Left atrial enlargement and AF are commonly encountered together. The literature suggests a 30% or higher risk for AF in the presence of left atrial enlargement than in a normal-sized atrium even after controlling for other AF risk factors. The risk of AF increases proportionally with the extent of left atrial enlargement. This may be because left atrial enlargement and AF commonly occur together with valvular pathologies that create a pressure and/or volume overload of the LA. Increasing wall stress causes dilatation and subsequent myocardial fibrosis and associated functional impairment. The response of the LA to these pressure, volume, and pathological changes is explained by Laplace’s law, T = P × R M , wherein T is wall tension, P is the pressure differential across the wall, R is the radius of the cylinder (i.e., the LA), and M is the wall thickness. Kuppahally et al. demonstrated an inverse relationship between fibrosis and wall strain (i.e., functional impairment) using echocardiography and delayed-enhancement magnetic resonance imaging (MRI).

Dilatation of the LA contributes to AF in two ways. First, fibrosis disrupts the electrical connections between left atrial muscle bundles. In a normally functioning atrium, these connections are important for normal synchronized atrial contraction, and disruption of these connections can lead to heterogenous electrical and mechanical atrial activity. Second, fibrosis effectively slows electrical conduction through the atrium, resulting in an increase in the cycle length of a potential reentrant circuit. The altered conduction characteristics caused by fibrosis results in an increased likelihood that multiple macro-reentrant circuits can be sustained and function as the drivers of persistent fibrillatory activity in AF.

However, even in the absence of valvular or known structural heart disease, left atrial enlargement is still associated with an increase in AF. For example, paroxysmal AF can be encountered in young, tall, athletic males who have a large LA simply because of their body size and high exercise capacity. Larger atrial size can accommodate macro-reentrant drivers that are too large to develop in a normal-sized atrium even with fibrotic pathology.

Moreover, there is evidence that AF begets left atrial enlargement, and left atrial enlargement begets AF (see Chapter 8 ). Because of the interdependence of these two factors, the modern surgeon must address both AF and left atrial enlargement for effective treatment of patients with AF. It is well known that successful treatment of AF can lead to “reverse remodeling” of the LA that can be measured by reduction of left atrial size and return of left atrial function after therapy. ^, Pharmacologic conversion, electrical cardioversion, and mitral valve repair have all been shown to decrease left atrial size, improve atrial function, and decrease AF over time. ^,

It is well-documented that LA enlargement is a risk factor for failure of surgical ablation. The landmark Atrial Fibrillation Follow-up Investigation of Rhythm Management (AFFIRM) trial found that an enlarged atrium was associated with recurrent AF after ablation in 46% of patients after 1 year and 84% of patients after 5 years. Comparatively, in patients with a non-enlarged atrium, 1% of patients at 1 year and 5% of patients at 5 years had recurrent AF. Studies evaluating predictors of recurrent AF after surgical ablation consistently find that left atrial enlargement is associated with significantly higher odds of recurrent AF. Traditionally, a left atrial diameter of 65 mm is considered to be the threshold size at which ablation becomes much less likely to be successful.

As discussed in the preceding section, left atrial enlargement is largely the result of fibrosis, which disrupts the atrium’s normal electrophysiology. Some have suggested that surgical ablation of AF in an enlarged LA requires increased ablative energy, more extensive lesions, and longer aortic cross-clamp times to be successful. ^, Because surgical AF ablation is underused, the thought that more extensive lesion sets are needed in patients with an enlarged LA and the perceived difficulty of performing a complete Cox Maze-III/IV lesion set even in normal-sized left atria may serve as additional barriers to attaining a durable, successful ablation in the presence of left atrial enlargement.

There are several possible explanations for the failure of surgical AF ablation in a large LA. First, the lesion sets that are used for surgical ablation may be inadequate to eradicate the macro-reentrant drivers of AF because the normal atrial and conduction architecture is disrupted. Second, with atrial enlargement, there is a much greater surface area between the lesions of a Cox Maze-III/IV available to participate in the formation of macro-reentrant circuits that drive and sustain AF. ^, Together, these factors make successful ablation of AF in enlarged LA a challenging problem.

In the Multi-Ethnic Study of Atherosclerosis (MESA), large right atrial size was found to be associated with a greater incidence of AF. Some studies suggest that the adverse effects of right atrial enlargement are similar to those of the LA. Emerging data support the concept that right atrial enlargement and biatrial enlargement are important predictors of suboptimal outcomes for AF ablation. ^, In a cohort of 519 patients with AF, Kong et al. showed that biatrial enlargement was associated with an increased need for reablation independent of left atrial enlargement.

The American Society of Echocardiography (ASE) recommends transthoracic echocardiography (TTE) for the evaluation of left atrial size. Several echocardiography metrics are used in the preoperative diagnosis of LA enlargement including left atrial diameter, left atrial volume, and left atrial volume index (LAVI).

