Anatomy of the Left Atrial Appendage

The left atrial appendage (LAA) is a long, hook-like true diverticulum of the left atrium (LA). While parallel-running pectinate muscles are contained within the tubular LAA (Fig. 1), the body of the LA is a smooth-walled structure.1-3 The LAA lies within the pericardium, next to the superior lateral aspect of the main pulmonary artery, and superior to the left ventricular free wall.3 It is often multilobed. Veinot et al.4 defined a lobe anatomically as: (1) a visible “outpouching” from the main tubular LAA, often demarcated externally by a “crease,” (2) able to accept a 2-mm probe internally, (3) may be associated with a change in the main tubular LAA direction, and (4) could lie in a different plane from the main tubular LAA (Fig. 2 and Video Clip 1). Synthetic resin cast of the LA demonstrates that the LAA (appendage) contains pectinate muscles, while the body of the LA (body) is a smooth-walled structure. Pulmonary venous component = pulmonary veins; vestibule = vestibule of the mitral orifice. (With permission from Anderson et al.1). A. Demonstration of the multilobed anatomy of the LAA by multidetector computed tomography (3D volume-rendered cardiac CTA using Toshiba Aquilion 64 CFX, 0.5-mm slices of the heart acquired in 7 seconds with Vitrea EP software ). B. Imaging is shown from a left anterior oblique and cranial angulation to highlight the LAA. One lobe (1) is long and curvilinear, while the second (2) is “flat and wide.” Also shown are the left anterior descending coronary artery (LAD) with its diagonal branches and the circumflex coronary artery (Cx). LAA structure varies significantly. An autopsy study of 220 cases with resin casts of the LAA found a range of volumes from 0.7 to 19.2 ml, minimum diameter from 5 to 27 mm, maximum diameter from 10 to 40 mm, and a variation in length from 16 to 51 mm. In 70% of the cases, the long axis was significantly “bent” or spiral-shaped.5 A subgroup (n = 55) of patients was in atrial fibrillation prior to death. They had a larger LAA volume, larger orifice, and fewer “lobes.” Similar LAA findings were noted by multidetector computed tomography (MDCT) when comparing patients with atrial fibrillation to those in sinus rhythm.6 An autopsy study (n = 500) with 25 males and 25 females for each decade of life (age 1–100 years) was performed in patients without a history of heart disease.4 Over 97% had pectinate muscles of >1mm in size. Those with pectinate muscles of <1mm in size were noted only from the first or last decade of life. Most had two lobes (54%), followed by three lobes (23%), one lobe (20%), and four lobes (3%). Results from the Stroke Prevention: Assessment of Risk in a Community (SPARC) study, in which the LAA was evaluated by transesophageal echocardiography (TEE), revealed a single lobe in 29.1%, two lobes in 49%, and the remainder (22%) to have multiple lobes.7 It appears that the LAA is more distensible than the LA, holding a relatively larger volume of blood as LA pressure increases.3, 8, 9 Clamping of the LAA during surgery will result in noticeable LA distension, along with an increase in transmitral and pulmonary diastolic flow velocities.10 When investigating the LAA by TEE, it is important to keep in mind that the LAA is a three-dimensional (3D), multilobed structure.11 Therefore, evaluation should include imaging in multiple planes, including orthogonal views, in order to image the entire 3D complex structure. Pectinate muscles should not be confused with thrombus (Fig. 3 and Video Clip 2). Measurement of the two-dimensional (2D) LAA area is not reproducible or helpful, in view of the complex structure.12 TEE in the horizontal plane (0°) demonstrates a normal LAA with prominently noted pectinate muscles (arrows). Located between the ascending aorta (Ao), pulmonary artery (Pa), and the LAA is the transverse sinus (*) (see text). (Modified with permission from Kerut et al.20, p. 273). The transverse sinus lies between the anterior LA and posterior wall of the ascending aorta and pulmonary artery, above the level of the aortic sinuses (Fig. 3). It is also anterior to the superior vena cava13-18 (Fig. 4). It may contain fluid with or without echo dense fibrinous material and be mistaken for thrombus (Fig. 5). By rotating the TEE transducer, and “following” the echo free space, one will note that this space is not “attached” to the LA.19-21 When pericardial fluid is within the transverse sinus, the LAA itself may appear as a mass. Rotating the transducer and/or changing the transducer imaging angle will this time reveal that the structure becomes the LAA and opens into the LA (Fig. 6 and Video Clip 3). Schematic drawings of the transverse sinus in (A) a sagittal plane and (B) transection through the aorta. Pericardial reflections are black colored in these images. The transverse sinus (long black arrow) runs between the anterior aspect of the left atrium and the posterior wall of the ascending aorta and main pulmonary trunk (PT). It is located above the level of the aortic sinuses. AC recess = aortocaval recess; AMV = anterior mitral valve leaflet; AZV = azygous vein; CS = coronary sinus; IVC = inferior vena cava; L = left coronary cusp aortic valve; LV = left ventricle; P = noncoronary cusp aortic valve; RA = right atrium; RAL = right anterolateral pulmonary valve; RPA = right pulmonary artery; RV = right ventricle; SVC = superior vena cava. (With permission from: McAlpine Wallace A. Heart and Coronary Arteries: An Anatomical Atlas for Clinical Diagnosis, Radiological Investigation, and Surgical Treatment. Springer-Verlag, Berlin, 1975, pp. 128–129). TEE of the transverse sinus with material noted within the space. This may simulate a LAA clot, but by rotating the transducer, and following the echo free space, one notes that it is not “attached” to the LA. (With permission from Kerut et al.20, p. 273). TEE in the vertical plane (90°) demonstrates a fluid-filled transverse sinus in which the LAA appears in cross-section as a circular object (arrow). By changing the transducer imaging angle (see Video Clip 3), this “circle” will become readily evident as the LAA and open into the LA. LA = left atrium; Pa = pulmonary artery; Ao = aortic root. The summarizing points about LAA anatomy include: The LAA is a 3D structure, most often having two or more lobes. Pectinate muscles should not be confused with pathology. Evaluation of the LAA should include multiple planes so as to evaluate each lobe. Echo dense fibrous material within the adjacent transverse sinus should not be confused with LAA thrombus. When pericardial fluid is present within the transverse sinus, one should not confuse a normal LAA lobe seen in cross-section as a “mass.” Video Clip 1. Video of the same patient in Figure 2 demonstrating the complex 3D structure of the LAA. Video Clip 2. Video of a normal LAA with pectinate muscle. Video Clip 3. TEE in the vertical plane (90°) demonstrates the transverse sinus is fluid filled, with the LAA visualized as a "circle" within it. By changing the angle to 75° and on to 60° and 45°, the LAA becomes readily apparent. It appears bilobed and is seen to be "attached" to the LA. Please note: Blackwell Publishing are not responsible for the content or functionality of any supplementary materials supplied by the authors. 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