Aortic stenosis is becoming an increasing health care problem as the population ages. Surgical aortic valve replacement remains the gold standard but is associated with high mortality and morbidity rates in elderly patients and those with multiple comorbidities. The authors explore transcatheter aortic valve implantation as an attractive alternative therapy in this high-risk population and outline its limitations and future directions, with a special emphasis on the role of echocardiography.
The evolution of alternative approaches to the management of severe, symptomatic aortic stenosis (AS) in elderly and high-risk patients, which incorporate percutaneous transcatheter techniques, has generated profound interest in recent years. Although randomized data comparing surgical aortic valve replacement (SAVR) and transcatheter aortic valve implantation (TAVI) are eagerly awaited, the superiority of TAVI over medical therapy for many patients deemed unsuitable for surgery now seems clear with the release of the Placement of Aortic Transcatheter Valve (PARTNER) trial. It seems likely then that we will see an exponential increase in the use of TAVI over the next decade. In addition to reviewing the increasing health burden of severe AS and evaluating the different approaches to TAVI, we focus upon the role of echocardiography (with comments on complementary imaging techniques where appropriate) throughout the entire process, from patient selection, through procedural guidance to postprocedural follow-up.
The Emerging Epidemic of Valvular Heart Disease
As the burden of rheumatic disease began declining in the developed world, it was felt that valvular disease would no longer constitute a major public health concern. However, increasing life expectancy has resulted in an increase in the prevalence of clinically significant degenerative valvular disease. In 2006 Nkomo et al. examined epidemiologic data with well-characterized population samples and prospectively gathered echocardiographic measurements. They characterized the burden of degenerative valvular disease within the US population as being significant, with a national prevalence of moderate or severe valvular disease of 2.5% (mitral regurgitation [MR], 1.7%; aortic regurgitation [AR], 0.5%; AS, 0.4%; mitral stenosis, 0.1%). They demonstrated the correlation between increasing age and degenerative valvular disease, with moderate to severe valvular disease being present in one of eight people aged > 75 years. By contrast, the Euro Heart Survey found that valvular AS was the most common disorder among patients with diagnosed valvular disease (33.9%).
AS can be valvular, subvalvular, or supravalvular in location; however, only valvular AS is addressed in this review. AS results in obstruction to left ventricular (LV) outflow and can manifest symptomatically as syncope, heart failure or angina. Symptomatic AS is associated with a high risk of death. The present standard of care for symptomatic AS is relief of the valvular obstruction with SAVR. SAVR offers symptom relief and good long-term outcomes in most patients with symptomatic AS, with reasonable mortality and morbidity. Significant survival benefit has furthermore been demonstrated in patients aged > 80 years compared with medical therapy for symptomatic AS. SAVR is nonetheless still associated with considerable mortality and morbidity in this older group of patients and those with multiple comorbidities. Kohl et al. demonstrated a mortality rate of 13% (SAVR with or without coronary artery bypass grafting), with high morbidity rates and prolonged intensive care and hospital stays. It should also be remembered that elderly patients referred for surgery or deemed suitable for aortic valve replacement constitute a selected population; many other elderly patients are poor candidates for SAVR. When applied across the general elderly population, the reported morbidity and mortality rates for SAVR are likely to be underestimates because of selection bias: older and higher risk patients and those who decline or are not offered surgery will not appear in registries. The natural history of AS in those who decline or are rejected for SAVR has been understudied, but indications are that survival is dismal. The recently released PARTNER study results demonstrate the high mortality rates of those patients deemed unsuited for SAVR and treated medically, with a 1-year mortality rate of 50.7%. Less invasive options for valve replacement therefore are fast becoming an attractive alternative to SAVR or medical or palliative care in the group of patients deemed to be inoperable or at excessive risk.
Types of Percutaneous Valves
Currently two types of percutaneously implantable aortic valves are in clinical use in North America and Europe: the balloon expandable Edwards SAPIEN series of valves (Edwards Lifesciences Inc., Irvine, CA; Figure 1 ) and the self-expandable CoreValve ReValving System (Medtronic, Luxembourg City Luxembourg; Figure 1 ). While the SAPIEN series is available on an investigational basis in the United States, the CoreValve ReValving System is not currently available for commercial or investigational use in the United States. Both the SAPIEN series of valves and the CoreValve are Conformité Européenne certified in Europe for use, with both valves also being available in Canada on a compassionate or investigational basis. In addition to the percutaneous aortic prostheses that have been approved for clinical use in North America and Europe, there are a number of other devices that have first-in-human applications, which include the Paniagua percutaneous heart valve (Endoluminal Technology Research, Miami, FL), the ATS 3f percutaneous heart valve (ATS Medical, Minneapolis, MN), the AorTx percutaneous heart valve (Hansen Medical Inc., Mountain View, CA), the Perceval-Percutaneous Valve (Sorin Group, Arvada, CO), the JenaValve (JenaValve Technology, Wilmington, DE), the Lotus Valve (Sadra Medical Inc., Campbell, CA), and the Direct Flow Medical aortic valve (Direct Flow Medical, Inc., Santa Rosa, CA) ( Figure 2 ). There are also a number of other potential percutaneous aortic prostheses being evaluated in preclinical settings.
