Reply to Bauer et al.

Abstract

We appreciate the comments from Bauer et al. regarding our previously published article in this journal [1, 2] and would like to take the opportunity to clarify some important points of our study. The experimental set-up of our study included a sample closed perfusion system without a reservoir and without an air removal device on the venous side, as the main goal was to analyse pressure dynamics during cardiopulmonary bypass (CPB) and their effects on the formation of gaseous microemboli (GME) using a minimized closed perfusion circuit (MPC). We did not focus on the minimal extracorporeal circulation (MECC) system with a venous bubble trap (VBT) and with an oxygenator with an integrated arterial filter. To the best of our knowledge, this is the first study providing continuous recordings of venous line pressures every 250 ms during CPB. Independent of the further development of CPB techniques, in our opinion, it remains very important to document these pressure changes. The observed pressure differences reflected mostly volume requirements and underlined the need for an appropriate sensitive sensor to detect them. As Bauer et al. comment, this is the case in most current systems. However, venous pressure monitoring is not always sensitive enough to detect these differences [3]. In addition, the current automatic pump flow regulation involves a delay and under certain circumstances, is not fast enough to compensate negative pressure peaks sufficiently, leading to flow reduction. Even experienced perfusionists and anaesthesiologists, carefully handling the volume management, cannot provide 100% safety. To address this problem, our group developed a new device for automatic pressure and volume compensation during MPC, which was also evaluated in this experimental model, and these data are currently in submission. A large number of references within the comment of Bauer et al. state that MPC may be more vulnerable to accidental air introduction—the main trigger that develops de-airing devices [1]. In the in vitro study of Kutschka et al. [4], clinical aspects such as volume depletion were not considered. The better neurocognitive outcome after MPC when compared with CPB by Anastasiadis et al. [5] may be related to differences in cerebral perfusion that are influenced by various factors. However, the used MPC system had no VBT and the presence of GME was not investigated. All components of our experimental perfusion system are nowadays in routine clinical use. We used the same CPB equipment in order to make a realistic comparison between MPC and CPB. We agree with Bauer et al., that modern MECC systems with de-airing devices are safe and show a clinical benefit in daily routine use. Nevertheless, some aspects regarding these specific devices such as the use of more priming volume, more foreign surface contacts and additional costs should be studied further. Our newly developed compensation device may ameliorate or avoid these effects. Finally, we would like to mention that, following our experimental results, we plan to clinically compare MPC with clampless off-pump strategies, as the latter are well known for excellent postoperative neurological outcomes [6].