A retrospective comparative study of minimally invasive extracorporeal circulation versus conventional extracorporeal circulation in emergency coronary artery bypass surgery patients: a single surgeon analysis^† Free

Abstract

OBJECTIVES

At the moment, the main application of minimally invasive extracorporeal circulation (MiECC) is reserved for elective cardiac operations such as coronary artery bypass grafting (CABG) and/or aortic valve replacement. The purpose of this study was to compare the outcome of emergency CABG operations using either MiECC or conventional extracorporeal circulation (CECC) in patients requiring emergency CABG with regard to the perioperative course and the occurrence of major adverse cardiac and cerebral events (MACCE).

METHODS

We analysed the emergency CABG operations performed by a single surgeon, between January 2007 and July 2013, in order to exclude the differences in surgical technique. During this period, 187 emergency CABG patients (113 MiECC vs 74 CECC) were investigated retrospectively with respect to the following parameters: in-hospital mortality, MACCE, postoperative hospital stay and perioperative transfusion rate.

RESULTS

The mean logistic European System for Cardiac Operative Risk Evaluation was higher in the CECC group (MiECC 12.1 ± 16 vs CECC 15.0 ± 20.8, P = 0.15) and the number of bypass grafts per patient was similar in both groups (MiECC 2.94 vs CECC 2.93). There was no significant difference in the postoperative hospital stay or in major postoperative complications. The in-hospital mortality was higher in the CECC group 6.8% versus MiECC 4.4% (P = 0.48). The perioperative transfusion rate was lower with MiECC compared with CECC (MiECC 2.6 ± 3.2 vs CECC 3.8 ± 4.2, P = 0.025 units of blood per patient).

CONCLUSIONS

In our opinion, the use of MiECC in urgent CABG procedures is safe, feasible and shows no disadvantages compared with the use of CECC. Emergency operations using the MiECC system showed a significantly lower blood transfusion rate and better results concerning the unadjusted in-hospital mortality.

INTRODUCTION

Coronary artery bypass grafting (CABG) with the help of cardiopulmonary bypass (CPB) is an established, safe and effective procedure for coronary revascularization. Today, it is widely accepted that the use of extracorporeal circulation (ECC) may trigger a systemic inflammatory response and cause myocardial, renal, pulmonary and neurological dysfunction. Over the past 14 years, the minimally invasive extracorporeal circulation (MiECC) has been developed with the aim of reducing the side-effects of the conventional extracorporeal circulation (CECC), incorporating the advantages of CPB circuits while overcoming the limitations of off-pump coronary artery bypass (OPCAB) surgery.

Our department started performing myocardial revascularization procedures with MiECC in 2004 and after this 10-year experience we can most certainly affirm that the use of MiECC is a safe alternative for CABG. From 2007 until now, 6387 surgeries were performed in our clinic. Out of 4009 CABG procedures, the contingent of emergency CABG patients was 1278, corresponding to 31.8%.

While several studies [1–3] compared the impact of minimally invasive versus conventional extracorporeal circuits in elective CABG patients, only one study did so in emergency patients [4]. When analysing the results of the study, a clear and precise statement regarding major adverse cardiac and cerebral events (MACCE) and moreover postoperative mortality is not possible. Considering the positive results and experience from our everyday practice, while taking into account the ongoing debate in the field of cardiac surgery, we decided to investigate the feasibility of MiECC in emergency CABG patients.

MATERIALS AND METHODS

We conducted a retrospective analysis from January 2007 to July 2013 on 187 emergency patients undergoing a CABG procedure using either MiECC or CECC. The surgeries were performed by a single surgeon in order to eliminate the differences that may occur in surgical technique. The individual patient’s consent was waived because of the study’s retrospective design and data collection from routine patient practice. Only patients with emergency presentation due to acute coronary syndrome (unstable angina, non-ST segment and ST segment elevation myocardial infarction, elevated myocardial markers), severe three vessel coronary artery disease and severe stenosis of the left main coronary artery who underwent surgery within 24 h after hospital admission were included in the study. Acute myocardial infarction was diagnosed by conventional 12-channel electrocardiography, elevated myocardial enzymes and confirmed by immediate coronary angiography. CABG procedures were performed with CECC or MiECC under general anaesthesia. A complete surgical revascularization was the aim in all patients.

The exclusion criteria were redo surgery or combined procedures.

