Transesophageal echocardiography (TEE) is useful for technically complicated or challenging cases, when annular size is uncertain or close to established cutoffs and for determining the number of cusps when transthoracic imaging is equivocal [3, 9, 15]. A mid-esophageal long-axis image at 120–140° traditionally provides adequate views of the LVOT and annulus in a sagittal plane similar to TTE (Fig. 24.3). Single-plane short-axis views at 35–60° can also demonstrate the shape of the annulus but should not be used in isolation due to limitations in identifying the identical plane obtained from long-axis views. Employing biplane imaging to obtain a short-axis image, however, augments the usefulness of this view to demonstrate the elliptical shape of the annulus (Fig. 24.4a–c). TEE-based annular measurements generally correlate with TTE (r = 0.89) although TTE-based measurements are usually larger with mean difference of 0.22 mm and absolute difference between TEE and TTE of 0.6 ± 0.8 mm [16]. TEE measurements demonstrate higher intra- and interobserver concordance than TTE and computed tomography and have been shown to alter prosthesis selection in 16 % of cases compared to dual-source computed tomography [17, 18].

There is no gold standard for noninvasive assessment of aortic annulus dimensions. Whether it is TTE, TEE, or another noninvasive method, the ultimate decision will fall to the operator depending on local expertise and availability. Each technique has its advantages and disadvantages [4]. It is important, however, to recognize the unique characteristics of echocardiography. The sagittal plane measurement by TTE and TEE usually approximates the minor axis of the elliptically shaped annulus as measured by multi-detector computed tomography (MDCT) scanning. The difference between MDCT and TTE (1.22 ± 1.3 mm) or TEE (1.52 ± 1.1 mm) is larger than the difference between TTE and TEE [16]. In addition, traditional TTE or TEE measurement of the annulus uses long-axis images of the aorta in which the cusp opening is centered in the aorta; however, this view does not bisect the aorta at its largest dimension but rather images a plane just lateral to the maximum dimension which may cause significant underestimation of the true maximal annular diameter [2]. This is the most likely cause of undersizing of the annulus by echocardiographic measurement. Preprocedural MDCT-based measurements of annular diameter and post-procedural comparisons of THV area to annulus area predict paravalvular aortic regurgitation slightly better than TEE annular diameter (area under the curve: 0.81 compared to 0.70, respectively) [19]. Newer MDCT-based approaches which employ three-dimensionally derived cross-sectional measurements of the aortic annulus affect device sizing and patient selection and reduces paravalvular aortic regurgitation for the Edwards SAPIEN™ THV compared to traditional two-dimensional TEE [20]. This underscores the added value of routinely employing three-dimensional techniques for aortic annulus sizing such as by MDCT, cardiac magnetic resonance imaging (CMR) [21], or three-dimensional TEE [18, 22].

Echocardiography, however, has limitations in some patients being considered for TAVR. Most notable is the acoustic shadowing that can obscure interpretation of the annular complex due to heavily calcified aortic leaflets or root. With TEE, mitral annular calcification can also cause acoustic shadowing over the LVOT. In addition, with two-dimensional imaging, the imaging field is in a single plane that can lead to different measurements compared to three-dimensional real-time imaging and multiplanar reconstruction which allows complete morphologic analysis [23].

The major implications of aortic leaflet morphology for TAVR are related to the number of cusps present, the degree of leaflet calcification, and the height of the leaflets with respect to the coronary ostia. The latter is addressed in a separate section. Bicuspid aortic valves are considered a contraindication for TAVR due to concerns of poor seating, asymmetrical stent expansion, and/or paravalvular aortic regurgitation due to severe distortion of the native valve leaflets [10, 11]. Although there are now numerous reports of successful TAVR procedures in patients with congenitally bicuspid aortic valves [24–28], incomplete deployment may lead to aortic regurgitation or impairment of valvular function [29].

