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J Thorac Cardiovasc Surg 2007;133:389-396
© 2007 The American Association for Thoracic Surgery
Surgery for Acquired Cardiovascular Disease |
Division of Cardiothoracic Surgery, Washington University School of Medicine, Barnes-Jewish Hospital, Saint Louis, Mo.
Read at the Eighty-sixth Annual Meeting of The American Association for Thoracic Surgery, Philadelphia, Pa, April 29-May 3, 2006.
Received for publication April 27, 2006; revisions received September 22, 2006; accepted for publication October 9, 2006. * Address for reprints: Ralph J. Damiano, Jr, MD, Washington University School of Medicine, Barnes-Jewish Hospital, Suite 3108 Queeny Tower, 1 Barnes-Jewish Hospital Plaza, Saint Louis, MO 63110. (Email: damianor{at}wustl.edu).
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Abstract |
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METHODS: From April 1992 through July 2005, 242 patients underwent the Cox-maze procedure for atrial fibrillation. Of these, 154 patients had the Cox-maze III procedure, and 88 had the Cox-maze IV procedure. Logistic regression analysis was used to identify covariates among 7 baseline patient variables. Using the significant regression coefficients, each patient’s propensity score was calculated, allowing selectively matched subgroups of 58 patients each. Operative outcomes were analyzed for differences. Late follow-up was available for 112 (97%) patients. Freedom from atrial fibrillation recurrence and survival was calculated at 1 year by using Kaplan-Meier analysis.
RESULTS: The Cox-maze III procedure had significantly longer crossclamp times. There was no significant difference in intensive care unit and hospital stay, 30-day mortality, permanent pacemaker placement, early atrial tachyarrhythmias, late stroke, and survival. Freedom from atrial fibrillation recurrence was greater than 90% in both groups at 1 year.
CONCLUSIONS: The use of bipolar radiofrequency ablation has simplified the Cox-maze procedure, making it applicable to virtually all patients with atrial fibrillation undergoing concomitant cardiac surgery. The Cox-maze IV procedure produces similar surgical outcomes to the Cox-maze III procedure at 1 year of follow-up.
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Introduction |
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Atrial fibrillation (AF) affects more than 2.2 million persons in the United States, with an increased incidence within the elderly population.1
The presence of AF is an independent risk factor for stroke. When compared with AF-free patients, there is a 5-fold increase in the incidence of stroke.2 Even after adjusting for preexisting cardiovascular conditions, AF is associated with a 1.5- to 1.9-fold increase in mortality risk.3 Additional consequences of AF include increased hospitalizations, thromboembolic events, hemodynamic compromise, and decreased quality of life.4 In addition to the increased patient morbidity and mortality, AF is associated with significant hospital costs.5
The Cox-maze (CM) procedure was introduced in 1987. It is a set of atrial incisions aimed at interrupting the macrore-entrant circuits within the atrial tissue, which were believed to be the cause of AF.6 The final iteration of the procedure was the CM III procedure.7 Although the CM III procedure produced excellent results and was considered the gold standard for curing AF,8,9 its technical complexity prevented it from being incorporated in the daily practice of the majority of cardiac surgeons.
A variety of energy sources have been used to replace the incisions and simplify the CM III procedure, including unipolar and bipolar radiofrequency (RF), microwave, cryoablation, laser, and ultrasonography.10-18 At our institution, bipolar RF ablation was chosen to replace many of the CM incisions. Our preference for this energy source was based on experimental data from our laboratory showing that bipolar RF always created reliable transmural lesions, had short ablation times, and resulted in a focused delivery of energy that prevented collateral injury, enhancing the safety margin of ablation.10-12 Early results in a multicenter trial showed bipolar RF ablation to safely and effectively electrically isolate atrial tissue.19 This newest version, the CM IV procedure, combined bipolar RF ablation and cryoablation to replace most of the incisions of the CM III procedure. In a small group of 40 patients, this new operation was found to be simpler and shorter than the CM III procedure, with equivalent short-term efficacy.20 However, no direct comparison of matched patients undergoing the CM IV versus the CM III procedure has been performed. The objective of this study was to compare the outcomes of patients undergoing the CM IV procedure with propensity-matched cohorts from the historical CM III group.