The investigated parameters were as follows: logistic European System for Cardiac Operative Risk Evaluation (EuroSCORE), standard EuroSCORE, acute myocardial infarction, left ventricular ejection fraction, total number of bypass, use of the left internal thoracic artery as graft material, use of an intra-aortic balloon pump (IABP), number of transfused units of red blood cells (RBC), acute renal impairment with the need of renal replacement therapy, length of intensive care unit (ICU) stay, postoperative hospital stay, rethoracotomy, postoperative neurological events including transient/prolonged ischaemic neurological deficit and stroke and in-hospital mortality. The inflammatory systemic reaction (white blood cell count, C-reactive protein), renal function (serum creatinine, serum urea) and cardiac enzymes (troponin T, creatine-kinase) were monitored at three time points (T1–T3): T1, preoperatively; T2, 24 h; and T3, 48 h after surgery.

Anaesthetic and surgical management

Patients received standard premedication (clonidin, noctamid) 1 h before arrival at the operating theatre. After induction (propofol, sufentanil and pancuronium), anaesthesia was maintained both with inhalational (sevoflurane) and intravenous (propofol, remifentanyl) agents. A median sternotomy was performed. Parallel to the harvesting of the left internal thoracic artery, the great saphenous vein was prepared for bypass grafting. After systemic administration of heparin, a standard cannulation of the ascending aorta and of the right atrium (venous triple-stage® Slim cannula) was performed. The dosage of heparin was calculated by the Heparin Management System (Hepcon HMS; Medtronic, Minneapolis, MN, USA) in order to achieve a minimum level of 4.1 units of heparin per ml patient blood. The ascending aorta was cross-clamped and warm (37°C) blood cardioplegia (Calafiore) was applied. The coronary vessels were exposed and anastomosed. The proximal anastomoses were performed under partial clamping of the aorta. The patients were weaned off ECC and heparin was antagonized by systemic administration of protamine. After surgery the patient was transferred to the ICU, where all patients received standard haemodynamic monitoring and mechanical ventilation support.

Extracorporeal circulation

Maquet HL30 heart lung machines and customized extracorporeal circulation (ECC) sets (Maquet Cardiopulmonary, Hirrlingen, Germany) were used in both groups.

Conventional extracorporeal circulation -set-up

The clinical CECC routine practice at the MediClin Heart Centre Coswig includes the use of a membrane oxygenator Quadrox®, a centrifugal pump Rotaflow®, an open perfusion system containing the VHK 2001® venous hardshell cardiotomy reservoir and the Quart® arterial line filter (Maquet Cardiopulmonary, Hirrlingen, Germany).

For each patient, the CPB system was set-up 1 hour prior to the operation. The standard priming included three solutions: 500 ml ringer acetate (Jonosteril®), 500 ml mannitol 10% solution and 500 ml hydroxyethyl starch solution (Volulyte®, HES 6%). Each perfusion set was flushed with carbon dioxide for 3 min before priming procedure was initiated. The patient’s body surface area was determined and an individual ECC flow was calculated based on the cardiac index of 2.5/l/min/m². Anticoagulation strategy was performed with heparin concentration-based anticoagulation management (Hepcon HMS; Medtronic, Minneapolis, MN, USA). The heparin dose–response test was performed before skin incision, aiming at an activated clotting time (ACT) of 480 s. However, as the calculation of the volume of every individual according to body surface area is an approximation, particularly in cardiac surgery, the ‘pump’ heparin was added to the patient as a bolus to create a safety window. The CECC circuit was primed with an additional 10,000 international units of heparin.

CPB was performed at normothermic level at 36°C. Retrograde autologous priming (RAP) was performed for all patients with stable haemodynamic conditions, leading to a reduction of the priming volume. The CECC flow was set as required in order to maintain a mean arterial pressure (MAP) between 50 and 75 mmHg. Patients received transfusion of red blood cells at a haematocrit level <24%.

Minimally invasive extracorporeal circulation-set-up

The standard MiECC system at our institution is a customer-modified standard configuration including the membrane oxygenator Quadrox®, the venous bubble trap VBT®, the centrifugal pump Rotaflow® and the arterial filter Quart® (Maquet Cardiopulmonary, Hirrlingen, Germany).

The system is Bioline®-coated, offering a homogeneous surface which reduces thrombocyte adhesion and thrombus formation, as well as the adherence of leucocytes and cell clusters. A triple-stage® Slim (28/32 Fr.) venous return catheter was used in all MiECC cases. This venous cannula is specially developed for application with the minimally invasive ECC to optimize the venous drainage under active kinetic drainage due to the centrifugal pump and also to reduce the aspiration phenomena that may encounter when using other venous cannulas [5].

The preparation and the priming procedure were the same as for CECC. Calculations in terms of flow, heparin and preoperative data were equal to the CECC procedure. CO₂ flush over 3 min was also performed. We used 500 ml hydroxyethyl starch (Volulyte®) and 500 ml mannitol 10% priming solution.

