Preprocedural imaging - Clinical Tree

Introduction

The importance of precise preprocedural multimodality imaging with echocardiography and cardiac multislice computed tomography (MSCT) before transcatheter aortic valve implantation (TAVI) cannot be overemphasized. Both echocardiography and MSCT imaging are the foundations of quality outcomes in TAVI and are key tools for patient selection. Participation of sonographers, technicians, radiologists, and the imaging cardiologist within a multidisciplinary heart team is vital in acquiring key information to guide patient diagnosis, prognostic risk, and aortic valve intervention strategy.

Patients with symptoms and signs of aortic stenosis (AS) initially undergo transthoracic echocardiography (TTE) to assess severity. TTE remains the standard approach for diagnosis and often is performed in the community setting before specialist referral to the heart team. ^, Once a patient is considered for TAVI, a dedicated cardiac MSCT of the heart and peripheral arterial vasculature is performed to determine anatomic suitability, valve sizing, and access route. Furthermore, MSCT in particular provides the operator with essential data for the procedure in order to risk stratify cases to minimize complications and allow for optimal transcatheter aortic valve (TAV) delivery.

This chapter outlines the role of echocardiography and MSCT in the workup of a patient with severe AS for TAVI and describes situations where additional adjunctive imaging may be useful.

Echocardiographic assessment and diagnosis of severe aortic stenosis

Transthoracic echocardiography

TTE assesses valvular morphology of the aortic valve and severity of stenosis ( and ).
Senile calcific AS is the most common pathology where calcium deposits on the aortic valve leaflets lead to decreased excursion and a reduced opening orifice area.
In young patients, AS typically occurs secondary to congenital abnormalities of the aortic valve with only one cusp (unicuspid) or two cusps (bicuspid).
Rheumatic heart disease can affect patients of all ages, is more common in endemic areas, and causes fusion of the commissures with a triangular orifice area with thickening and calcification noted on the edges of the aortic valve cusps and commissures.
Bicuspid aortic valve disease is found in high proportions in those of Southeast Asian ethnicity. It is associated with bulky leaflet calcification and concurrent aortic regurgitation (AR) causing mixed aortic valve disease and is often diagnosed in the presence of a variety of aortopathies.

Diagnosis of severe AS

Table 4.1 describes the echocardiographic features of severe AS.

TABLE 4.1

Severe AS Diagnosis by TTE

	Mild AS	Moderate AS	Severe AS
Peak aortic velocity (m/s)	<3	3–4	≥4
Mean gradient (mm Hg)	<20	20–40	>40
AVA (cm ² )	>1.5	1.1–1.5	≤1.0
Indexed aortic valve area (AVA/BSA − cm ² /m ² )	>0.85	0.65–0.85	≤0.60
Dimensionless index (DI)	>0.50	0.25–0.5	<0.25

AS, Aortic stenosis; AVA, aortic valve area; BSA, body surface area; DI, dimensionless index; TTE, transthoracic echocardiography.

