Reply Letter to “Reassessing the Chill: A Critical Perspective on SherpaPak Versus Ice Storage in Heart Transplantation”

To the Editor: We thank the authors for their interest in our article. Our study is retrospective and has limitations which are highlighted explicitly in the limitation section of the article. Although it is difficult to adjust for all possible confounding variables, including the impact of United Network for Organ Sharing (UNOS) allocation policy changes, propensity matching was used to minimize the effects of confounding variables. This included matching for the era, which refers to the timing around the UNOS changes in October 2018. The comment regarding underlying risk adjustment precision1, we respectfully point out that we did in fact use the era in the propensity score adjustment as noted above, after which the baseline variables such as IMPACT score and extracorporeal membrane oxygenation (ECMO) bridging were well-matched. Following propensity matching, the results found in the unmatched analyses were still observed. Right ventricular dysfunction was determined based on echocardiogram assessment and can be subject to variation. Echocardiogram assessments were not centralized or blinded in the GUARDIAN registry. The limitation of using echocardiogram for right ventricle dysfunction and possibility of variation in echocardiogram assessment was highlighted and discussed in the limitation section of the article. In the study, propensity matching on era was included to adjust for some of the potential time-related bias which can arise due to reverse consecutive enrollment of the ice control cases, which were limited to 3 years before SherpaPak cases. While the limitation in the timeframe of the historical ice controls to 3 years was meant to limit some of the potential biases around changes or advances in treatment and practice, matching on era also reduced some of these potential biases. This is also highlighted in the article, including in the limitations. We understand the implications of an industry-funded registry, but as you pointed out, it provides unique and valuable insight to outcomes in heart transplant recipients who used SherpaPak for donor transportation. In the methods section of the article, we were transparent, specifically stating that the “GUARDIAN registry is funded and administered by Paragonix Technologies, Inc. (Waltham, MA)” so that readers are aware of this association. Additionally, the data were shared with an independent contract statistician for validation of the analyses, and this independent statistical validation by K. Najarian from MediCEPT was described in the acknowledgments. Finally, we agree that it would be interesting to see SherpaPak outcomes outside of the GUARDIAN registry. Hence, we did a retrospective study at our center (outside of GUARDIAN registry) which showed that despite having longer ischemic times in the SherpaPak group, the outcomes were similar to the traditional ice storage group.2 We agree that further studies are needed to validate our findings to aid in the widespread use of the SherpaPak Cardiac Transport System.