Fig. 3.1
(a) Shows the epicardial aspect of a heart specimen while (b) shows the endocast from a similar heart with common left pulmonary vein (LPV). Both are viewed from left lateral perspective to show the appendage pointing antero-cepahalad. The red broken line marks the ostium and the black broken line marks the distal margin of the neck. (c, d) are two different views of the same endocast of a left atrial appendage to demonstrate the changes in morphology. Double arrows mark the bends and the asterisks indicate the lobes. Ao = aorta; LPA = left pulmonary artery, Os = ostium, PT = pulmonary trunk; t = tail of appendage
Generally, the appendage is a flattened projection from between the anterior and lateral aspects of the left atrial chamber. Importantly, there are considerable variations in size, shape, and relationship with adjacent cardiac and extra-cardiac structures that can affect efficacy and safety of interventional procedures. The appendage tip is directed antero-superiorly with its floor (lower surface) usually overlapping the left border of the right ventricular outflow tract or the pulmonary trunk and the main stem of the left coronary or the circumflex artery (Figs. 3.1 and 3.2). In some hearts its tip is directed backwards with its body folding back upon itself while in a few hearts the tip portion passes rightward behind the arterial pedicle to sit in the transverse pericardial sinus (Fig. 3.2). The epicardial surface on heart specimens shows crenellations, or lobes. Owing to its tubular shape, its junction with the left atrium is narrow, often defined by a waist, corresponding to the ostium (or orifice) on the endocardial surface (Fig. 3.1). The position of the ostium varies, as does the length of the appendage (Fig. 3.2).
Fig. 3.2
Approximately left lateral perspectives of the left atrial appendage to show the variability in morphology, lobes (*), location of the ostium (red broken line), and length of the appendage’s neck (black broken line denotes distal margin). (a, b) have anteriorly directed ostium related to the orifice of the left superior pulmonary vein (LSPV) whereas (c) has a laterally directed ostium. The lower three panels show variations in the tip(t) portion of the appendage. (d) A bifid appendage. (e) The appendage body is folded over. (d) the tip is in the transverse sinus behind the aorta. Abbreviations as in Fig. 3.1
3.1.2 Important Surrounding Structures
The fibrous pericardium surrounds the heart. Relevant to manoeuvres in the pericardial space to reach the appendage is the course of the left phrenic nerve which descends on the fibrous pericardium in the vicinity of the roof of the appendage [1, 2]. A study on cadavers revealed that the course of the left phrenic nerve and its accompanying pericardiophrenic vessels in the fibrous pericardium was overlying the tip of the appendage in 59% and over the neck of the appendage in 23% [2].
The major branches of the left coronary artery also run on the epicardial surface and overlap the appendage to a varying extent. Furthermore, on the epicardial aspect the left coronary lymphatic channel formed from the union of the anterior interventricular trunk and the obtuse marginal trunk passes beneath the left atrial appendage close to the ostium. The anterior interventricular cardiac vein on its ascent continuing as the great cardiac vein that runs into the atrioventricular groove also passes underneath the appendage but its course tends to veer away from the ostium.
3.1.3 The Left Atrial Appendage
The juncture of the appendage with the atrial body is the ostium (or orifice) so the appendage is deemed to be the structure extending from the ostium to the tip. Veinot and colleagues [3] described a lobulated body portion and a tail portion. The lobes are defined as protrusions from the appendage body whereas the tail portion may bend but not produce new lobes at the point of flexion (Fig. 3.2). Eighty percent of the 500 heart they examined had two lobes or more. Although the appendage has a general flattened shape, the lobes and tail are not all aligned on the same plane (Fig. 3.2).
3.1.3.1 Ostium
When viewed from inside the left atrium, the ostium leading to the appendage is not perfectly round (Fig. 3.3). Instead, it is commonly oval in shape or tear-drop shaped with mean long diameter of 17.4 ± 4 mm and short diameter of 10.9 ± 4.2 mm as measured on postmortem hearts [1]. On the endocardial surface, the superior and posterior borders of the ostium are well demarcated by the left lateral ridge separating it from the orifices of the left pulmonary veins. Lacking a ridge, the anterior and inferior borders are less well-defined. Nevertheless, there is a perceptible narrowing of the left atrium in this region. Notably, the smooth atrial wall is often punctuated by pits and troughs in this pre-ostium region. The floor of these indentations is extremely thin, comprising of little more than a few myocytes sandwiched between epicardium and endocardium [1].
