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Ann Thorac Surg 1991;52:1000-1004
© 1991 The Society of Thoracic Surgeons

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Articles

Not all neonatal hearts are equally protected from ischemic damage during hypothermia

Division of Cardiovascular Surgery, Department of Surgery, St. Michael's Hospital, University of Toronto, Toronto, Ontario, Canada

^_* Address reprint requests to Dr Wittnich, Cardiovascular Surgery Research, St. Michael's Hospital, 30 Bond St, Room 806F, Toronto, Ont, Canada M5B 1W8.

Despite hypothermia, pediatric cardiac surgeons continue to experience difficulties in providing adequate myocardial protection in newborns. This study examines the effects of deep hypothermia on neonatal heart tolerance to ischemia by measuring metabolic responses and the time to onset of ischemic contracture, or "stone heart." After control right ventricular biopsy specimens were obtained, hearts of newborn pigs (n = 36) were excised and placed in temperature-regulated baths: 37.5 ° ± 0.5 °C (n = 9), 19.0 ° ± 0.5 °C (n = 14), and 12.0 ° ± 0.5 °C (n = 13). With a compliant balloon in the left ventricle to measure pressure, time to onset of ischemic contracture (> 2-mm Hg rise) was recorded, and sequential biopsies were done. Data indicated hypothermia significantly (p < 0.001) prolonged time to onset of ischemic contracture from 29.5 ± 1.7 minutes (mean ± standard error of the mean) at normothermia to 150.0 ± 6.4 minutes at 19 °C and to 283.8 ± 46.4 minutes at 12 °C. Lactate buildup at 30 minutes of ischemia was significantly reduced by 70% with hypothermia. Decline in adenosine triphosphate level was significantly reduced by 50% (19 °C) and 75% (12 °C) with hypothermia. More importantly, a subgroup of hearts in each hypothermia group (n = 5 per group) was identified by 38% to 48% lower adenosine triphosphate stores before ischemia compared with the group means. These hearts had a significantly (p < 0.01) shorter time to onset of ischemic contracture than the group means (19 °C, 65.0 ± 8.0 minutes; 12 °C, 167.0 ± 4.0 minutes). These "at risks" hearts would not be afforded the same safe time for repair expected with hypothermia, and this requires consideration if we are to improve outcomes in this young patient population.

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