The Left Atrium and the Left Atrial Appendage
SECTION 1 THE LEFT ATRIAL APPENDAGE
 FIGURE 1.1 Three-dimensional computerized tomographic arteriogram demonstrated above. Volume rendering has been done, and the mitral annular region has been removed by cropping. The left atrial appendage is in red. The pulmonary veins and their branches appear like “horns” on the left atrium. |
ANATOMY OF THE LEFT ATRIAL APPENDAGE
Interest in the left atrial appendage (LAA) has risen during recent years because of its role in thrombus formation in relationship to stroke. When thrombi are present in hearts not affected by the rheumatic process, ˜90% have their origin in the LAA.1
The LAA is a slender fingerlike projection from the atrial body that has various shapes and sizes. The os of the LAA is narrow compared to that of the right atrial appendage and lies just above the atrioventricular groove. The atrioventricular groove itself at this point contains the circumflex coronary artery and the great cardiac vein and is also related to the left phrenic nerve in the overlying fibrous pericardium.
The LAA has a smooth lining in the area of the os and then extends to a variable cavity that contains muscle bands and pectinate muscles, which have a whorl-like configuration.
From the exterior of the heart, the LAA lies in between the pulmonary artery and the left ventricle filling in the gap between these structures. From superior to inferior in an anterior view, it is the third of the four “bumps” on the cardiac silhouette. First is the aorta, then the pulmonary artery followed by the LAA, and finally the left ventricle, all components of the left border of the heart from the anterior perspective. The fibrous pericardium over the LAA contains the phrenic nerve.
From the inside of the left atrium, the LAA is separated from the pulmonary veins by a prominent ridge termed the “Coumadin ridge” or the broad left lateral ridge.
Imaging of this structure gave rise to the descriptive sign named the “Q-tip sign.”
From the exterior of the heart, the LAA usually wraps around the pulmonary artery but can have a tip that is even turned posterior. It has epicardial fat and multiple crenellations on the exterior that are associated with one or more lobes.2,3,4
Wang et al. have studied the shape of the LAA with multidetector computerized tomography (MDCT) to aid in preoperative planning for implantation of a closure device. In this group of patients, four different morphologies were defined. “Windsock,” “cauliflower,” “chicken wing,” and “cactus” types were identified (Fig. 1.2).5 In another study of patients with drug refractory atrial fibrillation who were about to have ablation, the “chicken wing” configuration was the most common, and in those who had a history of transient ischemic attack (TIA), the “cactus” configuration was the most common and the “chicken wing” was the least common.5,6,7
The Shape of the Left Atrial Appendage
 FIGURE 1.2 The left atrial appendage comes in a variety of shapes. It has been described by Wang as part of preoperative planning before device implantation to have four general shapes as viewed on computerized tomography. It is important to know the shape of the left atrial appendage, the diameter of the ostium, the orientation of the major axis of the appendage, and the depth of the appendage before a closure is performed with any device or procedure. The frequency of the anatomic types found in the individuals studied by Wang is listed below.5 A: The “chicken wing” type with a short neck and a bent toward the tip (18.3%). B: The “windsock” type, which has a short neck (46.7%). C: The “cauliflower” type (29.1%). D: The “cactus” type (5.9%). (Source: DiBiase L, Santangeli P, Anselmino M, et al. Does the left atrial appendage morphology correlate with the risk of stroke in patients with atrial fibrillation? Results from a multicenter study. J Am Coll Cardiol. 2013;60:531-538.) |
FUNCTION OF THE LEFT ATRIAL APPENDAGE
The LAA has similar functions to the LA. It acts as a reservoir, a conduit, an active contractile chamber, and a suction source. Experimentally, occlusion leads to increased left atrial pressure. The mechanical function of the LAA can lead to augmentation of stroke volume. Since the LAA is more distensible that the remainder of LA in can also act as a reservoir.