Fig. 3.3
(a, b) simulate TEE long axis and short axis cuts respectively to show the relationship between the left lateral ridge(*) and the left pulmonary vein orifice. The proximal (red broken line) and distal margin of the neck (black broken line) are indicated. The arrow in (b) points to the circumflex artery. The double arrows mark the plane of the atrial septum. Note the proximity of the aortic root to the anterior margin of the septum. (c) Shows endocardial aspect of the ostium. Proximal to the ostium the free left atrial wall without any trabeculae or pectinated muscles is visible. The ridge between the superior and posterior borders marks the ostium (red broken line). The muscle bundles distal to the ostium located within the appendage are clearly visible. (d) A histologic section through the appendage wall shows the paper-thin area in between the muscle bundles prone to perforation
Commonly known as Coumadin ridge, the left lateral ridge is actually an infolding of the left atrial wall which contains within it the vein or ligament of Marshall and also a left atrial artery that occasionally supplies the sinus node of the heart (Fig. 3.3). This so-called ridge varies in extent, size and in cross-sectional profile. In cases with narrow ridges, there is increased risk of dual-disk device implants obstructing drainage of the adjacent pulmonary vein. From heart to heart, the ostium of the appendage varies in its relationship with the pulmonary venous orifices. Most common is the ostium situated in an antero-superior position relative to the left superior pulmonary vein. Least common is the location of the ostium antero-inferior to the left inferior pulmonary vein orifice whereby the distance between the anterior and inferior margins of the ostium to the mitral annulus is reduced [4].
3.1.3.2 Neck
There is usually a tubular portion of the appendage leading from the ostium to the wider lobulated body. This neck region varies in length and in morphology (Fig. 3.2). Those with short necks, with or without an early lobe close to the ostium, or has sharp angulation of the appendage body, may not be suitable for implanting devices that plug into the appendage. In hearts with a funnel shaped neck, its distal margin may be interpreted as the appendage ostium instead [4]. On imaging, however, the ostium is sometimes taken at the level of Coumadin ridge whereas the neck (and the landing zone) is deeper into the appendage at the level of the circumflex artery (Fig. 3.3).
3.1.3.3 Appendage Body
The body is lobulated (Fig. 3.2). Its cavity is lined by a complex arrangement of muscle bundles of varied thicknesses interspersed with paper thin atrial wall in between (Fig. 3.3). At the interface between upper and lower walls the bundles may be strut-like with variations of thin bundles and thicker branching bundles. On imaging, some thicker bundles appearing like thrombi or intraatrial masses need careful evaluation.
3.1.4 The Atrial Septum
Crossing the atrial septum to reach the ostium of the appendage requires understanding of the septal components. The thin flap valve of the oval foramen that is fused to the muscular rim of the foramen represents the site of the true atrial septum. This area is better seen on the right atrial side as a depression surrounded by a slightly raised rim in many cases although the distinction between the two structures are not clear in approximately 18% of individuals [5]. The rim is a muscular filled with fibro-fatty tissues of the epicardium and termed the interatrial groove (or septal raphe). Crossing though the thicker tissues of the fold may result in a less maneuverable catheter than crossing through the thin flap valve. Care should be taken not to exit the heart into the transverse pericardial sinus or into the aortic root when crossing anteriorly outside the rim. The variable locations of the ostium (Fig. 3.2) may require careful assessment of the optimal position for puncture in order to reach the target site.
