Original Article

Incidence of Atrial Fibrillation after Off-pump and On-pump Coronary Artery Surgery: Current Best Available Evidence

S Raja, G Dreyfus

Keywords

atrial fibrillation, cardiopulmonary bypass, off-pump coronary artery bypass surgery, opcab

Citation

S Raja, G Dreyfus. Incidence of Atrial Fibrillation after Off-pump and On-pump Coronary Artery Surgery: Current Best Available Evidence. The Internet Journal of Thoracic and Cardiovascular Surgery. 2003 Volume 6 Number 2.

Abstract

Coronary artery bypass graft (CABG) surgery is an effective form of treatment for patients with ischemic heart disease. This method is well tolerated by majority of patients; however it can cause some complications. The early postoperative atrial fibrillation (AF) is among the most common ones. Incidence of postoperative AF varies from 5 to over 40% according to definition of the arrhythmia, patients' characteristics, type of operation and method of heart rhythm monitoring. Some investigators consider postoperative AF to be a benign and self-limited arrhythmia. It rarely has a fatal outcome, however may lead to instability of the patient, prolongs hospital stay and increases costs. In some cases AF can be the reason of perioperative myocardial infarction, stroke, and persistent congestive heart failure. The use of cardiopulmonary bypass (CPB), the influence of cardioplegia and myocardial ischemia are possible factors responsible for postoperative occurrence of AF. For last few years off-pump coronary artery bypass (OPCAB) surgery on the beating heart, without cardiopulmonary bypass has become very popular. Rapid development of technology for OPCAB, especially stabilizing devices, has made it possible to approach almost all surfaces of the beating heart. OPCAB has excellent short-term results, however is not completely free from complications. The problem of atrial fibrillation in patients after beating heart surgery appears to be controversial. This review article analyses the available evidence to try and solve this controversy.

 

Introduction

Atrial fibrillation (AF) is one of the most common arrhythmias to occur after conventional coronary artery bypass grafting (CABG) with cardiopulmonary bypass (on-pump CABG).1,2 Postoperative AF has been associated with increased morbidity and prolonged hospitalization after on-pump CABG surgery. Several clinical factors have been associated with AF after on-pump CABG.3,4,5,6 Available evidence suggests that surgical “over-manipulation” of the right atrium7, surgical thoracic trauma8, use of cardioplegic solutions1, cross-clamping of the aorta1, withdrawal of β-blockers administered preoperatively9, structural changes in the heart such as those related to age, as well as the effects of postoperative hypoxia, hypovolemia, and electrolyte imbalance7 could trigger AF in patients undergoing on-pump CABG. Clinical factors that predispose persons to AF may act through a common denominator: “the dispersion of refractoriness,” a term used to describe the heterogeneity of local atrial refractory periods.10,11

Electrophysiologic mechanism of postoperative atrial fibrillation

The electrophysiologic mechanism of postoperative AF is believed to be reentry that results from dispersion of atrial refractoriness.10,12,13 When adjacent atrial areas have dissimilar or nonuniform refractoriness, a depolarizing wavefront becomes fragmented as it encounters both refractory and excitable myocardium.10,12,13 This allows the wavefront to return and stimulate previously refractory but now repolarized myocardium leading to incessant propagation of the wavefront or reentry.10,12,13 Currently, there is not an adequate explanation for why some patients develop postoperative AF whereas others having the same surgical interventions remain in sinus rhythm. Individuals vulnerable to AF are speculated to have the electrophysiologic substrate (nonuniform dispersion of atrial refractoriness) before operation that is then aggravated by surgical perturbations.10

It is widely believed that enhanced sympathetic nervous system activity increases susceptibility to postoperative AF.14,15,16,17,18,19 Sympathetic activation, however, is highest the first 24 hours after operation, whereas the onset of AF usually occurs between the second and third postoperative days.1,3,7,20 Furthermore, the atrial electrophysiologic effects of autonomic nervous system stimulation are complex. In contrast to the ventricle where sympathetic activation decreases and vagal stimulation increases the threshold for tachycardia and fibrillation, both sympathetic and parasympathetic activation alter atrial refractoriness, possibly contributing to the arrhythmia substrate.21,22 Heightened vagal tone has been demonstrated before AF in nonsurgical patients.23 Evaluation of cardiac sympathovagal balance before the onset of AF in patients recovering from CABG showed either higher or lower measures of heart rate variability before AF, a finding consistent with divergent autonomic conditions before arrhythmia onset.24 The latter findings support the possibility that in some patients heightened sympathetic tone is present before AF but in others, either higher vagal tone or dysfunctional autonomic heart rate control is present before arrhythmia onset.24

The reason for the delay in the onset of AF more than 2 to 3 days after operation is not clear. One possibility is that the onset of AF is related to an exaggerated inflammatory response especially involving the pericardium.25,26 Mechanical stretching of the atrium can alter cellular electrophysiologic properties suggesting that increased intravascular volume due to postoperative mobilization of interstitial fluid could contribute to the development of AF.27 Tachycardia or brief episodes of AF lead to shortening of the atrial effective refractory period (electrophysiologic remodeling) promoting the maintenance of AF.28,29,30,31 Alterations in calcium-handling proteins have been suggested to be an important mechanism for this electrophysiologic remodeling.29,30,31,32 Downregulation of mRNA for L-type calcium channels and for sarcoplasmic reticular calcium–ATPase have been demonstrated in atrial tissue obtained before cardiac operations in patients with preexisting AF and perhaps these mechanisms contribute to susceptibility to postoperative AF.33 The hypothesis that postoperative AF is related to altered gene expression is an attractive explanation for varying individual susceptibility and for the time lag between operation and the onset of the arrhythmia.