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Methods |
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Propensity Score Analysis
The nonrandomness of procedure assignment was addressed by propensity matching to provide a more reliable assessment of outcomes based on procedure type. Logistic regression analysis was used to identify covariates among 7 baseline patient variables that were imbalanced in the 2 groups of interest (SPSS 11.0 for Windows; SPSS Inc, Chicago, Ill). Variables included age, sex, preoperative left ventricular ejection fraction, New York Heart Association (NYHA) classification, diagnosis of AF (persistent, permanent, or paroxysmal AF), duration of AF before surgical intervention, and type of operative procedure performed (lone CM, CM with a concomitant coronary artery bypass graft [CABG], or CM with a concomitant valve procedure with or without a CABG). Persistent AF was defined as continuous AF. Permanent AF was defined as persistent AF of more than a 6-month duration that had failed either electrical or chemical cardioversion. Paroxysmal AF was defined as sinus rhythm with intermittent episodes of AF. By using the significant regression coefficients, a propensity score was calculated for each of the 242 patients who underwent a CM III or IV procedure. The total population was ranked by propensity score, and the patients were closely matched on the basis of this score. The short- and long-term outcomes of the patients were blinded during the matching process. Resulting matched patients were analyzed for differences in selected early and late outcomes: crossclamp time, cardiopulmonary bypass time, intensive care unit (ICU) length of stay, hospital length of stay, 30-day operative mortality, postoperative permanent pacemaker placement, incidence of early tachyarrhythmias, late stroke, survival, and late recurrence of AF.
Surgical Technique
The surgical procedure for the CM III procedure has previously been described in detail.21 Most patients underwent a median sternotomy and cardiopulmonary bypass with bicaval cannulation. On the beating heart, right atrial incisions included excision of the right atrial appendage, followed by a free wall incision, a linear incision from the orifice of the superior vena cava to the orifice of the inferior vena cava, and a perpendicular, or T, incision to the level of the tricuspid valve annulus. A second incision to the tricuspid annulus was made from the right atrial appendage. At the tricuspid annulus, a 3-mm cryoprobe (Frigitronics CCS200, Trumbell, Conn) was applied.
The heart was arrested, and access to the left atrium was through a standard left atriotomy with amputation of the left atrial appendage. The remaining left atrial incisions included an atrial septal incision and encirclement of the pulmonary veins with extension to the mitral valve annulus. A cryoprobe was used between the appendage amputation site and the 2 ends of the pulmonary vein encircling the incision, as well as over the coronary sinus, and at the mitral valve annulus.
The bipolar RF system consisted of the ablation-sensing unit and the Atricure Isolator (n = 57; Atricure, Inc, Cincinnati, Ohio) or the Medtronic Cardioblate BP Surgical Ablation System (n = 1; Medtronic, Inc, Minneapolis, Minn). For the Atricure device, the energy was applied at 75 W and 750 mA between the jaws of the instrument. The generator continuously monitored voltage, current, temperature, and conductance. Tissue temperature was measured 1 mm from the electrode edge. Two seconds after conductance decreased to less than 0.025 siemens, an indicator light flashed, and an audible tone was heard, signifying full-thickness coagulation and termination of the ablation. Total ablation time and maximum tissue temperature were recorded for every lesion.
The Medtronic irrigated bipolar RF surgical ablation device consisted of a hand piece with embedded electrodes and an RF generator. The device was irrigated with saline solution to improve conduction of the delivered energy. The RF generator continuously monitored tissue impedance, current, voltage delivered, and the duration of ablation. Tissue was considered fully ablated when impedance reached a stable plateau. Initially, moderate power was applied to the tissue. The derivative of impedance (dZ/dt) was calculated every 200 ms. When impedance achieved a stable plateau, the power was increased by a step function of 5 W. If the plateau in impedance was not sustained, then the algorithm determined that transmurality had not been achieved, and ablation continued until another plateau in impedance was detected. This process was repeated until an impedance plateau was sustained after an increase in power. When this end point was reached, the microprocessor determined that transmurality had been achieved, and a signal was provided to the user.