At a glance, the size of the LAA suggests that the left atrium may be dilated. Also, occurrences within the left atrium influence the function of the LAA. The presence of atrial fibrillation (AF) is associated with larger volumes and orifice sizes of the LAA sometimes being up to 3 times the size in those who have normal sinus rhythm. Furthermore, in individuals with AF, the endocardial surface of the dilated LAA is smoother and has more endocardial fibroelastosis than in the normal appendage.2,4,8,9,10
In addition, the LAA has endocrine function in that it contains stretch receptors that can influence heart rate and natriuretic peptides (ANP) that may account for ˜30% of all ANP that in turn can influence diuresis, natriuresis, and heart rate.8
Analysis of the mechanical function of the LAA using echocardiography centers around the measurement of pulsed-wave Doppler flow signals into and out of the appendage using TEE. Once adequate images are obtained, the pulsedwave Doppler sample is placed within the first one-third of the LAA with flow as parallel as possible to the cursor. A good color Doppler signal from this site has a characteristic pattern. Four distinct flow waves are usually identifiable in the presence of normal rhythm and flow. First, there is a positive wave produced by flow out of the appendage toward the transducer in atrial systole (red on color Doppler) with a velocity usually in the 60 cm/s range. This positive wave is followed by a negative or filling wave of the LAA and is of similar velocity although in the opposite direction (blue on color Doppler). In individuals with normal or high velocities of filling and emptying, a variable number of reflection waves follow before the end of systole. Early ventricular diastole is associated with a smaller positive wave and then a negative wave that follows mitral valve opening (see Fig. 1.3). Normally, the most prominent wave is the emptying wave, which is positive and ranges from 50 to 80 cm/s; the corresponding filling wave is in the 45-60 cm/s range.4 Emptying velocities of <40 cm/s in individuals with nonvalvular atrial fibrillation have a limited chance of maintaining normal rhythm at 1 year.11
Spontaneous echo contrast (SEC) is associated with low flow velocities and increasing risk of thrombus formation. Sometimes, the SEC is so dense that it is difficult to discern from the jellylike thrombi that occur in the apex of the LAA. Echo contrast agents can sometimes be used to separate SEC from jellylike thrombus. In addition, when the SEC is very dense and the flow velocities are very low or absent, it sometimes assumes a tornadiclike appearance. All of these visual findings are associated with very low velocities in the LAA.
One can imagine the effects of various arrhythmias on the velocity profiles of flow into and out of the LAA. Extremely low-velocity flow leads to the formation of SEC, a precursor of thrombus. Atrial fibrillation, atrial flutter, and AV dissociation alter the velocity profile in a diagnostic manner. Other abnormalities such as diastolic dysfunction, mitral stenosis, and mitral regurgitation have significant effects on the anatomy and function of the LAA.
Normal Pulsed-Wave Doppler Velocities in the Left Atrial Appendage
 FIGURE 1.3 Pulsed-wave Doppler flow pattern in a normal individual with vigorous LAA contraction and normal relaxation. The first wave (1) is very prominent with a velocity of about 90 cm/s with a negative filling wave that follows of similar velocity (2). In diastole, there are two waves with the first being in early diastole (3) and second that reflects from the early diastolic filling wave (4). The normal emptying velocity for the appendage is >50 cm/s. In the presence of atrial fibrillation, very low velocities <20 cm/s often indicate potential for thrombus formation and decreased chances of success with cardioversion. Very low velocities also are often associated with smoke within the LAA. Good velocities suggest that the mechanical integrity of the LAA is still intact and that the duration of the rhythm may not have been so long. Additionally, the potential for thrombus formation is lessened and the chances of successful cardioversion are enhanced.4,24 |
IMAGING OF THE LEFT ATRIAL APPENDAGE
The Left Atrial Appendage on Transthoracic Echo
The LAA can be visualized from several views including the A2C view, the high cross-sectional short axis view, and less commonly the subcostal views (Fig. 1.4A and B). It is there to be visualized but often is overlooked. Although the appendage can be visualized from TTE, the views are incomplete, and the thorough investigation that is needed to exclude thrombus can only be done with TEE. When a small pericardial effusion is present, the outer borders of the LAA are often outlined. This pericardial fluid can extend around posterior to the pulmonary artery and the aorta in the space that is called the transverse sinus. Sometimes, just the tip of the appendage is visualized in this small fluid-filled space and can give the appearance of a mobile mass. The right atrial appendage also is better outlined in the setting of a small pericardial effusion.
