curve to the left, i.e. tighter binding of oxygen with hemoglobin. Other potential factors that limit cerebral oxygen delivery during cardiopulmonary bypass include reduced perfusion flow rate of the cardiopulmonary bypass ump, a significant reduction in oxygen delivery secondary to hemodilution, a shift to the left of oxyhemoglobin dissociation secondary to hypothermic blood temperature, and run off the systemic circulation to the pulmonary circulation through collaterals and bronchial vessels. In addition venous cannulation in neonates and young infants can be tenuous. The fact that young children are free of arteriosclerosis potentially makes them an ideal population for studying the effects of cardiopulmonary bypass on neurodevelopmental outcome. On the other hand the heterogeneity of congenital heart defects as well as genetic associations such as microdeletion of chromosome 22 can complicate the interpretation of clinical studies in neurodevelopmental outcome after cardiac surgery. The results of several prospective and retrospective studies of neurological and developmental outcome after pediatric cardiac surgery as well as laboratory studies have led us to the following inferences: 1] the use of circulatory arrest should be minimized; 2] pH stat strategy is preferred for infant cardiopulmonary bypass over the alpha stat strategy; 3] a higher hematocrit, e.g. 30% may be preferable to 20%; 4] a hyperoxic perfusion strategy is preferred over a normoxic strategy; and 5] postoperative fever should be strenuously avoided