Atrial fibrillation (AF) is cardiac arrhythmia resulting from grossly disorganized atrial electrical activity, due to multiple and continuous intra-atrial reentrant circuits, characterized by impaired atrial contraction, followed by absent atrial systole.1 Certain pathological heart conditions, such as those evolving with structural changes, or surgical manipulation of the organ, may increase the risk of developing AF.2 Arrhythmia occurs in about 30% of patients in the postoperative of coronary artery bypass graft surgery and in up to 60% of postoperative patients submitted to concomitant valvular surgery.3^,4

The consequences of AF, especially when it is irreversible, are clearly associated with the risk of thromboembolic phenomena and mortality, affecting 11% of the elderly population, up to 30 days after the diagnosis.5 In addition, cognitive changes, heart failure and worsened quality of life have been reported.6^,7

Having tools for predicting the risk of developing AF in the postoperative period of cardiac surgery (POAF) may contribute to the implementation of measures designed to prevent and improve the monitoring of patients at risk. Different models have been tested to that end,4^,8^,9 such as the CHADS₂ and CHA₂DS₂-VASc scores.9^-11

Although these scores have been originally developed to assess the risk of thromboembolism in patients with AF, their items include risk factors for the onset of arrhythmia itself.1^,2^,12 However, the results of studies that have evaluated the predictive capacity of the CHADS₂ and CHA₂DS₂-VASc scores in relation to POAF are controversial.9^,10^,13

Recently, a study demonstrated that left atrial (LA) size was the best variable to discriminate between patients with or without POAF.14 To our knowledge, no study has assessed the contribution of LA size evaluation to the predictive ability of those scores. The objectives of this study were to verify the predictive capacity of the CHADS₂ and CHA₂DS₂-VASc scores for the onset of POAF in patients submitted to coronary artery bypass grafting and/or valvular surgery; and to assess the contribution of LA size to the predictive capacity of these scores.

This is a retrospective cohort study. Data were collected in the period from June 2017 to October 2017 at the Heart Institute of the Clinics Hospital of the School of Medicine of the University of São Paulo (INCOR-HC-FMUSP).

We selected 144 medical records of patients submitted to coronary artery bypass grafting and/or valvular surgery in 2015, half of whom had developed POAF. The demographic and clinical characteristics of interest, registered in the preoperative period, are described in Table 1.

Although table 1 does not show statistically significant difference between patients with and without POAF, in relation to age, it is possible to consider that, clinically, the age is associated with the onset of arrhythmia in the postoperative period (p = 0.050).

In relation to medication taken on a regular basis at home, the use of different classes was observed, among which, antihypertensives, antiarrhythmics, statins, antiplatelet agents, anticoagulants and diuretics. However, we only found statistically significant difference between patients with or without POAF in relation to the use of digitalis (0%, n = 0 vs 6.9%, n = 5, respectively; p = 0.026).

Laboratory tests results conducted in the preoperative period were not analyzed because they were not available for many patients. Table 2 describes the minimum and maximum values observed in laboratory tests results collected in the first 24 hours after surgery.

The other intra- and postoperative data of interest for this study are described in Table 3.

The CHADS₂ mean scores for patients with and without POAF were, respectively, 1.82+1.05 and 1.49+1.18 (p = 0.077). The CHA₂DS₂VASc mean scores, on the other hand, were, 3.38 + 1.53 and 2.96 + 1.56, in this sequence (p = 0.109). In order to evaluate the predictive capacity of these scores, alone or combined with LA size, we established the cut-off point of each score to better identify the patients who developed POAF (Chart 1).

Chart 1

Table 4 describes the predictive capacity of these scores, either alone or associated with LA size, to determine the onset of POAF.

Tables 5 and 6 show logistic regression models of the CHADS₂ and CHA₂DS₂VASc scores alone and in association with left atrial size.

Chart 2 shows the ROC curve area for the scores alone or in association with LA size.

Chart 2

The present study described the prediction capacity of the CHADS₂ and CHA₂DS₂VASc scores, alone or in association with LA size, to determine the onset of POAF in patients undergoing myocardial revascularization surgery and/or valvular surgery. As far as we know, no other study had added the contribution of left atrial size to the prediction capacity of those scores. Having tools that allows us to predict the risk of POAF in an easy and reliable way is important because it could help identify patients at risk, who would benefit from more careful monitoring in the postoperative period, as well as, for the institution of prevention measures.

Patients with POAF were older compared to those who did not develop arrhythmia. Advanced age is, admittedly, a major risk factor for AF in patients in general16^,17 and in those undergoing cardiac surgery.18^,19 In fact, population aging, especially in emerging countries, will play an important role in changing the incidence of this type of arrhythmia in the general population, as well as in patients undergoing cardiac surgery.5

The mean ejection fraction in both groups was considered within normal limits, although LA size was slightly increased in both groups.20 When AF is addressed in the context of heart failure, arrhythmia is more frequent in patients with normal ejection fraction, due to volume overload and pressured atrial chambers.21 Atrial remodeling, caused by a structural rearrangement of myocardial fibers, is directly linked with the presence of AF.21 Other authors have reported that LA size is associated with the occurrence of arrhythmia2^,4^,9^,22 and seems to be the best discriminant between postoperative patients who developed POAF and those who did not.14 Considering the important role played by the LA in the physiopathology of AF, and the results of previous studies, we decided to test the predictive capacity of the scores alone and combined with this measure, even though no difference was observed between the groups with and without AF in relation to the size of this heart chamber. This will be further discussed below.

