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Division of Cardiothoracic Anesthesia, Department of Anesthesiology, and the *Cardiovascular Division, Department of Medicine, Washington University School of Medicine, St. Louis, Missouri
Address correspondence and reprint requests to Charles W. Hogue, Jr., MD, Department of Anesthesiology, Washington University School of Medicine, 660 South Euclid Ave, Box 8054, St. Louis, MO 63110. Address email to hoguec{at}notes.wustl.edu
| Abstract |
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Implications: Transesophageal echocardiography performed immediately before coronary arterybypass graft (CABG) surgery is not useful for prediction of susceptibility todevelop atrial fibrillation postoperatively. Postoperative atrial fibrillationcommonly occurs after CABG surgery in the absence of preoperative atrialenlargement or Doppler derived functional abnormalities.
| Introduction |
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Several electrophysiologic-based measurements are associated with risk for postoperative atrial fibrillation, but routine use of these techniques have limitations, including the need for specialized equipment and the added time needed for testing (710). Echocardiography has been successfully used to identify cardiac abnormalities associated with susceptibility to atrial fibrillation in longitudinal studies of the general population and for patients with left ventricular dysfunction (11,12). Atrial dimensions and left ventricular mass, in particular, are related to risk for atrial fibrillation, whereas other data suggest that altered left ventricular diastolic filling dynamics measured with trans-mitral valve pulsed-wave Doppler echocardiography identify susceptibility for atrial fibrillation in some patients (11,12). There have been conflicting findings regarding whether risk for postoperative atrial fibrillation in CABG surgical patients is associated with preexisting cardiac structural abnormalities especially left atrial enlargement (2,13,14). Further, these data were obtained using transthoracic M-mode echocardiography and diastolic filling dynamics measured with Doppler echocardiography were not reported. Transesophageal echocardiography (TEE) is increasingly used during cardiac surgery, and this method might provide a practical means of routine screening for cardiac abnormalities that could be related to atrial fibrillation risk after surgery. The purpose of this study was to evaluate whether preexisting abnormalities of cardiac structure or function detected with TEE are prevalent in patients later developing atrial fibrillation after CABG surgery.
| Methods |
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After tracheal intubation a multiplane TEE probe (Hewlett Packard, Andover, MA) was inserted and connected to an intraoperative echocardiography system. Echocardiographic examination was systematically performed before incision. Color and pulsed-wave Doppler echocardiography evaluations of the cardiac valves were performed to exclude valvular regurgitation or stenosis. Pulsed-wave Doppler echocardiographic measurements of trans-mitral valve blood flow velocity were performed by sampling at the tips of the mitral valve leaflets from the four-chamber view. Pulsed-wave Doppler examinations of pulmonary blood flow were performed after the left superior vein was identified with color flow Doppler and two-dimensional TEE imaging and Doppler data were acquired 1 cm proximal to the confluence of left superior pulmonary vein with the left atrium. All TEE images were recorded on videotape for later analysis. Cardiopulmonary bypass was conducted with nonocclusive roller pumps using a membrane oxygenator (Cobe, Denver, CO) and a single two-stage cannula inserted directly into the atrium. Catheters for venting the left ventricle were inserted via the right superior pulmonary vein. High potassium, cold (3°C5°C) blood cardioplegia was administered after aortic cross-clamping under mild systemic hypothermia (venous temperature >32°C). Patients received standard institutional care postoperatively, including the resumption of ß-adrenergic receptor blocking drugs for patients receiving this therapy before surgery, but there were no other drugs given for arrhythmia prophylaxis. Potassium and magnesium were administered postoperatively to keep serum concentrations within the normal range. After surgery, the patients were continuously monitored until the time of hospital discharge for arrhythmias with telemetry electrocardiography. The definition of atrial fibrillation was irregular heart rhythm with a fluctuating baseline without well-defined P wave. The diagnosis was made by attending medical staff based on telemetry findings confirmed with 12-lead electrocardiograms. Only sustained arrhythmia episodes were considered.
Off-line analysis of the recorded echocardiographic images and pulsed-wave Doppler results were performed by an investigator blinded to the patient outcomes. Measurements were performed using the calculation software package of the echocardiography machine (Hewlett Packard, Andover, MA). Left atrial dimensions were measured from the four-chamber view immediately before diastolic valve opening. The horizontal diameter of the atrium was defined as the maximal distance from the inner portion of the mid-septum to the inner portion of the midlateral atrial wall. Left ventricular wall thickness was measured from the posterior ventricular wall using images from the midpapillary short axis view at end-diastole. Left ventricular area was measured from this same view at end-systole and end-diastole. Transmitral valve blood flow velocity curves were analyzed for the following variables: early filling (E) wave (the highest initial velocity in early diastole) and late filling (A) wave caused by atrial contraction. The peak E and A velocities, E/A velocities ratio, and the E and A velocity-time integrals were calculated. The deceleration time of the E-wave was the time from the peak E-wave until the velocity crossed zero. This was extrapolated when the A-wave velocity was superimposed on the E-wave. Pulmonary venous blood flow velocity curves were analyzed for the following variables: peak reverse velocity at atrial contraction (PV-A wave), highest peak forward velocity during ventricular systole (PV-S wave), which was further defined as two distinct peaks, an early (SE) and a late (SL) one, and diastole (PV-D wave) (15). The systolic (including both SE and SL) and diastolic integrals were measured, and the S/D ratio was calculated using the S and D velocities. All TEE and Doppler echocardiography measurements were made from three consecutive cycles at end-exhalation during a period of relatively stable R-R intervals corresponding to the baseline heart rate.