3.2 Pre-Procedural Imaging of the LAA
As the broad variety in LAA anatomy impacts device selection, implantation strategies and procedural efficacy an accurate assessment of anatomic LAA characteristics is mandatory before a LAA occlusion procedure. The orifice of the LAA is typically located laterally and superiorly with the LAA body typically extended between the anterior and lateral walls of the left atrium (LA) and the tip directed antero-superiorly. There are substantial variations in LAA anatomy to consider that impact device selection and procedural success. The LAA ostium is generally oval in shape with diameters ranging from 10–40 mm [1, 3]. A very eccentric LAA ostium implanted with a round-shaped device may have an increased risk of peri-device leakage. In comparison with normal sinus rhythm, atrial fibrillation (AF) is associated with larger LAA volumes by up to three times normal size [6], therefore patients with chronic AF will most likely need larger device sizes.
Determining the depth and orientation of the main anchoring lobe is also an important aspect for LAA occlusion procedures. The number and origin of additional LAA lobes also influences implantation strategy. More than 50% of patients have two or more lobes [3]. If a lobe originates very close to the LAA ostium it may remain unsealed after device placement. When there are two large lobes with a bifurcation close to the ostium or a chicken wing morphology is present specific implantation strategies are often needed.
Imaging modalities used to evaluate the LAA routinely include transthoracic (TTE) and transesophageal echocardiography (TEE) and occasionally multidetector computed tomography (MDCT) or magnetic resonance imaging (MRI).
3.2.1 Transthoracic Echocardiography
Two-dimensional (2D) and three-dimensional (3D) TTE are not adequate to characterize the LAA anatomy, but they are useful prior to a planned procedure to assess LA dimensions, volumes and function. In addition, left ventricular (LV) function has to be evaluated, as it is an important factor for stratifying the risk for thromboembolism as determined by the CHA2DS2-VASC score [7]. Contraindications for LAA closure that can also be identified by TTE include rheumatic mitral valve (MV) disease resulting in valvular AF, the presence of thrombi in the cavities of the LA or LV or pericardial effusion that is more than small in size. Patients with severe mitral stenosis or other valvular diseases requiring surgical repair or patients with mechanical heart valves who require chronic anticoagulation therapy should not be considered for LAA closure procedures.
3.2.2 Transesophageal Echocardiography
As the LAA morphology cannot be definitely assessed by TTE a TEE is essential and is considered as the main imaging modality in the pre-procedural evaluation of the LAA. In the majority of patients the close anatomical relationship of the LA and the esophagus results in excellent imaging quality and allows for a detailed LAA assessment [8].
First, a thrombus in the LA or LAA has to be excluded by TEE [9, 10]. In most cases 2D TEE, which has a higher spatial resolution compared to 3D, is sufficient to exclude a thrombus, but in some cases 3D TEE may be helpful in differentiating a thrombus from a large pectinate muscle [11]. Figure 3.4 shows an example of a 2D TEE view and an enface 3D view of the LAA that verifies the presence of a prominent tissue bridge formed by pectinate muscles, thus allowing the exclusion of a thrombus.
Fig. 3.4
Differentiation of a thrombus from pectinate muscles in the LAA by 3D TEE. In (a) a 2D TEE 90° view shows an echodense structure in the distal part of the LAA (marked with a yellow star). The corresponding 3D TEE enface view (b) clarifies, that this structure is a pectinate muscle bridge. A thrombus could therefore be excluded
Reverberation artifacts have also been described in the LAA. It could be demonstrated that 23% of patients had structures in their LAA which were interpreted as artifacts [12]. All artifacts were located twice the distance from the transducer as from the Coumadin ridge. The position and echogenicity markedly differs from LAA thrombi. Unlike the artifacts, all thrombi found in the LAA were attached to the LAA wall and had a uniform consistency and different texture to that of the LAA wall. None of the artifacts were attached to the LAA wall and the consistency was not uniform (Fig. 3.5). A prominent Coumadin ridge (ligament of Marshall) can also be misdiagnosed as thrombus. The lack of mobility and the typical location, most commonly best seen in a mid-esophageal two-chamber view, usually helps to distinguish it from an abnormal structure as seen in Fig. 3.5.