Regarding the use of continuous medication, comparison between the groups with and without AF showed that only the use of digitalis was different. Although this drug was prescribed with the aim of treating heart failure, its action on cell membranes and increased vagal tonus5 could contribute to reduce the onset of POAF.

It is known that the risk to develop AF in the postoperative of cardiac surgery may vary depending on the type of surgery. In a multicenter study, the risk of POAF was higher in patients who had undergone coronary artery bypass grafting combined with valvular surgery (OR: 1.8; CI 95%: 1.2 - 2.7) or valve surgery alone (OR: 1.4; CI 95%: 1.1 - 1.9) compared with coronary artery bypass grafting surgery alone.23 Although studies have associated the use of ECC with increased postoperative complications, among them, the occurrence of atrial fibrillation,24 this study did not find significant differences between the two groups. In addition, we did not observe any relation with the type of surgical procedure, nor in relation to the other intraoperative variables in the groups with and without POAF.

Several studies have demonstrated the association between arrhythmia and adverse postoperative outcomes.23^,25 A meta-analysis23 with over 69 thousand patients showed that the length of stay was significantly higher among patients with and without POAF (11.0 vs 8.9 days, respectively; p < 0.00001). In coherence with the literature data, POAF patients, in this study, remained hospitalized for longer periods.

Nevertheless, mortality rates were higher among patients without POAF. These results were surprising, but some hypothesis can help us understand them. Short-term follow-up compared to other studies who had followed the patients after hospital discharge can contribute to decrease the number of new-onset and recurrent cases of POAF, as well as the number of deaths. In a meta-analysis, the recurrence rate of POAF ranged from 61% to 100% within two years.26

With respect to the estimation of the predictive capacity of the scores, the cut-off values of the CHADS₂ and CHA₂DS₂VASc scores that best discriminated patients with and without POAF were > 0.5 and < 3.5, respectively. In other words, patients would be at risk of developing POAF even if they had relatively few risk factors. In this situation, it is possible that the scores would not be very useful for identifying patients that should receive more intensive monitoring in the postoperative period or other prophylactic measures. Corroborating that perspective, the sensitivity of the CHADS₂ score increased, but specificity was extremely low; and for CHA₂DS₂VASc, they remained far below adequate.

When assessing the effect of LA size on the predictive capacity of these scores, although the sensitivity of the CHADS₂ score has decreased, all the other measures of specificity, PPV and NPV improved, but did not seem to be satisfactory. Similarly, for the CHA₂DS₂VASc score, in spite of a decrease in the specificity value, there was an improvement in all the other measures, which is not enough to ensure its adequacy.

Moreover, the regression models failed to show that the scores, alone or in association with the LA, are predictors of POAF. Therefore, it can be asserted that the models analyzed in this study are not good predictors of POAF. The literature is controversial regarding the determination of the predictive capacity of the CHADS₂ and CHA₂DS₂VASc scores.8^,9^,10^,13

Some studies have shown that the risk of POAF increased as the CHADS₂ and CHA₂DS₂VASc scores increased, but they did not demonstrate their predictive capacity.9^,10 Recently, researchers13 have analyzed the predictive capcity of different risk scores (Society of Thoracic Surgeons risk of mortality score, Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE)-AF score, POAF score and CHA₂DS₂VASc) and age for the new- onset of AF after coronary artery bypass graft operation. They noted that none of the variables analyzed performed well. The ROC area for CHARGE-AF was 0.6796 (CI 95%: 0.6672-0.6920), whereas for CHA₂DS₂VASc it was 0.5917 (CI 95%: 0.5782-0.6052). Different results were obtained in another study,8 which showed good estimations on sensitivity (84.21), specificity (84.54) and negative predictive value (97.23) of the CHA₂DS₂VASc score to predict the risk of AF after coronary artery bypass, with a ROC area of 0.87.

The present study has limitations relating to sample size and to the fact that this is a single-center study. Furthermore, the short follow-up period did not allow for the identification of new- onset and recurring AF after hospital discharge, which might have contributed to poor score performance in predicting arrhythmia, since the items of both scores take into account chronic conditions that can affect myocardial structure and stability in the long term after cardiac surgery.

The CHADS₂ and CHA₂DS₂-VASc scores alone were not good predictors of POAF in patients undergoing coronary artery bypass graft and/or valvular surgery in this study. Although LA size has improved the estimation of sensitivity, specificity, PPV and NPV, it was not enough to improve the predictive capacity of the scores.