Why are newborn hearts vulnerable to global ischemia? The lactate hypothesis.

Whether newborn hearts are more vulnerable to ischemic injury than adult hearts remains controversial. Our recent canine studies showed that newborn hearts had a shorter time interval between the beginning of global ischemia and the onset of contracture than the adult hearts. To reconcile this finding with the known better tolerance of newborn hearts to hypoxic perfusion, we hypothesized that the greater anaerobic glycolytic capacity of immature myocardium leads to greater accumulation of lactate during total ischemia, resulting in tissue acidosis and earlier irreversible damage. In this study, newborn and adult rat hearts were excised and incubated in normothermia for 0, 5, 10, and 15 min. The newborn myocardial lactate contents were significantly higher (p less than .05) than those in the adult hearts. In another series of rats, 20 meq/liter of KCl was injected to produce cardioplegia before the excision of the hearts. The differences in lactate levels were even greater. The data are consistent with the lactate hypothesis, and suggest that in clinical neonatal cardiac preservation, providing sufficient washout may be beneficial.