Fig. 3.5
Left atrial appendage artifact (reverberation) seen inside the LAA. No clear attachment to the LAA wall of the reverberation artifact can be observed. The measurements (red and yellow brackets) show, that the artifact is twice the distance from the transducer as the Coumadin ridge. Note, the prominent Coumadin ridge itself may be misdiagnosed as a thrombus in this patient. The lack of mobility and the typical location helps to distinguish it from a thrombus
As demonstrated in Fig. 3.6, the use of color flow Doppler with a reduced pulse repetition frequency (PRF) or an ultrasound contrast agent may also be helpful in determining the presence of a thrombus. Tissue Doppler has also been used to distinguish true thrombus from pectinated muscle or tissue: thrombus appears with altered movement from the surrounding tissue when analyzed image by image.
Fig. 3.6
(a, b) Imaging with color flow Doppler and ultrasound contrast agent for thrombus detection. In (a) a thrombus (yellow arrrow) within the LAA cavity is seen (left) that is spared out in color Doppler imaging with a reduced pulse repetition frequency (PRF = 15.4 cm/s) (right). (b) gives an example of a thrombus (yellow arrow) in the very distal part of a chicken wing morphology that is spared out from ultrasound contrast in X-plane imaging
If a thrombus in the LA or LAA is verified, the procedure should be postponed in order to avoid iatrogenic thromboembolism caused by clot dislodgement by a catheter or a device during device placement. In the presence of a LA or LAA clot anticoagulation therapy should be given until the thrombus has resolved as determined by repeat TEE imaging (a TEE is usually repeated after 4-6 weeks). If a thrombus persists, case reports have been presented that describe safe LAA occlusion by using carotid protection devices like the Claret device (Sentinel).
Other potential sources of systemic embolism such as cardiac masses and aortic arch atheroma also need to be excluded. 3D TEE is a very useful adjunct to conventional 2D imaging as it renders more accurate information on surface features, diameters, volumes, mobility, spatial relationship to neighboring structures and exact location and origination of such intracardiac masses [13–16].
A systemic infection as well as an active endocarditis constitutes a contraindication for LAA closure and must be ruled out by laboratory parameters and TEE imaging [17].
As transseptal puncture (TSP) is most commonly used to place the catheter and LAA occlusion device into the LAA, careful evaluation of the interatrial septum in regard to mobility, thickness, the presence of an interatrial septum aneurysm, atrial septal defect(s) (ASD) or a patent foramen ovale (PFO) is necessary. The left superior pulmonary vein (LSPV) should be assessed for obstruction and the mitral valve for any kind of pathology. Figure 3.7 shows that these two structures are adjacent to and located in close proximity to the LAA ostium and have a low but potential risk of interference with a LAA occlusion device.
Fig. 3.7
(a, b) Neighboring structures. The relationship of the LAA to neighboring structures is shown in a 90° 2D TEE (a) and a 3D wide angle enface view from the left atrium (b). LCx = left circumflex coronary artery; MV = mitral valve; LAA = left atrial appendage; LSPV = left superior pulmonary vein
3.2.2.1 Anatomical Imaging of the LAA
In order to understand the complex 3D morphology of the LAA a careful multiplane 2D TEE analysis should be performed. Scanning through the LAA by rotating the TEE probe from 0° to 180° [18] with the probe position in the high or mid-esophageal region is recommended. It is important to define the number as well as the exact origination of additional lobes and identify and characterize all lobes accurately.
The depth of the LAA is evaluated by angiography (preferably by using an atraumatic pigtail catheter) and by TEE imaging. The view that shows the maximal depth of the LAA is typically optimal for the evaluation of the long axis of the LAA. It is usually found in a short axis TEE plane between 45° and 70°. The left circumflex coronary artery (LCx) can be identified in this view at the medial side of the LAA and the limbus (also known as Coumadin ridge) to the left superior pulmonary vein which defines the lateral border of the LAA (Fig. 3.8). These structures play a role as important landmarks for the sizing process as described later. In cases where the LAA originates more anteriorly, the longest depth is usually seen in an image plane <45° and the aorta will be visible in the view. In cases where the LAA originates more laterally the long axis and consecutively the longest depth of the LAA can be appreciated best in planes >60° (up to 90°). In these imaging planes the aorta is not in the imaging plane (and thus is not seen), but the MV comes into the view. The origination and the course of the LAA in antero-lateral orientation should be evaluated as it has impact on the choice of the TSP site and the selection of an appropriate curve of the delivery sheath as illustrated in Fig. 3.9. With 3D TEE imaging this information can be obtained in a single wide-angle enface view that includes the MV (positioned in the middle of the image), the interatrial septum on the medial side and the LAA on the lateral side of the MV as demonstrated in Fig. 3.8.