The CM IV procedure differed from the CM III procedure because it isolated the right and left pulmonary veins as 2 islands, preserved the right atrial appendage, and in some cases left the posterior left atrium in electrical continuity with the remaining atrium.22 After either median sternotomy or right thoracotomy, patients underwent a pericardiotomy and were started on cardiopulmonary bypass. If patients were not in normal sinus rhythm, intraoperative direct-current cardioversion was performed. Before the ablation procedure, the pacing threshold was recorded from the pulmonary veins. Bipolar RF ablations were completed around the right and the left pulmonary veins. After ablation, electrical isolation was confirmed by means of bipolar pacing at 20 mA from both the superior and inferior pulmonary veins. If atrial capture was present, the ablation was repeated until electrical isolation was achieved. The remainder of the operation has been described in a prior publication.23
In summary, the right-sided lesions were created by making a simple atriotomy that extended from the intra-atrial septum to near the atrioventricular groove at the acute margin of the heart (Figure 1). All the other incisions of the traditional cut-and-sew method were replaced with bipolar RF ablation lines. Two cryolesions were placed at the tricuspid annulus by using a linear cryoprobe. The left-sided lesions involved only a simple atriotomy extending onto the dome of the left atrium and inferiorly around the orifice of the right inferior pulmonary vein. It intersected the encircling right pulmonary vein ablation. A connecting lesion was performed from the inferior aspect of the left atrium into the left inferior pulmonary vein. In atria larger than 5 cm in diameter, a second connecting lesion was placed from the superior aspect of the incision into the left superior pulmonary vein. A bipolar RF ablation line was performed from the inferior end of the incision down to the mitral annulus. A final cryolesion was placed at the mitral annulus with a 15-mm bell probe. The left atrial appendage was amputated, and a final bipolar RF ablation was performed between the left atrial appendage and the left superior pulmonary vein.
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Follow-up for the patients in the CM III group consisted of a retrospective cross-sectional analysis performed in 2001. This included a mailed questionnaire or telephone interview, as well as contact with either their cardiologist or primary care physician regarding recurrence of AF. In patients who complained of arrhythmia recurrence, copies of an electrocardiogram and Holter monitoring were obtained. Patients in the CM IV group were followed prospectively and had visits scheduled for 1, 3, 6, and 12 months postoperatively. Patients were then followed annually. At all follow-up visits, a history, physical examination, and electrocardiogram were obtained. For patients who could not return to our institution, telephone questionnaires were performed, and electrocardiograms were obtained from referring physicians to document the heart rhythm. In patients with symptoms of palpitations, an electrocardiogram or prolonged Holter recording (>24 hours) was obtained to assess their rhythm status.
Recurrence of AF was documented if a patient required a cardioversion or atrial ablation greater than 3 months after the procedure or the presence of AF was seen on prolonged Holter monitoring or electrocardiography. Any episode of recurrent AF after 3 months was classified as a permanent failure.
Data Analysis
Data were represented as frequency distributions and percentages. All continuous data were expressed as means ± standard deviation. Categoric data were expressed as counts and proportions. Comparisons were done with paired, 2-tailed t tests for means of normally distributed continuous variables and the Wilcoxon rank sum test for skewed data. Fisher exact or 
2 tests were used to analyze differences among the categoric data. Freedom from AF recurrence and survival was calculated at 1 year by using Kaplan-Meier analysis. Statistical analysis of data was conducted with the SPSS system for statistics (SPSS 11.0 for Windows, SPSS Inc).
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Results |
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This process matched 58 of the 154 patients who underwent a CM III procedure with 58 of the 88 patients who underwent a CM IV procedure. Thirty of the 88 patients in the CM IV group were not able to be matched with the patients in the CM III group because their propensity scores were extreme outliers.
Selected preoperative patient characteristics for the CM III and CM IV groups are listed in Table 1. There was no significant difference between the 2 groups in age, sex, left ventricular ejection fraction, preoperative diagnosis (persistent, permanent, or paroxysmal AF), preoperative AF duration, operative procedure (lone CM, CM with concomitant CABG, or CM with concomitant valve and CABG), or NYHA classification. Late follow-up was available for 112 (97%) patients. Late follow-up was 100% in the CM IV group and 93% in the CM III group. Mean follow-up was significantly longer in the CM III group (4.3 ± 2.7 years) versus that in the CM IV group (1.2 ± 0.8 years). Median follow-up was 5.1 years and 1.1 years in the CM III and CM IV groups, respectively.
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Discussion |
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It consisted of a complex set of incisions designed to prevent macrore-entrant circuits from forming in the atria, which was the prominent theory at that time regarding the mechanism of AF.6,24,25 The CM procedure was difficult, required long crossclamp times, and was associated with significant morbitiy.9 Its technical complexity prevented the CM procedure from being adopted widely by cardiac surgeons.