Transthoracic echo considerations

Although aortic velocity and aortic valve area (AVA) are key measures for the diagnosis of severe AS, an integrated approach with Doppler echocardiography is recommended to determine severity ( Fig. 4.1 ).
Further useful data acquired from TTE include indexed stroke volume, left ventricular (LV) size, LV wall thickness, ejection fraction, and assessment of multivalvular disease and pulmonary hypertension to determine the severity and stage of the disease process.
In equivocal cases, the dimensionless index (DI) may also guide a diagnosis. The DI is defined as a ratio of the LVOT time-velocity to that of the aortic valve jet (see Table 4.1 ).
The use of a smaller nonimaging Doppler probe for continuous wave Doppler measurements at the apex, right parasternal edge, and suprasternal notch should be routinely performed in all cases to ensure the maximum velocity and gradients across the valve are being obtained in the evaluation of AS severity.
In patients with atrial fibrillation, variable Doppler signals are common and an average of five consecutive continuous wave Doppler signals is recommended to assess severity. In these cases, the maximum Doppler signal should also be recorded on the echocardiography report, as well as the average mean and peak gradients.
To account for differences in body size an indexed AVA may be calculated by dividing the AVA by body surface area (BSA) and can be an adjunctive measurement in an AS diagnosis. This has been found to increase the prevalence of severe AS in patients initially thought to have moderate AS.
Potentials for error in the calculation of the AVA by the continuity equation can occur when measuring the LV outflow tract (LVOT) diameter by 2D TTE. Guidelines from the European Association of Cardiovascular Imaging and American Society of Echocardiography provide instruction on how to optimize LVOT measurement, with recent consensus that the LVOT diameter should be measured in midsystole directly below the aortic valve cusps rather than 1 cm below because of the muscular and elliptical shape below the aortic annulus ( Fig. 4.2 ).
The continuity equation states that in the absence of a shunt, blood flow is equal within one area (the LVOT) to a second area (AVA). In echocardiography the AVA is calculated using the velocity time integral (VTI), which is the most accurate method and preferred. The flow through the LVOT can be calculated by measuring the LVOT diameter (in cm), squaring that value, multiplying the value by 0.78540 (which is π/4) giving a cross-sectional area of the LVOT (in cm ² ), and multiplying that value by the LVOT VTI (in cm), measured on the spectral Doppler display using pulsed-wave Doppler. From these, it is easy to calculate the area (in cm ² ) of the aortic valve by dividing the LV stroke volume (in cm ³ ) by the aortic valve VTI (in cm) measured on the spectral Doppler display using continuous wave Doppler ( Fig. 4.3 ):

\begin{matrix} {Aortic valve area (in cm}^{2}) \\ = \frac{{LVOT diameter}^{2} × 0.78540 × LVOT VTI}{Aortic valve VTI} \end{matrix}

$\begin{matrix} {Aortic valve area (in cm}^{2}) \\ = \frac{{LVOT diameter}^{2} × 0.78540 × LVOT VTI}{Aortic valve VTI} \end{matrix}$

Differential diagnosis

It is important to discern AS from subvalvular stenosis caused by a subaortic membrane ( Fig. 4.4 ), or hypertrophic obstructive cardiomyopathy defined as an increased velocity with Doppler or color measurements within the LVOT.
Supravalvular stenosis is very rare and often confined to patients with congenital abnormalities. In selected cases where there is discrepancy between TTE measurements, correlation with cardiac magnetic resonance imaging derived velocities or invasive hemodynamic assessment during cardiac catheterization may be considered.
Patients with severe AS unlikely to benefit from TAVI include patients with cardiac amyloidosis characterized on TTE by increased LV wall thickness ( Figs. 4.5 and 4.6 ), abnormal tissue Doppler measurements, LV strain ( Fig. 4.7 ) and a characteristic “speckled” pattern as the amyloid protein is more echogenic than standard myocardium (see Figs. 4.5 and 4.6 ).

Identifying potential complications

Detailed and intricate information on the anatomy of the aortic valve and surrounding structures can aid heart teams and TAVI operators to avoid complications. Examples include the following:

Prominent septal bulge, which may increase the risk of valve embolization.
LV thrombus, which would mandate the need for cerebral protection, perioperative management of anticoagulation, and consideration of careful LV guidewire positioning during valve deployment and favor the need for right ventricular pacing.

Heavily calcified annulus or left ventricular outflow tract (see Case Study 4.1 ).

CASE 4.1

AR, Aortic regurgitation; AS, aortic stenosis; LVOT, left ventricular outflow tract; MSCT, multislice computed tomography; TTE, transthoracic echocardiography.

