Asian Cardiovasc Thorac Ann 2008;16:292-297
© 2008 Asia Publishing EXchange Ltd
Predictors of Sinus Rhythm after Radiofrequency Maze and Mitral Valve Surgery
Suchart Chaiyaroj, FACS,
Tachapong Ngarmukos, FACC,
Panuwat Lertsithichai, MD
Cardiothoracic Surgery Unit, Department of Surgery, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Bangkok, Thailand
For reprint information contact: Suchart Chaiyaroj, FACS, Tel: 66 2 201 1315, Fax: 66 2 201 1316, Email: schaiyaroj{at}gmail.com, Cardiothoracic Surgery Unit, Department of Surgery, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Bangkok 10400, Thailand.
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ABSTRACT
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The prevalence and predictors of postoperative sinus rhythm in patients undergoing a radiofrequency ablation maze operation and mitral valve surgery were assessed in 63 patients of whom 54 (86%) were in permanent atrial fibrillation. Operative mortality was 3.17%. At a median follow-up of 18 months, 54 (88.5%) patients were in sinus rhythm and 7 (11.5%) were in atrial fibrillation. The probability of sinus rhythm at 30 months was 92%. Left atrial diameter was the most significant prognostic factor for sinus rhythm on multivariate proportional-hazard regression analysis. The cutoff value of preoperative left atrial diameter for predicting persistent atrial fibrillation at 6 months was 6 cm (100% sensitivity and 73.6% specificity). The radiofrequency ablation maze operation can be performed in addition to mitral surgery with a high rate of successful conversion to sinus rhythm. Preoperative left atrial diameter < 6 cm is an important prognostic factor for sinus rhythm conversion.
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INTRODUCTION
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Atrial fibrillation (AF) increases the risks of morbidity and mortality in the cardiac surgical patient.1 The rate of spontaneous conversion to sinus rhythm (SR) in patients with AF undergoing mitral valve surgery alone is 8%–28%.2 Reports show 80%–90% rates of SR conversion after mitral valve surgery with intraoperative radiofrequency ablation (RFA).2,3 Prognostic factors for failure to convert to SR are not well defined. This study evaluated the prevalence of postoperative SR and its prognostic factors in patients undergoing the RFA maze operation with concomitant mitral valve surgery. The thresholds of prognostic factors were determined for clinical application.
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PATIENTS AND METHODS
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The study included 63 consecutive patients with AF who underwent mitral valve surgery and an intraoperative RFA maze procedure between June 2004 and December 2006. The inclusion criteria were persistent and permanent AF; paroxysmal AF was excluded. Clinical data were obtained from the hospital database and reviewed retrospectively. The study protocol was approved by the ethical committee of Ramathibodi Hospital. Patient characteristics are summarized in Table 1
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A median sternotomy was performed, and the patient was placed on cardiopulmonary bypass in the standard fashion. The biatrial maze operation followed the concept of the modified Cox’s maze procedure, using an irrigated RFA device (Cardioblate; Medtronic, Minneapolis, MN, USA) which included a Cardioblate generator, unipolar surgical ablation pen, and a bipolar device. The saline irrigation rate was 5 mL·min–1 to cool the endocardial surface. The patient was grounded with a different electrode applied to the skin. The power generator was set at 30 W. When utilizing the Cardioblate pen, the electrode was placed on the endocardial surface without applying pressure. The tip was slowly oscillated 10 times over the area to create a continuous endocardial lesion. Time spent on the tissue depended on the estimated thickness of the tissue and the energy level. The algorithm was confirmed by visual changes in the tissue. The Cardioblate bipolar device creates epicardial ablation with a transmural feedback mechanism. It was applied in the larger or longer areas to complete the lesion lines. Lesion patterns were similar to those of the modified Cox’s maze procedure.3–5 We produced 7 lesions on the left atrium and 9 on the right. The bipolar ablation device was used for most lesions except the mitral isthmus line, tricuspid valve lines, and right isthmus line. The left atrial appendage was excised and closed in all patients. Valve surgery was undertaken during the procedure. Mitral valve repair was achieved using multiple techniques of chordal replacement, secondary chordal transfer, posterior annuloplasty, and posterior leaflet quadrangular resection. A bileaflet mechanical prosthesis was chosen for mitral valve replacement. Valve surgery data are given in Table 2.