Statistical Analysis
Continuous data were analyzed with analysis of variance using Bonferroni post hoc test when appropriate and with Wilcoxons rank sum test when the data were nonparametrically distributed. Categorical data were analyzed with Fishers exact test. The relationship between left atrial size and risk for postoperative atrial fibrillation was evaluated both as a continuous variable and as a dichotomous variable. For the latter, patients were categorized based on atrial size in 0.1-cm increments. Stepwise logistic regression analysis was used to assess for the potential of any echocardiographic variable to be of value in predicting postoperative atrial fibrillation. In this analysis, age and duration of aortic cross-clamping were included because of the strength of these variables to identify risk for this arrhythmia in other series. All values are expressed as mean ± SD. A significant difference was considered to exist for P
0.05.
| Results |
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| Discussion |
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Identifying patients predisposed to atrial fibrillation after CABG surgery would allow for more appropriate targeting of patient electrocardiogram monitoring and preventative drug therapy. It is hypothesized that patients susceptible to postoperative atrial arrhythmias have a preexisting electrophysiologic substrate that becomes vulnerable as a result of surgical perturbations (16). Experimentally, atrial distention promotes the electrophysiologic conditions for atrial fibrillation by reducing action potential duration, decreasing the effective refractory period and shortening the wavelength of depolarization (1719). Further, myocardial stretching can induce ectopic beats and after-depolarizations that may promote the initiation of arrhythmias (20,21). The mechanisms for these processes, termed mechanoelectrical feedback, appears to be activation of selective and nonselective sarcolemmal ion channels as a result of myocardial stretch (stretch-activated ion channels) as well as volume-sensitive ion channels (2225).
Experimentally, thus, there is an electrophysiologic association between increased atrial dimensions and the electrophysiologic conditions conducive to the development and maintenance of atrial fibrillation. Further, in the general population, there is a well appreciated relationship between atrial enlargement and atrial fibrillation, although a causal role of atrial size in arrhythmogenesis (as opposed to secondary enlargement from the arrhythmia) has not always been clear (2628). Other prospective longitudinal studies of elderly, nonsurgical patients initially in sinus rhythm, have shown that left atrial enlargement, left ventricular wall thickness and left ventricular fractional shortening are predictive of the development of atrial fibrillation after a mean follow-up of 7.2 years (11).
Although there appears to be a plausible basis to suspect that patients prone to develop atrial fibrillation after CABG surgery might have preexisting cardiac structural or functional abnormalities, our results suggest that such cardiac abnormalities are uncommon for many patients developing this arrhythmia. These findings are similar to those previously reported where transthoracic M-mode echocardiography was used to measure left atrial size (2,14). Others, however, have reported that left atrial size detected before surgery with two-dimensional transthoracic echocardiography was associated with postoperative atrial fibrillation (13). In that report, patients with a history of atrial fibrillation were included in the analysis. Atrial fibrillation induces cardiac structural changes including hypocontractility or atrial "stunning" that could confound the results (2931). Our study included only patients without valvular heart disease and excluded those with a history of atrial fibrillation.
Doppler echocardiography allows for the assessment of the dynamics of cardiac blood flow that provide insight into atrial and ventricular diastolic function and compliance (32). There are few data, however, on whether Doppler echocardiography derived assessments are related to susceptibility for postoperative atrial fibrillation. In nonsurgical populations such abnormalities have been reported (12). Further, in a study of patients undergoing pulmonary resection, transvalvular Doppler echocardiography indices after surgery were independently associated with risk for postoperative supraventricular arrhythmias (33). Our results showed that although some patients may have had diastolic dysfunction as indicated by the ratio of systolic-to-diastolic pulmonary venous wave ratio >1.0 and transmitral early wave-to-atrial wave ratio <1.0 (Tables 4 and 5), there were no differences in the frequency of such abnormalities or in the mean blood flow velocity patterns between patients with and without postoperative arrhythmias. We did find that, compared with patients developing atrial fibrillation, the patients remaining in sinus rhythm had significantly longer transmitral deceleration time of the early diastolic wave. These findings would be compatible with abnormalities in diastolic relaxation of the left ventricle. Nonetheless, when adjusting for patient age and duration of aortic cross-clamping, there were no differences in any other Doppler derived indices suggesting that abnormal left atrial and ventricular filling dynamics are not prevalent in patients susceptible to postoperative atrial fibrillation.
Our TEE measurements were obtained after induction of anesthesia and institution of positive pressure ventilation, both of which could alter cardiac preload and, thus, the TEE cardiac structural and functional measurements. Although these results might not necessarily be readily extrapolated to the awake state, the conditions of this study are similar to the usual clinical conditions where TEE might potentially be implemented for detecting any abnormalities associated with postoperative atrial fibrillation. Further, in this study we did not report right atrial size or hepatic venous and tricuspid valve Doppler measurements. The latter are difficult to consistently obtain in patients with TEE because of the position of these structures in relation to the posterior and rotated position of the TEE probe. Despite the increased prevalence of postoperative atrial fibrillation in this series, the sample size of this study is a limitation in light of the small differences for many of our TEE findings between patients with and without atrial fibrillation. Consequently, we cannot exclude a Type II error for failing to distinguish these two groups with our TEE assessments. Based on our results, a study of nearly 3500 patients would be required to exclude the latter with confidence. Thus, our results do not eliminate that patients with preexisting left atrial enlargement or other cardiac abnormalities may be prone to postoperative atrial fibrillation. Our findings do suggest, though, that atrial fibrillation commonly occurs in the absence of atrial enlargement or Doppler derived cardiac functional abnormalities. The data imply that the use of TEE immediately before surgery would be an insensitive means for routinely identifying many patients susceptible to this arrhythmia.
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