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Department of Cardiovascular Surgery, University Hospital, Berne, Switzerland
Accepted for publication May 29, 2007.
* Address correspondence to Dr Immer, Department of Cardiovascular Surgery, University Hospital, Berne, 3010, Switzerland. (Email: franzimmer{at}yahoo.de).
Presented at the Forty-third Annual Meeting of The Society of Thoracic Surgeons, San Diego, CA, Jan 29–31, 2007.
| Abstract |
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Methods: One thousand fifty-three consecutive patients underwent CABG surgery using the MECC perfusion technique. Subgroup analyses focused on perioperative myocardial markers (cardiac troponin I [cTnI]), incidence of atrial fibrillation (AF), and perioperative evaluation of inflammatory markers and data were compared with those of patients who underwent CABG using CECC. A propensity score analysis was performed.
Results: Patient characteristics and distribution of EuroSCORE risk were similar in both groups. Severity of coronary artery disease and extent of revascularization were also comparable in both groups (number of distal anastomoses: 3.2 ± 1.1 in CECC vs 3.2 ± 0.9 in MECC; p = not significant [ns]). The cTnI was significantly lower in the MECC group (11.0 ± 10.8 µg/L in MECC vs 24.7 ± 25.3 µg/L in CECC; p < 0.05). Incidence of AF was 11.1% in MECC and 39.0% in CECC (p < 0.05). Inflammatory markers (interleukin-6, SC5b-9) were lower in MECC patients (p < 0.05). Propensity score analysis confirmed faster recovery in MECC patients and lower incidence of AF.
Conclusions: Minimal extracorporeal circulation is a safe perfusion technique for CABG and may therefore concurrence OPCAB and traditional CABG under CECC.
| Introduction |
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Cardiac troponin I (cTnI) was shown to be a sensitive and specific marker of myocardial damage after open heart surgery [8–10]. It is also a reliable indicator of clinical outcome. In pediatric cardiac surgery, for example, it was shown that high cTnI values are associated with an adverse midterm outcome [11]. Coronary artery bypass grafting on a beating heart (off-pump; OPCAB) has gained popularity as an alternative to CABG with CECC, and it is argued that a substantial number of CECC-related complications may be eliminated. A few studies have shown, for example, that OPCAB significantly reduces postoperative cTnI levels as compared with on-pump coronary surgery [12]. Nevertheless, a traditional CECC seems to remain preferred by most cardiac surgeons, possibly due to the higher technical comfort obtained with this approach.
Minimized extracorporeal circulation (MECC) may represent an attractive alternative to CECC. It integrates a centrifugal pump, a membrane oxygenator, and in our model an optoelectrical suction system. The circuit has a very short tubing length. The cardiotomy reservoir and the conventional suction device have been eliminated. The priming volume is therefore reduced to 900 mL. In the present study, we analyzed the potential benefits of MECC in patients undergoing CABG and compared the results with those of CECC.
| Patients and Methods |
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Measurement of Inflammatory Markers
Inflammatory response was compared between both systems. However, in order to avoid a possible selection bias, we focused our analysis on 60 consecutive patients prospectively randomized to be operated either with CECC or MECC. Serial venous blood samples were taken preoperatively: baseline, after heparin administration, ten minutes after initiation of cardiopulmonary bypass, after protamine sulfate administration (in a 1:1 ratio), as well as two and 24 hours after surgery. The enzyme-linked immunosorbent assay kits were used for measurement of interleukin-6 (IL-6) (Endogen, Human IL-6; Pierce Biotechnology Inc, Rockport, IL), SC5b-9 (Quidel, San Diego, CA), and human lactoferrin (HL) (Bioxytech; OXIS Health Products Inc, Portland, OR).
Measurement of Myocardial Markers
Serial venous blood samples were taken preoperatively in both groups as well as after aortic declamping at 6, 12, and 24 hours. The cTnI concentration was analyzed by immunoassay using the Stratus II analyzer (Dade Behring, Eschborn Germany), the upper normal limit being 0.6 µg/L. Creatine kinase isoenzyme MB (CK-MB) was also measured preoperatively and at 6, 12, and 24 hours after aortic declamping. A 12-lead electrocardiogram (ECG) was recorded preoperatively as well as 6 and 24 hours after surgery. Patients with perioperative myocardial infarction (PMI) were excluded from the present analysis. Criteria used for definition of PMI were the following: new Q waves of more than 0.04 ms and a reduction in R waves of more than 25% in at least two leads in the ECG, as well as echocardiographic criteria according to the guidelines reported by the Society of Thoracic Surgeons. The CK-MB diagnosis criteria for PMI were CK-MB values of more than 60 IU/L at 6, 12, or 24 hours postoperative.
