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a Department of Cardiothoracic Surgery, Medical University of Vienna, Vienna, Austria
b Department of Radiology, Medical University of Vienna, Vienna, Austria
c Department of Cardiology, Medical University of Vienna, Vienna, Austria
Accepted for publication May 22, 2007.
* Address correspondence to Dr Wisser, Department of Cardiothoracic Surgery, University of Vienna, Währinger Gürtel 18-20, Vienna, A-1090, Austria (Email: wilfried.wisser{at}meduniwien.ac.at).
Presented at the Poster Session of the Forty-third Annual Meeting of The Society of Thoracic Surgeons, San Diego, CA, Jan 29–31, 2007.
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Abstract |
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Methods: Nineteen patients (mean age, 63.1 ± 11.7 years) being at least six months after surgical ablation procedure, with stable sinus rhythm, were selected for the study. They underwent cardiac MRI. End-diastolic and end-systolic volumes were measured using Simpson’s rule. The presence of visual contraction was visually assessed.
Results: In MRI evaluation mean end-diastolic volume of the right atrium and left atrium after an ablation procedure was 127 ± 45 mL and 163 ± 50 mL, respectively. Mean stroke volume was 23 ± 15 mL and 26 ± 12 mL for the right and left atrium. Mean ejection fraction of the right atrium was 0.19 ± 0.14 and 0.17 ± 0.1 for the left atrium. An atrial kick of both atria was observed in 8 of 19 (47%) patients. An atrial kick of only the right atrium was observed in an additional 13 of 19 (68%) patients.
Conclusions: The anticipated events after a surgical ablation procedure are the restoration of atrial contractility and the associated atrial kick, thereby enhancing cardiac output and decreasing the risk of thromboembolism. Evaluation of atrial function after an ablation procedure using SSFP MRI is feasible and allows a standardized documentation of postoperative atrial function, thus allowing evaluation of the surgical outcome in a reproducible way. Echocardiographic evaluation seems to underestimate the transport function of the atrium.
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Introduction |
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The maze procedure, initially developed by James Cox in 1991, is an established surgical procedure for restoration of sinus rhythm and reestablishment of atrial function in patients with chronic atrial fibrillation. Although the original Cox maze procedure restores sinus rhythm in 90%, this technique has not gained widespread application due to its complexity. Therefore, alternative energy sources such as radiofrequency, microwave, and cryothermy for the creation of atrial lesions have emerged in order to avoid the traditional cut and sew maze. Success rates are highly variable and not easy to compare because of different study endpoints [2–5].
The aim of treating atrial fibrillation is the restoration of a regular rhythm so that a normal cardiac output is maintained. Especially, the mechanical transport function of the atria is of importance for optimum cardiac output. Despite the presence of atrial transport function in most of the patients after the maze procedure, more important is whether the amount of mechanical function is sufficient to provide atrial contraction, which in turn is mandatory to eliminate the risk of thrombus formation. It was the aim of our study to assess the presence of atrial kick by means of SSFP MRI.
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Material and Methods |
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TopAbstractIntroductionMaterial and MethodsResultsCommentReferences |
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Indications for surgery were the following: mitral valve insufficiency (n = 6), mitral + tricuspid valve insufficiency (n = 1); mitral valve stenosis + tricuspid valve insufficiency (n = 1); mixed mitral valve pathology (n = 3); aortic valve stenosis (n = 1); aortic valve insufficiency (n = 1); aortic valve stenosis + mitral valve insufficiency (n = 2); coronary artery disease (n = 2); coronary artery disease + mitral valve insufficiency (n = 1); and coronary artery disease + mitral + tricuspid valve insufficiency (n = 1).
Routine transthoracic echocardiographic controls were done preoperatively as well as 3, 6, and 12 months postoperatively using standard parasternal and apical views. Measurement of the A wave and atrial diameters were done according to the guidelines of the American Society of Echocardiography [6].
All patients underwent cardiac magnetic resonance imaging (MRI) (Philips Intera 1.5T, Philips Medical Systems, Austria) using steady-state free precession (SSFP) pulse sequence 12 months after surgery. Standard two-chamber and four-chamber long axis series as well as a complete set of short axis cine acquisitions were obtained (8 to 10 slices, thickness 6 mm, 15 to 20 phases) in order to capture both atria entirely. Two experienced readers evaluated the images independently using a workstation and commercially available dedicated software. End-diastolic and end-systolic volumes were measured using Simpson’s rule and were obtained from measuring the biplane area length in orthogonal long axis, two-chamber, and four-chamber views by manual segmentation by one of the two observers. The presence of visual contraction (atrial kick) was visually assessed. Normal values (MRI variables) in patients with sinus rhythm are given in the current literature [7] with a left atrial ejection fraction of 0.39 ± 0.06 and a right atrial ejection fraction of 0.28 ± 0.06.
