How to Design a Study Comparing Pulsatile and Nonpulsatile Perfusion During Cardiopulmonary Bypass?

To the Editor: We read the study conducted by James et al.1 very carefully. This single-center retrospective study involved 76 heart transplantation (HT) patients. The authors concluded that pulsatile flow generated by intra-aortic balloon pump (IABP) during cardiopulmonary bypass (CPB) may improve end-organ perfusion as suggested by lower postoperative lactate levels. We would like to discuss a couple of issues upon design of the study. First, in this article, the author did not mention that when preoperative IABP switched to an automatic 80 beats/min mode and when switched back to a 1:1 IABP mode postoperatively. Therefore, it was completely unclear how long pulsatile perfusion lasted during CPB. This data were very important when you compare pulsatile and nonpulsatile mode during CPB. In addition, short CPB time in both groups may not be sufficient to detect the impact of pulsatile perfusion. Second, another issue regarding research design is that the sample size involving 76 HT patients was very small. The authors attempted to use propensity score matching to reduce baseline differences between the two groups, but this resulted in extremely small number of patients being ultimately included (only 18 patients in each group). The primary outcome of this study was peak lactate 24 hours postoperatively. However, the propensity matching was only according to age, organ ischemia time, and CPB time, and laboratory tests about preoperative systemic perfusion and renal function were not described. Considering the retrospective study, the indication for using IABP during CPB in IABP-On group was unclear. Perhaps it was because the perfusion of this group was worse, and pulsatile CPB was used to improve their prognosis. Therefore, based on these issues, there may be the existence of selection bias. Thus, the topic of pulsatile and non-pulsatile perfusion during CPB in the literature is still controversial.2,3 The questionable study design may increase confusion over the benefits of pulsatile and nonpulsatile flow during CPB. We suggest that researchers should consider optimizing experimental design in future investigations on pulsatile perfusion.