Comparison of mild and deep hypothermia: do they provide similar protection in all neonatal hearts?

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

Calcium and the heart: an essential partnership

Calcium ions are important in many aspects of normal cardiac function as well as in the response to certain pathologic states. The contribution that myocardial calcium influx makes to the cardiac action potential and the pharmacologic efficacy of compounds designated as calcium channel blockers is examined with respect to current knowledge regarding the structure and characteristics of cardiac sarcolemmal calcium channels. Once intracellular, calcium provides the link between cardiac electrical activity and actual mechanical shortening of cardiomyocytes through a complex interaction of regulatory and structural contractile proteins. This is followed by calcium clearance from the cytosol; the mechanisms by which this occurs are manipulated by drugs such as the digitalis glycosides to enhance myocardial contractility. The importance of intracellular 'second messengers' (eg, cyclic AMP) in constituting a final common pathway for the effects of certain cardiotonic agents is defined. The significance of abnormal calcium homeostasis under conditions of heart failure, myocardial infarction, ventricular fibrillation and cardiomyopathy is examined. The role of calcium in the mediation of myocardial damage under conditions of ischemia and secondary to a phenomenon known as 'the calcium paradox' is discussed. The finding that neonatal hearts are more vulnerable to ischemic contracture than adult hearts may be partially explained by differences between neonatal and adult myocardial calcium handling. Understanding of the interactions that exist between the calcium ion and the cardiomyocyte requires a sound knowledge of this essential partnership by both the physiologist and the practising physician

Relative vulnerability of neonatal and adult hearts to ischemic injury

To clarify the controversy over whether the neonatal heart is more or less susceptible to global ischemia than the adult heart, the time interval between the onset of ischemia and the beginning of contracture (TIC) was compared in neonatal (n = 6, 3 to 5 days old) and adult (n = 6, 4 to 5 months old) pig hearts. A comparison of the myocardial concentrations (mumol/g wet weight) of ATP, glycogen, lactate, and glucose-6-phosphate (G-6-P) was also done. The anesthetized animals underwent a sternotomy, and control right ventricular myocardial biopsy samples were taken. The heart of each was rapidly excised and placed in a 37 degrees C substrate-free Krebs-Henseleit bath. A compliant balloon was placed in the left ventricle to record pressure changes. Time to onset and peak TIC were recorded and additional proximal right ventricular biopsy samples were taken. Data were recorded as mean +/- SEM. Results demonstrated that neonatal hearts had a significantly shorter (p less than .05) TIC (29.5 +/- 1.7 min) than adult hearts (43.0 +/- 2.9 min) and exhibited more rapid lactate accumulation (2.5 +/- 0.5 to 22.1 +/- 3.0, p less than .001) and an increase in G-6-P (0.07 +/- 0.03 to 0.34 +/- 0.07, p less than .01) with a nonsignificant decline in myocardial glycogen (23.3 +/- 12.1 to 17.4 +/- 4.0, p greater than .05). Our findings indicate that neonatal hearts are more sensitive than adult hearts to global ischemia, which has potential implications for myocardial protection in pediatric cardiac surgery