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Ann Thorac Surg 2003;75:505-507
© 2003 The Society of Thoracic Surgeons

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Original article: cardiovascular

Off-pump coronary artery bypass grafting does not decrease the incidence of atrial fibrillation

^a Division of Cardiothoracic Surgery, The Ohio State University Medical Center, Columbus, Ohio, USA

Accepted for publication August 14, 2002.

* Address reprint requests to Dr Brown, Midwestern Cardiac Surgery, Inc, 680 Park Ave West, Suite 203, Mansfield, OH 44906, USA
e-mail: dabrown{at}mwcsinc.com

	Abstract

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BACKGROUND: Postoperative atrial fibrillation remains a significant source of morbidity after coronary artery bypass grafting. We reviewed the data on 2,569 patients to determine if the absence of cardiopulmonary bypass resulted in a lower incidence of atrial fibrillation.

METHODS: All patients undergoing coronary artery bypass grafting without cardiopulmonary bypass from January 1, 1997 through June 30, 2001 were evaluated for postoperative atrial fibrillation. The data of 252 patients with no cardiopulmonary bypass (group 1) were reviewed and compared with three other patient groups. Group 2 consisted of 1,470 patients using cardiopulmonary bypass during the same study period. Group 3 consisted of 841 patients with a similar number of grafts as the study group but using cardiopulmonary bypass. Group 4 consisted of historical data for 847 patients operated on using cardiopulmonary bypass collected from January 1995 through December 1996. Prophylactic ß-blockade was instituted in January 1997. Groups 1 to 3 received this treatment, but group 4 did not.

RESULTS: Group 1 had an incidence of atrial fibrillation of 8.8%. Groups 2, 3, and 4 had incidences of atrial fibrillation of 11.6%, 9.4%, and 28.0%, respectively. When compared with group 1, the incidence of atrial fibrillation in group 4 was statistically different (p <. 0001).

CONCLUSIONS: Avoiding cardiopulmonary bypass did not aid the reduction of atrial fibrillation at our institution.

	Introduction

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The incidence of atrial fibrillation after coronary artery bypass grafting (CABG) consistently has been reported as between 20% to 40% in large groups of patients [1–3]. This clinical problem continues to remain a significant source of morbidity. Increased risk of stroke, length of stay in the intensive care unit, total hospital stay, need for antiarrhythmics and permanent pacemakers, as well as total cost of patient care have all been attributed to postoperative atrial fibrillation [1, 4–6]. Numerous preoperative, intraoperative, and postoperative risk factors have been identified and targeted as possible culprits for triggering postoperative atrial fibrillation. The use of cardiopulmonary bypass has routinely been implicated as a primary cause of postoperative atrial fibrillation. In this study we compared the incidence of postoperative atrial fibrillation in patients who underwent coronary revascularization using cardiopulmonary bypass (CPB) with a large group of patients who were revascularized without using CPB. Minimally invasive direct coronary artery bypass (MIDCAB) and off-pump coronary artery bypass (OPCAB) techniques were used to determine whether the absence of CPB resulted in a lower incidence of postoperative atrial fibrillation.

	Material and methods

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Patient selection
Data were gathered for patients undergoing CABG at our institution from January 1995 through June 2001. All patients with concomitant procedures were excluded from the data. Patients undergoing revascularization with the use of CPB from January 1995 through December 1996 (n = 847) compromised a historical comparison group (group 4). The remaining patients were divided into other groups depending on the surgical approach and whether CPB was used. Three principal surgeons operated on the patients, with each surgeon performing both on-pump and off-pump operations. Surgeon preference and patient suitability determined the type of surgical approach. One surgeon stopped using the MIDCAB approach because of personal experience with increased patient discomfort. Off-pump operations were attempted on any patient in which it was believed to be hemodynamically stable and with virtually all single-vessel operations.

Group 1 consisted of 252 patients who underwent CABG without the use of CPB. Group 2 consisted of 1,470 patients who underwent CABG using CPB. Group 3 was a subgroup of group 2, consisting of 841 patients undergoing revascularization using CPB, but with a similar number of grafts (1 to 2 distal grafts) compared with group 1.

Operative technique
Off-pump operations
These operations were performed either by using the MIDCAB or OPCAB approach, again depending on surgeon preference. The MIDCAB procedures were performed through a left anterior thoracotomy incision with harvesting of the left internal mammary artery by either direct or thorascopic techniques. The OPCAB procedures were performed through a median sternotomy approach with standard left internal mammary artery dissection. Other conduits for revascularization included the right internal mammary artery, the radial artery, or saphenous vein grafts, or a combination thereof, dependent on patient need. The type of stabilization device used was determined by the discretion of the individual surgeon. Proximal anastamoses were performed either directly to the left internal mammary artery as T-grafts or directly to the aorta with partial occlusion clamping. Patients undergoing MIDCAB and OPCAB operations did not receive epidurals or magnesium.

