Diameter measurement of the aortic valve annulus for transcatheter bioprosthesis size selection

Nicolo Piazza, MD, FESC, FRCPC, Ruediger Lange MD, PhDGerman Heart Center, Munich

Surgeons commonly define the “aortic valve annulus” as the semilunar crown-like ring demarcated by the leaflet attachment line that runs across the aortic root. For purposes of transcatheter aortic valve implantation (TAVI), the enigmatic “aortic valve annulus” corresponds to a virtual ring formed by joining the basal attachment points of the leaflets within the left ventricle. This plane represents the inlet from the left ventricular outflow tract into the aortic root.
Transcatheter aortic bioprostheses are typically oversized by 5-30% relative to the diameter measurement of the aortic valve annulus (see Table 1). The intention of oversizing is to create enough interference between the prosthetic valve and aortic valvar complex to ensure adequate anchoring and to avoid paravalvular aortic regurgitation.
A number of imaging modalities can be used to obtain the “aortic valve annulus diameter”. These include transthoracic echocardiography (TTE), transesophageal echocardiography (TEE), multislice computed tomography (MSCT), contrast aortography, and magnetic resonance imaging (MRI). As a result of the elliptical shape of the aortic annulus and the fact that the annulus can be transected across multiple planes, diameter measurements may differ across various imaging modalities.

As a general rule of thumb: the aortic annulus measurement obtained from the MSCT oblique saggital plane corresponds to the parasternal long axis measurement obtained during echocardiography. On the other hand, the aortic annulus measurement obtained from the MSCT coronal plane corresponds to the anteriorposterior measurement obtained during contrast aortography.
The MSCT coronal measurement is invariably larger than the MSCT oblique saggital measurement. In fact, the mean difference between the maximum and minimum diameters of the aortic annulus is approximately 6.5 mm. In some extreme cases, this difference can be as high as 9 mm as shown in Figure 1. Thus, measurements obtained by echocardiography usually underestimate the maximum diameter obtainable from MSCT.
The elliptical shape of the aortic valve annulus creates the following clinical conundrum and take home message of this communication: How do we interpret and apply a 2-D diameter measurement of the aortic annulus for purposes of transcatheter aortic valve size selection well knowing that we are underestimating and overestimating the true maximum and minimum diameter of the aortic annulus, respectively?

There is significant inter-hospital heterogeneity in practice patterns – the number and type of imaging modalities employed, the method for measuring an elliptical aortic annulus in the presence of dense calcifications, and finally the interpretation and application of these measurements for transcatheter valve size selection. Up until now, manufacturers have provided sizing guidelines based on echocardiography. Results after transcatheter aortic valve implantation are characteristically described as being “good” or “acceptable”. We would like to challenge this by noting that after TAVI, 70-90% of patients have some degree of paravalvular aortic regurgitation (mostly grade 1-2), 10-20% of patients may require a post-implant dilatation, and 1-5% of patients need a valvein-valve. There is emerging evidence that increasing annulus size alone and greater annulus size to nominal prosthesis size correlate with the severity of paravalvular aortic regurgitation and post-implant dilatation. This would seem to suggest that we might be systematically undersizing transcatheter aortic valves during our daily practice. Using multi-planar reconstructions, MSCT can provide axial (short-axis) images of the aortic root. The aortic valve annulus can be found at a level just below the basal attachment points of the 3 leaflets (Figure 2).
Axial images allow measurements of the maximum and minimum diameter of the annulus to pass through the center of the ellipse. Some investigators advocate the use of the mean diameter (maximum + minimum/2), area of the ellipse or perimeter of the ellipse for sizing of transcatheter aortic valves.
Although common sense would agree with this, there is still much to be learned about the interference between the aortic annulus and transcatheter prosthesis, especially in the context of a balloon-expandable or self-expandable frame. Furthermore, using the mean diameter or cross-sectional area will inevitably narrow the range for annulus criteria and at the same time increase the number of eligible patients for TAVI.
We must not become too complacent and accept current TAVI results as “acceptable”. We must continuously question and challenge our “conventional” and “institutional” methodologies with the aim of improving patient safety and outcomes. Together with engineers and post-implantation imaging, we need to better understand how the “device interacts with the patient” and how the “patient interacts with the device” – the device-patient interface.
As manufacturers begin to offer more valve sizes, we will need to better understand the 3-D nature of the aortic annulus – 2D measurements will not suffice!