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Ann Thorac Surg 2001;71:1491-1495
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

Atrial fibrillation: prevalence after minimally invasive direct and standard coronary artery bypass

^a Department of Acute/Tertiary Care, School of Nursing, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
^b Medical Archival System, Inc, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
^c Department of Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
^d Frances Payne Bolton School of Nursing, Case Western Reserve University, Cleveland, Ohio, USA

Accepted for publication January 17, 2001.

Address reprint requests to Dr Hravnak, Department of Acute/Tertiary Care, School of Nursing, University of Pittsburgh, 314 Victoria Bldg, 3500 Victoria St, Pittsburgh, PA 15261
e-mail: mhra{at}pitt.edu

	Abstract

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Background. This study identified and compared the prevalence of new-onset atrial fibrillation (AFIB) following standard coronary artery bypass grafting (SCABG) with cardiopulmonary bypass (CPB) and minimally invasive direct vision coronary artery bypass grafting (MIDCAB) without CPB. A further comparison was made between AFIB prevalence in SCABG and MIDCAB subjects with two or fewer bypasses.

Methods. This is a retrospective, comparative survey. Patients with new-onset AFIB who underwent SCABG or MIDCAB alone were identified electronically using a triangulated method (International Classification of Diseases, 9th revision, Clinical Modification [ICD-9 CM] code; clinical database word search; and pharmacy database drug search).

Results. The total sample (n = 814; 94 MIDCAB, 720 SCABG) exhibited a trend toward lower AFIB prevalence in MIDCAB (23.4%) versus SCABG (33.1%) subjects (p = 0.059). AFIB prevalence in the SCABG subset with two or less vessel bypasses (n = 98; n = 18 single vessel, n = 80 double vessels) and MIDCAB subjects (n = 94; n = 90 single vessels, n = 4 double vessels) was almost identical (SCABG subset 24.5% versus MIDCAB 23.4%, p = 0.860). Slightly more than half (56.9%) of new-onset AFIB subjects were identified by ICD-9 CM codes, with the remainder by word search (37.7%) or procainamide query (5.4%).

Conclusions. In this sample, the number of vessels bypassed seemed to have a greater influence on AFIB prevalence than the application of CPB or the surgical approach. Retrospective identification of AFIB cases by ICD-9 CM code grossly underestimated AFIB prevalence.

	Introduction

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Atrial fibrillation (AFIB) is the most common complication following coronary artery bypass grafting (CABG), with an incidence ranging from 5% to 50% [1–4]. Although post-CABG AFIB is not associated with a significant increase in mortality, it has been noted to result in increased morbidity related to postoperative stroke, hemodynamic compromise, ventricular dysrhythmias, and iatrogenic complications associated with treatment. Most commonly, post-CABG AFIB lengthens hospitalization [5–6].

Prior research suggests that the incidence of postoperative AFIB may be increased as a consequence of surgical manipulation and, possibly, myocardial ischemia and inflammatory reactions related to perioperative cardiopulmonary bypass (CPB) [4]. If so, patients who undergo off-pump procedures such as minimally invasive direct vision coronary artery bypass (MIDCAB) with limited left thoracotomy and no CPB should have a lower incidence of AFIB than patients undergoing standard CABG (SCABG) with CPB. To date, this potential has not been extensively explored.

The purpose of this study was to examine the prevalence of new-onset AFIB in patients who underwent CABG alone in a university-affiliated medical center. We examined the influence of surgical approach and CPB (SCABG versus MIDCAB) on AFIB prevalence for the entire sample. In addition, we compared AFIB prevalence in MIDCAB subjects and a subset of SCABG subjects with less than or equal to two vessels bypassed.

	Material and methods

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Following Institutional Review Board approval (March 13, 1998), data were retrospectively obtained for a 25-month interval (May 1, 1996 to May 31, 1998) from the Medical Archival System (MARS) at the University of Pittsburgh Medical Center–Health System. The MARS is a repository for information forwarded from the health system’s electronic clinical, administrative, and financial databases. MARS is indexed on every word and will recover all encounters with a given patient between specific dates [7]. All subjects were over 18 years of age and underwent SCABG or MIDCAB alone (International Classification of Diseases, 9th revision, Clinical Modification [ICD-9 CM] procedure codes 36.10 to 36.16 from the medical records discharge abstracts). 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. All subjects participated in the routine AFIB prophylaxis strategy, in effect during the study period, that included: magnesium supplementation in the OR, upon intensive care unit (ICU) admission, and on the 1st postoperative day; and beta-blocker administration beginning on the 1st postoperative day, if permitted by heart rate, blood pressure, and cardiac index evaluation.

