Artificial heart valves
As recently as 1950, little could be done for serious defects of the heart valves, caused by congenital malformation, infection or degenerative diseases of the cardiovascular system. Symptoms included breathlessness, dizziness, and fainting, and deaths from congestive heart failure or stroke were not uncommon.
The development of the heart lung machine and safe ways to stop and restart the heart raised the possibility of replacing defective heart valves. From the 1950s onwards there were many attempts to build artificial valves, that mimicked the anatomy of heart valves, from artificial materials. A team at the University of Minnesotaestablished the structure of heart valves taken from cattle and human cadavers. While the valves appear very delicate, the collagen in their structure makes them enormously strong.
Attempts were made to manufacture valves structured in much the same way using various polymers. Testing these valves in dogs, it was soon discovered that copying the natural design was not the answer. It was difficult to get the anchoring mechanism right, and the lack of strength was a major problem. Blood clotting around sutures was a problem with early designs.
For durability, more robust materials and designs were clearly required. Woven materials such as knitted Teflon increased post-operative survival, with some dogs living for several weeks. This in turn led to the possibility of studying the problem of clotting, which is inevitably caused by foreign materials or damaged surfaces in the heart. It was established that the valve design should not have any nooks or crannies where blood could stagnate and tend to clot. Even when this problem is avoided, long-term anticoagulant drugs are necessary.
Stronger designs were still needed if these valves were to be used in patients. In the late 1950s/early 1960s, Albert Starr
Obtaining long term dog survivors without anticoagulant treatment encouraged Starr to try replacing mitral valves in patients. By October 1961, of the 12 patients who had received artificial mitral valves, two had died from unrelated causes, and three from infections. The remaining patients were well and two had returned to work.
The Starr-Edwards caged-ball valve is still in use. Its success encouraged development and testing, in calves and dogs, of further designs, such as the caged disc valve and the tilting disc valve. The Bjork-Shiley tilting disk valve was introduced in 1969, and bi-leaflet, pivot designs were introduced in the 1970s. These are now the most common type of mechanical valve.
The possibility of using transplanted valves was also explored in dogs and the best methods of preparation and storage were established. Human valves from cadavers were transplanted into human patients, but it became clear that there were logistical problems in maintaining an adequate supply. By the mid 1960s it was realised that the answer lay in transplanted valves from other species.
Xenograft valves, from pigs, sheep, calves and goats were transplanted into dogs in the early 1970s. Blood clots were not a problem, but durability and rejection needed to be tackled. Following work in rabbits, guinea pigs and rats, a biologically inert, functional and durable valve was produced by washing, denaturing and tanning processes. Such 'bioprosthetic' valves, usually from pigs, have been used successfully in many human patients.
The future for replacement heart valves may lie in tissue engineering. The perfect replacement would obviously be formed from tissue from the patient, tailored to the right shape. Tissue from the artery of a lamb has been grown on a matrix of the correct shape in an artificial culture medium, and the resulting valves have been transplanted into sheep.
- Schimert G et al (1961) in Prosthetic Valves for Cardiac Surgery. Ed Merendino K. CC Thomas, Springfield
- Frater R & Ellis L (1961) In Merendino, ref 1
- Starr A (1961) J Thoracic Cardiovasc Surg 42, 673
- Starr A & Edwards M (1961) Ann Surg 154, 726
- Bjork V & Henze A (1979) In Tissue Heart Valves, Ed Ionescu M. Butterworths, London
- Paton B (1972) in Biological Tissue in Heart Valve Replacement. Ed Ionescu M, Ross D and Wooler G. Butterworths, London
- Carpentier A et al. (1969) J Thoracic Cardiovasc Surg 58, 467
- Pharmaceutical Business News (1995) 15 June