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Ann Thorac Surg 2007;83:1368-1373
© 2007 The Society of Thoracic Surgeons

Original Articles: Cardiovascular

On-Pump Beating-Heart Coronary Artery Bypass: A Propensity Matched Analysis

Shinichi Mizutani, MD*, Akio Matsuura, MD, Ken Miyahara, MD, Tadahito Eda, MD, Akemi Kawamura, MD, Teruaki Yoshioka, MD, Katsuhiko Yoshida, MD

Division of Cardiovascular Surgery, Aichi Cardiovascular and Respiratory Center, Ichinomiya, Aichi, Japan

Accepted for publication November 3, 2006.

* Address correspondence to Dr Mizutani, Department of Cardiovascular Surgery, Social Insurance Chukyo Hospital, 1-1-10, Sanjo, Minami-ku, Nagoya, Aichi 457-8510, Japan (Email: smizutan{at}cf7.so-net.ne.jp).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 References
 
Background: It remains unclear how cardioplegic arrest affects surgical results after coronary artery bypass grafting surgery (CABG). This study compares early outcomes after on-pump beating-heart CABG and conventional CABG.

Methods: From 2002 to 2005, 114 patients underwent on-pump beating-heart CABG. Multivariate logistic regression revealed five characteristics according to which technique is liable to be used: history of cerebral infarction, urgent or emergent operation, lower ejection fraction, preoperative creatine kinase, and lower number of diseased vessels. The early clinical outcome for these patients was compared against 114 conventional CABG patients, matched using a propensity score constructed with these five significant variables and with two nonsignificant variables: history of diabetes mellitus and hypertension.

Results: On-pump beating-heart CABG significantly reduced the duration of operation and cardiopulmonary bypass, total blood loss, and peak creatine kinase (p < 0.05). The number of patients requiring additional intra-aortic balloon pump support was significantly lower in the on-pump beating-heart CABG group (2 versus 13, p < 0.01). No patients required percutaneous cardiopulmonary support after on-pump beating-heart CABG, whereas 4 patients needed it after conventional CABG. Complete revascularization was significantly lower (42.1% versus 77.2%, p < 0.0001), but in-hospital mortality was less in the on-pump beating-heart CABG group (2.6% versus 9.6%, p < 0.05). No significant difference was found in morbidity including stroke, renal failure, mediastinitis, and prolonged ventilation.

Conclusions: On-pump beating-heart CABG can be performed safely, including on high-risk patients. Use of cardiopulmonary bypass and the elimination of cardioplegic arrest may be of most benefit to hemodynamically unstable patients.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 References
 
Cardioplegic arrest and the use of cardiopulmonary bypass (CPB) have led to tremendous progress in coronary artery bypass grafting (CABG), but the unphysiologic nature of this technique may lead to mortality and morbidity. Off-pump CABG has the potential to reduce several adverse effects of conventional CABG. Many authors have reported the superiority in early or midterm outcome of this procedure, compared with the conventional approach, in an unselected or a specific subgroup of patients [1–5]. However, these two techniques differ significantly, in particular in the use of CPB and cardioplegic arrest. Furthermore, the chance of hemodynamic collapse during off-pump CABG remains a limitation of this procedure. Because of its hemodynamic stability, on-pump beating-heart CABG has been preferred in our institute over off-pump CABG, especially in high-risk patients. The present work evaluates the early outcome of this procedure in comparison with the conventional approach utilizing cardioplegic arrest. A propensity score analysis was performed to match the two distinct groups and permit comparison.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 References
 
The Institutional Review Board approved this study and waived the requirement for individual consent because of its retrospective manner.

Patients
From January 1999 to March 2005, 811 patients underwent CABG at our institute. Of these, 445 underwent conventional CABG alone under cardioplegic arrest. Since December 2002, our methodology for on-pump beating-heart CABG has been standardized, and 114 patients underwent CABG using this technique. A total of 559 patients were therefore included in this study. Patients who required concomitant procedure or percutaneous cardiopulmonary support preoperatively were excluded from this study. In the on-pump beating-heart CABG group, patients who underwent insertion of a left ventricular (LV) venting tube were also excluded.