Incidence of Postoperative AF
Postoperative AF was considered in patients until discharge, providing electrolyte levels had been controlled and alterations corrected. In cases where AF was suspected a 24-lead electrocardiogram was recorded. Patients with preoperative elevation of cTnI and [or] history of intermittent or persistent AF were excluded. Maximal body weight increase after surgery was recorded and compared between the groups. Overall, 752 patients (71.4%) from the MECC group fulfilled the criteria and were compared with data from 278 patients (74.5%) who underwent isolated CABG surgery with CECC.
Operative Technique
Our minimized cardiopulmonary bypass (MECC; Jostra AG, Hirrlingen, Germany) included a centrifugal pump and a Quadrox oxygenator (Jostra). The circuit had no cardiotomy reservoir and its priming volume was 900 mL (900 mL Ringer lactate), whereas it was 1,800 mL for the CECC used in the current study (900 mL crystalloid and 900 mL colloid solutions). In our setting air entry into the venous side is detected by an ultrasound probe and immediately stops the perfusion. Up to now only one air emboli has been detected in 1,053 patients operated with the MECC system. Air removal was rapidly performed without consequence. To prevent additional damage to the blood cells a new suction device, the SmartSuction (Cardiosmart AG, Muri, Switzerland), was integrated into the MECC system. Aspiration is controlled by an optoelectrical sensor placed at the tip of the suction cannula and is activated only when in direct contact with a liquid interface. Aspirated blood is automatically retransfused into the venous line of the circuit and therefore no additional suction pump or reservoir is required. In the CECC group a classical suction unit was used, driven by a roller pump, and the aspirated blood was collected into the cardiotomy reservoir.
Surgery was performed through a median sternotomy. After heparinization (MECC, 250 IU/kg; CECC, 400 IU/kg) cardiopulmonary bypass was conducted with an arterial cannula in the ascending aorta, and a two-stage venous cannula inserted through the right atrium or a bicaval cannulation technique in case a patent foramen ovale needed to be closed. In patients where a MECC system was used the venous cannula(e) was (were) secured with two snares to ensure proper fixation and stabilization, and to minimize the risk of air aspiration into the circuit. The rationale for applying two different levels of systemic anticoagulation is based on the difference of both circuits. Indeed, the closed MECC circuits prevent air-blood contact and therefore require a reduced degree of systemic anticoagulation. Neither heparin-coated tubing nor other "biocompatible" materials were used in either group. However, the heparin-bonded oxygenators were similar in CECC and MECC patients. In our practice the target activated clotting time (ACT) is 480 seconds in MECC and 700 seconds in CECC perfusions. After baseline examination ACT is monitored every 20 minutes. Flow rates were adjusted according to cardiac index (CI) and body surface area (BSA): in MECC perfusion flow rate is calculated by 1.8 CI x BSA and in CECC by 2.4 CI x BSA. Both groups underwent surgery in moderate hypothermia (32°C). Myocardial protection was performed with a single shot crystalloid cardioplegia in both techniques followed by high-potassium cold blood cardioplegia during five minutes in the aortic root immediately after cross-clamping the aorta, and repeated every 20 to 30 minutes in the CECC group. In the MECC group crystalloid cardioplegia was repeated only if mechanical activity was observed. A one-minute warm reperfusion with 37°C oxygenated blood with a flow rate of 200 to 300 mL/minute was performed immediately after the last distal anastomosis while the mammary artery(ies) was (were) unclamped. In both groups proximal graft anastomoses to the ascending aorta were performed during rewarming.
Statistical Analysis
Data are presented as mean values ± their first standard deviation. The Mann-Whitney U test and the
2 test were used for comparison between groups of continuous and nominal variables, respectively. A p value of less than 0.05 was considered significant. The statistical analysis was performed using the SPSS 11.0 software (SPSS Inc., Chicago, IL). We used the propensity score technique to confirm and validate our results and accordingly extracted 373 patients from the MECC group and compared them with data obtained from matched patients from the CECC group as in the following steps.
Step 1
The CECC and MECC groups were compared and the appropriate statistical tests were used (Student t test for unpaired data and the Pearson
2 test) to identify significant differences regarding preoperative and intraoperative variables. Variables related to the different surgical technique were excluded from the analysis. Six variables were found to be different: age, weight, ejection fraction, history of myocardial infarction, history of cerebrovascular accident, and diabetes.
Step 2
These six variables were further analyzed in a multivariable stepwise logistic regression, taking CECC as dependent variable. The final predictive model included four independent variables: age (p < 0.031), history of myocardial infarction (p < 0.001), history of cerebrovascular accident (p = 0.039), and diabetes (p < 0.001).