 FIGURE 1.4 A very large left atrium in addition to a very large left atrial appendage shown on transthoracic echo (yellow arrow). The size of the left atrial appendage is related to the duration of atrial fibrillation and the left atrial pressure. TTE, A2C view. |
 FIGURE 1.5 On the high cross-sectional views, the appendage can also be visualized; however, artifact is often prominent in this view preventing accurate determinations about the presence or absence of thrombus (yellow arrow). TTE, high cross-sectional view. |
The Left Atrial Appendage on Transesophageal Echo
Transesophageal echo (TEE) is the primary method for analysis of the LAA. Multiplane imaging makes it possible to visualize all segments of the LAA. Threedimensional echocardiography can also be utilized. This type of investigation is needed to evaluate for the presence of thrombus in the setting of atrial fibrillation, precardioversion, stroke, peripheral embolization, and others.3,4
The etiology of embolic stroke in the setting of atrial fibrillation focuses on the presence or absence of thrombus in the LAA. In individuals with nonvalvular atrial fibrillation if thrombus is present, it will be in the LAA 90% of the time and not in the body of the LA.1 The cul-de-sac configuration of the LAA and the alterations of flow in the LAA that occur in atrial fibrillation combine to make thrombus formation more likely. Atrial fibrillation alters the normal contractility of the LAA and leads to stagnation of flow.
The anatomy of the LAA varies from one individual to another. Wang et al. using CT angiography have imaged and described the shape of the LAA in four categories (see Fig. 1.2). These very descriptive categories are the “windsock,” “cauliflower,” “chicken wing,” and the “cactus” types in the order of their frequency. Evaluating the shape of the LAA and the size of the ostium is important as one plans for application of a LAA closure device.5 Some of these configurations are more common in those with TIA (the cactus type).6
Imaging of the LAA using the multiplane TEE requires using the entire spectrum of angles. As one approaches the appendage from the midesophageal region, it is easily identified from the 60°-70° view (see Figs. 1.6 and 1.7). This is always a good starting point. It is worth noting also that the general sequence of performing TEE should be the same each time. There may be variations from operator to operator, but a repeatable sequence is important from the standpoint of not missing a view or an area of the LAA or the heart for that matter. During the procedure, when adjacent abnormalities are identified, it is tempting to move to that area, but this risks changing the sequence and leaving out something. On the other hand, when time is constrained, one has at times to go for the “money shots” first.
A thorough investigation of the lobes of the LAA is always indicated. Thrombi can be tucked away in small areas and require a search to identify. Once the “comma-shaped” LAA is identified at 60°-70°, the angle can be advanced to interrogate the appendage to higher angles at about 15° intervals and then back from about 40°-0°. At higher angles, one notes the pectinate muscles assume linear; partitioning shapes and lobes are defined. These are easily mistaken for thrombi. Usually there are two lobes, but sometimes small side lobes are present and extend superior toward the transverse sinus.
At a glance, the size of the LAA tells a story. High left ventricular filling pressures, mitral regurgitation, and stenosis all often are associated with large appendages. Similar to the left atrium, the LAA also serves as a reservoir.