Surgical Technique
All procedures were performed using CPB, established in most cases by aortic inflow cannulation. For outflow cannulation, the bicaval technique was mainly used for conventional CABG, and single two-staged cannula was mainly used for on-pump beating-heart CABG. Left ventricular venting was performed only with the conventional CABG. In the conventional CABG, all anastomosis was performed under cardiac arrest using tepid blood antegrade or retrograde cardioplegia, or both. The CPB flow was maintained at about 2.5 L · min–1 · m–2 during cardioplegic arrest. In the on-pump beating-heart CABG, all coronary anastomosis was performed using a commercially available stabilizer and heart positioner. A shunt tube was also used for coronary-graft anastomosis in many cases. Proximal anastomosis for the free graft in the on-pump beating-heart CABG was performed with partial aortic clamping. The CPB flow was maintained at 1.5 to 2.0 L · min–1 · m–2 during distal anastomosis for the left anterior descending artery, or at 2.0 to 2.5 L · min–1 · m–2 during distal anastomosis for the lateral, inferior, or posterior wall. When the heartbeat disappeared on the electrocardiogram during apical retraction, the heart was repositioned so as to keep beating.

Propensity Analysis
The choice of technique (beating-heart or conventional arrested heart) was made by the operating surgeon. Table 1 lists preoperative patient characteristics. The characteristics of the two groups are quite dissimilar. The most striking differences were in urgency of the operation, ejection fraction, and the level of creatine kinase (CK). Because of the substantial differences in baseline characteristics of patients in the two groups, a propensity score analysis was performed to adjust for treatment selection bias [6].


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  		Table 1 Preoperative Clinical Characteristics
 
First, to identify the character with which on-pump beating-heart CABG was liable to be selected, multivariate logistic regression was used. Of 18 defining baseline variables, 13 were not significant in the logistic regression analysis. These were sex, age, diabetes mellitus, hypertension, hyperchoresterolemia, chronic renal insufficiency, body mass index, history of smoking, the extent of aortic calcification, chronic obstructive pulmonary disease, congestive heart failure, prior cardiac operation, and peripheral vascular disease. Logistic regression analysis identified five variables as significant predictors of the performance of on-pump beating-heart CABG. These five were having a history of cerebral infarction, urgent or emergent operation, lower LV ejection fraction, log CK, and lower number of diseased vessels (Table 2).


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  		Table 2 Multivariate Logistic Regression Analysis for the Selection of On-Pump Beating-Heart Coronary Artery Bypass Grafting Surgery
 
Second, a propensity score was calculated for all 559 patients using these significant regression coefficients. To minimize the selection bias of the groups, two nonsignificant variables, history of diabetes mellitus and hypertension, were also used to construct a propensity score. Each on-pump beating-heart CABG patient was then closely matched with a conventional CABG patient having the same or nearest propensity score. The matching was performed in the total population. This process successfully matched the 114 patients who underwent on-pump beating-heart CABG with 114 patients who underwent conventional CABG with arrested heart. The resulting 228 matched patients are the focus of the outcomes analysis in this study.

Patient Characteristics After Matching
Preoperative clinical patient characteristics for the on-pump beating-heart CABG and the conventional CABG groups after matching are listed in Table 3. All characteristics that differed significantly between two groups before matching became similar.


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  		Table 3 Preoperative Patient Characteristics After Matching
 
Statistical Analysis
Continuous data are quoted as mean ± SD unless indicated, and were compared between groups using Student’s t test. Binary variables were compared using the {chi}2 test or Fisher’s exact test for expected cell counts less than 5. For CK levels and intensive care unit stay, a logarithmic transform was applied to the data before analysis, because the variance tended to be too large. Statistical significance was defined as p less than 0.05. Statistical analysis was performed using Stat View 5.0 (Abacus Concepts, Berkeley, California).


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 References
 
Perioperative Surgical Characteristics
Perioperative and postoperative patient characteristics are shown in Tables 4 and 5. Go On-pump beating-heart CABG was associated with a shorter duration of the operation and CPB, less total blood loss, and lower log peak CK. Thirteen patients in the conventional CABG group required additional intra-aortic balloon pump initiation intraoperatively or postoperatively, whereas only 2 patients required this in the on-pump beating-heart CABG group (p < 0.005). Four patients in the conventional CABG group required percutaneous cardiopulmonary support postoperatively owing to difficulty with cessation of the CPB or low cardiac output; no patients needed this in the on-pump beating-heart CABG group. The number of bypass grafts and the rate for complete revascularization were lower in the on-pump beating-heart CABG group (p < 0.0001).


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  		Table 4 Perioperative Surgical Characteristics
 

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  		Table 5 Early Operative Outcome
 
Early Operative Outcome
The incidence of paroxysmal atrial fibrillation and of atrial flutter was lower in the conventional CABG group (p < 0.005). Other complications, including incidence of cerebrovascular accidents, new requirement for hemodialysis, mediastinitis, and prolonged ventilation for more than 72 hours, did not differ between the two groups. The duration of intensive care unit stay was shorter in the on-pump beating-heart CABG group (p < 0.05). The LV ejection fraction of surviving patients at discharge was lower in the on-pump beating-heart CABG group (p < 0.05). In-hospital mortality was lower in the on-pump beating-heart CABG group than in the conventional CABG group (2.6% versus 9.6%, p < 0.05).