The heparin management, temperature, arterial pump-flow and transfusion conditions were similar to CECC perfusion. Retrograde autologous priming (RAP) was performed for all patients with stable haemodynamic conditions, reducing the priming volume to 730 ± 110 ml. All mini-circuits comprised a venous bubble trap (VBT®) used to safely deair the system in case of air entry at the venous line.

Statistical analysis

All data were stored in a database, using Microsoft Office Excel (Microsoft Corporation, Redmond, WA, USA) and the SPSS 18.0 software for Windows (SPSS, Chicago, IL, USA). The Shapiro–Wilk test was used in order to evaluate whether the variables were after a Gaussian model distributed. Normally distributed continuous data were expressed as means [± standard deviation (SD)], whereas non-normally distributed continuous data were presented as median (interquartile range). The Student’s t-test was used to verify the comparability of the two groups and to determine potential differences in measured laboratory parameters and the time spent in ICU. Categorical data, for example, transfused red blood cell products, were analysed based on quantity, using an χ²-test or Fisher’s exact test. A P-value of <0.05 indicated statistical significance. Mann–Whitney U-test was used for non-parametric continuous variables. odd ratios (ORs), calculated as the probability of an occurring event divided by the non-occurrence of an event, and their two-sided 95% confidence intervals (CIs) were presented within each subgroup.

RESULTS

All emergency CABG patients operated on by a single surgeon between January 2007 and July 2013 were identified and investigated according to the used perfusion technique: study group MiECC (n = 113) vs control group CECC (n = 74).

Demographic data and predicted operative mortality are given in Table 1. Despite a higher number of patients presenting with acute myocardial infarction at admission in the MiECC group (76.1% vs 68.9%), the logistic and standard EuroSCORE were higher in the CECC group (Table 1).

Investigating the perioperative data, the rate of units of blood transfused per patient was significantly lower in the MiECC group. The number of grafts and the cross-clamp time were similar in both groups, though the left internal mammary artery was used as a bypass graft more frequently in the group of patients operated using MiECC (Table 2).

The data from the postoperative hospital course are gathered in Table 3.

To better match the two groups, we conducted a sub-analysis including only patients with positive troponin T (>14 ng/l), acute myocardial infarction at presentation and logistic EuroSCORE between 8 and 40. Considering these criteria the CECC group included 31 patients and the MiECC group 46 patients (Table 4).

The two groups were comparable with regard to the demographic data and the calculated risk of the procedure. The collected data from the perioperative and postoperative course as well as the analysed laboratory parameters were similar in both groups (Table 5).

In the group of patients operated using CECC, we found a higher number of patients having needed catecholamine therapy, and/or IABP support, before surgery. In addition, there was a higher incidence of patients presenting with cardiogenic shock or post cardiopulmonary resuscitation (CPR) in the CECC group. The P-value shows a significant difference in the catecholamine therapy and also in the group of patients who entered the operating theatre receiving CPR (Table 6).

The in-hospital mortality was investigated when comparing the overall groups (MiECC n = 113, CECC n = 74; P = 0.48), in the subgroup of patients presenting with acute myocardial infarction (MiECC n = 83, CECC n = 50; P = 0.045) and in the group of patients with positive Troponin T (>14 ng/l), acute myocardial infarction at presentation and logistic EuroSCORE between 8 and 40 (MiECC n = 46, CECC n = 31; P = 0.68). After excluding the 12 patients who needed cardiopulmonary resuscitation before surgery, the difference in in-hospital mortality between the two groups was 3.5% (MiECC) vs 4% (CECC).

DISCUSSION

Over the past 7 years, the work group at our institution have achieved excellent clinical results with regard to the use of minimally invasive extracorporeal circuits. The continuous effort to optimize the clinical implications and related options in patient care at our institution have led to a shift in the treatment of CABG patients, reaching a 81% application of MiECC circuits in 2013 [5–8]. Wiesenack et al. [9] had already postulated in 2004 that MiECC systems may serve as an alternative and less invasive approach compared with the conventional perfusion circuits in elective CABG surgery. In a propensity score analysis, Immer et al. [1] confirmed faster recovery in MiECC patients and a lower incidence of atrial fibrillation.

In our retrospective analysis, we focused only on emergency patients considering currently available data supporting MiECC in patients undergoing CABG. Though there were 76.1% patients in the MiECC vs only 68.9% in the CECC group with acute myocardial infarction at presentation, the data on in-hospital mortality are in favour of the MiECC system in the whole group as well as in all other investigated subgroups (not statistically significant). The same conclusion was published by Ried et al. [4, 10].

