Accurate Measurement of Left Ventricular Outflow Tract Diameter: Comment on the Updated Recommendations for the Echocardiographic Assessment of Aortic Valve Stenosis

Accurate echocardiographic calculation of aortic valve area (AVA) is integral to the appropriate management of patients with aortic stenosis. The greatest potential for error in the calculation of AVA by the continuity equation is left ventricular outflow tract (LVOT) diameter, which must be squared to calculate LVOT area. In the recent update of the European Association of Cardiovascular Imaging and American Society of Echocardiography recommendations for the assessment of aortic stenosis severity, the discussion and illustrations of LVOT measurement raise questions that deserve attention. These include (1) the location of the measurement for calculation of left ventricular (LV) stroke volume and (2) the imaging plane that yields the correct LVOT diameter in the setting of ectopic calcification.

Regarding the first issue, Baumgartner et al. state that it may be necessary to move the sample volume apically by 0.5 to 1.0 cm to obtain a laminar flow curve without spectral dispersion, and thus in this situation, LVOT diameter can be measured at the same distance from the valve as the Doppler sample volume position. Historically, however, LVOT measurement has been made at the annulus. There are strong outcomes data using this method for calculating AVA. In addition, the LVOT below the annulus is in large part composed of the basal muscular septum and as such is more elliptical and irregular as well as dynamic, particularly in the setting of a septal bulge that is highly prevalent in patients with aortic stenosis. Because the measured aortic annulus is composed primarily of the fibrous interleaflet trigones or triangles, this measurement, on the other hand, varies less with the cardiac cycle and is more accurate and reproducible. In the legend to Figure 5 of the focused update, the authors state that “in many patients, as in this case, the LV outflow tract is relatively rectangular within 1 cm of the aortic annulus,” such that the LVOT diameter is about the same at the annulus as at 1 cm below. In our experience and in view of the literature, this is in fact rarely the case, and most patients with aortic stenosis, especially those with severe stenosis, have a septal bulge and therefore hourglass-shaped LVOT inflow. Mehrotra et al. showed that the LVOT is also less distensible in patients with severe aortic stenosis with greater peak systolic ellipticity (smaller sagittal plane and larger coronal plane) and greater LVOT cross-sectional area underestimation (relative to normal control subjects) if using a single long-axis (sagittal plane) measurement. The annulus, on the other hand, is more fibrous, exhibiting less dynamic changes and thus a more stable, circular area whether by single-plane measurement or direct planimetry. The ellipticity of the LVOT may also explain recent findings of a larger planimetered LVOT area compared with planimetered annular area. In fact, Caballero et al. found that measurements at the level of the annulus resulted in the best correlation between two-dimensional and three-dimensional transesophageal echocardiographic dimensions. Numerous authors have shown that AVA calculation using direct planimetry of the LVOT or annulus results in larger valve areas using the continuity equation. A recent head-to-head comparison of multidetector computed tomographic measurement of LVOT area (AVA CT ) and traditional echocardiographic methods (single sagittal diameter measurement) (AVA echo ) showed that although hybrid AVA CT was larger than AVA echo , it did not improve the correlation with transvalvular gradient, the concordance of gradient-AVA, or mortality prediction compared with AVA echo . In fact, long-term survival was predicted by larger cut-point values for severe aortic stenosis if AVA CT (<1.2 cm 2 ) is used rather than AVA echo (<1.0 cm 2 ).

With regard to the correct measurement of the annular measurement in the setting of ectopic calcification within the LVOT, the updated recommendations suggest off-axis imaging to avoid the ectopic calcification on the anterior mitral valve leaflet. Two important anatomic facts should be considered. First, the aortic annulus that is measured on echocardiography is a virtual plane defined by the lowest hinge points of the aortic cusps, with more than half of the annulus composed then of the interleaflet triangles. For the most accurate measurement of the LVOT, one should always aim for the image that bisects the largest dimension of the aortic annulus, which, from the long-axis or sagittal plane, bisects the right coronary cusp hinge point anteriorly and the interleaflet triangle between the left and noncoronary cusps posteriorly ( Figure 1 ). Second, recent computed tomographic studies have shown that in the setting of severe aortic stenosis, protruding calcium into the LVOT is most frequently seen from the left coronary or the noncoronary sinus regions. Because the mitral-aortic curtain is in fibrous continuity with the entire noncoronary leaflet, the posterior part of the left coronary leaflet, and the interleaflet triangle between the two, a common location for ectopic calcium is the LV side of the A2 scallop. Thus, seeing the protruding calcium may actually be an indication that one is on axis for the largest sagittal plane annular measurement. When two aortic valve cusps appear centered within the aortic root, as in Figure 7B of the update, the image may be tangential to the largest annular diameter. In Figure 1 , the diameter measurements that should be performed in the setting of ectopic calcification is shown using Figure 7 of the update. Figures 1 A and 1C have been adjusted so that the scales are exactly the same ( red arrow along the edge of the sector), with the yellow arrow showing the correct diameter measurement of the annulus superimposed on both the on-axis image of the LVOT ( Figure 1 A) and the off-axis image of the LVOT ( Figure 1 C). The correct measurement should exclude the ectopic calcium ( Figure 1 A) and measure onto the mitral-aortic curtain in the plane that bisects the right coronary cusp hinge point anteriorly and the interleaflet triangle between the left and noncoronary cusps posteriorly ( Figure 1 B). Because the plane of the image in Figure 1 C images two cusps opening in the center of the aorta, the image is tangential to the actual largest sagittal diameter ( Figure 1 D) and thus will incorrectly measure a smaller annulus.

Figure 1

Accurate measurement of the annulus. Figure 7 from the recommendations is reproduced below (panels A and C represent the original panels A and B ) with the same scale shown ( red arrows at edge of sector).The original figure legend suggests that measurement in panel A would result in an incorrectly small annulus, but the largest diameter of the annulus is obtained from panel A ( yellow double arrow ) by excluding the ectopic calcium ( red asterisk ) extending onto the anterior mitral valve leaflet. Panel B is a representation of where that measurement would be taken from the short-axis view. The same yellow arrow from panel A superimposed onto panel C is longer than the suggested measurement ( dashed blue arrow ). Panel D is a representation of where that incorrect measurement would be taken from the short-axis view, with the two calcified cusps centered in the aortic root. LCC , Left coronary cusp; LVOTd , LVOT diameter; NCC , noncoronary cusp; RCC , right coronary cusp.

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Apr 15, 2018 | Posted by in CARDIOLOGY | Comments Off on Accurate Measurement of Left Ventricular Outflow Tract Diameter: Comment on the Updated Recommendations for the Echocardiographic Assessment of Aortic Valve Stenosis

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