Fig. 3.8
(a–d)Anatomical imaging of the LAA. The recommended views for LAA evaluation (~0°,~45,° ~ 90°, ~135°) are easiest obtained by using X-plane imaging as demonstrated in (a, b). The 45° view and the 135° view allow for an evaluation of the LAA borders in an anatomical orientation (b). In (c) a 3D enface view demonstrates the axis of the 45° plane (green line—defines the lateral and medial border of the LAA) and the axis of the 135° plane (red line—defines the anterior and posterior border of the LAA) in the same patient. The longest depth can be observed in the 45° view (b) thus indicating a more lateral than anterior position of the LAA ostium which is confirmed in a 3D enface view from the LA (d). (e) Example of a left atrial appendage that originates anteriorly. The maximum length of this LAA is measured in a 10° view with the aorta in the view thus indicating that this LAA ostium is originating more anteriorly. This is confirmed in a 3D TEE enface view (bottom right). Ao = aorta; MV = mitral valve; LAA = left atrial appendage; LSPV = left superior pulmonary vein
Fig. 3.9
Schematic to illustrate the impact of the location of the LAA orifice on the transseptal puncture site (TSPS). In (a) the LAA is located more laterally. The chosen TSPS in the mid-fossa and the usage of a specific sheath curve (illustrated by a green arrow) result in a coaxial entry into the LAA. In (b) the LAA is located more anteriorly and the usage of the same TSPS and sheath curve does not allow for a coaxial entry into the LAA (illustrated by a red arrow). Coaxial entry into the LAA in this case can be achieved by choosing a more posteriorly located TSPS as illustrated with the green arrow. LAA = left atrial appendage; TSPS = transseptal picture site
The short axis of the LAA can be readily assessed using X-plane imaging (an imaging modality that comes with 3D imaging). When a 45° view optimally shows the long axis of the LAA a corresponding orthogonal 90° view is automatically rendered by the system and a 135° (45° + 90° = 135°) (Fig. 3.8) imaging plane of the short axis of the LAA is shown. The cropping of a 3D data set also can render these views. When X-plane imaging is not available the 135° view can also be achieved by conventional 2D imaging. To get optimal image quality it is thereby sometimes necessary to slightly pull back the TEE probe to position it slightly higher in the esophagus.
The 135° imaging plane is particularly important. First, larger ostium diameters can more frequently be found on imaging planes showing the short axis of the LAA between 120° to 135° rather than at 45° or 90° [19] due to the elliptical shape of the LAA ostium in most of the cases. As the largest measurement of the ostium diameter usually dictates the size of the selected device it is crucial to obtain measurements in this view. Secondly, in this view the anterior and posterior borders of the LAA can be appreciated (Fig. 3.8). The posterior part of the LAA is of major importance as this is the region where the protrusion of the devices from the LAA into the LA is more pronounced and where peri-device leakages after device deployment most commonly occur. Therefore it is mandatory to assess this region with particular caution after device placement.
Thirdly, we have found, that a chicken wing morphology is generally best evaluated using planes between 120° and 135°. Recognizing this specific morphology adequately may have implications on implantation strategies [20, 21]. The depth of the LAA is typically shortest in this imaging plane.
LAA morphologies which make device implantation more challenging should also be carefully identified before a planned procedure because this may have impact on the device selection and/or on the implantation strategies. Table 3.1 provides an overview of potentially challenging LAA morphologies.
