Mechanical Aortic Valve Replacement

Excerpt

Aortic valve stenosis is the most common type of valvular heart disease in the elderly. The incidence of aortic valve disease increases with age; one in eight people over the age of 75 have moderate to severe valvular heart disease. The natural history of aortic stenosis usually begins with an asymptomatic period with the development of symptoms once the valve is critically stenosed. The progression of stenotic valve and time of onset of symptoms varies from patient to patient. Symptomatic patients with severe stenosis have a poor prognosis. Ross first described the higher mortality rate after the onset of cardiac symptoms in 1968.

Aortic valve replacement (AVR) has shown to substantially improve survival in patients with symptomatic severe aortic stenosis, and this has formed the basis for recommending aortic valve replacement (AVR) in such circumstances. Currently, there are two techniques being used for aortic valve replacement; mechanical or bioprosthetic valve replacement via open-heart surgery and cardiopulmonary bypass and minimally invasive mechanical valve via a transcatheter (TAVR) approach.

The three main types of artificial heart valves are mechanical, bioprosthetic, and tissue-engineered valves. The mechanical heart valve is composed of strong, durable material, bioprosthetic valves are created from animal valves or part of animal tissue that is strong and flexible, and tissue-engineered valves are grown in-vitro using seeding human cells on to the scaffold. We will be discussing mechanical heart valves in this article.

There are three main types of mechanical valves that are used so far.

1. Caged ball valve: First artificial heart valve that used a metal cage to house a silicone elastomer ball. When the heart contracts and the pressure in the chamber exceed more than the atrial, the ball inside raises to the top of the cage and prevents blood flow. When the heart finishes contracting and the pressure inside the chamber drops, the ball falls back to the base of the cage and allows blood flow. Caged ball valves have a high tendency to form blood clots and imposed higher hemodynamic consequences, and are no longer used.

2. Tilting disc valve: This is a type of swing check valve, made by a metal ring, which holds the disc and is covered by the fabric where sutures are applied to place the valve. It opens when the chamber pressure drops to pump the blood out and closes to prevent the backflow of the blood. Medtronic-Hall model is the most common tilting-disc design in the US.

3. Bi-leaflet heart valves: These are the most commonly used mechanical valve prostheses. This valve consists of two semicircular leaflets. They are considered the least thrombogenic of mechanical valves and are characterized by washing regurgitant blood jets.

The major advantage of the mechanical heart valve is its durability, as it can last for approximately 20 years, but it increases the risk of blood clot formation and can cause mechanical hemolytic anemia.