Heavily Calcified Annulus and LVOT Causing Severe Paravalvular AR

An 83-year-old man presented with symptomatic severe AS and moderate AR. TTE revealed bulky and extraordinarily heavily calcified leaflets (see Fig. 4.1 ), confirmed on MSCT with calcification extending into the LVOT. This caused significant difficulty in MSCT sizing; however, the measurements suggested the annulus perimeter (79.6 mm), diameter (24.9 mm), and valve area (488 mm ² ) were on the borderline between a size M and L self-expanding Accurate Neo (Boston Scientific, MA, USA) valve (see Table 4.5 ). Because of the risk of significant annular and LVOT calcification, a size M was deployed to try to minimize the risk of annular rupture ( ). Unfortunately, postdeployment there was severe paravalvular aortic regurgitation ( ) despite postdilation with a 22-mm noncompliant balloon. As the patient was hemodynamically unstable, the decision was made to perform a valve-in-valve procedure with a 26-mm Sapien-3 (Edwards Lifesciences, CA, USA) valve to nominal pressure. There was residual mild paravalvular AR ( ) and the patient was discharged without complications.

TABLE 4.5

Components of the MSCT TAVI Report

Aortic root	Phase used for evaluation (%) and scan quality
Basal annulus ring	Major × minor diameter (mm) Area (mm ² ) Area derived diameter (mm) Perimeter (mm) Perimeter derived diameter (mm) Height to left mainstem (mm) Height to right coronary (mm) Eccentricity index (%) Valve morphology Calcification at BAR and extension
Aortic root	Sinus of Valsalva (mm) Sinotubular junction (mm) Ascending aorta (mm) Calcification description
Abdominal aorta	Size, calcification, angulation, tortuosity, thrombus
Right access	Right common iliac (mm and calcification) Right common iliac at bifurcation (mm and calcification Right external iliac (mm and calcification) Right common femoral (mm and calcification) Angulation description Tortuosity index: ratio
Left access	Left common iliac (mm and calcification) Left common iliac at bifurcation (mm and calcification Left external iliac (mm and calcification) Left common femoral (mm and calcification) Angulation: description Tortuosity index: ratio
Preferred access site	Right or left
Alternative access suitability	Transapical, transcaval, subclavian
Extracardiac findings	Radiologist review
Summary	Recommendations

BAR, Basal annular ring; MSCT, multislice computed tomography; TAVI, transcatheter aortic valve implantation.

Prior mechanical mitral valve replacement, which may interact with balloon expandable TAVI devices on deployment.

Transesophageal echocardiography

Transesophageal echocardiography (TOE) can play a complementary role for TAVI planning in the following instances:

Valve morphology, aortic root size, and ascending aortic dimensions, particularly in patients with bicuspid aortic valve disease.
TOE may be considered in patients with significant kidney disease to minimize the risk of contrast-induced nephropathy.
Assessment of coexistent mitral valve pathologies is useful to allow for comprehensive heart team assessment.
Where dedicated MSCT cannot be performed, TAV sizing with the use of 3D TOE can be performed by experienced operators with accurate and reproducible results.

Magnetic resonance imaging

Dedicated cardiac magnetic resonance (CMR) imaging is a noninvasive, nonionizing radiation diagnostic tool that provides an accurate assessment of LV volumes, mass, function, and presence of myocardial fibrosis and scarring.

CMR can provide a complementary assessment of aortic valve pathology; however, it should not be used in substitution for echocardiography as TTE remains the key gold standard diagnostic tool. This is because the temporal resolution of CMR is inferior to TTE, and flow velocities are often underestimated compared with echocardiography.
CMR can be useful in patients with poor TTE windows or those with a contraindication to MSCT (risk of contrast anaphylaxis or end-stage renal failure), and can provide an accurate assessment of aortic valve morphology, the aortic annulus, and concomitant aortopathy.
Furthermore, CMR can assess the LV stroke volume, LVOT area, and AVA by planimetry and can provide further myocardial characterization when concomitant cardiomyopathy is suspected.
CMR remains an expanding imaging modality and may provide additional information and clarification in challenging cases of AS diagnosis where there is discrepancy in TTE measurements.

Stress testing

Exercise stress testing may be used to reveal symptoms in patients with moderate or severe AS. A “positive” test is defined as a failure of the blood pressure to rise during the test or the development of symptoms.
Stress echocardiography in patients with symptomatic severe AS is contraindicated and not required for diagnosis or patient workup where concurrent ischemic heart disease is suspected.
Dobutamine stress echocardiography (DSE) is a test performed under medical supervision to assess contractile reserve in patients with low-flow low-gradient (LFLG) aortic valve disease (see Section 4.3). The protocol for a DSE includes low-dose infusion 2.5 μg/kg/min with incremental doses of infusion to a maximum rate of 20 μg/kg/min and ceased when a result is obtained and/or there is an adequate chronotropic and inotropic response.

