The LAA is an anterolateral structure within the left atrium that originates anterior to the left pulmonary vein ostia (Fig. 4.2). The body of the atrial appendage typically extends anteriorly toward the right ventricular outflow track, so it may also lay over the left main and left anterior descending coronary artery. In a small number of patients, the LAA is directed laterally and posteriorly or superiorly toward the transverse sinus [6]. The LAA may course behind the pulmonary trunk and thus prohibit use of pericardial snare-type devices. Internally, the os of the LAA is anterior to the pulmonary veins, separated by a left lateral ridge of tissue. This tissue represents an infolding of the left atrial wall, forming a lateral ridge approximately 5 mm in width. In a third of cases, the LAA is either superior or inferior to the pulmonary veins [7]. Inferiorly, a muscular band separates the LAA from the mitral valve (Figs. 4.1 and 4.2). The circumflex coronary artery runs along the mitral annulus, underneath the LAA, near the LAA neck (Fig. 4.1).

The general relationship of these structures to each other is typically preserved but their orientation in space is often displaced due to rotation of the heart or enlargement of the left atrium. There is also significant variation in the distances between these structures. Dilation of the left atrium increases with aging, AF, hypertension, and valvular disease, and such changes may exaggerate these variations [1]. Persistent AF is associated with dilation of the left atrium, pulmonary veins, and LAA [8]. It is also notable that in 25 % of patients, there is a single left pulmonary trunk, instead of separate left superior and inferior pulmonary veins [8]. In such patients, there may be a greater inferior separation between the pulmonary and LAA ostia.

The course of the circumflex and the location of the pulmonary veins are important to recognize given possible trauma to these structures during LAA occlusion procedures. The circumflex and great coronary vein typically run underneath the neck of the LAA and can be compressed during intracardiac LAA occlusion.

While LAA thrombus is often associated with strokes, several components of the left atrium, including the body, pulmonary veins, vestibule, and left side of the atrial septum all play an important role in AF, which may lead to stroke as detailed in prior reviews [7, 9, 10]. These not only described the nonuniform orientation and thickness of the left atrial musculature and its variable extension into the pulmonary veins as sheaths, which is more prominent superiorly, but also related this anatomy to the processes thought to occur in AF [9, 10].

The left phrenic nerve runs anteriorly over the LAA and can be damaged during ablation procedures within the LAA or open surgical procedures, but to date has not been described as a complication of transcatheter LAA occlusion. In one anatomic study, the phrenic nerve ran over the neck of the LAA in 23 % of patients [7]. The esophagus runs posteriorly to the left atrium and pulmonary veins, so is not affected by LAA procedures, but can be damaged during ablation procedures [11].

A complete understanding of the anatomy of the atrial septum and the LAA is crucial to performing both left atrial ablations and LAA occlusion. The intra-atrial septum separates the anteriorly positioned right atrium from the laterally and posteriorly located left atrium. The face of the septum therefore typically points laterally and posteriorly, toward the left pulmonary veins and slightly away from the LAA (Fig. 4.2B2). As the left atrium enlarges and the septum bows toward the right atrium, the plane of the intra-atrial septum may be displaced, exaggerating this posterior rotation, and a steeper left angulation of the image intensifier and posterior rotation of the transseptal needle are often needed to achieve perpendicularity to the septal plane (Fig. 4.3a). Conversely, right atrial enlargement requires a more lateral rotation of the transseptal needle (Fig. 4.3b) [12].

In order to engage the LAA from a transseptal approach, a posterior and inferior puncture will allow for the most direct trajectory of wires and delivery sheaths to the anterior LAA (Fig. 4.2B2). Although a puncture through the center of the fossa or foramen ovalis is the standard technique for transseptal punctures, there is significant variability in the location of the fossa despite its traditional depiction as being in the center of the atrial septum [13]. Additionally, the patent foramen ovale tends to be located superiorly, and catheter crossing from such a high position can make engagement of the LAA challenging.

While endeavoring to perform an inferoposterior septal puncture, it is nevertheless critical to be aware of the surrounding “rims” of the septum, to avoid perforations through the right atrial wall or pulmonary veins, that could lead to pericardial effusion or tamponade [14]. Given the variability in septal anatomy and location of the fossa, real-time echocardiographic imaging is strongly recommended to define the ideal transseptal location, and guide the transseptal puncture.