Step 3
Patients in the CECC group were divided into quintiles according to their propensity score. For each quintile a corresponding number of patients from the MECC group were randomly assigned. The homogeneity of both groups was then verified, comparing preoperative and intraoperative variables, as well as the primary operating surgeon, and revealed no significant differences.
| Results |
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Subsequently, 1,257 patients could be analyzed (CECC: n = 326 [87.4%]; MECC: n = 931 [88.4%]; p = ns). Preoperative and intraoperative characteristics were similar in both groups. Postoperatively, the MECC group showed significantly lower concentrations of cTnI 11.0 ± 10.8 vs 24.7 ± 25.3 µg/L (p < 0.05) in the CECC group 24 hours postoperatively (Fig 2).
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Propensity Score Analysis
The control group created through the propensity score analysis was homogeneous for all preoperative and intraoperative variables. Data have not been corrected to the different CPB techniques, taking into account the lower priming volume in the MECC group and consequently a significantly higher nadir of the ACT value on CECC. The multivariable stepwise forward logistic regression analysis confirmed the findings of data analysis of the total collective with a better outcome for MECC patients in terms of early extubation, duration on ICU, and postoperative incidence of AF (Table 2).
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| Comment |
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The principle of MECC was developed and clinically successfully introduced by Wiesenack and colleagues [14], and today several models are proposed on the market. Our personal setting proposes an implementation of the system by using the SmartSuction system (Cardiosmart). Technically, distal anastomoses are performed on the arrested heart similarly as for the conventional CECC approach and the technique is not limited by possible hemodynamic instability. In our opinion, MECC is technically less demanding than OPCAB surgery and allows maintaining a constant and stable peripheral perfusion. Interestingly, the MECC approach allowed reducing the ECC and ACC times. This is due mainly to the difference of time necessary to administer cardioplegia (single shot cardioplegia immediately after ACC in MECC patients versus a traditional five minutes initial cardioplegia administration repeated every 20 to 25 minutes in CECC patients). In fact, because the aorta is not vented in our MECC setting the heart is not completely unloaded during the procedure, and consequently a slight coronary flow persists in the majority of patients. This minimal residual perfusion avoids the presence of air in the coronary system and thus may explain the improved myocardial protection we observed. Indeed, in our study, cTnI concentration and CK-MB values in the MECC group were comparable with those recently reported by Alwan and colleagues [12] in patients who underwent beating heart surgery, as well as within the same range as a smaller collective of MECC patients from a previous report from our group [15]. The improvement of myocardial protection may also be reflected by the fact that defibrillation to restore a sinus rhythm (SR) after opening the aortic clamp was much less frequently required (in only 23 patients [2.2%] of the mini-ECC-group) as spontaneous restoration of a SR occurred in all other patients. Conversely, 178 patients (47.7%) of the CECC group necessitated to be defibrillated after aorta declamping (p < 0.05). Importantly, it is less than probable that the highest cardiac enzyme we observed in the CECC group is explained by a corresponding higher rate of perioperative defibrillation, as it was previously shown that defibrillation provokes none or only minor cTnI elevation [16, 17]. The superiority of MECC was also indirectly reflected by a perioperative, noticeably improved myocardial contractility after aorta declamping. Overall, the summation of these small advantages may account for an earlier recovery as confirmed by a reduced ICU and hospital length of stay.
Elimination of blood-air interaction is probably a major advantage of our MECC setting. Indeed, the elimination of the venous reservoir, combined with the modified optoelectrical suction system and the avoidance of aortic venting, allows minimizing the interaction of blood with air. This probably explains the reduced early postoperative inflammatory activation in the MECC as compared with CECC patients. Previous studies [18, 19] have also confirmed an advantage of eliminating the interaction of air and blood.
The largely reduced priming volume of our MECC system, and consequently the possibility to maintain a normal hematocrit during the CPB processing, not only contribute to the reduced postoperative body weight changes, but certainly explain the decreased blood transfusion requirement, a factor clearly associated with postoperative outcome in cardiac surgical procedures. The postoperative reduction of AF (with an incidence of only l1% in the MECC group of patients) without additional preoperative prophylactic medication might also be explained by a decrease in the usual volume compartment shift, traditionally observed in patients undergoing CECC (Fig 3).
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We conclude that the MECC system, at least in the setting we developed, is a safe procedure and seems to improve the ECC-related adverse effects. Because it still allows hemodynamic support during the surgical procedure, it may then constitute an attractive alternative to CECC or OPCAB.
| Discussion |
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DR IMMER: We dont use actually the MECC system for open-heart procedures. There is one group in France who reported results for aortic valve replacement. We just use the MECC system in coronary artery bypass grafting. What we perform, actually, is closure of the PFO, or atrial septal defect, on the MECC system because we have a closed circuit. We are typically an on-pump clinic, so we perform around 90% on-pump. We have the amount of 10% beating heart procedures. Thats a part of our philosophy. We think that improving the pump could reduce the complication associated with the classical pump. And we think that we should perform arterial bypasses, good quality and good exposition, so we are not typically a beating heart clinic.
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