Mild preischemia hypothermia adversely affects postischemic myocardial function in the neonatal piglet heart

During cardiac surgery, operative hypothermia has been shown to be beneficial in certain situations, although in children perioperative hypothermia has been associated with several physiologic alterations that have proven detrimental to their postoperative function. Little attention has been given to the effects of mild (34.5 degrees C) perioperative hypothermia on postischemic myocardial function in the pediatric population. It was hypothesized that mild hypothermia would be detrimental to postischemic ventricular function in the neonatal heart.

Neonatal (0-2 days old) piglets were subjected to mild perioperative hypothermia without rewarming (HT-only, n = 6), hypothermia followed by rewarming (HT-RW, n = 6), or continuous normothermia (NT, n = 8). The hearts were rapidly excised, suspended on an isolated perfusion apparatus, and allowed to spontaneously beat while being perfused with an asanguinous solution. All hearts were subjected to 20 min global, normothermic, zero-flow ischemia followed by 45 min oxygenated crystallite buffer reperfusion (I-R).

Compared to that of NT piglets, there were significant (P < 0.05) reductions in recovery of left ventricular (LV) diastolic and systolic function following ischemia and reperfusion in HT-RW animals. When the hearts were rendered ischemic without first rewarming, the degree of myocardial dysfunction was not as severe. In contrast to the NT piglets, HT-RW animals demonstrated significant (P < 0.05) reductions in the final recovery of LV developed pressure (71 +/- 6 vs 105 +/- 6 in NT), LV rate pressure product (52 +/- 4 vs 102 +/- 9 NT), and LV end diastolic pressure (32 +/- 7 vs 3 +/- 1 in NT) following I-R. When compared to the HT-RW group, HT-only piglets did not exhibit significant differences in systolic function, although diastolic function was minimally altered initially as evidenced by the slight elevation of LV end diastolic pressure at 5 min, with reperfusion in the HT-only group (P < 0.05).

In this newborn piglet model, mild hypothermia significantly reduces recovery of systolic and diastolic left ventricular function when followed by an episode of global myocardial ischemia-reperfusion only when the animals are returned to normothermia prior to the ischemic insult. When hypothermia is immediately followed by the ischemic event, left ventricular function is unaffected.