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Abstract
Atrial fibrillation (AF) occurs in one quarter to one third of patients after coronary artery bypass graft surgery (CABG). Conventional CABG uses cardiopulmonary bypass, a process that is itself associated with a systemic vascular inflammatory response that contributes to postoperative morbidity. The avoidance of cardiopulmonary bypass is associated with a significant reduction in the inflammatory response and in the release of markers of myocardial necrosis when compared with conventional CABG. There is speculation that off-pump CABG may reduce the incidence of postoperative AF through reduced trauma, ischaemia, and inflammation. Current data, however, do not emphatically answer the question of whether the incidence of post-CABG AF is reduced by off-pump surgery. The evidence from both observational and randomised studies is conflicting and many studies have weaknesses in design, conduct, or interpretation. It remains an attractive hypothesis that postoperative AF is reduced by off-pump CABG but more robust data are required.
- atrial fibrillation
- off-pump coronary artery bypass surgery
- AF, atrial fibrillation
- CABG, coronary artery bypass graft surgery
- CPB, cardiopulmonary bypass
- MIDCAB, minimally invasive direct coronary artery bypass
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- AF, atrial fibrillation
- CABG, coronary artery bypass graft surgery
- CPB, cardiopulmonary bypass
- MIDCAB, minimally invasive direct coronary artery bypass
Approximately 27 000 people in the UK and 500 000 people in the USA undergo coronary artery bypass graft surgery (CABG) annually, 25–30% of whom develop atrial fibrillation (AF) postoperatively. AF is associated with increased morbidity and expenditure, and prolonged hospital stay after CABG.1 Advances in surgical techniques have not been accompanied by a decrease in the incidence of post-CABG AF. However, the use of “off-pump” CABG may provide a novel solution to this problem. This review analyses the current evidence regarding the incidence of AF after off-pump CABG.
PATHOGENESIS OF POST-CABG AF: A ROLE FOR THE “POST-PUMP SYNDROME”?
The pathogenesis of post-CABG AF is incompletely understood. Evidence suggests that the patients at greatest risk of developing the arrhythmia have an identifiable electrophysiological substrate.2 The nature of this substrate may be slow atrial conduction or dispersion of atrial refractoriness.3 A number of perioperative factors, such as inadequate myocardial protection, electrolyte imbalance, β blocker withdrawal, change in autonomic tone, or the response to cardiopulmonary bypass (CPB), may precipitate AF in these patients.
Conventional CABG employs CPB during the period of cardiac arrest. This provides for extracorporeal oxygenation of blood before returning it to the systemic circulation, thus ensuring adequate delivery to the vital organs. CPB is itself associated with a systemic inflammatory response syndrome that contributes to postoperative morbidity.4 Post-pump syndrome is characterised by leucocytosis, increased capillary permeability, accumulation of interstitial fluid, and organ dysfunction. It is thought to be triggered by surgical trauma, lung reperfusion, and contact of blood with the “foreign” extracorporeal bypass circuit.
Currently only circumstantial evidence suggests a causal relation between CPB and postoperative AF, a minority of investigators having reported an association between bypass time and post-CABG AF.5 Nevertheless, it remains an attractive hypothesis that the avoidance of CPB will reduce the incidence of post-CABG AF.
DOES OFF-PUMP CABG REDUCE THE INCIDENCE OF POST-CABG AF? THE TRIAL EVIDENCE
There is considerable interest in beating heart surgery without CPB and minimally invasive direct coronary artery bypass (MIDCAB), both of which may reduce postoperative morbidity. Off-pump CABG is associated with a significant reduction in the inflammatory response6 and in the release of markers of myocardial necrosis7 compared with conventional CABG. Furthermore, there are now data assessing the incidence of AF in patients revascularised by these techniques (tables 1 and 2).8–52
Incidence of AF after conventional and minimally invasive direct CABG
Incidence of AF after conventional and off-pump CABG by median sternotomy
In two series, post-CABG AF occurred in 18 (5.9%) of 306 patients who underwent left internal mammary artery grafting to the left anterior descending artery by MIDCAB23 and was recorded in only 34 (1.7%) of 2052 patients who had off-pump CABG by median sternotomy.25 In a study of patients who underwent CABG by MIDCAB (n = 93) or conventional operation (n = 74), AF was less frequent (7% v 31%, respectively; p < 0.05) in the group without CPB.10 By contrast, different series have reported rates of AF of up to 37% despite no CPB,26 while other observational studies have shown no difference in AF incidence post-CABG with and without CPB.16,20
Theoretically, any reduction in AF incidence after off-pump CABG may be attributed to the less invasive nature of MIDCAB procedures than of median sternotomy rather than to the absence of CPB. In 969 patients who underwent CABG without CPB, for example, MIDCAB was associated with a lower incidence of AF (14% v 25%; odds ratio 0.4, 95% confidence interval 0.3 to 0.7; p < 0.001) than a median sternotomy approach.19 Specific information about the impact of CPB on postoperative AF therefore comes from studies comparing patients undergoing median sternotomy with and without CPB. Thus, AF occurred in 23% of 3171 patients who underwent conventional CABG compared with 14% of 350 off-pump operations performed through sternotomy (p < 0.001).33 Objective comparison is, however, difficult since the off-pump patients received fewer grafts, had shorter surgery, and were ventilated for a shorter period. Furthermore, in other studies, the incidence of AF was not significantly different in groups undergoing CABG by median sternotomy with and without CPB.26,48
The importance of considering differences in clinical characteristics is illustrated by a recent study in which the incidence of post-CABG AF was significantly lower (23% v 39%; p = 0.02) among 101 MIDCAB patients than in 108 on-pump patients.22 The MIDCAB patients were younger, had a lower frequency of previous cardiac surgery, had less extensive coronary disease, received fewer grafts, and had a shorter postoperative hospital stay. After adjustment for these differences, surgical method was no longer a significant predictor of AF.
To elucidate the role of CPB properly in post-CABG AF, randomised data are required. In one study, 200 low risk patients were randomly assigned to off-pump or conventional CABG.32 Sustained AF (> 10 minutes) was significantly less common (8% v 39%; p = 0.001) in the off-pump group. Indeed CPB was the only independent predictor of postoperative AF. These data, however, contrast with those from a more recent trial in which 281 patients undergoing first, elective, isolated CABG were randomly assigned to on- or off-pump surgery.51 All patients received sotalol prophylaxis. Off-pump CABG resulted in reduced creatine kinase MB fraction release, reduced use of blood products, and a shorter hospital stay compared with on-pump surgery, but nevertheless the incidence of AF (20% v 21%, respectively; p = 0.79) was similar.
DISCUSSION
Post-CABG AF continues to affect morbidity, hospital stay, and financial resources. There is speculation that off-pump CABG may reduce the incidence of AF, but current data do not answer this hypothesis. The evidence is conflicting. Interpretation of studies is confounded by patient selection, inappropriate control groups, and poorly defined arrhythmia monitoring.
Alternative strategies for reducing both the inflammatory response to CPB and the incidence of post-CABG AF exist. β Blockers are generally accepted to prevent AF.53 Some studies have reported a reduction in the serum concentration of inflammatory markers after CPB using heparin coated extracorporeal circuits, or by high dose aprotinin or prednisolone.54 Whether these findings translate into clinical benefit, specifically a decrease in the incidence of post-CABG AF, remains uncertain.
Conclusion
Off-pump techniques offer several potential advantages over conventional CABG (shorter operation times, more rapid recovery times, reduced neurocognitive impairment, reduced need for blood products). It is uncertain whether a reduction in the incidence of postoperative AF can be added to this list. More robust data are required.
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