Off-pump coronary artery surgery and incidence of postoperative atrial fibrillation: Current best available evidence

Recently, there has been a renewed interest in performing CABG on the beating heart.34,.35,36,37 Several studies have reported improved myocardial and renal protection,34,38 minimal inflammatory response,39 excellent patency rate of the grafts,36 and decreased costs.7,40 Furthermore, a reduction in postoperative AF has been reported in off-pump series,35,37 although this remains an area of controversy.

Only a logical and comprehensive approach to evaluate available evidence on this issue can resolve this controversy. A logical and comprehensive approach to evaluating clinically relevant research incorporates many different types of evidence (including randomized clinical trials [RCTs], nonrandomized clinical trials [non-RCTs], and experimental data) and analyzes the information's content for consistency, coherence, and clarity. A useful metric for the assessment of clinical research is shown in Table 1.

Figure 1
Table 1: Grading of recommendations and level of evidence

Table 2 presents a summarized comparison of incidence of atrial fibrillation after conventional CABG and OPCAB.

Figure 2
Table 2: Incidence of atrial fibrillation after Conventional CABG and OPCAB

In the hierarchy of clinical evidence, a meta-analysis of all the randomized controlled trials and large retrospective studies can be regarded as the gold standard for assessing the efficacy of a therapeutic modality (e.g. OPCAB). To date only two meta-analyses of all the available literature on safety and efficacy of off-pump coronary artery surgery have been done.57,58 Interestingly in their meta-analysis of available evidence to test whether there are differences between CABG and OPCAB in terms of early outcomes for the patients needing multivessel myocardial revascularization, Parolari et al57 did not analyse the impact of these two surgical techniques on the incidence of postoperative atrial fibrillation. On the other hand, Reston et al58 did a more comprehensive meta-analysis of all randomized and nonrandomized controlled studies which satisfied the following selection criteria:

  1. they had to be controlled studies that compared OPCAB and CABG;

  2. they had to report results obtained from patients receiving OPCAB through a sternotomy separately from results of patients receiving related procedures (e.g., MIDCABG);

  3. they had to include at least 15 patients in each treatment arm (they adopted this criterion because almost all of the morbidity outcomes they evaluated had occurrence rates below 10%).

  4. studies had to report patient-oriented outcomes (such as mortality or various types of morbidity);

  5. studies had to report whether their study population consisted of patients with single-vessel disease, multivessel disease, or a mix of both. When different studies reported results for the same group of patients, only the most recent and most comprehensive publication was included to avoid double counting of patients. Studies that used pharmacologic stabilization of the heart were excluded.

They performed meta-analyses for short-term (30 days or less) and midterm (3 to 25 months) outcomes in this report. Short-term outcomes included length of hospital stay, operative mortality, and the following operative morbidities: myocardial infarction (MI), stroke, reoperation for bleeding, atrial fibrillation (AF), renal failure, and wound infection. Midterm outcomes included need for reintervention with percutaneous transluminal coronary angioplasty (PTCA) or CABG, angina recurrence, and overall mortality. They also empirically determined whether potential biases that might result from differences in study design or patient characteristics actually biased a study's results. In addition, they also conducted sensitivity analyses and tested for publication bias.

Reston and colleagues58 identified 53 studies that met the inclusion criteria. They comprised 10 RCTs, five prospective controlled studies, and 38 retrospective controlled studies. Eighteen studies were from the United States and 35 were from non-U.S. centers. Collectively, these trials enrolled 46,621 patients who received OPCABG.

Patient selection criteria differed somewhat among the individual studies. Most studies (55%) included patients with single and multivessel disease, whereas the remaining 45% included only patients with multivessel disease. Five studies (9%) included only patients receiving elective surgery. Patient exclusion criteria were reported in 36 studies (68%). The most frequently reported exclusion criteria were repeat operation (26%), renal dysfunction (21%), emergency operations (15%), low ventricular ejection fraction (13%), and prior stroke or ischemic attack (11%).

The meta-analysis of AF exhibited heterogeneity that could not be explained by meta-regression of study quality or patient characteristics. However, random-effects meta-analysis showed a statistically significant reduction in atrial fibrillation among patients receiving OPCAB.

Conclusion

On the basis of current best available evidence it can be concluded that the incidence of postoperative atrial fibrillation with OPCAB is no worse and may be significantly better than that accompanying conventional CABG (Grade A).

Acknowledgement

Table1 reprinted from Journal of Thoracic and Cardiovascular Surgery 124(4): 655-9, Mack MJ, Duhaylongsod FG: Through the open door! Where has the ride taken us? Copyright 2002, with permission from Elsevier.

Correspondence to

Dr. Shahzad G. Raja, MRCS Department of Paediatric Cardiac Surgery Alder Hey Hospital, Liverpool L12 2AP, United Kingdom Tel: +44(0)151 252 5635 Fax: +44(0)151 252 5643 Email: drrajashahzad@hotmail.com

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Author Information

Shahzad G. Raja, MRCS
Specialist Registrar, Department of Paediatric Cardiac Surgery, Alder Hey Hospital

Gilles D. Dreyfus, MD, PhD
Professor of Cardiac Surgery, Department of Cardiac Surgery, Harefield Hospital

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