First performed in humans seven years ago, transcatheter aortic valve implantation (TAVI) for the treatment of aortic stenosis (AS) is a dynamic field of research and development: two devices have been CE marked and are commercialised.
As a general principle, the selection of candidates for TAVI and the performance of the procedure should involve multi-disciplinary teams including cardiologists, surgeons, imaging specialists and anaesthesiologists, all with experience in the management of valve disease.
I - Selection of candidates
TAVI should only be performed in patients with severe symptoms that can definitely be attributed to severe AS, and be restricted, for the time being, to patients at high-risk or with contraindications for surgery.
The key element to establishing the risk for surgery is clinical judgement, in association with a more quantitative assessment, based on the combination of several scores (e.g, expected mortality > 20% with the Logistic EuroSCORE and > 10% with STS score). This approach allows the frequent risk factors such as chest radiation, previous aorto-coronary bypass with patent grafts, porcelain aorta, and liver cirrhosis, not covered by scores to be taken into account.
TAVI should not be performed in patients whose life expectancy is less than 1 year, these patients are instead to be managed conservatively.
When TAVI is considered, the following steps should be taken to assess feasibility :
- Coronary angiography should be performed, and if associated coronary artery disease (CAD) 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.
- Correct sizing of the valve is crucial to minimise the potential for paravalvular leakage and to avoid prosthesis migration after placement. A “gold standard” method of measurement has yet to be established. Transthoracic echocardiography is the most popular method and transoesophageal echocardiography is needed in borderline cases. Multislice computed tomography (MSCT) is also increasingly used.
- Echocardiography is the preferred tool for assessing the morphology of the LV outflow tract and, before implanting self-expandable devices, the dimensions of the aortic root, as well as the other valves.
- MSCT using different views is the most accurate method to evaluate the size, tortuosity, and calcification of peripheral arteries.
II - General contraindications for TAVI are :
- Aortic annulus < 18 mm or > 25 mm for balloon expandable and < 20 or > 27 for self expandable devices.
- Bicuspid valves because of the risk of incomplete deployment of the prosthesis.
- The presence of asymmetric heavy valvular calcification, which may compress the coronary arteries during TAVI.
- Aortic root dimension > 45 mm at the sino-tubular junction for self-expandable prostheses.
- Low position of coronary ostia (<8 mm from the aortic annulus).
- Severe organic mitral regurgitation.
- Apical LV thrombus.
There are specific complications for the transfemoral approach:
- Iliac arteries: Severe calcification, tortuosity, small diameter (<6 to 9mm depending on 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: severe respiratory insufficiency, chest deformity, or previous surgery of the LV using a patch.
III - Performance of TAVI
The performance of TAVI should be restricted to high volume centres which have both cardiology and cardiac surgery departments, and expertise in structural heart disease intervention.
IV - Devices
Two devices are under clinical investigation. One is the Edwards-Sapien balloon-expandable transcatheter heart valve, for the transfemoral and the transapical approaches. The other is the Medtronic CoreValve System which has a self-expanding, nitinol frame for transfemoral and, more recently, transaxillary use.
V - Implantation techniques
Two different approaches are currently used: In the transfemoral approach, the common femoral artery can either be prepared surgically or approached percutaneously. The transapical approach requires an antero-lateral mini-thoracotomy. Preliminary cases using the axillary approach have been reported. Both approaches 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, balloon valvuloplasty is performed to pre-dilate the native valve.
The position of the prosthesis at the aortic valve can be assessed by aortography and echocardiography. When positioning is considered correct, the prosthesis is released, under rapid pacing with the balloon expandable, but not with self-expanding devices.
Immediately after TAVI, aortography and, whenever available, TEE, are performed to assess the location and degree of aortic regurgitation, the patency of the coronary arteries, and to rule out complications.
After the procedure, patients should stay in intensive care for at least 24 hours and be closely monitored for several days.
VI - Results
Since the first in-man TAVI by Alain Cribier in 2002, over 8000 high-risk patients with severe symptomatic AS have been treated using this technique. Two thirds were performed using the transfemoral approach. Patients were mostly over 80 years old, and at high risk. The procedural success, i.e. correct positioning of the prosthesis, is currently over 90% in experienced centres. Valve function is good with a final valve area ranging from 1.5 to 1.8cm². Thirty-day mortality ranges from 5 to 18%. Acute myocardial infarction occurs in 2 to 11%. Mild to moderate aortic regurgitation, mostly paravalvular, is observed in around 50% whilst severe aortic regurgitation is seen in <5%. Prosthesis embolisation or malpositioning occurs in 1 to 3% of cases, and coronary occlusion in <1%. Stroke ranges from 2 to 9%. In the transfemoral approach, vascular complications range from 11 to 15% with the balloon expandable device, decreasing to 2-4% with the self expandable ones, which have a lower profile. Finally, atrioventricular block necessitating pacemaker implantation occurs in 4 to 8% with the balloon expandable device and in up to 30 % with self-expandable devices.
All reports consistently showed a learning curve effect both on the success rate and the incidence and severity of complications. Long-term results, up to 6 years (though only 1 to 3 years in most studies), are reported in a limited number of patients. The survival rate is approximately 70% at 2 years with a significant improvement in clinical condition in most cases. The majority of late deaths are due to comorbidities. No case of prosthetic dysfunction has been reported so far.
There are currently no studies available comparing either one device against the other or one approach against the other.
Case reports have shown the feasibility of a valve in valve implantation, for either acute failure of TAVI or for degenerated valve prostheses.
VII - Perspectives
As well as enrolment in randomised controlled trials, data should be accumulated in registries to assess safety and durability.
Progress in delivery systems and valve manufacturing has lead to 18F introducers as well as a wider range of prosthetic valve dimensions and improvements in device manufacturing to produce better durability. Furthermore, improved imaging could facilitate valve placement.
Fig. 1 - Aortic annulus measurement
Top panel: transthoracic echocardiography
Bottom panel: Multislice computer tomography
Fig.2 - Prosthesis and currently used approaches for transcatheter aortic valve implantation.
Fig.3 - Prosthesis in place
Left : Edwards Sapien balloon expandable prosthesis.
Right : Medtronic CoreValve prosthesis.
In both cases a pace maker lead is positioned in the right ventricle and a transoesophageal probe is in place.
Fig. 4 – 3D transoesophageal view of the placement of the prosthesis at the level of the aortic valve. The arrows show the upper and lower edge of the prosthesis
Fig.5 - 3D transoesophageal view of the prosthesis
Left panel: diastole; Right panel: systole
Current results suggest that TAVI is feasible and provides haemodynamic and clinical improvement for up to three years in patients with severe symptomatic AS at high risk or with contraindications for surgery. Pending questions mainly concern safety and long-term durability. Surgeons and cardiologists must work as a team to select the best candidates, perform the procedure, and, finally, evaluate the results. Today these techniques are targeted at high-risk patients but they may be extended to lower risk groups in the future, if the initial promise holds true after careful evaluation.