Introduction

Surgical aortic valve replacement has been the mainstay of management of severe aortic stenosis (AS) for decades (see Chapter 7 for a detailed description of this procedure). However, a high operative risk in a significant proportion of patients has precluded them from operative management. The mean survival of only two years for untreated symptomatic severe aortic stenosis (Horstkotte & Loogen 1988) has made it necessary to find less invasive methods of treating this condition.

Balloon aortic valvuloplasty, pioneered by Cribier et al. in the mid-1980s (Cribier et al. 1987), was associated with a significant mid-term improvement in quality of life (Letac et al. 1988). However, the recurrence rate of 80% at one year and lack of survival benefit (O’Neill 1991; ‘Percutaneous balloon aortic valvuloplasty. Acute and 30-day follow-up results in 674 patients from the National Heart, Lung, Blood Institute [NHLBI] Balloon Valvuloplasty Registry’ 1991) led to a substantial decline in its use. After a number of years of technological development, the first transcatheter heart valve was implanted by Cribier et al. in 2002 in an inoperable patient with severe AS in whom bicuspid aortic valve had failed (Cribier et al. 2002). The procedure, which necessitated the trans-septal approach due to severe iliofemoral disease, was a complete success.

Since that time, new devices for transcatheter aortic valve implantation (TAVI) have been developed, with notable advancements, including reductions in the profile of delivery systems, improved sealing around the valve, and greater ability to recapture and reposition the valve. Furthermore, a greater number of arterial access routes are being utilised, and they currently include transfemoral (approximately 70% of cases) (Biasco et al. 2018), transapical, trans-subclavian/transaxillary, transaortic, transbrachiocephalic, transcarotid, trans-septal and transcaval.

TAVI was initially performed in inoperable and high-surgical-risk patients with severe AS – >10% predicted 30-day operative mortality (Shroyer et al. 2003), based on the outcomes of the original PARTNER (Placement of Aortic Transcatheter Valves) trials (Leon et al. 2010). In inoperable patients, TAVI resulted in lower rates of death and rehospitalisation, as well as fewer symptoms, compared to standard therapy which included BAV, at the expense of vascular complications and a higher rate of strokes (Leon et al. 2010; Makkar et al. 2012).

Two trials were published in 2019 comparing TAVI to SAVR in low-surgical-risk patients with severe aortic stenosis (‘<4% predicted operative mortality at 30 days’): PARTNER 3 and Evolut Low Risk. PARTNER 3 continued the trend seen in the previous two trials by using a balloon-expandable valve. In PARTNER 3, TAVI using the Edwards Sapien 3 valve met non-inferiority as well as superiority criteria for the primary endpoint of a composite of death, stroke or rehospitalisation at one year (Mack et al. 2019). Evolut Low Risk, a trial designed to assess the safety and efficacy of transcatheter aortic valve replacement (TAVR), used the self-expanding CoreValve, Evolut R or Evolut PRO valves by Medtronic. TAVI met the non-inferiority endpoint of a composite of death or disabling stroke at 24 months (Popma et al. 2019). Notable in Evolut Low Risk, patients undergoing TAVI had higher rates of moderate or severe aortic regurgitation (3.5% vs 0.5%), and a high incidence of pacemaker implantation (17.4% vs 6.1%). Patients with bicuspid aortic valve disease were excluded from all these trials. It is likely that TAVI will in future expand into the low-surgical-risk population based on the results of these trials, and ultimately become first-line therapy for treatment of severe AS.

With the aging population, the prevalence of AS continues to increase, and there will be an increased demand for TAVI. It is predicted that in future TAVI will also become a standard of care for younger patients with severe aortic stenosis. However, at present there is a lack of robust data regarding TAVI valve longevity (Salaun et al. 2018). The development of TAVI has been a boon for the management of severe aortic stenosis, and it is one of the most rapidly evolving fields of cardiology.

Patient selection

Patients referred for consideration for TAVI are evaluated by physicians or surgeons who perform TAVI. At present the decision to offer surgical aortic valve replacements (SAVRs), TAVI or medical therapy, with or without bicuspid aortic valve is largely based on a quantitative and qualitative assessment of operative risk. The Society of Thoracic Surgery (STS) risk score (Shroyer et al. 2003) and EUROSCORE (Nashef et al. 1999) are the two most commonly used risk scores for predicting 30-day operative mortality. It is accepted that a number of factors that predict poor outcomes cannot be reliably measured, such as frailty (Xie et al. 2017) – a qualitative assessment of suitability for surgery is essential. Patients are discussed by a multidisciplinary heart valve team (HVT) and a consensus decision is made regarding the best management for each patient.

