Transcatheter aortic valve implantation: indications

Introduction

After several years of animal studies, the first in vivo transluminal aortic valve implantation (TAVI) was performed by Alain Cribier in Rouen in 2002 [1]. The procedure is feasible and safe. The success rate is high. The possible procedure-related complications are stroke, need for a new pacemaker (more frequently for the self-expanding system than for the balloon-expandable system) and paravalvular regurgitation which, when moderate to severe, is associated with a higher mortality rate [2]. Currently, TAVI is indicated in patients considered not suitable for aortic valve replacement. Ongoing studies will evaluate whether the procedure is indicated in patients with intermediate risk. Guideline updates require appropriately designed and executed studies providing significant results.

Confirmation of severe aortic stenosis

AS is considered to be severe when peak aortic velocity is >4 m/sec or mean pressure gradient is ≥40 mmHg [3] . Transvalvular pressure gradients are flow-dependent. A valve area <1.0 cm² is considered severe and should be indexed to body surface area (cut-off value <0.6 cm²/m²) in patients with a small body surface area. In patients with low-flow, severe AS can be present with lower velocities and lower valve gradients. In the presence of symptomatic severe low-flow/low-gradient and reduced left ventricular ejection fraction (LVEF), dobutamine echocardiography should be performed at low to moderate doses (up to 20 µg/kg/min). An aortic valve area ≤1.0 cm² with peak velocity ≥4 m/sec at any flow rate confirms severe AS. This implies the presence of contractile reserve, defined as a >20% increase in stroke volume [4] . In the absence of contractile or flow reserve, the severity of AS is more difficult to ascertain [5] . If dobutamine induces an increase in valve area with no or minimal increase in mean gradient, the patient has pseudo-severe AS and should not be submitted to an intervention for valve disease. Patients with low-gradient and normal LVEF called paradoxical low-flow (stroke volume index <35 ml/m²), low-gradient severe AS have a guarded prognosis in the absence of surgical aortic valve replacement [6] . Severity is confirmed by an increased LV relative wall thickness, small LV dimensions, high BNP values and severe calcifications of the valve, easily quantitated with computed tomography. It is important to remember that the aortic valve calcium score suggesting severe AS is different in men and women, >2,065 AV and >1,275 AV, respectively.

The major pitfall in the assessment of AS severity is an underestimation of stroke volume and thus aortic valve area by the continuity equation which implies a circular LV outflow tract. LV outflow tract is, however, frequently oval and the minor dimension is measured by echocardiography. An underestimation of the diameter is squared in the continuity equation. Other imaging studies may be useful, such as three-dimensional echocardiography, computed tomography or cardiac magnetic resonance imaging.

Risk assessment

Risk assessment should combine risk estimates, procedure impediment, major organ dysfunction and frailty. An STS score >8-10% or a EuroSCORE II >15-20% indicates high risk. However, the EuroSCORE has been shown to overestimate the risk of aortic valve replacement but it is still the scoring system of choice according to the ESC/EACTS guidelines of 2012 [7] . The parameters used to calculate and predict risk are indicated in Table 1. However, none of the risk scores was designed for candidates of TAVI. Current surgical risks correlate poorly with the observed outcome of patients undergoing TAVI, even for the transapical approach [8] . Other models are available, such as the university health system consortium (UHC) which demonstrates better performance for postoperative complications after surgery but should not be used for TAVI indications [9].

Table 1. Patient-related factors in risk scores.

Procedure impediment may represent an indication for TAVI even if the risk scores are not very high - heavily calcified ascending aorta, porcelain aorta, mediastinal radiation damage or coronary bypass grafts [10] . Major organ compromise includes severe cardiac dysfunction, severe chronic obstructive pulmonary disease with forced expiratory volume <1 L/s, renal failure ≥ stage 3, cancer, liver cirrhosis and cerebral dysfunction.

Several frailty indices are available to determine whether frailty is moderate to severe.

