Interactions between preischemic hypothermia and cardioplegic solutions in the neonatal lamb heart

Hypothermia is believed to improve the tolerance to both ischemia and cardiopulmonary bypass and is commonly used during heart operations, particularly in the neonate. However, hypothermia also causes calcium to accumulate in the myocyte experimentally, and an increase in intracellular calcium during ischemia may worsen the effect of ischemia and impair the postischemic recovery of function. This effect of hypothermia on intracellular calcium has generally not been considered in experiments that attempt to optimize the composition of cardioplegic solutions. We have evaluated the impact of hypothermia before cardioplegic ischemia on the efficacy of two common cardioplegic solutions, one with calcium (St. Thomas' Hospital cardioplegia) and the other without calcium (glucose-potassium cardioplegia), in 37 isolated blood-perfused neonatal lamb hearts. Left ventricular maximal developed pressure, positive maximum of the first derivative of left ventricular pressure, left ventricular stiffness constant at 10 mmHg end-diastolic pressure, coronary blood flow, and myocardial oxygen consumption were measured before and 30 minutes after 2 hours of cold ischemia. After baseline measurements were made, two groups of hearts (ST-C and GK-C) had perfusion-cooling for 10 minutes to 17 degrees C myocardial temperature, and two other groups (ST-NC and GK-NC) had the same period of normothermic perfusion. Then the hearts were arrested with 4 degrees C St. Thomas' cardioplegic in groups ST-C and ST-NC and with glucose-potassium cardioplegia in groups GK-C and GK-NC. In the groups without preischemic cooling, both St. Thomas' (group ST-NC) and glucose-potassium (group GK-NC) cardioplegia resulted in a similar recovery of function compared with baseline levels (group ST-NC: developed pressure = 91.3% +/- 9.2%, dP/dt = 88.1% +/- 8.9%, left ventricular stiffness constant = 96.1% +/- 3.3%; group GK-NC: developed pressure = 89.3% +/- 6.9%, dP/dt = 82.6% +/- 8.8%, left ventricular stiffness constant = 99.4% +/- 2.0%; data are mean plus or minus the standard deviation). However, with preischemic cooling, St. Thomas' cardioplegia (group ST-C) resulted in a significantly reduced recovery of both systolic and diastolic function (developed pressure = 81.6% +/- 6.2%, dP/dt = 75.1% +/- 8.4%, left ventricular stiffness constant = 103.7% +/- 2.7%) compared with that for both glucose-potassium cardioplegia (group GK-C: developed pressure = 92.4% +/- 8.7%, dP/dt = 83.7% +/- 6.0%, left ventricular stiffness constant = 100.5% +/- 2.1%) and St. Thomas' cardioplegia without preischemic cooling (group ST-NC) (p < 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)