Aortic stenosis (AS) is currently the most frequent native valve disease in Europe and is most often seen in elderly patients with co-morbidities.1 Valve replacement is the definitive therapy for patients with severe AS who have symptoms or objective consequences such as left ventricular (LV) dysfunction.2,3 However, the risk of surgery may be higher in elderly patients with significant co-morbidities.4,5 In addition, several registries show that referring physicians often do not propose surgery, as was the case in the Euro Heart Survey with 33% of patients with severe valve disease and severe symptoms not being considered for surgery.6 Thus, despite the good results of valve surgery, there may well be a role for less invasive alternatives. Balloon aortic valvuloplasty (BAV) is now rarely used, mainly due to its limited long-term efficacy.2,3 This article will address the potential indications for TAVI and the various steps of the procedure, and will summarise the results that are currently available.
Potential Indications for Transcatheter Aortic Valve Implantation
The selection of candidates for TAVI, and especially risk assessment, should involve multidisciplinary consultation between cardiologists, surgeons, imaging specialists and anaesthesiologists, all having experience in the management of valve disease.8 TAVI should be performed only in severe AS. Echocardiography is the preferred tool for assessing the severity of AS according to a combination of measurements of the valve area and flow-dependent indices. In patients with a low LV ejection fraction and low-gradient, low-dose dobutamine, echocardiography is useful to differentiate between severe and the rare ‘pseudo-severe’ AS.2,3
At present, TAVI should be proposed only in patients with severe symptoms that can definitely be attributed to valve disease. This may be difficult in the elderly. TAVI should currently be restricted to patients at high risk or with contraindications to surgery. The evaluation of the risk of surgery is based on the assessment of cardiac and extra-cardiac factors.9,10 Risk scores, such as the EuroScore9 and the STS Predicted Risk of Mortality score,10 are of interest. However, they share similar limitations: predictive ability is reduced in high-risk patients, who represent only a small proportion of the population from which the scores were elaborated, and highrisk patients form a particularly heterogeneous group in which it is difficult to capture all of the co-morbidities.11 The key element to establishing whether patients are at high risk for surgery is clinical judgement, which should be used in association with a more quantitative assessment based on the combination of several scores (for example, expected mortality >20% with the Logistic EuroScore and >10% with the STS score). This approach allows us to take into account risk factors that are not covered in scores but are often seen in practice, such as chest radiation, previous aorto-coronary bypass with patent grafts, porcelain aorta and liver cirrhosis, etc.
TAVI should not be performed in patients whose life expectancy, related to age and the presence of co-morbidity, is less than one year; these patients should instead be managed conservatively. In addition to clinical evaluation, semi-quantitative scoring systems such as those used in geriatrics may be helpful.12
When TAVI is envisaged, the following steps should be taken to assess its feasibility:
- Coronary angiography should be performed. If associated coronary artery disease requires revascularisation, the question of whether to proceed, as well as the chronology of interventions, should be the subject of individualised discussion based on the patient’s clinical condition and anatomy. If necessary, due to severe proximal stenosis, percutaneous coronary intervention (PCI) is ideally performed a few weeks before TAVI. The position of the coronary arteries relative to the aortic cups can be assessed using aortography or multislice computed tomography (CT).13
- Correct sizing of the valve is crucial to minimise the potential for paravalvular leakage and to avoid prosthesis migration after placement. Transoesophageal echocardiography (TEE) has been found to show larger values than transthoracic echocardiography; thus, it should be performed if borderline values lead to doubt regarding the feasibility of the procedure.14 Multislice CT13 is also increasingly used for this purpose. A ‘gold standard’ method of measurement has yet to be established.
- Echocardiography is the preferred tool for assessing the morphology of the LV outflow tract as well as the other valves and, before implanting self-expandable devices, the dimensions of the aortic root.
- Multislice CT using different views is the most accurate method for the evaluation of the size, tortuosity and calcification of peripheral arteries.15
Some contraindications are general, while others are approach- or device-specific. General contraindications for TAVI are:
- an aortic annulus of <18mm or >25mm for balloon-expandable and <20mm or >27mm for self-expandable devices;
- bicuspid valves due to the risk of incomplete deployment of the prosthesis;16
- the presence of asymmetrical heavy valvular calcification, which may compress the coronary arteries during TAVI; the bulk and distribution of calcification in the valve may be assessed by fluoroscopy and multislice CT. Finally, the risk of coronary compression can be anticipated during BAV;
- an aortic root dimension >45mm at the aorto-tubular junction for self-expandable prostheses;
- low position of the coronary ostia (<10mm from the aortic annulus);
- severe organic mitral regurgitation; and
- apical LV thrombus.