As mentioned earlier, the functional analysis of LAA centers around Doppler flow patterns into and out of the appendage. Once adequate images are obtained, the pulsed-wave Doppler sample volume is placed within the first one-third of the LAA or about 1 cm within the ostium of the LAA with flow as parallel as possible to the cursor (see Fig. 1.6). A good color Doppler signal from this site has a characteristic pattern. Analysis of flow patterns should be performed from a direction that is as parallel to flow as possible. Four distinct flow waves are usually identifiable in the presence of normal rhythm and flow. First, there is a positive wave that produced positive flow toward the transducer (red in color) usually in the 60 cm/s range. This positive wave is followed by a negative or filling wave of the LAA and is of a similar velocity although in the opposite direction (blue in color Fig. 1.7). In individuals with normal or high velocities of filling and emptying, a variable number of reflections waves follow before the end of systole. Early diastole is associated with a smaller positive wave that follows mitral valve opening (see Fig. 1.3).
One can imagine the effects of various arrhythmias on the imaging of the LAA and the velocity profiles. Atrial flutter and atrial fibrillation (see Fig. 1.10) leave their mark on the profile as does AV dissociation. These flow patterns are of physiologic significance but also can be of diagnostic value. In the setting of atrial fibrillation, low flow velocities predict a lower chance of long-term maintenance of normal rhythm.11
Low flow velocities are associated with SEC and increasing risk of thrombus formation. Sometimes the SEC is so dense that it is difficult to discern from the jellylike thrombi that occur in the apex of the LAA. Echo contrast agents can sometimes be used to separate SEC from jellylike thrombus. In addition, when the SEC is very dense and the flow velocities are very low or absent, it can assume a tornadic appearance. All of these visual findings are associated with very low velocities in the LAA. In the setting of nonvalvular atrial fibrillation (see Figs. 1.11 and 1.12) as mentioned, thrombi when they occur are found in the LAA in contrast to rheumatic valvular disease, and atrial fibrillation thrombi are found mural and in the LAA (see Fig. 1.13).
Cursor Location for Measurement of Velocities Into and out of the LAA
 FIGURE 1.6 Measurement of the velocity of flow into and out of the left atrial appendage should be done about 1 cm from the ostium of the appendage and as parallel as possible to flow usually in the 70° view. Yellow line represents the pulsed-wave Doppler beam and the red dot the position of the sample volume. |
 FIGURE 1.7 From the 70° view, the color Doppler image on stop frame shows a blue color or flow toward the transducer. This position is good for sampling of the pulsed-wave Doppler signal for analysis. |
CASE PRESENTATION
The Left Arial Appendage in Arial Flutter
A 47-year-old gentleman presented to the emergency room with 5 days of palpitations and irregular heartbeat. In the ER, he was discovered to have atrial flutter with variable conduction. TEE was done, and he subsequently had ablation.
 FIGURE 1.8 The EKG from the 47-year-old gentleman who presented with atrial flutter. The blue arrows point to flutter waves at a rate of slightly <300 BPM, and the red arrow points to a QRS complex that obscures the flutter wave. |
 FIGURE 1.9 Pulsed-wave Doppler sampling in the first third of the atrial appendage shows rhythmic waves consistent with atrial flutter (yellow arrows). There is alternation of the velocities associated with the position of the mitral valve. TEE, 72° view. |
The Left Atrial Appendage in Atrial Fibrillation
 FIGURE 1.10 Sometimes the atrial rhythm is not obvious from the electrocardiogram. For example, in the setting of ventricular pacing, the atrial rhythm is often ignored. The loss of atrial kick can be a significant clinical problem. If an atrial lead is present, then interrogation of the device can make this determination; however, the TEE can make the diagnosis. In the above example, the typical atrial fibrillation Doppler velocity waves are present (yellow arrows) and demonstrate good velocities suggesting that the situation is favorable for long-term maintenance of sinus rhythm. The smaller velocities are related to mechanical activity, while the mitral valve is closed. |
Thrombus in the Left Atrial Appendage
 FIGURE 1.11 Transesophageal echo from 0° showing an even larger mural thrombus (yellow arrow) in the left atrial appendage appearing to be firmly attached to the wall. TEE, 32° view. |
 FIGURE 1.12 Small thrombus in a second lobe of the LAA (yellow arrow). TEE, 114° view. |
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