Low-flow low-gradient aortic stenosis

The most difficult group with aortic valve disease are patients with LFLG disease states.

Fig. 4.8 provides a guide on how to interpret TTE findings in the context of severe AS. LFLG AS with reduced left ventricular ejection fraction (LVEF) is defined on echocardiography as an AVA of <1.0 cm ² , mean gradient <40 mm Hg, LVEF <50%, and an indexed stroke volume (SVI) of ≤35 mL/m ² .
DSE is useful in this setting to determine severe AS from pseudosevere AS (defined as an increase in AVA >1 cm ² after dobutamine infusion) often accompanied by an increase in LV contractile reserve (defined as an increased in stroke volume of 20% or more).
LFLG AS with preserved LVEF often affects elderly females and is characterized by a small LV cavity, systemic hypertension, and LV hypertrophy. It is defined as an AVA <1.0 cm ² , mean gradient <40 mm Hg, SVI ≤35 mL/m ² , and an LVEF ≥50%. In this group measurement of aortic valve calcium score by dedicated cardiac CT Agatston units is useful in determining severity and outcome.

Current guidelines suggest that a score of >2000 units in men and a score of >1200 units in women are likely diagnostic for severe LFLG AS ( Table 4.2 ). The ESC/EACTS guidelines state that all patients evaluated for severe AS will fall into one of four categories ( Table 4.3 ) :

1.
High-gradient severe AS: AVA <1.0 cm ² , mean gradient ≥40 mm Hg.
2.
LFLG AS with reduced ejection fraction: AVA <1.0 cm ² , mean gradient <40 mm Hg, LVEF <50%, SVI ≤35 mL/m ² . DSE is required to determine whether LFLG severe AS or pseudosevere AS (increase in AVA >1.0 cm ² after dobutamine infusion often accompanied by LV contractile reserve).
3.
LFLG AS with preserved ejection fraction: AVA <1.0 cm ² , mean gradient <40 mm Hg, LVEF ≥50%, SVI ≤35 mL/m ² , and aortic valve calcium score by CT recommended to determine likelihood of severe AS.
4.
Normal-flow low-gradient AS with preserved ejection fraction (moderate AS): AVA <1.0 cm ² , mean gradient <40 mm Hg, LVEF ≥50%, SVI >35 mL/m ² .

TABLE 4.2

Aortic Valve Severity by Calcium Score From MDCT

	Men	Women
Severe AS very likely	≥3000	≥1600
Severe AS likely	≥2000	≥1200
Severe AS unlikely	<1600	<800

AS, Aortic stenosis; MDCT, multidetector computed tomography.

TABLE 4.3

AS Diagnosis Dependent on Multimodality Imaging

AS Category	Aortic Velocity (m/s)	AVA (cm ² )	Mean Gradient (mm Hg)	LVEF	SVI (mL/m ² )	DSE	MSCT AV Calcium Score
High-gradient severe AS	≥4.0	≤1.0	≥40	Irrelevant	Irrelevant	NR	Irrelevant
LFLG AS with reduced ejection fraction	3–4 m/s	≤1.0	<40	<50%	≤35	Yes AVA >1.0cm ² with contractile reserve Pseudosevere AS	NR
LFLG AS with preserved ejection fraction	3–4 m/s	≤1.0	<40	≥50%	≤35	NR	Yes Men >2000 Women >1200 Severe AS likely
Normal-flow LG gradient AS with preserved EF (moderate AS)	3–4 m/s	≤1.0	<40	≥50%	>35	NR	NR

AS, Aortic stenosis; AVA, aortic valve area; DSE, dobutamine stress echocardiography; EF, ejection fraction; LG, low gradient; LVEF, left ventricular ejection fraction; MSCT, multislice computed tomography; NR, not required; SVI, indexed stroke volume.

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