Bypass operations
The more standard CABG procedure used a median sternotomy incision. Single two-stage venous and aortic cannulations were used. Revascularization was performed during a period of moderate systemic hypothermia (30°C to 34°C) and cardiac arrest. After aortic cross clamping, cardioplegic arrest was achieved and maintained with individual surgeon preferences including antegrade and retrograde cardioplegic techniques, as well as either crystalloid or blood cardioplegia. Proximal anastamoses were performed under single cross clamp or side-biting clamp depending on surgeon preference. Two grams of magnesium were routinely given after release of the cross clamp. The sternotomy was closed in a standard fashion.

All patients were continuously monitored postoperatively for the duration of their hospital stay by telemetry. Postoperatively all patients were kept within the normal range for magnesium and potassium, which were replaced as needed. In 1997 the routine use of prophylactic postoperative ß-blockade (metoprolol 25 mg twice daily, titrating to 50 mg twice daily for a heart rate > 60) was instituted in all patients undergoing CABG. Therefore the patients in group 4 preceded this institutional change.

For the purpose of this study, atrial fibrillation was defined as sustained atrial fibrillation requiring pharmacological intervention or electrical cardioversion, or both. Other possible measures included percentage of reoperation, preoperative use of ß-blockers, and ejection fraction.

Statistical analysis
All analyses were performed using the Statistical Package for the Social Sciences, Version 10.1 (SPSS Inc, Chicago, IL). Age data are presented as mean (± standard deviation) and compared between groups using the Student’s t test. Differences in incidences between the groups were analyzed using analysis of variance. All p values less than 0.05 were considered significant.

	Results

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Preoperative demographic comparison of all four groups are outlined in Table 1. There were no statistically significant differences between any of the patient groups with respect to mean age or to age more than 70 years. Groups 2 and 4 had significantly less percentage of reoperative procedures compared with groups 1 and 3. Group 1 had significantly less males compared with group 3. The ejection fraction was significantly different between all groups. Group 4 was statistically different from the other groups when considering the use of preoperative ß-blockers.

	Comment

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Postoperative atrial fibrillation remains the most common cause of morbidity after CABG [3] and carries a risk of hemodynamic instability and increased risk for thromboembolic events [1–3], as well as a significant economic impact. Need for further intervention both pharmacologically as well as surgically, longer intensive care unit stays, and longer hospital stays have all translated into increased hospital and patient charges [1, 4–6].

Although the cause of postoperative atrial fibrillation in patients undergoing coronary revascularization remains unknown, multiple risk factors and triggering events have been suggested. Several reports have targeted the use of CPB as a significant factor. Atrial manipulation and cannulation, cardioplegic arrest, prolonged aortic cross-clamp times, poor preservation of atrial tissue, and release of systemic mediators have also been implicated [1, 4, 6, 7]. This study was designed to determine if eliminating CPB would lower the incidence of postoperative atrial fibrillation.

Data regarding the use of CPB and the incidence of atrial fibrillation is conflicting. Previous reports with small numbers of patients have demonstrated no decrease in atrial fibrillation in comparing these two patient populations [8–10]. Other studies have shown decreased rates of atrial fibrillation in patients who have undergone CABG without the use of CPB [11–12]. Kilger and colleagues [11] actually suggest that the use of CPB was responsible for an increased rate of atrial fibrillation. In this study, there was no statistical difference in the incidence of atrial fibrillation between groups 1, 2, and 3. Preoperatively, these three groups were quite similar except that the CPB group 2 had less reoperative patients and group 3 had slightly more males compared with group 1.

Avoiding CPB in CABG patients has not led to the reduction of atrial fibrillation at our institution in the last 3 years. There may have been a lower incidence of atrial fibrillation in the operations performed using cardiopulmonary bypass because of the use of magnesium, however the effects of intravenous magnesium should be considered transient at best.

A statistically significant difference was identified in all groups from 1997 to 2001 when compared with the historical CPB group (group 4). Increased circulating catecholamines have repeatedly been postulated as an important pathogenesis for triggering postoperative atrial fibrillation [13]. In reviewing changes instituted at our program that could have led to such an improvement, the addition of routine ß-blockade prophylaxis for atrial fibrillation for all postoperative CABG patients was identified as a significant change. This would support what has already been described about the efficacy of ß-blockade in reducing postoperative atrial arrhythmias [14–15]. The increased use of preoperative ß-blockers and its subsequent withdrawal probably also contributed to the difference in incidence in group 4 when compared with the other groups.

Atrial fibrillation remains a difficult postoperative concern and the exact cause is uncertain. In our institution we have not lowered the incidence of atrial fibrillation by simply removing cardiopulmonary bypass.

	References

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