Patients were initially identified via procedure codes, then review of operative reports to identify SCABG versus MIDCAB, and number and location of vessels bypassed. To identify new-onset AFIB, we first identified patients who had an ICD-9 CM code for AFIB (427.31) from the administrative database using MARS. Because ICD-9 CM code assignment does not differentiate between new-onset and preexisting AFIB, discharge summaries for all patients with code 427.31 were reviewed to verify that AFIB had occurred and determine if AFIB was a newly acquired or prior problem (full chart review if necessary). Second, the clinical database for all remaining patients (no ICD-9 CM code 427.31) who underwent SCABG or MIDCAB was subjected to a MARS word search (AFIB, atrial fib, atrial dysrhythmia, or word variations) and a determination made for new-onset or preexisting AFIB as described for positive searches. Third, the pharmacy database was queried for procainamide administration for all patients not identified as having new-onset AFIB in the prior two steps, and a positive search evaluated as described. Patients with a questionable AFIB status were eliminated from the study. A total of 997 patients were identified as having SCABG or MIDCAB surgery within the 25-month time frame. Of these, 63 were excluded due to preexisting AFIB (active or inactive) and 120 due to either more than one surgical procedure during the admission, or death during or within 12 hours of operation. The triangulated method identified new-onset AFIB in a total of 260 subjects: a) ICD-9 CM code, n = 148; b) word search, n = 98; c) procainamide query, n = 14. The remaining 554 subjects who met entry criteria did not experience AFIB (NAFIB). Therefore, the final sample was comprised of 814 subjects (NAFIB = 554; AFIB = 260).

Demographic, clinical, and fiscal data and information regarding the age, gender, and race were extracted from the MARS database. To identify past medical problems, ICD-9 CM codes within similar disease categories were clustered and only diseases with cumulative frequencies greater than 75 selected for analysis. Hypertension was excluded from this process since it was individually coded in 74% of the sample. The number of vessels bypassed was obtained by review of the operative report (obtained from MARS). Administrative and finance database components of the MARS were used to identify operating room charge hours.

All information was transferred from the MARS database into the Microsoft Excel (Microsoft Office 97, Microsoft Corp, Redmond, WA) research database utilizing unique subject identifiers later stripped to preserve confidentiality. Using SPSS (Statistical Package for the Social Sciences, SPSS Inc, Chicago, IL), between-group differences were analyzed using Student’s t tests for continuous variables, and ² analysis for categorical variables. A value of p less than 0.05 was considered significant. When several variables within a category were analyzed, a Bonferroni correction was made and the alpha divided evenly across multiple variables. Differences in AFIB prevalence between groups were examined using the ² test for proportions.

	Results

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Subject characteristics
The study group consisted of 814 subjects, 720 of whom underwent SCABG and 94 MIDCAB (Table 1). There were no significant differences between SCABG and MIDCAB subjects for age, gender, race, or past medical problems. As anticipated, SCABG and MIDCAB subjects differed in regard to number of vessels bypassed (p = 0.0004) and OR charge hours (p = 0.0004).

View larger version (53K): [in this window] [in a new window]   Fig 1. Comparison of atrial fibrillation (AFIB) prevalence in the total sample of standard coronary artery bypass graft (SCABG) and minimally invasive direct vision coronary artery bypass (MIDCAB) subjects, and a subset of MIDCAB and SCABG subjects with less than or equal to two vessels bypassed.

View larger version (66K): [in this window] [in a new window]   Fig 2. Trends in atrial fibrillation (AFIB) prevalence over a 25-mo study interval in standard coronary artery bypass graft (SCABG) and minimally invasive direct vision coronary artery bypass (MIDCAB) subjects.

