The Devil Is in the Details: A Proof of Concept Study for Pulsatile Perfusion During Cardiopulmonary Bypass

To the Editor: We appreciate the commentary in response to our publication, "Intraoperative Use of Intra-Aortic Balloon Pump to Generate Pulsatile Flow During Heart Transplantation: A Single-Center Experience."1 The authors raise several points, which we would like to clarify. This study was designed to examine the impact of pulsatility, generated by an intra-aortic balloon pump (IABP), during cardiopulmonary bypass (CPB) for patients undergoing heart transplantation (HT). Patients are brought to the operating room with underlying cardiac function augmented by IABP. As described in Materials and Methods, the IABP was allowed to continue counterpulsation (rate: 80 beats/minutes) after CPB initiation, throughout cross-clamp and subsequent reperfusion. During CPB weaning, pulsatility is generated by the underlying rhythm of the newly implanted heart, and IABP continued in a 1:1 ratio unless the heart rate was over 100 beats/minute, in which case the IABP was set at 1:2 ratio. Therefore, pulsatile perfusion time is equal to the CPB time in the IABP-On group. We stated within Limitations that overall CPB times were relatively short in both groups, which may have dampened the impact of pulsatility during CPB on measures of end-organ perfusion. We acknowledge this paper is a retrospective review from a single institution with a limited study population; it is intended to be hypothesis-generating. Propensity score matching was used to eliminate significant differences in baseline characteristics that may have been confounders for elevated lactate postoperatively. Patient age, organ ischemia time, and length of CPB all impact systemic perfusion, which is why they were selected for propensity matching. Including additional criteria for matching would have further reduced the study population, limiting the value of the analysis. The authors incorrectly assert preoperative renal function was not described, when it is noted in Table 3 as an outcome before and after propensity score matching. Notably, there was not a significant difference in creatinine on admission for either the original or propensity matched cohorts. Finally, the authors claim the indication for IABP during CPB in the IABP-On group was unclear and posit this group of patients had worse underlying perfusion at baseline and pulsatile CPB was used to improve their prognosis. We reiterate what is described in Materials and Methods that beginning in May 2022 all patients bridged to HT with IABP were maintained with pulsatile CPB intraoperatively. This represented a change in our management strategy that was applied to all patients. While there may be an "era effect" of other simultaneous changes in management strategies that cannot be quantified, the assertion that selection bias exists is dubious. We agree with the authors of this Letter to the Editor on two points. Firstly, study design is important and the limitations for broader applicability should be acknowledged. Secondly, the importance of pulsatile flow during CPB remains an area of significant controversy in the literature and warrants further investigation. Our experience using an existing IABP to generate pulsatility during CPB for HT found reduced perioperative lactate levels. Hopefully, larger prospective studies can be conducted to confirm or refute these findings.