Echocardiography can evaluate the extent, location, and distribution of AoV calcification and provide important information helpful for the success of TAVR. Severe, asymmetric calcification of the AoV leading to asymmetry of leaflet excursion may also result in incomplete expansion of the THV, limited orifice size and paravalvular regurgitation. One grading system used for surgical AVR which discriminates calcium being confined to leaflet tips (mild), calcium extending from the leaflet tips to the leaflet bases (moderate), and bulky calcification that extended from tip to base in all three leaflets (severe) has shown modest correlation with the actual weight of the excised native valve (Box 24.1) [30]. Bulky AoV calcification is a risk factor for left main coronary artery occlusion during THV deployment, and in rare cases, with creation of a membranous septal defect due to inferior displacement of extensive annular or basal leaflet calcifications during stent expansion [31]. Valvular calcification poses a risk with either the SAPIEN™ or CoreValve® THV; however, positioning of the CoreValve® is more independent of valvular calcification owing to its constrained waist and larger length.

Coronary artery compromise is an important but fortunately infrequent complication of TAVR occurring in <1 % of patients in clinical trials and single-center reports [37, 38]. The SAPIEN™ Aortic Bioprosthesis European Outcome (SOURCE) Registry reported a 0.6 % incidence of coronary obstruction over 30 days in their post-marketing registry [39]. The possible mechanisms for coronary ostial obstruction during TAVR have been described and include (1) impingement of the coronary ostia by the THV support structure; (2) inappropriately high positioning of the sealing cuff of the THV; (3) embolization of atheroma, calcium, thrombus, air, or vegetation; (4) an oversized THV; (5) aortic root dissection; and (6) displacement of the native and bulky aortic leaflets towards the coronary ostia (Box 24.2) [37, 40–42]. Webb described risk factors for coronary ostial obstruction which include a low-lying coronary ostium <10–12 mm from the basal leaflet insertion to the coronary ostium, bulky calcification of native leaflets, and a narrow aortic root [42]. These risk factors have been present in many of the reported cases of coronary ostial obstruction during TAVR [41, 43] although no definite criteria exist to exclude patients on the basis of risk for coronary obstruction.

The normal adult aortic cusps vary in height even in apparently tricuspid valves without congenital malformation. Vollebergh et al. described the natural variation in leaflet height with right, noncoronary, and left coronary leaflets measuring on average 14.1, 14.1, and 14.2 mm, respectively. In adults with calcific AS, the average right, noncoronary, and left coronary leaflet heights are slightly smaller: 13.9, 13.6, and 14.0 mm, respectively [44]. Minimum recommended annular-ostial heights for the 23 and 26 mm SAPIEN™ THVs are >10 and >11 mm, respectively [4], although some suggest a more cautious approach using >14 mm as a general guideline for a safe annular-ostial distance [31]. The annular-ostial height is not relevant for the CoreValve® THV because of its unique structure and constrained waist which sits within the SoV to avoid “jailing” the coronary ostia [3]. For the self-expanding valve, however, the height of the SoV is an important consideration, with a preferred SoV height of >15 mm. SoV heights of <10 mm are generally considered so low as to preclude successful THV implantation [45].

The sinotubular junction (STJ) describes the upper most ring of the aortic root complex representing the interface between the aortic root and the tubular ascending aorta and plays an important role in maintaining competence of implanted prosthetic valves [47]. Echocardiographic studies demonstrate the dynamic nature of the aortic root during the cardiac cycle with the STJ width increasing by 3.4 ± 1.9 mm from diastole to systole [48]. Echocardiographic and ex vivo studies have also demonstrated that the STJ and ascending aorta are usually larger in patients with calcific AS compared to patients with normally functioning valves with a mean difference of 1.3 mm [2, 49]. For TAVR, STJ width and the presence and degree of calcification appear to be critical and may be related to the incidence of patient-prosthesis mismatch (PPM) [50, 51]. Small and heavily calcified STJs may predispose patients to balloon migration, THV embolization, or aortic root rupture during implantation of the THV and is associated with more severe atherosclerotic aortic arch disease [3, 52–54]. Although consideration of the STJ width and degree of calcification appear critical for correct and lower-risk positioning, quantitative data on the impact of the STJ and tubular aorta size and morphology on TAVR outcomes are limited [55].