Types of Percutaneous Valves
Currently two types of percutaneously implantable aortic valves are in clinical use in North America and Europe: the balloon expandable Edwards SAPIEN series of valves (Edwards Lifesciences Inc., Irvine, CA; Figure 1 ) and the self-expandable CoreValve ReValving System (Medtronic, Luxembourg City Luxembourg; Figure 1 ). While the SAPIEN series is available on an investigational basis in the United States, the CoreValve ReValving System is not currently available for commercial or investigational use in the United States. Both the SAPIEN series of valves and the CoreValve are Conformité Européenne certified in Europe for use, with both valves also being available in Canada on a compassionate or investigational basis. In addition to the percutaneous aortic prostheses that have been approved for clinical use in North America and Europe, there are a number of other devices that have first-in-human applications, which include the Paniagua percutaneous heart valve (Endoluminal Technology Research, Miami, FL), the ATS 3f percutaneous heart valve (ATS Medical, Minneapolis, MN), the AorTx percutaneous heart valve (Hansen Medical Inc., Mountain View, CA), the Perceval-Percutaneous Valve (Sorin Group, Arvada, CO), the JenaValve (JenaValve Technology, Wilmington, DE), the Lotus Valve (Sadra Medical Inc., Campbell, CA), and the Direct Flow Medical aortic valve (Direct Flow Medical, Inc., Santa Rosa, CA) ( Figure 2 ). There are also a number of other potential percutaneous aortic prostheses being evaluated in preclinical settings.
Patient Selection
At this stage, patient selection is on an individual basis with the type and approach of TAVI being largely determined by the experience and usual practices of the individual institution. Currently, patients with severe symptomatic AS who are considered too high risk for SAVR due to comorbidities, prior cardiac surgery or deemed inoperable due to factors such as porcelain aorta are potential candidates for TAVI. Until further data are obtained, advanced age per se is not considered an indication for TAVI unless there are associated factors increasing the patient’s risk for SAVR. Patients are excluded if a reasonable quality of life or life expectancy is not anticipated despite valve replacement.
With the SAPIEN series, contraindications to TAVI include an aortic annulus that is too small (<18 mm) or too large (>29 mm) to accommodate the presently available prostheses, or aortic valve and root anatomy that is unsuitable for TAVI. For the CoreValve, patients with femoral and iliac arteries <6 mm in diameter, severe tortuosity and arterial calcification, and aortic annuli <20 and >27 mm and those with ventricles that are too horizontal compared with the aortic root are deemed unsuitable for this procedure. As the superior aspect of the CoreValve frame needs to be secured within the ascending aorta, an aortic diameter > 43 mm is a contraindication for implantation of this device. The presence of greater than moderate MR is also a contraindication for implantation of the CoreValve.
With respect to mode of vascular access, the patient is considered for a transapical (TA) approach with a SAPIEN valve if the iliofemoral arterial system has significant vascular disease which renders the patient unsuited to a transfemoral (TF) approach. Similarly, subclavian artery access can be considered for CoreValve implantation if the femoral approach is unsuitable. In an effort to apply TAVI to a wider group of patients at each center, increasing interest is being focused on a complementary approach to treatment using either the CoreValve or the SAPIEN series.
Preprocedural Imaging
Multimodality imaging techniques including echocardiography, catheter-based angiography and fluoroscopy, and computed tomography are necessary for complete assessment, appropriate patient selection and successful treatment with TAVI. Echocardiography plays a fundamental role in TAVI, beginning with patient selection, prosthesis sizing, procedural guidance, assessment of complications and follow-up. With respect to the pre implantation evaluation, the traditional assessment of AS is performed usually with transthoracic echocardiography (TTE), with transesophageal echocardiography (TEE) being performed if the information is incomplete or suboptimal. In the first instance it must be verified that the obstruction to LV outflow is at the valvular level. Once hemodynamically significant valvular AS is confirmed, valve morphology is carefully assessed.