Frailty is not captured by the STS or EuroSCORE. Frailty indices predict surgical outcome [11], weight loss ≥10 pounds in the last year, decreased grip strength, exhaustion, low activity (weekly kilocaloric expenditure in the lowest 20th percentile for their gender [men <383 kcal/week; women <270 kcal/week]), slowed walking speed (walking speed <20th percentile, adjusted for gender and height [men: height and walk time ≤173 cm and ≥7 seconds or >173 cm and ≥6 seconds; women: ≤159 cm and ≥7 seconds or >159 cm and ≥6 seconds, respectively]). Other parameters can also be used: grip strength, standing balance, serum albumin [12] . Gait speed and STS score together predict outcome after surgery [13] .

TAVI has been shown to be superior to medical treatment in patients not suitable for surgery [14] and not inferior to surgical aortic valve replacement in high-risk patients [15] .

The heart team and heart valve clinic

The management of elderly patients with severe symptomatic AS should be decided by a Heart Team, including a cardiologist, a valve interventionalist, a cardiac surgeon, an imaging specialist, anesthesiologists, geriatricians and intensive care specialists. The patient, the family and the patient’s physician should be clearly educated by the Heart Team when several options for treatment are possible [7,10] .

Heart valve clinics and centers of excellence have developed. In the heart valve clinic the patient is evaluated by a cardiologist who is an expert in valvular heart diseases and who can select the appropriate tests required for the individual patient, e.g., imaging techniques, biological tests including renal function, coagulation status, natriuretic peptides. The patient is educated; follow-up and preoperative exams are organized [16] . The Heart Team integrates all information for an appropriate proposal.

Indications for TAVI

The recommendations produced by European and American guidelines are rather similar [7,10] . Both sets of guidelines indicate that a decision should be made by the multidisciplinary Heart Team. TAVI should only be performed in hospitals with cardiac surgery on-site. The European and American recommendations are indicated in Table 2.

Table 2. Indications for TAVI according to the ESC/EACTS and ACC/AHA guidelines.

ACC: American College of Cardiology; AHA: American Heart Association; AS: aortic stenosis; EACTS: European Association for Cardio-Thoracic Surgery; ESC: European Society of Cardiology; TAVI: transcatheter aortic valve implantation; TAVR: transcatheter aortic valve replacement

Adapted from Vahanian et al (Eur Heart J 2012) and Nishimura et al (JACC 2014).

Contraindications for TAVI

The European guidelines indicate that absolute and relative contraindications, both clinical and anatomical, should be identified [7] .

Absolute contraindications

Absolute contraindications include the absence of a Heart Team and no cardiac surgery on-site, appropriateness of TAVI not confirmed by the Heart Team, estimated life expectancy <1 year, comorbidity suggesting lack of improvement of quality of life, inadequate annulus size (<18 mm, >29 mm), active endocarditis, symmetric valve calcification, short distance between the annulus and the coronary ostium, and plaques with mobile thrombi in the ascending aorta.

Relative contraindications

Relative contraindications include inadequate vascular access for transfemoral or subclavian approach (such patients could be treated from the transapical approach), bicuspid valve (no longer applicable), haemodynamic instability, and severe LV dysfunction.

TAVI in intermediate-risk patients

Current guidelines do not yet recommend TAVI in intermediate-risk patients. Randomized studies are ongoing: the SURTAVI trial, PARTNER II trial (intermediate-risk cohort). The BERMUDA trial tested propensity-matched cohorts: the 1-year mortality was similar in patients submitted to TAVI (16.5%) or aortic valve replacement (16.9%) (hazard ratio with 95% confidence interval: 0.9 [0.57-1.42]; p=0.64) [17] . An important finding was that, during the course of the study, the matched STS fell from 6% to 4.3%. The Bern low-intermediate risk study showed a low 1-year mortality in intermediate (3-8) STS patients (16.1%), whereas the low STS (<3) patients had a 10.1% mortality and the high STS (>8) patients had a 34.5% mortality (low vs. high: relative risk=0.27 [0.09-0.77]; intermediate vs. high: relative risk=0.41 [0.24-0.67], p<0.001 for both) [18] .

Conclusions

The European guidelines will be updated. The results of ongoing studies could potentially widen the indications for TAVI to include patients with intermediate risk. However, many centers have already adopted TAVI in patients at intermediate risk in daily practice. Circumspection is needed with respect to terminal, irreversible diseases.

This article is supported in the form of an unrestricted educational grant by Edwards Lifesciences.