Patients were evaluated immediately postoperatively, at 1 and 3 months, and 3-monthly thereafter until 30 months after the operation. Heart rhythm was determined by 12-lead electrocardiogram. Echocardiographic data were recorded for left atrial diameter (LAD) and left ventricular function. Two-dimensional echocardiograms were obtained with a Hewlett-Packard Sonos 5500, using a 4.5-MHZ probe. Left atrial diameter was measured in M-mode parasternal long-axis view, and left atrial length was measured in 4-chamber apical view. No antiarrhythmic therapy was given immediately after surgery; if the patient was in rapid AF, amiodarone was administered, followed by electrical cardioversion if required. All patients received warfarin with a target international normalized ratio of 2.2–2.5. To explore the overall probability of SR occurrence with time after surgery, outcomes of patients with and without persistent AF for at least 6 months postoperatively were compared. Factors associated with persistent AF and their cutoff values were evaluated.
Continuous variables are given as mean and standard deviation or median and range. Categorical variables are given as counts and percentages. All analyses were performed using Stata version 7 software (Stata Corp., College Station, TX, USA). Statistical significance was defined as a p value of 0.05 or less. Continuous variables were compared using an unpaired t test or Wilcoxon rank-sum test, and categorical variables were compared using the chi-squared or Fisher’s exact test. The overall probability of SR conversion was estimated using survival analysis models for interval-censored data, with smoothing spline regression. Prognostic factors were also entered into the models, assuming proportional hazards. Estimates of the magnitude and significance of the effects of prognostic factors were undertaken using a model-based method. As a check on this model, Cox proportional-hazard regression was performed with the assumption that all events occurred precisely at the follow-up time. This assumption is unrealistic, but helps confirm the findings of the spline-based model if the estimates from both models are of similar magnitude or significance. Breslow’s method was used to handle tied observations. Factors associated with failure of the operation (persistent AF) were also determined using logistic regression analysis. Associations are reported as odds ratios and 95% confidence intervals.
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RESULTS
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There were 2 early non-cardiac-related deaths (3.17%) due to postoperative pneumonitis, sepsis, and respiratory failure. There was no reoperation for bleeding, no delayed cardiac tamponade, and no early or late neurological complications. One patient needed prolonged respiratory support because of chronic obstructive pulmonary disease and severe pulmonary hypertension. All 61 survivors were followed up. The median follow-up time was 18 months. Two patients received a permanent pacemaker for atrioventricular block or sick sinus syndrome at 1 month postoperatively. There was 1 late death due to acute prosthetic valve thrombosis at 3 months after surgery. Three patients had late recurrence of AF at 15, 17, and 21 months postoperatively; all of them were converted to SR. An estimate of the overall probability of SR conversion is shown in Figure 1. The probability of SR increases with time, from 40% at 1 month to 92% at 30 months after the operation. The probability of SR increases less steeply after 6 months. The characteristics and outcomes of the patients are presented in Table 1
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Significant prognostic factors associated with conversion to SR, identified by univariate analysis, are shown in Table 2. There was a significantly lower hazard ratio for SR conversion in patients with a longer duration of preoperative AF. Similarly, the permanent type of AF, larger LAD, and multiple valve procedures were associated with a lower rate of SR conversion. On the other hand, a higher ejection fraction (EF) was associated with a higher probability of SR conversion. Age and sex were not significant prognostic factors in this study. Similar results were obtained by Cox proportional-hazard regression analysis. On multivariate analysis, the only prognostic factor significantly associated with conversion to SR was LAD. After adjusting for age, sex, and other variables listed in Table 1, the hazard ratio for LAD was 0.903 (95% confidence interval, 0.824 to 0.990; p = 0.030), i.e., a 9.7% reduction in the probability of conversion to SR for each millimeter increase in LAD. In Table 3, baseline characteristics and outcomes in patients with and without persistent AF for at least 6 months postoperatively are compared. To determine which factors were significantly associated with persistent AF at 6 months, logistic regression analysis was performed. The results were similar to those of the time-to-SR occurrence analysis (Table 4). Sex and age were not significantly associated with treatment failure. Note that all patients with persistent AF at 6 months after surgery had combined valve surgery and a permanent type of AF prior to the operation. Hence these factors were not entered into the logistic regression models. As before, the only significant risk factor on multivariate logistic regression analysis was LAD. After adjusting for age, sex, and all variables in Table 4, the odds ratio was 1.65 (95% confidence interval, 1.12 to 2.44; p = 0.010). Table 5 and Table 6 show the cutoff values for preoperative AF duration, EF, and LAD, with the highest correct classification rate (highest accuracy) and the corresponding sensitivity, specificity, and 95% confidence intervals. These cutoff points were used in the time-to-SR curves to demonstrate the influence of these factors on the probability of conversion to SR. There were significant differences in probabilities of SR conversion between the 2 groups of patients at the cutoff values of prior AF duration, preoperative EF, and preoperative LAD (Figures 2, 3, and 4). The risk of AF after the operation was 3.02-, 1.67-, and 1.54-fold with preoperative LAD > 6 cm, preoperative EF < 40%, and duration of prior AF > 18 months, respectively.
