Artificial Heart Valves

History

"(a) First ball valve for animal implantation. (b) Sole surviving animal, surrounded by members of our team. (c) The shielded ball valve. (d) Autopsy specimen of a shielded ball valve showing a clot confined to the zone of implantation and covered by the Silastic shield." [A]

1952- First heart valve replacement using the caged-ball valve designed by a surgeon, Doctor Hufnagel. This valve did not replace the patient's diseased aortic valve but instead it was inserted in the descending aorta in an open-chest procedure.

1960- ball valves are introduced

1962- aortic homografts are introduced

1965- glutaraldehyde-preserved porcine 'bioprostheses'

1969- disc valves

1977- invention of bileaflet valves by St. John's Medical

Overview

Reasons for Valve Replacement

Damaged aortic valve [C]

Types of Artificial Heart Valves

Tissue/Bioprosthetic

Porcine heart valve preserved with glutaraldehyde [B]

Complications

With any surgery there is always a chance of complications. “Problems common to all heart valve replacement devices include small but persistent risks of endocarditis and paravalvular leak.”[1] Thromboembolism is also a common complication in heart valve replacements due to atrial fibrillation; homographs are an exception to this rule. Mitral valve replacements have the highest risk of thromboembolism. Artificial heart valves (like any foreign implantation in the body) inflict what is known as a neodisease[1] that can cause side effects such as rejection. “If valve replacement is successful and uncomplicated, most patients experience an improvement in their symptomatic state, and therefore in their quality of life.” [1] Mortality post implantation is very rare; in most cases death is due to patient-related factors. [1] The following complications have directed future research into the design of artificial heart valves. Some main focuses of current and future research include: engineering living tissue heart valves and further study related to the fluid mechanics of the heart to enhance the direction of future mechanical heart valves. [2]

Specific to Mechanical

"(a) The first animal implant: a bi-leaflet valve with a Dacron single-layer sewing ring. (b) A left atrial view of thrombotic occlusion two days after implantation in a dog. (c) A modified bi-leaflet valve with enhanced sewing ring and rearranged leaflets. (d) Thrombosis two days after implantation of the modified valve." [C]

Structural failure is a risk associated with all mechanical devices; some types of mechanical valves have an increased risk. Mechanical heart valves require anti-coagulants to prevent build-up of plaque around the device. These anti-coagulants pose the potential for bleeding complications. “Hemolysis is more common with some types of mechanical valves, but is usually subclinical in the absence of valvular malfunction”[1] “These complications are believed to be associated with non-physiological blood flow patterns”[2]. “Mechanical heart valve designs have evolved significantly, with the most recent designs providing relatively superior haemodynamics with very low aerodynamic resistance. However, high shearing of blood cells and platelets still pose significant design challenges and patients must undergo life-long anticoagulantion therapy” [2].

Specific to Tissue

Leaflet tearing is an issue for all tissue-based heart valves regardless of donor. [2] “Bioprosthetic or tissue valves do not require anticoagulants due to their distinct similarity to the native valve geometry and haemodynamics, but many of these valves fail structurally within the first 10-15 years of implantation.”[2] “Tissue valve disruption is virtually inevitable, provided the patient lives long enough” [1].

Transcatheter Heart Valves

Transcatheter heart valve replacement in the closed position. [D]

References

[1]Grunkemeir, Gary L. and Shahbudin H. Rahimtoola, “Artificial Heart Valves” Annual Review of Medicine 41 pp 251-63 (1990).
[2]Dasi, Lakshmi P. et al., “Fluid Mechanics of Artificial Heart Valves” Clinical and Experimental Pharmacology and Physiology 36 pp 225-237 (2009).