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J Am Coll Cardiol, 2006; 48:787-789, doi:10.1016/j.jacc.2006.05.036
(Published online 24 July 2006). © 2006 by the American College of Cardiology Foundation |
Cardiovascular Division, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts
* Reprint requests and correspondence: Dr. Warren J. Manning, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, Massachusetts 02215. (Email: wmanning{at}bidmc.harvard.edu).
Interest in LA size and structure as a marker of cardiac disease has increased steadily since Feigenbaum et al. (6) first reported that the unidimensional M-mode LA diameter correlates with angiographic LA size. Since then, ultrasound technology has advanced, adding 2-dimensional (2D) and, more recently, real-time 3-dimensional (RT3D) capabilities to its diagnostic armamentarium. Reflecting these advances, a recent position statement from the American Society of Echocardiography supported a transition to more volumetric approaches to cardiac chamber quantification (7).
In its normal, nondilated state, the LA is a spherical structure, and thus its volume may be well approximated by 4/3
r3, where r is a single measure of radius. However, even with minor LA distortion, the LA may enlarge asymmetrically because of the constraints of the surrounding structures, such as aorta and chest wall, as well as the eccentrically directed mitral regurgitation jets, making a unidimensional assessment inadequate (8). The assumption regarding the spherical nature of an enlarged LA may be partially tempered by using three separate radius measurements, or by the application of a biplanar area method (9). To further minimize geometric assumptions regarding the shape of the LA, true volumetric techniques can be used. These include cardiovascular magnetic resonance, cardiac computed tomography, and RT3D echocardiography. Cardiovascular magnetic resonance can accurately quantify LA dimensions, volume, and function, and can be performed with or without contrast agents (10,11). Cardiac computed tomography is occasionally used for anatomic imaging of LA around the time of pulmonary vein isolation (12), but exposes the patient to ionizing radiation and nephrotoxic contrast. Three-dimensional reconstruction of 2D images is a reliable estimate of LA volume, and RT3D echocardiography has proven promising in measuring LA volume directly (13). Although RT3D is now becoming more widely available, 2D echocardiographic methods are still easier to implement, and thus remain the mainstay of diagnosis in most laboratories. Therefore, current practice of LA volume estimation by echocardiography relies on its approximation from biplanar 2D measurements (14,15) with LA enlargement identified above a gender-neutral, body mass index-corrected volume of 32 ml/m2 (16).
Atrial arrhythmias, including AF, are encountered frequently after cardiac surgery. These arrhythmic complications are associated with an increased incidence of systemic thromboembolism and stroke, hemodynamic instability, and prolonged hospital stay and increased resource use (17). Clinical risk factors for postoperative atrial fibrillation include advanced age, obesity, mitral valve disease, and prior history of AF or congestive heart failure (17,18).
The feasibility and effectiveness of prophylaxis for AF among patients undergoing cardiac surgery has been previously shown. A Cochrane review of data from 58 trials, together enrolling over 8,500 patients, identified that beta-adrenergic blockers, sotalol, and amiodarone, as well as atrial overdrive pacing, all reduce the incidence of postoperative AF (19). In a recent PAPABEAR (Prophylactic Oral Amiodarone for the Prevention of Arrhythmias That Begin Early After Revascularization, Valve Replacement, or Repair) trial, perioperative amiodarone therapy reduced the postoperative risk of atrial tachyarrhythmias by approximately 50% overall, with a similar benefit in patients older and younger than 65 years (20). However, almost a week of pretreatment was prespecified in that study, which limits its applicability in the clinical setting. In addition, the strategy of routine preoperative administration of amiodarone exposes a large population of patients at variable risk for AF to potentially serious adverse effects of this therapy. Therefore, objective preoperative stratification or patients with respect to their risk of developing postoperative AF is desirable.
In this issue of the Journal, Osranek et al. (21) present the results of a prospective study of LA volume and incidence of postoperative AF. The LA volume was determined by 2D echocardiography using the biplanar area-length method (16,22), although using a slightly different formula from their recent publication on LA volume and cardiovascular outcomes, in which they used the mean of the 4-chamber and 2-chamber lengths rather than the shortest length (1). In a cohort of 205 patients undergoing cardiac surgery at the Mayo Clinic, the investigators show that preoperative LA volume index was an independent predictor of postoperative AF, with an LA volume index >32 ml/m2 conferring an almost 5-fold increased risk of postoperative AF (compared with the LA volume index 
32 ml/m2). Surprisingly, amiodarone or beta-blockade were not protective against development of postoperative AF (21). Data are also not provided for us to determine whether LA volume measures were superior to LA dimension or area measures in predicting AF incidence after cardiac surgery.
Echocardiographic LA volume estimated by the biplanar method has been used previously as a risk predictor of AF in older outpatients (23), as a marker of long-term adverse events in those with lone AF (24), and as a predictor of cardiovascular events (1). The current study by Osranek et al. (21) builds on these previous investigations and provides simple means for preoperative risk stratification of a large and common population of cardiac surgical patients. Although the investigators have chosen to correct LA volume for body size, they have not used gender-specific thresholds. An argument against any normalization is animal data supporting the theory of re-entry and AF that show that a minimum amount of atrial tissue is required (equal to the distance traveled by the cardiac impulse in one refractory period) and is critical to determination of the likelihood of re-entry (25). These data suggest that raw/non-normalized indexes may be more appropriate predictors.
Notably, in the study by Osranek et al. (21), in none of the studied patients younger than 65 years of age and with an LA volume <32 ml/m2 did postoperative AF develop, identifying a "low-risk" group in whom the risks of preoperative prophylaxis may in principle outweigh its benefits (21). The clinical implications of actually withholding prophylactic beta-blockers in younger patients with normal-sized atria before cardiac surgery will have to be assessed in formal clinical studies.
Old habits are often difficult to break. Parasternal LA dimension measurements have been a cornerstone of clinical echocardiography for almost 40 years. Advances in technology facilitated area measurement more than 2 decades ago. Osranek et al. (21) have now given us another good reason to think about the heart volumetrically, as echocardiography advances to the RT3D world.
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