The ostium is typically oval-shaped and obliquely oriented with respect to both the vestibule and annulus of the mitral valve, and the ridge of muscle that separates it from the left pulmonary veins (Figs. 4.1 and 4.2). The average diameter of the short axis is 20 mm and of the long axis is 30 mm, when measuring from the top of the limbus to the muscular band at the mitral valve [15], and tend to increase in size with age, gender, body size [1] and the presence of AF [8]. A more dilated and round profile has been noted in patients with AF [16]. There can be a threefold increase in the dimension of the LAA and its os, a feature that has been shown to recede following successful ablation for AF [5, 17]. There is also significant variation among studies reporting ostial dimensions, however, due to differences in definitions for autopsy versus echocardiographic and imaging landmarks of the ostium [1]. Furthermore, systolic measurements tend to be 15–20 % larger than diastolic ones [15], which is a critical consideration when selecting occlusion device sizes. Importantly, there is anatomic variation in the location (or height) of the ostium with respect to the body of the left atrium, which not only can affect access to the LAA following transseptal puncture, but also the shape of the left lateral ridge itself and proximity of the appendage to the pulmonary trunk or mitral valve. Reconstructions and morphologic studies have shown that the majority of LAA are located in line with the left superior pulmonary vein or just below, sometimes with only narrow separation between the pulmonary vein and the ostium via a narrow left lateral ridge [14, 18–20]. The ostium can also be located above the left superior pulmonary vein making it potentially more difficult to access for device occlusion and more likely that the lobar part of the LAA will run closer to the left side of the pulmonary trunk. Alternatively, an ostium that is adjacent to the body of the left atrium inferiorly, in line with the left inferior pulmonary vein, will be in closer proximity to the mitral valve and its vestibule [14, 18]. All these variations in height of the ostium may alter selection of device type, as well as adjustments in approach to the procedure.

In general the pectinate muscles within the LAA stop at the neck, but sometimes may extend from the neck and ostium along the inferior border of the left atrium. These extensions are often referred to as “pits or troughs” or by others as the limbus of the LAA. These regions are anatomically identical to the lobar region of the LAA in that they are composed of thin atrial wall interposed between fine pectinate muscle bundles [19, 21]. For endovascular occlusion, these regions are important to recognize prior to the procedure, not only because of the risk of perforation, but also because it may alter device choice and placement.

The narrowest part of the LAA, appropriately referred to as the “neck,” is typically located within the appendage just distal to the ostium (Fig. 4.1). This region is the part of the LAA that usually overlies the course of the circumflex coronary artery. There is wide anatomic variation in the distance between the ostium and the neck of the LAA, which sometimes gives the neck a considerable length that makes it favorable for endovascular closure. Conversely, when the neck is extremely short and is followed by a sharp change in angle within the subsequent lobar region (i.e., chicken-wing configuration), it can prevent the use of longer devices, such as the WATCHMAN that require a depth equivalent to the device diameter, or the Amplatzer Cardiac Plug (ACP, or Amulet) that requires a minimum of 10 mm depth for implantation [7, 22]. The neck width is also highly variable and neck dilation has been associated with an increased risk for stroke [23]. In those cases where the neck has become dilated such that it is of similar size to the ostium, significant device oversizing may be required, given the variation in neck length and diameter, it needs to be carefully imaged and measured when planning for device occlusion.

Although traditionally depicted as a narrow, finger-like structure, the lobar region demonstrates extreme anatomic variation. This variability has been noted anatomically, by computed tomography angiography (CTA) [24–26], magnetic resonance imaging [27], intracardiac echocardiography [28], and transesophageal echocardiography [15, 29]. A study of 500 human anatomic specimens showed 54 % of LAA having two lobes, 23 % having three lobes, 20 % having only one lobe, and 3 % with four lobes. There is also great heterogeneity within these groupings as each of these lobes can take a different course and are divided internally into successively smaller pockets [1] The endocardial surface is of the LAA is characterized by a network of pectinate muscles (Fig. 4.4). More extensive trabeculations may be associated with increased risk for strokes [30].

There is currently a trend to categorize this variability as seen by CTA and reconstruction into recognizable patterns, i.e., chicken-wing, cactus, windsock, cauliflower [25, 26]. An example of the chicken-wing morphology, which in its extreme form can be a challenge for device implantation for LAA occlusion is shown in Fig. 4.5. There are contradictory reports as to whether particular morphologies are seen more frequently in patients with thromboembolic events or stroke, or not [8, 25, 31–33]. A series of patients studied by Di Biase et al have found an association between the “cauliflower” morphology and an increased risk for strokes [25], but this finding was not borne out in all studies [31]. It has also been questioned whether such classifications are reproducible between observers. Undoubtedly, numerous other shapes can exist and have been described, reflecting the variable branching pattern of the LAA [34, 35]. The wide variation in LAA anatomy (Fig. 4.6) may not be easily separated into four discrete categories, and the anatomic features relevant to LAA occlusion pertain more to ostium diameter and the implant depth at the neck, rather than the distal LAA shape. The observed morphology also depends on the plane and modality in which the LAA is imaged [2, 36] (Fig. 4.4). An LAA seen as a single lobe in a particular view may be visualized as multilobed in another view, and this would support the concept of preimplant multidimensional reconstruction [26]. In reality, the lobar region of the LAA is a geometrically complex branching structure, akin to many others found within the heart and many other structures found in nature. Given the vast morphologic variation, it may prove more valid in future studies to describe the complexity of the LAA in numerical terms. Similar analyses, for example using fractal dimensions, have proved useful in analyzing the complex branching pattern of the myocardial trabeculations within the left ventricle [37].