   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
References
 
Despite its technical difficulty, off-pump coronary revascularization has been used widely in the present decade. Many investigators have reported the usefulness of this methodology in specific subgroup comparisons with the result of conventional CABG. The difference between these two techniques involves two major components, however. These are the use of CPB, and cardioplegic arrest. It is still not known which component is more important for good surgical outcome of off-pump coronary revascularization. Elimination of the CPB may in fact reduce the physiologic derangement caused by the systemic inflammatory response [7–10] and the direct complication that stems from the use of CPB. Despite the most modern cardioplegic techniques, acute postsurgical ventricular dysfunction is reportedly common [11, 12]. Although this dysfunction is usually resolved within hours to days after surgery, this depression may influence the result if the patient is significantly ill. Cardioplegic arrest may therefore have a direct adverse effect on the heart that gives rise to an increase in morbidity and mortality, especially for high-risk patients. On-pump beating-heart CABG can eliminate one of these components, and is reported to be an acceptable trade-off between conventional CABG and off-pump CABG [13]. Consequently, the present comparison between this technique and conventional CABG should clarify the effect of cardioplegic arrest on the surgical outcome.

The most notable characteristic of the population who underwent on-pump beating-heart CABG at our institute was their severity of disease. At least half of the cases underwent surgery with urgent or emergent status. The LV ejection fraction of these patients was significantly lower. The mean preoperative CK was as high as 562.6 IU/L, and 40.4% of the patients showed more than 200 IU/L of the preoperative CK levels, suggesting that this population includes many patients in the acute phase of myocardial infarction. Early surgical outcome after matching was better in the on-pump beating-heart CABG group. The number of patients who required subsequent intra-aortic balloon pump support was significantly lower. Also, no patients in the on-pump beating-heart CABG group needed percutaneous cardiopulmonary support in response to difficulty or impossibility of weaning from the CPB, whereas 4 patients needed it in the conventional CABG group. The peak CK value after surgery was significantly lower, suggesting lesser impact of the on-pump beating-heart technique on the myocardial injury in the perioperative period. In-hospital mortality was significantly lower in the on-pump beating-heart CABG group than in the conventional CABG group. These considerations may all be related to the elimination of cardioplegic arrest, which is the main difference between the two techniques. While criticism could be made that the mortality rate in the conventional CABG group was much higher than results reported from other centers for the same period, the total in-hospital mortality of this study population before matching was 3.0% (17 of 559). This finding shows that a significant number of patients who died in-hospital were selected by the propensity matching. Locker and colleagues [14] advocate not using CPB for coronary revascularization within 48 hours of the onset of acute myocardial infarction, based on comparison of conventional CABG and off-pump CABG. Patients in the present study population have diverse status, from stable angina to acute myocardial infarction, but on-pump beating-heart CABG was performed safely even in their emergent situation. Our results suggest that avoidance of cardioplegic arrest is more important than avoidance of CPB in such cases. Consistent with our finding, Izumi and colleagues [15] have reported that on-pump beating-heart CABG may be an acceptable option for acute myocardial infarction.

A powerful reason for preferring on-pump beating-heart CABG to off-pump coronary revascularization is hemodynamic stability during bypass grafting. A total of 113 patients underwent off-pump CABG during the same study period in our institute. Although the indication for off-pump CABG differed between cases, off-pump procedures were performed on relatively safe cases, taking into account lesions, target vessels, and preoperative hemodynamic stability. In cases in which hemodynamic collapse was expected during anastomosis, off-pump CABG was rarely indicated in our institute. As a result, we have seldom encountered situations in which emergent circulatory support was needed during off-pump coronary revascularization. Several authors have, however, reported that hemodynamic collapse and emergent conversion to CPB from off-pump CABG is associated with poor prognosis [16–18]. Our data suggest that on-pump beating-heart CABG can be performed safely even in patients who might suffer hemodynamic collapse during surgery. Since a report has suggested that high-risk patients benefit most from off-pump CABG [5], comparison between on-pump beating-heart CABG and off-pump CABG in patients of comparable risk is necessary to settle this issue.