Surgical Procedure
After median sternotomy, normothermic cardiopulmonary bypass was established in standard fashion. The venous drainage was accomplished by cannulating both venae cavae in case of mitral and tricuspid valve procedures, left atrial endocardial maze procedures, or by cannulating the right atrium with a two-stage cannula in the remaining cases.
The surgical ablation procedure was performed either endocardially with microwave (Lynx; Tektronix, Graz-Grambach, Austria) in six patients and with unipolar irrigated radiofrequency (Medtronic Pen, Medtronic, Minneapolis, MN) in eight patients, or epicardially with bipolar irrigated radiofrequency (Medtronic Cardioblate; Medtronic) in three patients and with focused ultrasound (Epicor: St. Jude Medical, Inc, St Paul, MN) in two patients. The lesion concept used was a biatrial endocardial and left atrial endocardial maze and epicardial pulmonary vein isolation in nine, five, and five patients, respectively. A detailed description of the ablation procedures has been published recently [8, 9].
Left atrial endocardial maze
The ablation lines were performed with a unipolar radiofrequency ablation tool. After onset of normothermic cardiopulmonary bypass, the venous inflow was occluded. Thereafter the aorta was cross-clamped and the heart arrested. Myocardial protection was achieved with cold blood cardioplegia infused antegradely and retrogradely. The left atrium was entered through the interatrial groove and the left-sided maze was performed. In contrast to the original maze procedure the left and right pulmonary veins were encircled separately, leading to two isolated isles that were connected at the back of the left atrium creating a figure "H." Then the remaining lesions to the posterior mitral annulus and the left atrial appendage were carried out. After completion of the maze procedure, the necessary heart surgeries were performed and the left atrium was closed by a double running suture.
Biatrial endocardial ablation
In cases of biatrial endocardial ablation, lesion patterns on the right side were added. Right sided lesions were created after atriotomy beginning from the superior vena cava to the inferior vena cava, then from the inferior vena cava to the sinus coronarius, and from the superior vena cava to the tricuspid valve and to the right atrial appendage.
Pulmonary vein isolation
The pulmonary veins were isolated with a bipolar radiofrequency ablation tool. After onset of normothermic cardiopulmonary bypass, right and left pulmonary veins were dissected free with the heart beating. After encircling of the pulmonary venous cuffs, the jaws of the pliers were positioned on the atrial wall and closed. Care was taken to apply the ablation on the atrial myocardium rather than on the pulmonary vein itself. No further connecting lines to the mitral annulus or the left atrial appendage were drawn. The left atrial appendage was left in place.
Epicor (St Jude Medical, Inc)
In patients where the Epicor system was used, the pericardial reflection lines around the superior and inferior venae cavae were dissected free to gain access to the transverse and oblique sinus. Thereafter a specially designed introducer-sizer was passed into the transverse sinus and guided into the oblique sinus, thereby encircling all four pulmonary veins. Afterward the UltraCinch (St Jude Medical) was passed in the same way and ablation was carried out for 10 minutes [9].
Follow-Up
Patients were evaluated 3, 6, and 12 months after operation and then on a yearly basis. Rhythm was determined on the basis of a 12-lead electrocardiogram and 24-hour Holter monitoring. Echocardiographic data were collected in respect to left and right atrial diameter and MRI was performed after 12 months.
Statistics
Statistical analysis was performed with SPSS 10.0 (SPSS, Chicago, IL). Continuous data were expressed as mean ± SD and compared using the Student t test for paired data. A p value less than 0.05 was considered significant.
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Results |
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Mean aortic cross-clamp time and extracorporeal circulation time was 77 ± 21 and 123 ± 40 minutes, respectively. There were no intraoperative complications. All patients experienced an uneventful postoperative stay and were discharged after a mean of 7 ± 5 days. A sinus rhythm was documented in all patients before discharge by ECG and echocardiography.
In echocardiography an A wave in both atria was observed in 4 of 15 patients. Unfortunately, in four patients the contractility of the right atrium was not measured. A significant correlation was found for the duration of preoperative atrial fibrillation. In the patients with an atrial kick it was a mean of 22 ± 39 months, whereas in the patients without an atrial kick (Fig 1) it was 46 ± 41 months (p = 0.016). An MRI examination at 12 months postoperatively showed the presence of an atrial kick of both atria in 8 of 19 patients (Fig 2). A significant correlation was calculated for the stroke volumes of the left and right atrium (p = 0.024) as well as for the MRI-defined left atrial kick and the echocardiographic-defined A wave through the mitral valve (p = 0.029).
