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By Leslie A. Hoffman, RN, PhD, and Marilyn Hravnak, RN, PhD, CRNP
About 400,000 adults undergo coronary artery bypass grafting (CABG) each year. Atrial fibrillation (AFib) is the most common complication following CABG, with an incidence ranging from 5-40%, and most commonly occurs on the second to fourth postoperative day. Although post-CABG AFib is not associated with a significant increase in mortality, it has been noted to result in increased morbidity related to post-operative stroke, hemodynamic compromise, ventricular dysrhythmias, and iatrogenic complications associated with treatment. Most commonly, post-CABG AFib lengthens hospitalization. Attempts to reduce the incidence of post-CABG AFib involve several strategies: 1) attempting to identify patients most at risk for development of this complication; 2) strategies for prophylaxis; and 3) strategies for management.
Predictors For Post-CABG Atrial Fibrillation
A variety of research studies have been conducted in an attempt to determine those patients most at risk for the development of post-CABG AFib, including several large multicenter trials. Older age is consistently the only preoperative risk factor predictive of this complication, with a 24% increase in prevalence for each 5-year incremental increase in age beyond 65 years.1 Gender has been implicated (increased prevalence in males), but inconsistently. A variety of pre-existing co-morbid medical conditions have also been implicated, such as chronic obstructive pulmonary disease (COPD), chronic renal insufficiency, and congestive heart failure. However, the relationship between pre-existing medical conditions and the development of post-CABG AFib has been inconsistent across studies, and in predictive models developed from the findings of these studies. Another potential risk factor, P wave duration on the preoperative surface EKG, is not associated with prediction, but the predictive sensitivity and specificity of this risk factor appears to be somewhat heightened when signal-averaged P wave duration data are used. There is some evidence that right coronary artery disease presents an increased risk for development of post-CABG AFib.
Perioperative and postoperative factors have also been examined for their predictive abilities. Prosthetic valve implantation is associated with increased risk, although it is unknown whether the etiology is the procedure itself, pre-existing atrial impairment, or the combination. Characteristics associated with cardiopulmonary bypass (CPB), such as pulmonary vein venting, bicaval cannulation, and extended cross clamp time have been implicated, and the effects of various types of cardioplegia solution have also been examined. However, the prevalence of post-CABG AFib has not consistently differed between patients who have off-pump CABG (OPCABG) or minimally invasive direct vision coronary artery bypass (MIDCABG) vs. patients who undergo standard CABG with CPB. Tamis and associates reported an AFib prevalence of 26% for MIDCABG (n = 42) and 33% for CABG with CPB (n = 33), a difference that was not significant.2 Similar findings were reported by Cohn and associates (AFib prevalence of 26% in MIDCABG and 34% in CABG with CPB; P = NS).3
AFib prevalence rates also seem to be similar when comparisons are made between CABG done without CPB (MIDCABG 25% vs OPCABG 29%).4 Although study findings appear to demonstrate a trend toward a lower prevalence of AFib following off-pump procedures, other characteristics may have produced these trends. Typically, patients who underwent CABG with CPB had more than 3 vessels bypassed, whereas patients who underwent off-pump procedures had less than 2 vessels bypassed.
We compared the incidence of AFib in a matched set of patients who underwent CABG with CPB to patients who underwent MIDCABG (without CPB). Subjects were similar in age and gender (P = NS) and number of vessels bypassed (CABG 1 vessel n = 18; 2 vessels n = 80) and MIDCABG (1 vessel n = 90; 2 vessels n = 4). AFib prevalence rates were almost identical (CABG with CPB 24.5% vs MIDCABG 23.4%, P = .860), suggesting number of vessels bypassed may be the more important determinant.5 Although postoperative characteristics have not been studied extensively, there is some evidence that postoperative fluid overload predisposes patients to this complication, even in noncardiac surgery cases.
To date, no studies have identified a strong prediction model for post-CABG AFib. Almassi and associates reported a model that found age to be the strongest predictor for AFib, followed by vein venting, COPD, use of digoxin, and heart rate less than 80 bpm.6 Aranki and associates also reported a model with age as the strongest predictor, followed by male gender, hypertension, intra-aortic balloon pump, postoperative pneumonia, prolonged mechanical ventilation, and ICU readmission.7 Neither study related a cumulative percentage of prediction for the model. Age was also found to be the best predictor of AFib in studies where signal averaged ECG data were used in the model.8,9 When Frost and associates performed regression analysis on a variety of variables in a study using P-wave duration and morphology on signal-averaged ECG, they noted that only increased age (> 60 years) and increased body weight (> 80 kg) were independent predictors of AFib, for a cumulative positive prediction of 37%.8 Fuller and associates reported a model based on age, gender, and beta-blockade that had a median predictive probability of 34% for subjects who had postoperative AFib, and 28% for those who did not.10
DeJong and Morton identified age and right coronary artery stenosis as correctly predicting AFib in 26.9% of AFib cases, although the sample size was small (only 52 subjects with AFib).11 We comprehensively examined pre, peri, and postoperative characteristics that might predict AFib in 730 patients undergoing CABG in a single medical center.12 Exclusion criteria were prior history of AFib (both active and inactive); prior or current heart, lung or heart-lung transplant; prior or current ventricular assist device; prior or current heart valve replacement or repair; any other surgical procedure during current admission; perioperative or postoperative myocardial infarction; and death in the operating room (OR) or within 12 hours of surgery. Prior CABG was not an exclusion criterion. The model correctly predicted AFib in 24% of subjects. Age provided the greatest percentage of prediction (15.3%), with body surface area adding an additional 8.1%, and entry of the location and number of bypasses adding only 1.2%. The goal of developing a sensitive and specific prediction model that would permit prospective identification of subjects likely to develop this complication remains elusive.
