An additional limitation is that this study is a single-center study. This has the disadvantage that there may be some aspects of patient selection, treatment or care that are unique, and therefore, the results may not extrapolate to all centers performing TMR. The potential advantage of a single-center study is in the uniform procedures for patient selection and treatment. Second, this is a retrospective analysis and it is well known that such results may not apply prospectively. Third, there are a relatively small number of patients and a relatively small number of deaths; therefore the study may be underpowered to detect the importance of all factors examined. For example, there was only a small percentage of women, so the effect of gender on outcome may not be accurate. Average ejection fraction was near normal and there were few patients with ejection fractions less than 30%; therefore, we have not determined the risk of mortality in patients with ejection fractions lower than this value. Accordingly, the results of the present study apply to the group of patients with reasonably well preserved ejection fractions. Fourth, there was a significant number of patients who were lost to follow-up after six months. Therefore, the accuracy of the Kaplan-Meier curves diminishes with time due to the reduction in number of available patients. Fifth, patients of this study were treated with a single laser system (CO2, The Heart Laser). Because the acute tissue effects of various lasers in clinical use (Ho:YAG, excimer) are different, it is possible that the risks of perioperative mortality may differ with other lasers. Finally, there may be other factors that contribute importantly to the risk of postoperative mortality that have not been considered.