Commentary: To cool, or not to cool? Still a hot topic

Central MessageContemporary controversies in temperature management during cardiopulmonary bypass highlight persistent knowledge gaps.See Article page 230. Contemporary controversies in temperature management during cardiopulmonary bypass highlight persistent knowledge gaps. See Article page 230. In this issue of the Journal, Bianco and colleagues1Bianco V. Kilic A. Aranda-Michel E. Dunn-Lewis C. Serna-Gallegos D. Chen S. et al.Mild hypothermia versus normothermia in patients undergoing cardiac surgery.J Thorac Cardiovasc Surg Open. 2021; 7: 230-242Google Scholar explore a question that continues to generate interest among cardiac surgeons: “to cool, or not to cool?” Dr Jennifer Lawton articulated this question in her commentary for the Journal in 2014.2Lawton J.S. To cool or not to cool?.J Thorac Cardiovasc Surg. 2014; 148: 2718-2719Abstract Full Text Full Text PDF PubMed Google Scholar Since then, more than 30 studies have been published on this topic. However, the entire canon is vastly broader, as PubMed searches for “hypothermia,” “normothermia,” and “cardiopulmonary bypass” yield upwards of 260 articles. Yet, while there is no shortage of scholarship on this topic, the controversy, alas, remains unresolved. To help resolve this controversy, Bianco and colleagues1Bianco V. Kilic A. Aranda-Michel E. Dunn-Lewis C. Serna-Gallegos D. Chen S. et al.Mild hypothermia versus normothermia in patients undergoing cardiac surgery.J Thorac Cardiovasc Surg Open. 2021; 7: 230-242Google Scholar report their single-center outcomes using 2 temperature strategies during cardiopulmonary bypass: mild hypothermia (32°C-35°C), and normothermia (≥36°C). After adjusting for confounding with propensity score matching, adult patients (N = 3202) undergoing coronary artery bypass grafting (CABG), isolated valve, and combined CABG-valve procedures were compared according to the cooling strategy. The primary outcomes were early and late survival, and the secondary outcomes were postoperative complications and hospital readmission. The authors found no differences in short-term or long-term survival between the 2 groups. Postoperatively, a greater proportion of patients in the mild hypothermia cohort experienced acute renal failure compared with patients in the normothermia cohort (3.7% vs 2.4%, respectively). In addition, patients in the mild hypothermia cohort had longer stays in the intensive care unit compared with patients in the normothermia cohort (46.5 hours vs 45.1 hours, respectively). There were no differences in readmission frequencies, blood transfusion requirements, postoperative strokes, reoperations, intra-aortic balloon pump use, or infectious complications between the 2 groups. The study reinforces the notion that elective cardiac surgery under normothermic conditions is safe and perhaps even preferable to the same surgery under the hypothermic method. This conclusion has been demonstrated in multiple studies, including several randomized controlled trials.3Randomised trial of normothermic versus hypothermic coronary bypass surgery. The Warm Heart Investigators.Lancet. 1994; 343: 559-563Abstract PubMed Scopus (278) Google Scholar,4Ho K.M. Tan J.A. Benefits and risks of maintaining normothermia during cardiopulmonary bypass in adult cardiac surgery: a systematic review.Cardiovasc Ther. 2011; 29: 260-279Crossref PubMed Scopus (35) Google Scholar Most notably, Bianco and colleagues1Bianco V. Kilic A. Aranda-Michel E. Dunn-Lewis C. Serna-Gallegos D. Chen S. et al.Mild hypothermia versus normothermia in patients undergoing cardiac surgery.J Thorac Cardiovasc Surg Open. 2021; 7: 230-242Google Scholar showed no significant differences in blood transfusion requirements, a common finding in previous studies.4Ho K.M. Tan J.A. Benefits and risks of maintaining normothermia during cardiopulmonary bypass in adult cardiac surgery: a systematic review.Cardiovasc Ther. 2011; 29: 260-279Crossref PubMed Scopus (35) Google Scholar Instead, the authors report a greater frequency of renal failure and longer intensive care unit stays in the mild hypothermia cohort. Although these differences are statistically significant, the magnitude of the difference is small and, therefore, may not be clinically relevant. Another limitation of the study is the failure to investigate differences in postoperative neurocognitive function—an important variable to consider in temperature management. Indeed, mild hypothermia may have a neuroprotective effect.5Nathan H.J. Wells G.A. Munson J.L. Wozny D. Neuroprotective effect of mild hypothermia in patients undergoing coronary artery surgery with cardiopulmonary bypass: a randomized trial.Circulation. 2001; 104: I85-I91Crossref PubMed Google Scholar Finally, one must consider the possibility that different temperature strategies in this study were simply a surrogate marker for different surgeons. Taken together, the findings in this study suggest that, at the very least, normothermia has similar outcomes compared with mild hypothermia. Limitations aside, the inconclusiveness of this study is a clear reminder that the question of whether to cool, or not to cool, remains unanswered and requires further investigation. Future research should take into consideration criteria such as demographics, indications, and relevant outcomes. Demographics could include cohorts of adult versus pediatric patients, or congenital versus acquired pathologies. Indications could include contrasting outcomes for CABG, valve, or CABG-valve procedures. In addition, analyses of many potentially relevant outcomes could include survival, transfusion requirement, myocardial protection and function, long-term neurocognitive deficits, inflammatory responses, and infection. The broader data set resulting from more targeted studies could finally lead to more meaningful conclusions to this query that has intrigued our profession for so long. Mild hypothermia versus normothermia in patients undergoing cardiac surgeryJTCVS OpenVol. 7PreviewTemperature during cardiopulmonary bypass (CPB) for cardiac surgery has been controversial. The aim of the current study is to compare the outcomes for patients with mild hypothermia versus normothermic CPB temperatures.