Post-CABG Prophylaxis Against Atrial Fibrillation
A variety of medications, electrolytes, and hormones have been examined for their efficacy in preventing post-CABG AFib. Postoperative administration of a beta blocker appears to be the most widely used strategy, with numerous individual studies and two meta-analyses supporting efficacy of this approach.13,14 Neither digoxin, oral verapamil, nor intravenous procainamide have been shown to be consistently beneficial in preventing this complication. Magnesium administration had been shown to have some benefit and, although the effects have not been dramatic, is used because of its favorable cost-benefit ratio and low side-effect profile. Ott and associates reported a study (n = 553) applying a multi- drug prophylactic regimen using triiodothyronine and thyroxine, magnesium, metoprolol, digoxin, steroids, and aggressive diuresis, resulting in AFib prevalence of 10.3%.15 Amiodarone and ilbutilide have shown some promise of efficacy, but outcomes of using these drugs have yet to be evaluated in large randomized, controlled trials. Lack of an adequate prediction model prevents prophylactic strategies that carry a higher risk of side effects from being broadly targeted to all patients. Therefore, the search continues for mediations with low toxicity and high efficacy that can be applied comprehensively to CABG patients.
The management of post-CABG AFib is centered upon 3 goals, which are sought in a stepwise fashion: 1) rate control conversion to sinus rhythm; 2) maintenance of sinus rhythm; 3) and if conversion cannot be achieved, anticoagulation presents an additional goal. Strategies described in the literature to achieve each of these goals are listed (see Table).
|Table: Management of Atrial Fibrillation after CABG|
|• Intravenous (IV) diltiazem
• IV esmolol
• IV amiodarone
|Maintenance of Sinus Rhythm|
|• Discharge on antiarrhythmic therapy for 4-6 weeks|
A variety of studies have demonstrated that most pharmacologic treatment regimens resulted in conversion to sinus rhythm within 24 hours: Hjelms and associates: procainamide converted 87% of patients in 40 minutes;16 Mooss and associates: 67% of diltiazem group and 13% of esmolol group converted within 6 hours, and 67% of diltiazem group and 80% of esmolol group converted within 24 hours.17 If pharmacological conversion is not achieved, electrical cardioversion is typically attempted within 48 hours of the onset of AFib. If electrical conversion is successful, patients are generally discharged on an antiarrhythmic agent for 6 weeks. If electrical cardioversion is unsuccessful, the patient may be discharged on a rate control agent and warfarin, and electrical cardioversion reapplied in 6 weeks. In many cases, however, spontaneous conversion is achieved prior to that time.
Although morbidity and mortality related to post-CABG AFib is low, this complication results in increased utilization of resources while treatment is applied, evaluated, and adjusted. Post-CABG AFib has been shown to increase the overall postoperative length of stay for these patients by 1-3 days, representing an additional $1500-7500 per patient for bed charges alone. Most studies have only examined bed charges. In studies that have more comprehensively evaluated costs, it has been noted that these additional days are also accompanied by consumption of additional resources from most hospital cost centers such as the pharmacy, laboratories, and respiratory care.12,18 Thus, the economic ramifications of post-CABG AFib are substantial.
Post-CABG AFib occurs frequently, and the search for a strong prediction model continues to be elusive. Prophylactic strategies have shown some success at diminishing the frequency, but not obliterating the development, of this complication. Treatment modalities are generally successful, but the need to use these therapies increases length of stay. Targeted preventive therapies that are widely accepted by providers and tolerated by patients have yet to be identified. Given inability to identify a strong predictive model, a more productive strategy may be to focus attention on testing protocols with the goal of identifying treatment that leads to the most rapid conversion to normal sinus rhythm, has acceptable efficacy, and is cost-effective. (Dr. Hravnak is Assistant Professor of Nursing and Director of the Acute Care Nurse Practitioner Program, University of Pittsburgh School of Nursing, Pittsburgh, Pa.) v
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