Despite various techniques of hypothermia it is still difficult to provide optimal myocardial preservation in neonatal cardiac surgery. The early effects of moderate and deep hypothermia were examined in neonatal pig hearts by studying metabolism over time and the tolerance of the hearts to global ischemia. The latter was measured by the time to onset of ischemic contracture, an indicator of irreversible ischemic damage. After control right ventricular biopsies were done, 56, 3-day-old neonatal hearts were excised and placed in one of four temperature-regulated baths (37 degrees C, 25 degrees C, 19 degrees C, 12 degrees C). A compliant left ventricular balloon measured onset time to ischemic contracture (TIC) (2 mm Hg increase in pressure). Biopsies were also done at 15 and 30 minutes of ischemia. Progressive hypothermia significantly (p less than 0.001) prolonged TIC but resulted in an increased standard error of the mean (SEM). Lactate accumulation was the least in deep hypothermia (12 degrees C, 19 degrees C) (p less than 0.05) and intermediate with moderate hypothermia (25 degrees C), and all were less (p less than 0.01) than that at normothermia. The decline of adenosine triphosphate was slowed more by hypothermia than normothermia. These observations can be used to improve current methods of myocardial preservation in neonatal hearts
