Intraoperative myocardial protection: still the cornerstone of cardiac surgery

The authors are to be congratulated on a well-conducted retrospective study [1] in the field of intraoperative myocardial protection, which represents 1 of the 4 vital fundamental principles of every cardiac surgical procedure. These 4 principles are [1] a perfect technical result [2], perfect intraoperative myocardial (and cerebral) protection [3], correct indication for a specific surgical procedure and [4] the preoperative status of the patient (e.g. number and severity of comorbidities, haemodynamic instability or shock, sepsis and neurological status). Based on the significant importance of myocardial protection, enormous research efforts in this field over the past decades have paved the way for modern cardiac surgery. We are now able to perform not only routine cases with short cross-clamp times very successfully but also complex cases with cross-clamp times >3 h. This gain in time has also allowed us to achieve much better technical success compared to previous decades. Good quality indicators to control the technical part of an operation (echocardiography, flow measurements, haemodynamic data) exist, and we use them regularly for intra- and postoperative monitoring. However, we rarely apply and report the known quality indicators for our intraoperative protection strategy, such as echocardiography (septal dysfunction, global left ventricular and right ventricular function), postoperative cardiac enzyme levels (CK-MB, troponin) and the need for haemodynamic support (inotropes, intra-aortic balloon pump, extracorporeal life support (ECLS)] [2]. Especially now, when ECLS devices can be easily implanted postoperatively, if the haemodynamics are not satisfactory, one gets the impression that intraoperative myocardial protection seems to be more and more a ‘forgotten modality’ [2]. In addition, the European Association for Cardio-Thoracic Surgery/European Association of Cardiothoracic Anaesthesiology/European Board of Cardiovascular Perfusion 2019 Guidelines on Cardiopulmonary Bypass [3] state that we still do not know which solution/strategy will result in the best outcome. As a result, the paper by Francica et al. [1] is important and directs our interest again to the field of myocardial protection. In their retrospective study, the authors report that, at their institution, adult patients scheduled for elective cardiac procedures undergo cardiac surgery with cardioplegic arrest using either cold-blood Buckberg depolarizing cardioplegia (BUCK.DEPOL group) or the newly developed adenosine-lidocaine-magnesium polarizing blood cardioplegia (ALM-POL group). The choice of the solution used for the elective patients was left to the operating surgeon. CustodiolR was used for all urgent/emergency/salvage adult cardiac procedures; the Buckberg solution was used for all paediatric procedures. Consequently, only elective adult cases were considered for this study. The primary end-point of the study was perioperative leakage of high-sensitivity troponin-I (Hs-TnI). The authors state that this is ‘in line with the 2018 Universal Definition of Myocardial Infarction’ [4]. However, it must be mentioned that the criteria for CABG-related myocardial infarction (MI) <48 h after surgery (type 5 MI) not only involve the (very sensitive) Hs-TnI values but also require (i) the development of new pathological Q waves, (ii) angiographically documented new graft occlusion or new native coronary artery occlusion and (iii) imaging evidence, e.g. new regional wall motion abnormalities [4]. Therefore, the primary end point in this study is not equivalent to the occurrence of an MI; it only represents a rise in Hs-TNI levels. In fact, no differences were found between the groups in the incidence of type 5 MI (Supplementary Material, Table S4). For the primary end point, there are significantly lower Hs-TnI values in the AML-POL group as well as lower Hs-TnIMAX values for both the overall population and the propensity score (PS) matched population (Table 1). Interestingly, the authors also analysed their enzyme data for patients with left ventricular dysfunction. In both subgroups (left ventricular ejection fractions of 30–50% and <30%) in the PS matched population, there were no differences in perioperative Hs-TnI leakage (Supplementary Material, Fig. S2). As a secondary end point, the authors collected data from 24 different parameters, listed in detail in their manuscript. In the PS matched population, the only highly significant difference between the groups was found in the spontaneous recovery of sinus rhythm (71% in the ALM-POL group and 53% in the BUCK-DEPOL group) (Supplementary Material, Table S4). There was a slight difference in the intraoperative use of the intra-aortic balloon pump; however, the number of patients (n = 4) was low. No differences between the groups in the PS matched population were noted for any of the other parameters. Therefore, taking all reported parameters into account, the conclusion of the study that 1 cardioplegic solution achieved superior protection compared to the other solution has to be questioned. The difficulty in objectively measuring the quality of 1 strategy (myocardial protection ‘strategy’ is more than only giving a pharmacologic agent, i.e. cardioplegic ‘solution’ into the coronary arteries) versus another one is one of the main problems in myocardial protection. To sum up, the main merits of this study are as follows: Direct our interest again to the important field of myocardial protection. Even though there were no deaths in either group in this study with elective patients (which can be expected today with different forms of myocardial protection in elective cases), we should remember that postoperative debriefing should not only include quality control measures of the technical aspects of the operation but also has to include a description of the quality of the intraoperative protection, such as echocardiography (septal dysfunction) and of the haemodynamic performance data and later (in the intensive care unit), enzyme leakage (Hs-TnI, CK-MB). Even though poor myocardial protection techniques may recover over time with the support of inotropes or ECLS in elective or semi-elective cases, it will have disastrous consequences for high-risk patients, whom we operate on today more and more often, and we will no doubt see this patient group even more frequently in the near future. Underscore the importance of further research in this field. Indeed, we have achieved a high standard of myocardial protection (if used correctly), but that does not that mean it cannot be improved further. Show that extremely good results, i.e. no deaths and low morbidity, can be obtained in elective patients. Direct our perspective to high-risk patients, in whom ‘perfect’ protection is of utmost importance. Over the past decades, many studies have been published describing the merits of a protection strategy that includes ante- and retrograde delivery of the cardioplegic solution, warm blood cardioplegia induction followed by cold induction and maintenance, intermittent cold blood perfusion and warm terminal blood cardioplegia reperfusion before removing the aortic cross-clamp. When considered together, this ‘integrated cardioplegia concept’ has generated a high standard of intraoperative myocardial protection [2, 5–8]. Strategy is more important than looking only to the ‘ideal’ cardioplegic solution [8]. Conflict of interest: FB is consultant to Dr. Franz Köhler Chemie GmbH, Bensheim, Germany.