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Figure 2. Comparison of probability of sinus rhythm (SR) conversion in 2 groups of patients at left atrial diameter (LAD) cutoff point of 6 cm.
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Figure 3. Comparison of probability of sinus rhythm (SR) conversion between 2 groups of patients at a duration of preoperative atrial fibrillation (DPAF) cutoff point of 1.5 years.
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Figure 4. Comparison of probability of sinus rhythm (SR) conversion between 2 groups of patients at a left ventricular ejection fraction (EF) cutoff point of 40%.
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DISCUSSION
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The RFA maze procedure successfully restores SR in patients with AF undergoing mitral valve surgery.4–7 The results are comparable to the cut-and-sew Cox’s maze procedure.8,9 The keys to success include complete transmural incision of the atrial tissue, a continuous line of block, and a good lesion pattern on the atrial wall.11,12 Our study found 92% probability of SR conversion at 30 months after surgery. Predictors of late recurrence of AF after the original cut-and-sew Cox’s maze have been described;12,13 however, few studies mention the reasons for RFA maze failure.14,15 We found that the larger the size of the left atrium, the less the chance of conversion to SR by the RFA maze operation in patients undergoing mitral valve surgery, as noted in other reports.14–16 Our study demonstrates that preoperative LAD > 6 cm, AF duration > 18 months, and EF < 40% are prognostic factors for failure to convert to SR after the RFA maze and concomitant mitral valve surgery. Chen and colleagues14 reported similar results. These findings may be applied in clinical situations to help choose the most beneficial treatment for this group of patients. Radio-frequency ablation maze and left atrial volume reduction surgery is associated with a higher rate of SR conversion.18,19 The indication and threshold of atrial size for atrial reduction plasty have not been defined. Our finding of a cutoff value of LAD at 6 cm may be useful as a guide in decision making for this extended maze procedure.
There are limitations to our study. Spot electrocardiograms at various intervals might underestimate the incidence of recurrent AF after surgical ablation.10,16,17 However, a symptomatic patient will be immediately detected and treated according to our protocol. Continuous rhythm monitoring by auto-capture devices would provide a better measurement of AF after surgical ablation.19
The exact causes of RFA maze failure, such as substrate in the atrial tissue or location of complex ganglionic plexi, are not clearly defined and need further investigation.10,11 Intraoperative nerve conduction testing, mapping, and ablation may improve the results.9,10 It was concluded that the RFA maze procedure is safe and effective as an additional procedure in patients with AF undergoing mitral valve surgery, having a high rate of SR conversion. The cutoff values of left atrial size, preoperative duration of AF, and preoperative EF can be used as prognostic factors for SR in the preoperative assessment and decision-making process for surgical treatment of AF to provide the most beneficial results.
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ACKNOWLEDGMENTS
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We thank our cardiac surgical team and electrophysiology team for their great effort, dedication, and spirit.
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REFERENCES
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ABSTRACT
INTRODUCTION
6 Months