	Comment

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Prevalence of AFIB
The overall prevalence rate for AFIB in our sample was 31.9%. A number of other studies have reported similar rates in the range of 30% to 39%, [3, 5] some have reported higher prevalence rates (40% to 50%) [8], but the majority reported rates of 20% to 29% [1, 4, 9], and hardly any with less than 20% [10]. Therefore, the prevalence rate for AFIB in our medical center is consistent to, or slightly above, that reported in the literature in the absence of an interventional trial. The multifaceted methodology used for case finding may have contributed to this result. Almost half (43.1%) of AFIB cases were identified when the search strategy was expanded beyond the use of ICD-9 CM codes. If our search strategy had been limited to use of ICD-9 CM codes alone, we would have mistakenly underestimated the overall AFIB prevalence rate as only 18.1%. Accordingly, this strategy has implications for retrospective and prospective studies that solely depend on ICD-9 CM codes to determine AFIB status.

Comparison of AFIB prevalence for SCABG and MIDCAB subjects
Some features that separate SCABG patients from the MIDCAB patients are the surgical approach, CPB, and the number of vessels bypassed. Several prior studies suggest that these differences may impact AFIB prevalence. Cross-clamp time and technique [9], bicaval venous cannulation and pulmonary vein venting [4], and the types and techniques of cardioplegia administration [11–13] have been shown to affect AFIB prevalence. Gu and colleagues [14] reported that SCABG subjects with single-vessel bypasses (n = 31) had elevated levels of inflammatory markers, whereas MIDCAB subjects (n = 31) displayed baseline levels of these substances.

Most centers reporting their experience with MIDCAB have not included information regarding AFIB prevalence [14–16]. In those that do, the variation between centers is great. Subramanian and coworkers [17] reported an AFIB prevalence of 7.5% in a series of 185 subjects, and Zenati and colleagues [18] reported a 6% prevalence rate in a series of 17 patients with single-vessel MIDCAB. Neither of these groups presented data regarding SCABG prevalence rates in their institutions. Higher AFIB prevalence has been reported in other MIDCAB series. Tamis and associates [19] reported an AFIB prevalence of 26% for MIDCAB (n = 42) and 33% for SCABG (n = 33), a difference that was not significant. Cohn and colleagues [20] related an AFIB prevalence of 26% in MIDCAB and 34% in SCABG, a rate that closely compared with that found in our study (MIDCAB, 23.4% versus SCABG, 33.1%).

In part, variations in AFIB prevalence for MIDCAB may be due to differences in the methods used for case finding. Further, experience with the MIDCAB procedure may have some effect. Our data included our earliest experience with MIDCAB, and showed a decline in AFIB prevalence over time. In a series of 77 MIDCAB patients, Possati and coworkers [21] noted that success with graft patency improved over time as the technique was perfected. These findings may be related.

Increased age is typically the only patient characteristics consistently identified as a risk factor for new-onset AFIB following SCABG [4–6]. Cohn and associates [20] compared SCABG and MIDCAB subjects (n = 55 in each group) who were matched on age (mean 62 ± 13 years), but not on number of vessels bypassed (MIDCAB, 1.1 ± 0.48 versus SCABG, 3.6 ± 1.1 vessels). They did not find different AFIB prevalence rates between the 2 groups, although there was a trend toward a lower rate for the MIDCAB group (MIDCAB 26% versus SCABG 34%, p = 0.64). In our study, the 98 subjects who underwent SCABG (with CPB) with one or two vessel bypasses and the 94 subjects who underwent MIDCAB (no CPB) were similar in all demographic variables including age. The AFIB prevalence rates were almost identical for both groups (24.5% SCABG subset versus 23.4% MIDCAB). Consequently, it appeared that the trend toward a lower prevalence of AFIB in MIDCAB subjects in our series resulted from having fewer vessels bypassed rather than surgical approach or elimination of CPB.

Limitations
The study was based on data obtained from an electronic medical record and, therefore, carries risks inherent to use of retrospective data. Measures were taken as described to enhance capture of AFIB cases.

Conclusions
Findings of this study showed a trend toward a higher AFIB prevalence rate for SCABG subjects compared to MIDCAB subjects when the groups did not have comparable numbers of vessels bypassed. There was a trend toward a lower prevalence of AFIB in MIDCAB subjects as experience with the procedure increased. However, there was no difference in AFIB prevalence when comparisons were made between MIDCAB and SCABG subjects who had a similar number (less than or equal to two) of vessels bypassed. In addition, the study demonstrated that use of a comprehensive methodology to determine AFIB status enhanced capability to accurately identify the prevalence of this complication from an electronic database.

	References

Top Abstract Introduction Material and methods Results Comment References

 

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