Over the past decade, a number of alternatives have been examined to simplify the surgical treatment of AF and reduce the associated morbidity. A central strategy of these efforts has been to replace the surgical incisions with linear lines of ablation by using a variety of energy sources, including unipolar and bipolar RF, microwave, cryoablation, laser, and ultrasonography.10-17 Although all of these have enjoyed some degree of success, experimental work by our laboratory and others have revealed that bipolar RF ablation resulted in the most consistent transmural lesions.10-13 Moreover, lesions were quicker to perform with bipolar RF than other energy sources. The injury also was contained within the jaws of the clamp, preventing the collateral damage seen with other unipolar devices.26-28 At our institution, this energy source was adopted to replace the majority of the CM incisions, and the procedure was termed the CM IV procedure.20 Early and late events were compared between the 2 groups to determine whether this new procedure produced similar outcomes as the CM III procedure. A propensity analysis was used to overcome selection bias.
In these matched patients the CM III group had significantly longer crossclamp times when compared with the CM IV group. Bipolar RF ablation replacing the surgical incisions simplified the procedure, allowing for shorter operative times in the CM IV group. The use of this ablation technology removed the major obstacle to widespread adoption of the CM procedure, its complexity and technical difficulty. This has significantly altered our own indication for AF operations in patients with concomitant heart disease. Although in the past the CM procedure was reserved for highly selected patients, it is now offered to virtually all patients with chronic AF referred for valvular or coronary surgery. This change in attitude is testified to by the fact that it took 9.2 years to accumulate the 58 patients in the CM III cohort compared with 3.2 years for the CM IV cohort.
Other outcomes were not significantly different between the 2 groups. ICU and hospital stay, 30-day mortality, permanent pacemaker placement, and atrial tachyarrhythmias and late stoke were similar. Thus the CM IV procedure resulted in similar operative outcomes with a shorter crossclamp time.
There was no difference in freedom from AF recurrence between the 2 groups on the basis of Kaplan–Meier analysis at 1 year. Both procedures yielded success rates of greater than 90%. This agrees with a prior report involving an unmatched comparison of 30 patients who underwent a CM III procedure with 40 patients undergoing a modified RF ablation maze procedure.29
A major difference between the CM III and CM IV procedures is the method of creating the lesions (cut-and-sew vs ablation). An incision creates a conduction block across the suture line 100% of the time. Although experimental studies have shown the ability to isolate targeted tissue and produce transmural lesions using the bipolar RF ablation device,10-13 it is possible that not all the lesions clinically produced conduction block or were transmural. Studies have shown that gaps as small as or smaller than 1 mm can allow the propagation of AF.30 Isolation of the pulmonary veins can be tested intraoperatively, but the remaining lesions cannot be verified. Furthermore, in the CM IV procedure less left atrial tissue is isolated, leaving more of the left atrium in electrical continuity. Further follow-up is needed to determine whether these small differences between procedures will have any late effect at greater than 1 year.
Antiarrhythmic drug use was different between the study populations at last follow-up. Although the data for the CM IV group at 1 year were available, postoperative data from the CM III group were collected in a cross-sectional manner and did not allow for analysis at 1 year but rather only at last follow-up. The median follow-up was quite different between the 2 groups, making it difficult to compare the CM III and CM IV groups in terms of need for antiarrhythmic drugs.
The limitations of this study included the relatively small number of patients in each group. However, with 116 patients, this series is the largest comparison of matched patients undergoing a cut-and-sew versus an ablation-assisted CM procedure in the literature. Moreover, with no prospective randomized studies either published or in process, this might be the most valid way to compare these 2 procedures.
The CM III patient information was retrospectively collected, whereas CM IV patient information was prospectively collected, and follow-up has been more complete. This might have contributed to the slightly higher failure rate. The CM IV procedure was more widely applied to higher-risk patients than the CM III procedure, making retrospective comparison difficult. For this reason, the propensity analysis was used to help overcome this shortcoming.
An additional weakness of this study is the means of assessing recurrence of AF. In the CM IV group an electrocardiogram was obtained at each follow-up appointment. If a patient in either the CM III or CM IV group complained of symptoms of palpitations, an electrocardiogram or prolonged Holter monitoring was obtained. If patients had implanted pacemakers, these were interrogated and used to determine the occurrence of AF. These limitations of follow-up were similar to all previous reports, and one strength of this study was the low percentage of patients lost to follow-up.
In summary, the CM IV procedure, using bipolar RF ablation, has simplified the procedure from a technical standpoint, making it applicable to most patients with AF undergoing concomitant cardiac surgery. Comparing matched populations, the operation can be done with a shorter crossclamp time and produces similar surgical outcomes to the cut-and-sew technique of the original CM procedure.
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Acknowledgments |
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Footnotes |
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