The multidisciplinary HVT typically consists of an interventional cardiologist, a cardiothoracic surgeon, an imaging cardiologist, a TAVI clinical coordinator (nurse), and often a radiologist. In most centres, patients must have severe aortic stenosis and have a predicted life expectancy of at least two years to be eligible for TAVI. Since the life expectancy of patients with symptomatic severe aortic stenosis is approximately two years, offering TAVI is not considered appropriate if other comorbidities will result in a life expectancy of less than this. Hence, one of the tasks of the multidisciplinary HVT is to liaise with other healthcare providers and coordinate further investigations if necessary, in order to provide this information. Furthermore, one of the radiologist’s most valuable roles is to identify extracardiac pathology on radiological investigations such as Computed Tomography (CT).

The severity of a patient’s aortic stenosis is not always clear, as measures of severity are commonly discordant. The imaging cardiologist plays a particularly valuable part by assessing stenosis severity and advising on further investigations if required. The cardiothoracic surgeon has numerous roles, including providing an expert opinion regarding eligibility for SAVR, providing surgical approaches for TAVI if percutaneous options are not suitable and in many institutions being a TAVI operator. The cardiothoracic surgeon has an exquisite knowledge of the structure and function of the aortic valve that is invaluable in cases of complex anatomy or challenging procedures.

The TAVI clinical coordinator processes TAVI referrals, coordinates clinical visits and investigations and has regular contact with patients. The interventional cardiologist is most commonly the TAVI team lead, having ultimate responsibility for patient management and the running of the TAVI service. As a physician, the interventional cardiologist has broad expertise in patient assessment and management and has extensive knowledge and skills in transcatheter cardiac procedures. With the TAVI clinical coordinator and cardiothoracic surgeon, they have direct contact with the referred patients and provide clinical management.

Patient eligibility

In addition to considering the severity of aortic stenosis, and a patient’s overall life expectancy, there are certain other considerations and contraindications that determine eligibility for TAVI.

Bicuspid aortic valve disease

Patients with bicuspid aortic valve disease (BAVD) have been excluded from most clinical trials of TAVI, so the technique’s efficacy has not yet been demonstrated in this population. Furthermore, this type of valve morphology presents technical challenges. BAVD is commonly associated with aortopathy, which renders the aortic root more fragile and prone to dissection upon valve expansion. The ascending aorta is often also dilated, which has a risk of aneurysm and dissection long-term, which is not addressed by TAVI.

Previous aortic valve replacement

Degeneration of surgical aortic valve replacements (SAVRs) may occur years after implantation, often at a time when patients are deemed high risk for a ‘redo’ operation. Degenerative SAVRs can stenose more rapidly than native valves, and risk factors for degenerative SAVRs include patient-prosthesis mismatch at the time of surgery (valve too small for the patient) and renal failure due to calciphylaxis. Valve-in-valve TAVI can be performed in previous TAVIs and in tissue surgical aortic valve repairs only and is dependent on both the geometry of the valve and the valve size. The new valve can be deployed within the old valve and then fully expanded, with the intention of displacing and possibly ‘cracking’ the scaffolding of the old valve. Certain tissue AVRs are not suitable for valve-in-valve TAVI. In the case of patient-prosthesis mismatch in the original prosthetic valve, the use of a supra-annular TAVI valve (such as those in the Medtronic range) allow a greater orifice area to be achieved.

Severe aortic regurgitation

TAVI can be performed in patients who have some degree of aortic stenosis and severe aortic regurgitation (AR). TAVI in pure AR can be performed, but valve embolisation is a significant risk, as the absence of valvular and annular calcification results in the lack of an anatomical ‘anchor’ for the valve. Patients with at least mild AS in combination with severe AR present a safer scenario, although this remains an ‘off-label’ indication. The aortic root is often dilated in AR, and in this case it may be difficult to obtain an adequate seal around the TAVI valve.

Vascular considerations

Previous aorto-femoral bypass or aortic endovascular repair essentially precludes the safe passage of a large-calibre delivery system transfemorally. The presence of an upper limb arteriovenous fistula generally precludes a subclavian approach on the ipsilateral side, due to a high risk of bleeding and compromise of the fistula. Previous left or right internal mammary artery bypass grafts also preclude a subclavian approach on the ipsilateral side due to risk of damage to the graft. Severe carotid disease significantly increases the risk of stroke with a transcarotid or transbrachiocephalic artery approach.

