Preoperative Clinical Risk Factors

Most preoperative clinical risk factors for the development of postoperative AF indicate (or even create) changes in the electrical substrate of the atrial myocardium. As such, the preoperative clinical risk factors most often evaluated are similar to AF unrelated to cardiac surgery.

2.1. Prior atrial fibrillation and other supraventricular arrhythmias

In a general cardiology population, the presence of prior atrial arrhythmias is strongly associated with the subsequent occurrence of AF; as would be expected, this association persists in patients following cardiac surgery. This association likely results from postoperative persistence of preoperative changes in the atrial structural substrate and of sensitivity to the same triggers which made AF possible previously. Other factors, such as the effect of the so-called "electrical remodeling" must also be considered [6, 7],

While patients with prior, preoperative AF are probably at greatest risk for the development of postoperative AF, supporting literature is less than overwhelming for several reasons. Firstly, a careful and precise definition of the term "prior AF" is absent from the literature. For example, patients with chronic AF prior to cardiac surgery would seem more likely to redevelop AF following that surgery. Some studies evaluating predictors of postoperative AF include these patients [8]; other studies clearly exclude these patients from analysis [9]). Unfortunately, most of the published literature fails to offer any definition of the term "prior AF". Secondly, many large trials evaluating predictors of postoperative AF intentionally exclude patients with preoperative arrhythmias [3,10] complicating analysis of its true role as a risk factor. Along these same lines, several other large trials neither explicitly excluded these patients nor did they include prior AF as a preoperative factor being analyzed [11,2].

Of the studies that do analyze the association of preoperative AF and its postoperative development, the largest study is from the Multicenter Study of Preoperative Ischemia Cardiac Surgery (MCSPI) database [80]. In this study, 2417 patients undergoing CABG with or without concomitant valvular surgery were randomly selected from 24 university-affiliated hospitals around the United States. Clinical variables were evaluated utilizing retrospective chart reviews and physician interviews. Of these patients, 332 had "prior AF" defined as "reported in the medical record or interviews" or present on preoperative ECG. 158/332 (48%) with

"prior AF" developed AF during their postoperative hospital stay as compared to 457/1933 (24%) without prior AF (P<0.01).

The nature of "prior AF" is at best incompletely characterized in the MCSPI study. In spite of this limitation, this data demonstrates that AF occurring prior to surgery is clearly associated with an increased occurrence of postoperative AF. This finding can be interpreted as demonstrating the important role of preexisting substrate in the occurrence of postoperative arrhythmias. However, an additional corollary observation can also be made from the MCSPI data: despite the clearly intentioned inclusion of patients with prior AF, only approximately 50% of those patients actually developed postoperative AF. These findings suggest that appropriate preventative therapy may not be futile and should be considered.

Other published reports clearly support the association of preoperative atrial arrhythmias with the occurrence of postoperative AF. Hashimoto et al [13] in a large, single-center study from the Mayo Clinic, demonstrated remarkably similar findings. In this study of 800 consecutive patients undergoing isolated coronary artery bypass grafting (CABG), postoperative AF occurred in 21/47 (45%) patients with "prior atrial arrhythmias" (a term not clearly defined in the manuscript) as compared to 165/749 (22%) without prior atrial arrhythmias (p<0.001). Interestingly, in this same population, the presence of premature atrial beats on preoperative ECG was also associated with an increased risk of postoperative AF. It is not inconceivable that in the appropriate postoperative setting, these premature beats provide the trigger for AF. Other smaller studies [9,14] have suggested similar findings.

As with AF unrelated to surgery, increasing age is an independent risk factor predictive of postoperative AF. The largest study from Leitch et al [3] included 5807 consecutive patients from a single institution. In this population, the overall rate of postoperative AF was 17.2%. For those patients under age 40 years the prevalence was 3.7%; for those aged 70 years and older the prevalence of AF was 27.7%. These data produced an odds-ratio per 10-year decile of 1.7; thus, for each 10-year increment in age, the risk of AF increased by 70%. Other large trials have demonstrated similar findings. Almassi et al [10], in a population of 3855 predominantly male patients from Veterans Administration Hospitals across the United States, demonstrated an odds ratio of 1.6 for each additional 10 years of age above 50 years (95% confidence intervals: 1.48-1.75, p<0.0001). Likewise, the MCSPI study cited above [8] demonstrated an odds ratio of 1.24 per 5 year increase in age. Aranki et al [12], in a study of 570 consecutive patients from Brigham and Women's Hospital, demonstrated an odds ratio of 2 for age 70 to 80 years old, 3 for age >80 years. Multiple smaller studies have also demonstrated a significant association of increasing age to postoperative AF [9, 15-19],

It has been hypothesized that age related changes in the atrial substrate, such as dilatation, myocardial atrophy and fibrosis, decreased conduction velocity, as well as related co-morbidities may explain the association of age with AF [12].

These changes in substrate exist independently of the event of cardiac surgery; as such, the age-related incidence of postoperative AF could theoretically parallel the age-related incidence of AF in the general population.

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