The PARTNER (Placement of Aortic Transcatheter Valves) trial was the first prospective, randomized controlled trial to investigate transcatheter heart valves in patients with severe, symptomatic AS. It consisted of two individually powered cohorts: PARTNER 1A and 1B. Cohort A compared TAVR to surgery among patients at high-surgical risk for operative mortality, while Cohort B compared TAVR to best medical management in patients with a prohibitive risk for surgery. PARTNER 1B showed that 1- and 5-year all-cause mortality rates were significantly lower in the TAVR arm compared with the conservative arm (31% vs 50% at 1 year and 72% vs 94% at 5 years, respectively; P < .001). Subsequently, PARTNER 1A demonstrated TAVR outcomes (all-cause mortality at 30 days, 1 and 5 years) to be noninferior to surgical AVR in patients at high risk for cardiac mortality.³

In the PARTNER 2A trial, patients considered to be at intermediate risk for surgical AVR were randomized to TAVR or surgical AVR. All-cause mortality or disabling stroke at 24 months was similar for both strategies: TAVR resulted in a significantly lower rate of death or disabling stroke in a prespecified analysis of the transfemoral access group, whereas similar outcomes to surgical AVR were seen in the transthoracic group. TAVR was associated with a lower rate of severe bleeding, acute kidney injury, and new-onset atrial fibrillation, although surgical AVR was associated with lower rates of paravalvular regurgitation and major vascular complications.⁴

Following publication of these results, the American Heart Association and American College of Cardiology (AHA/ACC) Valvular Heart Disease Focused Update recommended TAVR as an alternative to surgery for patients at intermediate surgical risk (Class of Recommendation IIA, Level of Evidence B).

The PARTNER 3 trial showed the superiority of transfemoral TAVR compared with surgical AVR for the primary composite endpoint consisting of death, stroke, or rehospitalization at 1 year in low-risk patients. Consequently, TAVR received US Food and Drug Administration (FDA) approval for low-risk patients in 2019.⁵

Structural deterioration of surgically implanted bioprosthetic valves is well established. Structural valve deterioration increases with time, particularly after the first 7 to 8 years following implantation and has an incidence of less than 1%, 10% to 30%, and 20% to 50% at 1, 10, and 15 years, respectively. Structural valve deterioration of bioprosthetic valves is strongly influenced by patient age at the time of implantation, with younger patients exhibiting accelerated valvular calcification and degeneration.⁶

As the criteria for TAVR are expanded, valve durability becomes an important consideration. In a study comparing TAVR and surgical AVR using a computational approach to assess leaflet fatigue, transcatheter valve leaflets sustained higher stresses, strains, and fatigue damage. Yet, mid-term outcomes appear encouraging.⁷ The 5-year follow-up of the PARTNER trial found no structural TAVR deterioration leading to hemodynamic compromise or valve replacement albeit only a minority of the study population had been followed for 5 years. The 5-year follow-up data from the Italian high-risk registry noted TAVR prosthesis failure in 1.4% and mild valve stenosis (mean gradient 20-40 mm Hg) in 2.8% although survivorship bias cannot be excluded.⁸,⁹ Long-term follow-up is necessary to assess TAVR durability.

In the SURTAVI (Surgical Replacement and Transcatheter Aortic-Valve Implantation) study, the rates of moderate or severe paravalvular regurgitation at 1 year were 5.3% in the TAVR group and 0.6% in the surgical AVR group.¹⁰ Moderate or severe paravalvular leaks were noted at 1 year in 6.8% of TAVR patients in the PARTNER A trial and only 1.5% of patients in the PARTNER S3i trial.¹⁰,¹¹ The decreased rate of paravalvular leaks is attributed to new device enhancements (low profile delivery system and an external skirt to prevent paravalvular regurgitation), improved case selection, increased operator experience, more precise valve positioning, and routine use of three dimensional (3D) CT reconstruction for accurate valve sizing.

The impact of paravalvular leak on mortality has yet to be fully elucidated. Assessing 2-year outcomes of the high-risk cohort in the PARTNER A trial showed even mild paravalvular leak was associated with increased mortality. On the other hand, among studies evaluating intermediate-risk groups, namely PARTNER 2A and PARTNER S3i, only moderate or severe paravalvular leak was associated with increased mortality. Patients with significant perivalvular leak should be evaluated for transcatheter occlusion device placement or surgical valve replacement.

Mechanical manipulation of the aorta and aortic valve with transcatheter device delivery and valve placement causes embolic debris to the brain in the majority of TAVR (and surgical AVR) patients. Rates of clinical stroke and transient ischemic attack were greater after TAVR than surgical AVR in the PARTNER A trial at 30 days (5.5% vs 2.4%, respectively;
P = .04) and at 1 year (8.3% vs 4.3%, respectively; P = .04).¹⁰ Conversely, in the CoreValve trial, the rates of stroke were similar with TAVR and surgical AVR at 30 days (4.9% vs 6.2%, respectively; P = .46) and 1 year (8.8 vs 12.6%, respectively; P = .10). In studies of intermediate-risk patients, similar rates of stroke were seen for TAVR and surgical AVR treatment.¹⁰ Although the incidence of cerebral infarctions following TAVR is high as assessed by diffusion-weighted magnetic resonance imaging (MRI), only a fraction of these lesions manifest significant clinical neurologic findings. The role of the routine uses of neuroprotection devices to mitigate the risk of stroke remains unclear at the present time.¹²

Two registries investigated the prevalence of subclinical leaflet thrombosis, valve hemodynamics, and clinical outcomes. Subclinical leaflet thrombosis occurred frequently in bioprosthetic aortic valves (12%), more commonly in transcatheter (13%) than in surgical valves (4%).¹³,¹⁴,¹⁵ Anticoagulation (both novel oral anticoagulants [NOACs] and warfarin), but not dual antiplatelet therapy, was effective in the prevention or treatment of subclinical leaflet thrombosis. Subclinical leaflet thrombosis was associated with increased rates of TIAs and strokes.

The true incidence of valve thrombosis, predisposing factors, clinical significance, the impact on long-term valve durability and rates of stroke, and the role of anticoagulation remain to be determined. Two ongoing pivotal trials of TAVR and surgical AVR include an FDA-mandated substudy of surveillance 4D CT in approximately one-third of the patients that will hopefully further inform the field.¹³ Additionally, four trials underway are evaluating post-TAVR anticoagulation strategies.¹⁶,¹⁷,¹⁸,¹⁹

Trials involving intermediate-risk patients show varying incidences of permanent pacemaker placement following TAVR compared with SAVR (PARTNER 2A 8.5% vs 6.7%; SURTAVI 25.9% vs 6.6%; PARTNER S3i 10.2% vs 7.3%, respectively). The self-expanding CoreValve has been associated with a higher need for permanent pacemaker placement because of the potential for deeper implantation into the left ventricular outflow tract with injury to the atrioventricular node or conduction bundles.¹¹ Early permanent pacemaker implantation is associated with a higher mortality and a higher composite of mortality and heart failure at 1 year. Advanced age, male gender, previous myocardial infarction, previous cardiac surgery, preexisting conduction abnormalities, and CoreValve utilization have all been identified as risk factors for new conduction defects following TAVR.