The specific contraindications for the transfemoral approach are:
- Iliac arteries: severe calcification, tortuosity, small diameter (<6–9mm according to the device used); previous aorto-femoral bypass.
- Aorta: Severe angulation, severe atheroma of the arch, coarctation; aneurysm of the abdominal aorta with protruding mural thrombus.
- The presence of bulky atherosclerosis of the ascending aorta and arch detected by TEE.
Contraindications for the transapical approach are rare:
- previous surgery of the LV using a patch, such as the Dor procedure;
- calcified pericardium;
- severe respiratory insufficiency; and
- non-reachable LV apex.
Performance of Transcatheter Aortic Valve Implantation
The performance of TAVI should be restricted to high-volume centresthat have both cardiology and cardiac surgery departments with expertise in structural heart disease intervention.8 The procedure requires the close co-operation of a team of specialists in valve disease including clinical cardiologists, echocardiographists, interventional cardiologists, cardiac surgeons and anaesthesiologists. The optimal environment for TAVI should be spacious and sterile, featuring high-quality imaging equipment. A hybrid suite is ideal as it fulfils the role of both an operating room and a catheterisation laboratory.
Two devices are under clinical investigation for use in TAVI (see Figure 1). One device is the Edwards-Sapien valve (Edwards Lifesciences Inc, CA, US), which consists of three pericardial leaflets – initially equine and currently bovine – mounted within a tubular, slotted, stainless-steel balloon-expandable stent. It is currently available in 23 and 26mm sizes, necessitating, for the transfemoral approach, 22 and 24Fr introducer sheaths, respectively, and 26Fr for the transapical approach. The other device is the CoreValve Revalving System (CRS™, CoreValve Inc, Irvine, CA, US), which has three pericardial leaflets – initially bovine and currently porcine – mounted in a self-expanding nitinol frame. It is available in 26 and 29mm sizes, which go through an 18Fr introducer for transfemoral use.
TAVI is currently carried out using two different approaches (retrograde transfemoral and antegrade transapical), which share the same main principles Most teams perform the procedure under general anaesthesia, although sedation and analgesia may suffice for the transfemoral approach. After crossing the aortic valve, BAV is performed to pre-dilate the native valve. Simultaneous rapid pacing decreases cardiac output, stabilising the balloon during inflation. The following imaging methods can be used to position the prosthesis at the aortic valve: fluoroscopy, to assess the level of valve calcification; aortography, to determine the position of the valve and the plane of alignment of the aortic cusps; and echocardiography – TEE is helpful, in particular in cases with moderate calcification. When the positioning is considered correct, the prosthesis is released.
Rapid pacing is used at this stage in the balloon-expandable but not the self-expanding devices. Immediately after TAVI (see Figures 2 and 3), aortography and, whenever available, TEE (see Figure 4) are performed to assess the location and degree of aortic regurgitation and the patency of the coronary arteries and to rule out complications. The haemodynamic results are assessed using pressure recordings and/or echocardiography. After the procedure, the patient should stay in intensive care for at least 24 hours and be closely monitored for several days ,especially regarding hemodynamics, vascular access, rhythm disturbances (especially late atrioventricular block) and renal function.
The specific issues related to the different approaches are as follows:
- For the transfemoral approach, close attention should be paid to vascular access. The common femoral artery can either be prepared surgically or approached percutaneously. The manipulation of the introductory sheaths should be careful and fluoroscopically guided. The closure of the vascular access can be affected surgically or percutaneously.
- The transapical approach requires an antero-lateral minithoracotomy, pericardiotomy, identification of the apex and then puncture of the LV using a needle through purse-string sutures. Subsequently, an introductory sheath is positioned in the LV and the prosthesis is implanted using the antegrade route.
