Unique responses of immature hearts to ischemia. Functional recovery versus initiation of contracture

Abstract

The characterization of unique responses of immature hearts to ischemic injury is important in devising better methods of myocardial protection for neonatal cardiac operations. Two end-points used to assess the vulnerability of immature myocardium to ischemic injury, namely, the time between onset of ischemia to the beginning of contracture and the functional recovery after reperfusion, had yielded results that appeared to be contradictory. In this study both the immature and adult rabbit hearts were used to study these two end-points in the same model, to assess their relationships and physiologic implications. Our data confirmed that, although immature hearts have greater capacity than adult hearts for functional recovery after identical periods of ischemic insult, their times to ischemic contracture are not prolonged, as could have been expected. A negative correlation between the rise in resting myocardial tension (i.e., contracture) and the recovery of ventricular function after reperfusion was noted both in the neonatal and in the adult hearts. However, reperfusion undertaken after "the onset of contracture" showed that the ventricle could still regain a measure of its function, which indicates that the "irreversibility" in global ventricular function is a gradual and progressive phenomenon. Biochemical studies of sarcoplasmic reticular calcium-adenosinetriphosphatase activity indicated that the immature myocardium has a significantly lower activity of this enzyme. Further depression of this enzyme activity after ischemia is seen in the immature hearts and may in part explain the earlier onset of contracture reported. A unifying concept to explain these unique responses of neonatal hearts to ischemia is proposed, based on the immaturities of certain key enzymes. The implications of these findings in the development of better protective techniques are also discussed.