Functional and cognitive status

Patients with poorer baseline functional status are likely to have a slower physical recovery postoperatively (Dronkers et al. 2013; Van Beijsterveld et al. 2019). A longer hospital stay can naturally predispose to nosocomial conditions that further prolong hospital stay and cause further physical deconditioning. Similarly, patients with baseline cognitive impairment are at a high risk of postoperative delirium (Adogwa et al. 2018) and this in itself can worsen cognitive impairment in the long term (Sprung et al. 2017). These physical and cognitive factors must be considered and discussed with patients and family, and in many cases a poor baseline state will preclude TAVI.

Several investigations are routinely performed as part of assessment for TAVI, and these relate to the practical considerations of the procedure. Firstly, a quality transthoracic echocardiogram is mandatory, and provides structural and functional information.

Transthoracic echocardiography

Transthoracic echocardiography (TTE) assesses left ventricular size and function as well as evidence of left ventricular hypertrophy. A small left ventricular cavity size limits the option of a transapical approach because of limited space for valve release and deployment. Severe basal septal hypertrophy increases the risk of vertical displacement of the TAVI valve, which can be mitigated by a slow valve release/expansion to increase friction. Severe left ventricular systolic impairment increases the risks associated with general anaesthesia. The aortic valve morphology can often be determined on TTE, although it may be obscured by extensive calcification.

Transoesophageal echocardiography

Coronary assessment

The presenting symptoms of aortic stenosis can be quite similar to those of ischaemic heart disease. As part of the workup for TAVI, it is necessary to determine whether significant coronary disease is present. This is commonly in the form of invasive coronary angiography but could also be performed with CT coronary angiography or stress imaging. If severe disease is identified, revascularisation is generally performed prior to TAVI for several reasons. Firstly, severe coronary disease predisposes to myocardial ischaemia during rapid pacing that is often performed during valve deployment. Secondly, severe coronary disease increases the overall procedural risk, especially in the case of general anaesthesia. Thirdly, coronary catheterisation after TAVI is technically challenging, especially when the TAVI valve’s stent frame overlies the coronary ostia. Fourthly, there is a risk of plaque shift into the ostia of coronary arteries during TAVI valve insertion, so this needs to be considered if ostial coronary plaque is already present.

12-lead electrocardiography

The important aspects of the 12-lead ECG for TAVI workup include the heart rate, rhythm and the presence of conduction disease. The ideal heart rate for a gated MDCT scan is 60 beats per minute or less. Heart rate control with beta-blockers and/or ivabradine is commonly commenced prior to MDCT. In addition, the artefact can be introduced in the presence of an irregular rhythm. Atrial fibrillation, frequent atrial and ventricular ectopic beats, and sinus arrhythmia are common causes of an irregular heart rhythm encountered on a routine ECG.

Patient preoperative preparation

Patients who are scheduled for TAVI are fasted for at least 6 hours prior to the procedure. They then have a set of routine blood tests, including a complete blood examination, electrolytes, renal function, and coagulation studies. As bleeding is a common and significant risk of TAVI, it is important to ensure that the patient is not significantly anaemic and that any conditions that predispose to bleeding (such as thrombocytopenia or coagulopathy) are adequately addressed preoperatively.

A history of recent gastrointestinal bleeding must be thoroughly investigated as it may represent angiodysplasia (Heyde’s syndrome, secondary to severe aortic stenosis) which is likely to resolve after TAVI, or a condition that will likely worsen after commencement of mandatory anti-thrombotic agents after TAVI, such as a bleeding malignancy. Anticoagulants are ceased preoperatively, although antiplatelet agents are typically continued.

In patients with renal impairment, rehydration with slow intravenous saline is routinely given. Patients must have their blood typed and cross-matched in advance of the procedure, to prepare for the possibility of needing a blood transfusion.

The areas of the body being operated on must be cleaned and shaved prior to the procedure. The patient must fully understand the risks and benefits of the procedure, including the risks of non-interventional management, before consenting.

TAVI valve systems

Below are brief descriptions of the current commercially available transcatheter heart valves used to treat severe aortic stenosis.

Edwards Sapien 3 valve (Edwards Lifesciences)