The findings related to the transfusion rate of units of blood per patient reveal a significant advantage (P = 0.025) of the MiECC group, probably due to the reduced amount of priming volume, smaller and shorter tubing lines and the shed blood management in MiECC procedures. When comparing the two defined subgroups, the difference in the transfusion rate of units of blood per patient is no longer statistically significant, most likely due to the smaller number of patients in each subgroup. Anastasiadis et al. [11] found in a prospective randomized study that the MiECC may attenuate the adverse effects of conventional circuits on the haematological profile of patients undergoing CABG surgery. Other randomized studies confirmed that MiECC patients need less blood during coronary bypass surgery [12].

Investigating high-risk CABG patients (EuroSCORE >10%), A. Haneya et al. [13] found a lower 30-day mortality, lower transfusion requirements, less renal and myocardial damage all in favour of MiECC. The same results were confirmed in a prospective study conducted on 40 EuroSCORE 6+ patients undergoing CABG [14] as well as in patients with reduced left ventricular function (LV-EF < 30%) [15].

As a potential next step, the treatment approach of patients in cardiogenic shock and the need for emergency myocardial revascularization therapy need to be discussed. A strategy could consist of preoperative implantation of an IABP, followed by beating heart revascularization while having the circulatory and haemodynamic support of the MiECC system. In patients with postoperative low cardiac output syndrome, the MiECC system could be alternatively used as an extracorporeal life support system in terms of a prolonged myocardial reperfusion strategy that could be further extended for application on ICU. This strategy is supported by the results published by Munos et al. [16] who found that on-pump beating heart surgery, using a minimized perfusion circuit without aortic cross-clamping, is the preferable method (included methods: MiECC, CECC, OPCAB, MiECC beating heart) for achieving complete revascularization and reduced postoperative multiorgan failure in high-risk patients (EuroSCORE >9) [17]. Speculations about maintaining native coronary blood to avoid global myocardial ischaemia as the optimal treatment for acute coronary syndrome patients whenever CABG surgery is indicated, need to be discussed [17].

In a 10-year experience using minimally invasive perfusion systems in CABG patients, the excellent survival rates, low transfusion requirements, low mortality and conversion rates were achieved in 2243 revascularization procedures by Puehler et al. [18].

After conducting a meta-analysis of randomized trials comparing the effectiveness of minimally invasive versus conventional CPB in adult cardiac surgery, Biancari and Rimpiläinen found that the MiECC was associated with lower mortality during the immediate postoperative period and with a significantly lower amount of postoperative blood loss, confirming safety and efficacy [2, 3, 19]. These results are comparable with ours with the important difference that the study comprised low-risk patients (EuroSCORE <3), whereas our study involved high-risk patients.

The results were confirmed in three other meta-analyses also reporting lower rates of postoperative atrial fibrillation and reduced rates of neurological events [20–22].

Rather sceptical findings also need to be acknowledged: Schöttler et al. [23] reported negative haemodynamic effects with more frequent dependence on norepinephrine after operations using miniaturized CPB, whereas Svitek et al. [24] found no clear clinical benefit of using minimally invasive extracorporeal circulation in CABG in low-risk patients.

Nevertheless, a learning curve has to be overcome when using minimally invasive extracorporeal circulation. Similar to OPCAB procedures, the MiECC technique requires an experienced team approach (surgeon, anaesthesiologist and perfusionist) before being routinely applied in all CABG patients.

CONCLUSION

The data in the current study support the thesis that the application of MiECC in emergency CABG patients provides a safe and feasible treatment option and are encouraging to start off a prospective randomized study with a larger target population of emergency CABG patients. Particularly in terms of lower blood transfusion rates and the related knowledge of reduced mortality after 5 years, study end-point as mortality at 30-day, 1-year and 5-year survival rates should be included in further investigations [25].

In our opinion, MiECC should be considered as a viable alternative to standard ECC in present day emergency CABG surgery.

Limitations

Our study reports results from a retrospective analysis of data from a single centre and surgeon. Even if both groups appear comparable and had a relatively equal distribution of the perioperative variables, they were not randomized, blinded or matched regarding the used ECC. Thus, defining indications and contraindications, considering various factors as patient selection, level of experience of the surgeon as well as of the operating team, may influence the patient’s outcome. In order to be able to find a significant difference in in-hospital mortality, a far larger study population would be needed. Thus, having a relative small study population, the non-statistical significant difference in mortality rate may be a type 2 error.

The statistical analysis was conducted only for the identified variables. Therefore, the possibility of interferences with non-quantified variables could not be completely excluded. The results were not adjusted for selection bias. Therefore, the gathered data could just be a reflection of the different patient populations treated with both procedures.

Conflict of interest: none declared.

REFERENCES

Author notes