The measurement of the aortic root annulus is necessary for the purposes of determining suitability for implantation of a SAPIEN or CoreValve prosthesis: choosing the prosthesis size may prove challenging because of the complex anatomy of the aortic valve and its relationship to the aortic root and the LV outflow tract (LVOT; Figure 3 ). Although the complex anatomy of the aortic valve and root has been long understood, the rapidly increasing availability of multidetector computed tomography (MDCT) and three-dimensional (3D) echocardiography has clarified conventional anatomic descriptions and extended our concepts of the 3D morphology of the valve. MDCT demonstrates the annulus as an oval-based coronet shaped structure with a significant difference between the longest and shortest axis. The aortic valve annulus as defined by TTE and TEE is therefore a nonanatomic structure represented by a plane subtending the most basal attachment points of the leaflets. In practical terms, the echocardiographic dimension used to determine candidacy for TAVI or to choose prosthetic size is the distance from the basal attachment of the noncoronary cusp to the basal attachment of the right coronary cusp. This is measured from a 2-dimensional parasternal long-axis image on TTE or midesophageal long-axis image between 120° and 140° plane rotation on TEE. We measure the maximal diameter at the aortic valve annulus: this is usually found in systole ( Figure 4 ). This anteroposterior measurement more closely approximates the minor rather than the major dimension of the elliptically shaped annulus as measured by MDCT. An aortic annulus diameter is first measured by TTE, with TEE being performed before the scheduled procedure only if an accurate measurement cannot be made with TTE (in our experience this occurs in about 10% of patients). Occasionally, TEE is required if the transthoracic echocardiographic images are of insufficient quality to describe aortic valve or root anatomy in sufficient detail. There is some debate as to the variability of annular measurement on TTE and TEE, with our experience being that annular measurements tend to be slightly larger on TEE than TTE, as reported by Moss et al. The discrepancy between annular measurements on TEE and MDCT is larger and more consistent. As current clinical experience and recommendations are based on echocardiographic annular measurements, annular measurements on MDCT are generally not used to determine patient suitability and prosthesis size in our institution. MDCT is nonetheless invaluable and is now performed in almost all cases before TAVI. MDCT can describe the shape and size of the sinuses of Valsalva (SoV), the height of the coronary ostia above the annular plane, the valve leaflets, and the sinotubular junction (STJ).
Accurate morphologic assessment of the aortic annulus, paying particular attention to eccentricity and the asymmetric distribution of calcium, may provide insights into the potential for paravalvular AR due to inadequate apposition of the fabric of the SAPIEN valve with the aortic annulus ( Figure 5 , Video 1 ). Special caution is warranted in assessing patients with small annular dimensions, and narrow and heavily calcified STJs, as these patients may be at higher risk for aortic root rupture. It is also thought that the presence of relatively flat SoV may predispose to aortic root rupture secondary to the effect of displaced calcium tearing the root ( Figure 6 , Video 2 ).
As the CoreValve differs fundamentally in design from the SAPIEN valve, being asymmetric and relatively long, the anatomic requirements for safe implantation are somewhat different and described in Tables 1 and 2 . Subaortic disease can affect implantation and must also be ruled out. The SAPIEN valve, by comparison, has a much lower profile and does not have SoV or STJ requirements but does require minimal distances between the aortic annulus and coronary ostia of >10 to 11 mm (depending on choice of prosthesis). In summary, a detailed knowledge of the morphology of the aortic valve and its associated structures, as well as the interaction between this anatomy and the particular device under consideration, is key to both the success of the procedure and to avoiding potentially catastrophic complications such as coronary ostial occlusion, device embolization and aortic rupture ( Figure 6 , Video 6 ).
| Diagnostic feature | Echocardiography | CT/MRI | Angiography | Selection criteria | ||||
|---|---|---|---|---|---|---|---|---|
| Left ventricle | Aortic root | Coronary | Vascular | Recommended | Not recommended | |||
| Atrial/ventricular thrombus | X | Not present | Present | |||||
| Subaortic stenosis | X | X | X | Not present | Present | |||
| LV ejection fraction | X | X | ≥20% | <20% without contractile reserve | ||||
| MR | X | ≤Grade 2 | >Grade 2 organic reason | |||||
| Vascular access diameter | X | X | X | ≥6 mm diameter | <6 mm diameter | |||
| Aortic and vascular disease | X | X | X | None to moderate | Severe vascular disease | |||
| Annular diameter | X | X | 20–23 mm for 26-mm valve; 24–27 mm for 29-mm valve | <20 or >23 mm for 26-mm valve; <24 or >27 mm for 29-mm valve | ||||
| Ascending aortic diameter | X | X | X | ≤40 mm for 26-mm valve; ≤43 mm for 29-mm valve | >40 mm for 26-mm valve; >43 mm for 29-mm valve | |||
| Diagnostic feature | Echocardiography | CT/MRI | Angiography | Selection criteria | ||||
|---|---|---|---|---|---|---|---|---|
| Left ventricle | Aortic root | Coronary | Vascular | Recommended | Moderate to high risk | |||
| LV hypertrophy | X | X | Normal to moderate (0.6–1.6 cm) | Severe (≥1.7 cm) | ||||
| CAD | X | X | None, mid, or distal (>70%) | Proximal lesions (>70%) | ||||
| Aortic arch angulation | X | X | Large radial turn | Sharp turn | ||||
| Aortic root angulation | X | X | <30° | 30°–45° | ||||
| Aortic and vascular disease | X | X | No or light vascular disease | Moderate vascular disease | ||||
| Vascular access diameter | X | X | >6 mm | Calcified and tortuous (<7 mm) | ||||
| SoV width | X | X | X | ≥27 mm for 26-mm valve; ≥29 mm for 29-mm valve | <27 mm for 26-mm valve; <29 mm for 29-mm valve | |||
| SoV height | X | X | X | ≥15 mm for both valves | <15 mm for both valves | |||
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