The most extreme indication for surgical downsizing of the LA is the presence of compressive symptoms on the enlarged LA from adjacent organs, commonly called “giant LA.” Fortunately, this problem is rare in most current clinical practices. There is ample evidence that left atrial reduction in patients with an left atrial diameter of 65 mm or larger who are undergoing mitral valve surgery and/or AF ablation alone increases the success rate of surgical AF ablation. This approach is supported by evidence showing that an LA larger than 65 mm in diameter rarely return to normal size after mitral valve surgery. ^,

Our primary surgical technique for downsizing of the LA is an atrial reduction plasty performed in combination with the Cox Maze procedure. After the patient is placed on standard cardiopulmonary bypass with bicaval cannulation, a left atriotomy is performed in the interatrial groove (“Waterston’s groove”) and extended around the pulmonary veins. The left atrial appendage (LAA) is inverted, excised, and oversewn at the stump. A resection of the inferior portion of the posterior left atrial wall in the left atrial isthmus is then performed using two incisions ( Fig. 18.1 ). One incision extends from the os of the right inferior pulmonary vein to the os of the left inferior pulmonary vein, and a second is placed parallel to the inferior pulmonary veins around the mitral annulus immediately above the atrioventricular (AV) groove fat pad. Finally, these two incisions are connected near the os of the left inferior pulmonary vein. Pulmonary vein isolation and LAA orifice isolation is then performed using radiofrequency ablation. The LA is closed, starting by sewing the pulmonary vein cuff to the remaining atrium above the mitral annulus.

Downsizing the left atrium (LA). Surgeons’ view of the internal LA through a standard left atriotomy in Waterston’s groove. After amputation of the left atrial appendage and closure of the resulting wall defect, the first lesion is created just beneath the two inferior pulmonary veins. A second lesion is then placed from the end of that incision near the left inferior pulmonary vein back toward the atrial septal side of the LA. These two lesions create a triangle of resected atrial wall in the left atrial isthmus between the inferior pulmonary veins and the left atrioventricular groove above the mitral annulus. LIPV, Left inferior pulmonary vein; LSPV, left superior pulmonary vein; RIPV, right inferior pulmonary vein; RSPV , right superior pulmonary vein.

Our standard technique also includes a right atrial reduction ( Fig. 18.2 ). This is accomplished in a similar fashion to the left atrial reduction, starting with excision of the right atrial appendage (RAA). An incision made from the excised RAA stump toward the inferior vena cava (IVC) and parallel to the right AV groove is connected to a second more posterior longitudinal incision running from below the IVC orifice to above the superior vena cava orifice, with care taken to avoid the sinoatrial node. These two incisions accomplish excision of a sizeable portion of the lateral right atrial free wall, which is then followed by radiofrequency ablation lesions isolating the tricuspid annulus and across the atrial septum.

Downsizing the right atrium. The right atrial appendage (RAA) is excised, and an incision is placed from the stump of the excised RAA parallel to the right atrioventricular groove down to the inferior vena cava (IVC). A separate incision is then placed from the IVC, back up to the RAA excision site near the superior vena cava, taking care to avoid the anatomic sinoatrial node.

Other described techniques include the following: (1) para-annular plication of the posteroinferior left atrial wall between the ostia of the inferior pulmonary veins and the posterior mitral annulus, superior plication between the right and left pulmonary veins, and then circumferentially around the left pulmonary veins toward the mitral annulus ( Fig. 18.3 ) ; (2) resection of three triangular segments of the left posterior wall ( Fig. 18.4 ) ; and (3) plication of both the posteroinferior wall and the superior wall of the LA through a combined superior and transeptal approach.

Downsizing the left atrium. Extensive plication of the left atrial wall in the areas designated. See text for further discussion.

Downsizing the left atrium. Resection of three areas of the left atrial wall. See text for further discussion.

A 2023 meta-analysis including 19 studies showed that left atrial reduction during the Cox Maze procedure was associated with higher restoration of sinus rhythm at discharge and fewer cases of perioperative stroke. In addition, a recent 2021 randomized clinical trial of 140 patients compared 70 patients who had mitral valve surgery plus a Maze procedure only (MVR + Maze) and 70 who had mitral valve surgery plus a Maze procedure plus left atrial volume reduction (MVR + Maze + LAVR). The left atrial volume reduction was accomplished by imbricating an extra portion of the LA lying just outside the roof and floor lesions of a Maze-IV procedure. Adding LA volume reduction to the Maze procedure had no impact on perioperative morbidity or mortality, but it improved the freedom from recurrent atrial arrhythmias, including AF, during a 24-month follow-up period.

The pathophysiology of AF and atrial enlargement are closely interrelated. The literature increasingly supports the concept that AF begets atrial enlargement and atrial enlargement begets AF. Given the negative prognostic effect of atrial enlargement on the success of ablation, it is important to consider addressing the atrial enlargement directly. In this setting, downsizing of the LA with resection and/or plication and downsizing the RA when necessary can be a valuable adjunct in the surgical treatment of patients with AF.