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Ann Thorac Surg 2000;69:1986
© 2000 The Society of Thoracic Surgeons
a Division of Cardiothoracic Surgery and Department of Medicine, University of Colorado Health Sciences Center, 4200 E 9th Ave, Denver, CO 80262, USA
e-mail: jcleveland{at}uchsc.edu
To the Editor
In a recent issue of The Annals of Thoracic Surgery, Ueno and colleagues [1] compared pulmonary allograft function and early and medium-term outcomes among three different groups with progressively longer graft ischemic times. The three groups of graft ischemic times were less than 5 hours, 5 to 8 hours, and greater than 8 hours, and the authors concluded based on their retrospective comparison that prolonged graft ischemic time did not adversely affect outcomes.
We have several concerns about the conclusions that these authors have drawn based upon their study design and statistical analysis of their data. The numbers of patients in each group were small, and as such, the probability of a type II (beta) error is high. To recall, the statistical power of a study is determined as 1- beta, with Beta representing the probability of a false-negative result. Thus, power actually is the probability of rejecting the false null hypothesis. For most studies, an acceptable power is 80% or greater.
We have calculated the power for each of the early outcomes after transplantation (blood loss, duration of ventilation, ICU stay, hospital stay, tracheostomy, and death). In all circumstances, the power for each outcome was under 50%, which means that there is a greater than 50% probability of a type II error being present. Thus, for the authors to state, "However, there was no significant difference in any other variables of early and medium-term outcomes among the three groups" is inaccurate. Clearly, this study lacked adequate power to detect a difference in early outcomes after transplantation, even if one existed.
Further, it is unclear how the authors chose their ischemic graft time thresholds. Were these thresholds chosen because of donor procurement allotments that caused their graft-ischemic time to be greater than 5 to 8 hours on occasion? If a statistical threshold was used to provide these breakpoints, then this analysis should be included in the manuscript as well. The thresholds chosen could greatly influence the results.
For example, we calculated the medium term mortality between group 1 and groups 2 and 3 combined using a Fisher’s exact test and the mortality was actually lower with shorter ischemic times, although not quite statistically significant (two-tailed p = 0.054). The observation that a difference might exist in medium-term mortality between short (less than 5 hours) and long (greater than 5 hours) graft ischemic time patients is also intuitively suggested by Figure 6 of this manuscript, when one compares the survival curves.
Lastly, these results should be viewed as representative of this center only, and it is possible that confounding center-specific factors contributed to the observed outcomes in this study.
We continue to believe that graft ischemic time bears an important relationship upon outcomes after bilateral sequential and single-lung transplantation. It is misleading, in our opinion, to conclude that graft ischemic time does not impact adversely upon outcomes when carefully conducted statistical analysis reveals that the probability of detecting differences is low. It would indeed be unfortunate to argue that prolonged graft ischemic times are therefore permissible, when the data from this manuscript are inadequate to support this conclusion, and indeed almost support the contrary viewpoint.
References
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