The influence of risk on the results of warm heart surgery: a substudy of a randomized trial

OBJECTIVE: The Warm Heart Investigators Trial randomized isolatedcoronary bypass patients to cold or warm cardioplegia, and demonstratedthat warm cardioplegia significantly reduced the prevalence of low outputsyndrome and myocardial infarction (as defined by CKMB enzyme release).This study was designed prospectively as a subanalysis of the originaltrial, to determine the effect of warm heart surgery on high risk patients,who were anticipated to derive the major benefit from warm cardioplegia.METHODS: The prespecified endpoint for this study was a composite outcomeof morbidity and mortality (death and/or low output syndrome and/orenzymatic myocardial infarction). Only patients with complete data for alloutcomes were included, totalling 1374 patients (692 warm cardioplegia, 682cold cardioplegia) who were randomized in the Warm Heart InvestigatorsTrial. High medium and low risk patients were identified by a multivariatemodel of predicted risk for the study outcome. RESULTS: Analysis of theindependent and interactive influence of cardioplegia technique andpredicted risk demonstrated that warm cardioplegia significantly reducedthe overall prevalence of morbidity and mortality (warm: 15.9 versus cold:25.2%, P < 0.01). However, no significant differences in warm-coldeffects were detected among risk terciles. Cardioplegia technique had asimilar differential influence on mortality and morbidity in low riskpatients (warm: 7.3, cold: 17.4%) as it did in high risk patients (warm:31.1, cold: 39.9%). CONCLUSIONS: Although our analysis confirms the overallbenefits of warm cardioplegia, our unanticipated finding in high risksubjects may be explained by the fact that morbidity and mortality in thatpatient subgroup is caused not only by poor myocardial protection, but byother clinical and technical factors. Further studies are necessary toidentify those patients who might benefit most from improved myocardialprotection techniques.