Preliminary cases using the axillary/subclavian approach have been reported.17
Since the first-in-man TAVI by Alain Cribier in 2002, over 8,000 high-risk patients with severe symptomatic AS have been treated using this technique.18–24 The number of peer-reviewed publications remains limited and, besides a few large single-centre series reporting series of around 100 patients, most of the studies come from industry-driven registries with all their inherent limitations. In addition, these reports often mix the results obtained with different generations of devices and at different levels of expertise. Finally, the definitions of success and complications may vary.
Over 6,000 cases have been performed using either the balloon-expandable or the self-expandable prosthesis.18–21 The patients treated were mostly over 80 years of age and at high risk. The procedural successrate – the correct positioning of the prosthesis – is currently over 90%. Valve function is good, with a final valve area ranging from 1.5 to 1.8cm2.
Mortality at 30 days ranges from 5 to 18%. Acute myocardial infarction occurs in 2–11%. Mild to moderate aortic regurgitation, which is mostly paravalvular, is observed in around 50% of cases, but severe AR in less than 5%. Prosthesis embolisation or malpositioning occurs in 1–3% of cases. Coronary occlusion is also rare (<1%). Vascular complications range from 11 to 15% with the balloon-expandable device and are 2–4% with the self-expandable device, which has a lower profile. The incidence of stroke ranges from 3 to 9%. Finally, atrioventricular block, necessitating pacemaker implantation, occurs in 4–8% with the balloon-expandable device,25 and in up to 30% with self-expandable devices.26 Long-term results up to six years (though only one to three years in most studies) are reported in a limited number of patients. They show a survival rate of 70–80% at one year, with a significant improvement in clinical condition in most cases. The majority of late deaths are due to co-morbidities.
The majority of the survivors experience significant functional improvement. Serial echocardiographic studies have consistently shown good prosthetic valve function with no structural deterioration of valve tissue.
Reported experience with transapical aortic valve implantation relates only to the balloon-expandable prosthesis,22–24 and comprises over 2,000 patients at even higher risk for conventional surgery than the patients treated via the transfemoral approach. The implantation success rate of the transapical procedure is also over 90%. Over 90% of cases are performed off-pump. Mortality rates range from 9 to 18%. The incidence of stroke is 0–6%. At mid-term, the one-year survival rate ranges from 50 to 70%, with most of the late deaths being due to co-morbidities. All reports consistently showed a ‘learning curve’ effect on both the success rate and the incidence and severity of complications. It should be noted that there are currently no direct comparative studies available comparing either one device over the other or one approach over the other. Finally, case reports have shown the feasibility of valve-in-a-valve implantation, either for the acute failure of TAVI or for degenerated valve prosthesis (see Figure 5). However, the data are very limited and do not allow for comprehensive analysis.27,28
The technique remains challenging, in particular with regard to vascular access for the transfemoral approach, device sizing and positioning. The major concerns regarding safety are:
- vascular complications with the transfemoral approach, which should decrease with smaller devices;
- long-term consequences of paravalvular leaks, even if mild to moderate regurgitation is not considered to have significant clinical consequences in the short term;
- atrioventricular block, the incidence, timing and predictors of which have to be identified more precisely – this is especially true when using self-expandable devices;
- mid-term clinical outcomes are encouraging, however, the long-term durability of these bioprostheses, their mode of failure, as well as the incidence of endocarditis or thromboembolic events, remains largely unknown; and
- the data on the feasibility of subsequent aortic valve intervention are very limited.29
A randomised trial – PARTNER US – comparing TAVI with aortic valve replacement or medical therapy according to the clinical status of the patients, is currently ongoing, where the primary end-point is all-cause mortality at one year. As well as enrolment in randomised controlled trials, data should be accumulated in registries, which will provide important data on safety and durability. Progress in delivery systems and valve manufacturing will lead to lower-profile (down to 18Fr) introducers as well as a wider range of prosthetic valve dimensions and improvements in device manufacturing, aimed at better durability. First-in-man cases have been performed with new devices that are repositionable and retrievable. Furthermore, improved imaging, such as online 3D reconstruction and stereo-taxis, could facilitate valve placement.
The currently available results of TAVI suggest that these techniques are feasible and provide haemodynamic and clinical improvement for up to three years in patients with severe symptomatic AS at high risk or with contraindications to surgery. Pending questions mainly concern safety and long-term durability. Surgeons and cardiologists must work as a team at all stages of the procedure. Today these techniques are targeted at high-risk patients, but they may be extended to lower-risk groups in the future if their initial promise holds true after careful evaluation.