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Mean end-diastolic volume of the right atrium (RA) and left atrium (LA) after surgical ablation procedure was 127 ± 45 mL and 163 ± 50 mL, respectively. Mean stroke volume was 23 ± 15 mL and 26 ± 12 mL for the RA and LA, respectively. Mean ejection fraction of the RA was 0.19 ± 0.14 and was 0.17 ± 0.1 of the LA. An atrial kick of both atria was observed in 8 of 19 (47%) patients. An atrial kick of only the RA was observed in an additional 13 of 19 (68%) patients (Fig 3). Mean interobserver variability for all parameters was 4.3 ± 1.5%
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Comment |
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However, the main question is whether the mechanical function of the atria is sufficient to provide an atrial contraction, which is mandatory for reducing the stasis of blood in the atria, thereby eliminating the risk of thrombus formation. Regarding the duration of atrial fibrillation, our findings were in accordance with a recent report of Isobe and Kawashima [11], who showed that patients with a longer duration of preoperative atrial fibrillation were less likely to recover atrial transport function after restoration of sinus rhythm [11].
In the current literature detailed examination of atrial transport function as well as long-term follow-up is still pending. This is attributed mainly to the limited ability of echocardiography to accurately evaluate atrial geometry, wall motion, and volume measurements due to the complexity of the atrial shapes [11–13] . There are some studies, particularly by Yashima and colleagues [10] and Ishii and colleagues [12], which report a slow recovery of atrial transport function or atrial function remaining at an unsatisfactory level, in a serial evaluation of echocardiographic studies [10, 12]. The results are somewhat more favorable in patients with lone atrial fibrillation, as demonstrated in a recent study by Hemels and colleagues [13], which showed a normal atrial contraction pattern in 41% of their patients after one year. As mentioned above, so far follow-up studies are mainly performed by echocardiography, which is often limited by suboptimal acoustic windows and thus impedes quantification of atrial dimensions [10–12].
Magnetic resonance imaging, on the other hand, is another noninvasive imaging technique and is currently the golden standard for assessment of left and right ventricular dimensions and function. The advantages of MRI over echocardiography are the superior image quality, the possibility of quantification of dimension and volumes, and the high reproducibility as well as the smaller sample size needed for statistical analysis [6, 11–13].
Especially, SSFP MRI has become the technique of choice for assessing regional and global cardiac function because of its superior blood tissue contrast that enables excellent visualization of cardiac motion in areas of slow blood flow [14–16].
Yamanaka and colleagues [17] have recently evaluated left atrial size by means of multislice computed tomography. They compared a series of patients with chronic atrial fibrillation and mitral valve disease with coronary artery bypass patients (who served as control). Multislice tomography accurately quantified left atrial volume and regional wall motion but revealed a diffusely low atrial mechanical function after the maze procedure despite recovery of sinus rhythm. However, one has to consider the disadvantage of radiation exposure to the patient.
The anticipated events after a maze procedure are the restoration of atrial contractility and the associated atrial kick, thereby enhancing cardiac output and decreasing the risk of thromboembolism. Our results indicate that echocardiographic evaluation seems to underestimate the transport function of the atrium.
We have found only one report existing in literature so far, by Bauer and colleagues [18], who evaluated 72 patients after the mini maze procedure with echocardiography as well as MRI in terms of diameter of the left atrium, left ventricular ejection fraction, and left ventricular end-diastolic and end-systolic diameter. They showed a higher restoration of atrial transport function rate in echocardiography (86%) versus MRI (78%) one year after surgery. They stated that echocardiography and MRI showed an excellent correlation. This might be based on the fact that they mainly evaluated left ventricular dimension and only the diameter of the left atrium, which are only two, two-dimensional evaluations. In our study however, we focused on quantification of atrial volumes and ejection fraction, which are three-dimensional evaluations, where MRI seems particularly useful. We are well aware of limitations present in this study, such as the small patient number, which allows only limited conclusions of statistical data.
Evaluation of atrial function after a maze procedure using SSFP MRI is feasible and allows a standardized documentation of postoperative atrial function, thus allowing to evaluate the surgical outcome in a reproducible way. Echocardiographic evaluation seems to underestimate the transport function of the atrium. Further studies are warranted to gain more insight into the exact evaluation of atrial transport function. Especially, the feasibility of MRI regarding other parameters of atrial transport function, such as atrial ejection fraction, might emerge as an indicator for intact transport function and therefore cessation of anticoagulation.
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