Table 3.1
Overview of some potentially challenging LAA morphologies
Morphology | Challenges | Potential strategy |
|---|---|---|
Seondary lobe originating close to the ostium | This lobe may not be covered after device placement | An Amulet device may be preferable as the proximal disc may cover the ostial area. A Watchman device should be anchored in the anterior/superior lobe with proximal expansion covering the inferior lobe |
Two large lobes with a bifurcation close to the ostium | Device may be captured in one of the lobes and the other lobe stays unsealed | An Amulet device may be used when the remaining depth is ≥10 mm. Alternatively a Watchman device can be implanted off axis in an oblique position |
Very short LAA | Not enough space to accomodate a device with standard implantation techniique | A Watchman device may be implanted with less depth by pushing distally after release of 50% of the device |
Very large LAA landing zone | A landing zone diameter >31 mm is larger than the required diameters for a Watchman occluder or an Amulet device | Two small Watchman devices (“kissing Watchman”) may be considered. A LARIAT suture system may also be an option (diameters <40 mm in CT evaluation are required) |
Very small LAA landing zone | A landing zone diameter <11 mm is smaller than the required diameters for a Watchman occluder or an Amulet device | Oversizing of an Amulet occluder or a Watchman device may be accepted. A LARIAT suture system may be considered |
Cone shaped LAA (high ostium/landing zone ratio) | The ostium diameters are considerably larger than the landing zone dimensions and the disc of an Amplatzer occluder may be too small for adequate sealing | A Watchman device may be preferable; if an Amulet occluder is used the device may need to be oversized to cover the ostium adequately |
Chicken wing morphology | Most likely small LAA depth, short neck, early bending may occur and anchoring may be difficult | A “sandwich technique” should be considered when using an Amulet device; distal anchoring with a Watchman device is another option |
Proximal LAA membranes or septae | Access to the LAA may be difficult or impossible with an endoluminal device | A LARIAT suture system may be considered |
3.2.2.2 Device Specific Measurements
Successful percutaneous LAA closure requires careful sizing of the dimensions at the plane where the device will be seated after release. This plane is commonly referred to as the “landing zone”. Optimal matching of LAA dimensions and device size ensures a stable device position, optimal sealing of the LAA and limits reposition of the device. An undersized device carries the risk of device migration or even embolization and increase risk of peri-device leakages [20]. Conversely, extreme oversizing of the device should also be avoided because of the risk of LAA perforation which may be followed by the development of a pericardial effusion as well as cardiac tamponade [22, 23]. Precise knowledge of the dimensions of the landing zone is therefore particularly important in selecting the appropriate device size (Table 3.2).
Table 3.2
Overview of applications for different imaging modalities in the assessment of the LAA
Imaging modality | Main imaging tasks | Application | |||
|---|---|---|---|---|---|
Patient selection | Procedure planning | Procedural guidance | FU | ||
2D/3D TTE | LA diameter, volume and function LV function for risk stratification (CHA2DS2-VASC Score) LA/LV thrombi/ intracardiac masses Evaluation of contraindications Pericardial effusion | yes | yes | no | yes |
2D/3D TEE | Thrombus exclusion (LA/LAA)- (less accurate with ICE) Exclusion of other sources of embolism (e.g.cardiac masses, aortic arch atheroma) (not possible with ICE) Evaluation of contraindications Number and origin of additional lobes Localisation of the LAA orifice (more anterior or more lateral) Evaluation of neighbouring structures (LSPV/ MV) Evaluation of the interatrial septum (mobility/ thickness/aneurysm/ PFO/ASD) Measurements for device selection Evaluation of challenging LAA morphologies Monitoring and guidance of procedural steps Monitoring of complications during the procedure (Pericardial effusion, tamponade, thrombi, device migration-embolization) Assessment of the final result after device positioning (device release criteria/stability) Assessment of TSP site Follow-up evaluation (pericardial effusion, thrombus detection, peri-device leaks, stable device position) | yes | yes | Yes (ICE can be used as alternative option during the procedure) | yes |
MDCT and MRI | Alternative imaging options to TEE (used for the same imaging tasks as TEE- see above)- can render additional or supplementary information Mostly used when TEE is not suitable Currently not used for procedural guidance | yes | yes | no | yes
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