Our methodology for on-pump beating-heart CABG has, as stated above, been standardized since December 2002. The main features of this technique are the use of CPB, the avoidance of cardioplegic arrest, use of the device for off-pump CABG, and the avoidance of LV venting. We applied LV venting during on-pump beating-heart CABG as well as conventional CABG up to December 2002, when we began our current method. A total of 38 patients underwent on-pump beating-heart CABG with insertion of a LV vent tube before December 2002. However, low cardiac output developed in several cases after bypass grafting. While several salvage CABG cases were included, the in-hospital mortality in these patients was extremely high (15.8%). We have since abandoned LV venting during on-pump beating-heart CABG, and such cases have become correspondingly rare. No comparisons have been made, because the number of cases is too small, but maintenance of some cardiac output and elimination of global ischemia might be important in preventing deterioration of cardiac function even if CPB is used. A future study will address this issue.

As expected, no significant benefit was detected for on-pump beating-heart CABG in relation to morbidity, including the incidence of stroke, renal failure, mediastinitis, and prolonged ventilation, which are all believed to be related to the use of CPB itself [19, 20], as used in all patients in this study. This might be the ultimate limitation to on-pump beating-heart CABG. Also, the rate for complete revascularization was significantly lower in the on-pump beating-heart CABG group. The shorter operation time and the shorter CPB time in the on-pump beating-heart CABG group may be related to this. Although it is not clear why incomplete revascularization was so frequent in this group, because it was not always stated in the medical record, a possible explanation is that the culprit lesion or jeopardized vessel might be specially targeted in emergent cases to reduce surgical invasiveness. This trend was significant in the early period of this study when we were early on the learning curve. The recent rate for complete revascularization in our institute is much higher than in this study period. Kilo and colleagues [21] have reported that incomplete target vessel revascularization of the culprit lesion is a promising option for selected high-risk patients, due mainly to lower perioperative mortality while their patients underwent off-pump CABG. However, poor late result after incomplete revascularization has been reported in both conventional and off-pump CABG [22, 23]. Future follow-up is needed to identify how this affects late survival.

This study is retrospective, and may therefore involve too many confounding factors to show any clear advantage of on-pump beating heart CABG. A prospective randomized trial will be needed to settle this issue. The small number of patients that were enrolled for matching is a further limitation. In summary, on-pump beating-heart CABG may ameliorate early outcome, especially of high-risk patients and urgent cases; less complete revascularization led to uncertainty over late results.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 References
 

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  2. Sabik JF, Gillinov AM, Blackstone EH, et al. Does off-pump coronary surgery reduce morbidity and mortality? J Thorac Cardiovasc Surg 2002;124:698-707.[Abstract/Free Full Text]
  3. Puskas JD, Williams WH, Duke PG, et al. Off-pump coronary artery bypass grafting provides complete revascularization with reduced myocardial injury, transfusion requirements, and length of stay: A prospective randomized comparison of two hundred unselected patients undergoing off-pump versus conventional coronary artery bypass grafting J Thorac Cardiovasc Surg 2003;125:797-808.[Abstract/Free Full Text]
  4. Al-Ruzzeh S, Nakamura K, Athanasiou T, et al. Does off-pump coronary artery bypass (OPCAB) surgery improve the outcome in high-risk patients?A comparative study of 1398 high-risk patients. Eur J Cardiothorac Surg 2003;23:50-55.[Abstract/Free Full Text]
  5. Stamou SC, Jablonski KA, Hill PC, Bafi AS, Boyce SW, Corso PJ. Coronary revascularization without cardiopulmonary bypass versus the conventional approach in high-risk patients Ann Thorac Surg 2005;79:552-557.[Abstract/Free Full Text]
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  8. Wan S, Izzat MB, Lee TW, et al. Avoiding cardiopulmonary bypass in multivessel CABG reduces cytokine response and myocardial injury Ann Thorac Surg 1999;68:52-56.[Abstract/Free Full Text]
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  10. Rastan AJ, Bittner HB, Gummert JF, et al. On-pump beating heart versus off-pump coronary artery bypass surgery—evidence of pump-induced myocardial injury Eur J Cardiothorac Surg 2005;27:1057-1064.[Abstract/Free Full Text]
  11. Breisblatt WM, Stein KL, Wolfe CJ, et al. Acute myocardial dysfunction and recovery: a common occurrence after coronary bypass surgery J Am Coll Cardiol 1990;15:1261-1269.[Abstract]
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  14. Locker C, Mohr R, Paz Y, et al. Myocardial revascularization for acute myocardial infarction: benefits and drawbacks of avoiding cardiopulmonary bypass Ann Thorac Surg 2003;76:771-776.[Abstract/Free Full Text]
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