Dr Uday Khanolkar, Senior Consultant Cardiologist, Narayana Health City gives insights on the evolution of novel transcatheter therapies for cardiac care
Novel transcatheter therapies for valvular heart disease have developed tremendously over the past decade. These innovative interventional methods are largely modeled from established surgical heart valve procedures, which have started to evolve to less-invasive approaches. Until a decade ago, interventional valve procedures included only balloon pulmonary, aortic, or mitral valvuloplasty, serving highly selected patients. In 2002, percutaneous valve therapy advanced greatly with the first catheter-based aortic valve replacement (AVR) procedure. Since then more than 55000 high-risk patients have had percutaneous aortic valve replacement worldwide.
Symptomatic aortic stenosis (AS) carries a poor prognosis. Medically treated patients with symptomatic AS have a one and five-year survival of 60 per cent and 32 per cent, respectively. Conventional open heart surgery is the first-line therapy for symptomatic aortic stenosis. In the ideal candidate, surgical AVR has an estimated operative mortality of four per cent. However, the mortality rate associated with AVR increases substantially with increasing age, the presence of left ventricular dysfunction, or other co-morbidities. These factors are considered one of the main reason for which one-third of patients with valve disease are not referred for surgery. Percutaneous transcatheter aortic valve implantation (TAVI) opened the possibility of treating patients who until now had been left untreated because it was believed that their operative mortality outweighed the benefits offered by traditional AVR.
Percutaneous heart valves are stent-based xenografts that are collapsed onto a catheter and are expanded at the time of implantation. Procedure is performed in a catheterisation laboratory, usually by placing a sheath in the femoral artery. The prosthetic stent valve is mechanically crimped onto a balloon catheter immediately before implantation. With a steerable guiding catheter, the balloon-mounted valve is passed retrograde through the aorta and positioned within the native aortic annulus. Positioning is confirmed by fluoroscopy, aortography, and transesophageal echocardiographic imaging. The delivery balloon is then inflated to expand the valved stent, thereby excluding and compressing the native aortic valve. The transapical approach is the most recently developed form of transcatheter AVR. The procedure involves a small left lateral thoracotomy and is performed in a hybrid operative suite. It requires a direct puncture and sheath insertion into the left ventricle. Patients who require AVR but have a ‘porcelain aorta’ or have peripheral vascular disease should be considered for the transapical approach. None of the procedure requires cardiopulmonary bypass or a sternotomy, and the femoral approach may not require general anesthesia. Because they are delivered via a catheter, percutaneous heart valves have the potential advantage of lower perioperative morbidity; mortality and lesser hospital stay (three to four days) than valves implanted using conventional surgical approaches. The procedure is reserved for those people for whom an open heart procedure is too risky. For that reason, most people who have this procedure are in their 70s or 80s and often have other medical conditions that make them a better candidate for this type of surgery.
Although a variety of mitral valve (MV) transcatheter therapies grew in parallel with aortic valve therapies, the MV therapies have had a slower development path. Challenges arising from the complex anatomy of the MV and mitral apparatus and the interplay of the MV with the left ventricle (LV) contribute to the greater difficulty in conceiving of and evaluating mitral devices. A number of transcatheter MV therapies have been adapted from surgical techniques and are being applied in patients at high-risk for surgery as a result of coexisting comorbidities.
Among all catheter-based mitral therapies, the leaflet repair MitraClip system to date has the largest clinical experience worldwide, with established and reproducible safety profile and effective reduction of MR with amelioration of symptoms and improved quality of life in high-risk surgical patients. The MitraClip is a mechanical clip that permanently opposes the middle of the anterior and posterior mitral leaflets. The procedure is performed in the cardiac catheterisation laboratory in the beating heart, under general anesthesia, and with fluoroscopic and transesophageal echocardiographic guidance.
Globally, the MitraClip has been used in more than 10,000 patients to mechanically reduce MR without the incisions and cardiopulmonary bypass needed for MV surgery. Although initial MV trials included patients with low to medium surgical risk, current usage and evidence suggest that a primary role for the MV device may be to treat symptomatic MR in patients who are either unsuitable or at high risk for MV surgery. The development of percutaneous MV replacement devices is in its early stages. The challenges for mitral replacement are more complex than for the aortic valve, and it is clear that the development and testing of these devices will take more time than with transcatheter aortic valve replacement. Device delivery and anchoring and the large size and eccentric geometry of the mitral orifice are the main complexities.
It is important to emphasise that these novel percutaneous techniques are not meant to replace surgical techniques in low-risk patients who are good candidates for surgery. They can be an effective option to improve quality of life in patients who otherwise have limited choices for repair of their valve.
Coronary angioplasty with stenting has revolutionised the treatment of coronary artery disease. The coronary stents have substantially evolved since their first use in 1980s and there are on-going studies to refine their design, structure and material. Drug-eluting stents were a breakthrough in the development of stents, with their ability to significantly reduce restenosis rates and the need for repeat revascularisation. Nevertheless, they are still associated with subacute and late thrombosis mostly associated with the polymer and necessitate prolonged antiplatelet therapy for atleast 12 months.
Latest revolution in the field of interventional cardiology is the introduction of biodegradable stents, a stent that does its job and disappears. The rationale for a fully bioresorbable scaffold is to provide the vascular scaffold (similar to a stent) for a defined period after Angioplasty; but these scaffolds are then gradually resorbed, so that the vessel will be free of any caging and can regain its normal function. The absence of any residual foreign material and restoration of endothelial coverage would also reduce the risk of stent thrombosis and the requirement for long-term anti-platelet therapy. Bioabsorbable implant stents can be used as a delivery device for agents such as drugs and genes, and will perhaps play a role in the treatment of vulnerable plaque. Also, bioabsorbable stents are compatible with MRI and MSCT imaging. Finally, it can help eliminate the concerns that a minority of patients have at the thought of having ‘an implant in their bodies for the rest of their lives.’ A number of different materials ranging from magnesium to a variety of polymers have been used to construct stents of different designs. Results from various trials show that bioabsorbable stents are the future of coronary artery disease treatment