Literature Review

cVivek Bhat MECH691X

March 4, 2008 Dr. Lyes Kadem Literature Review

Determining the effective orifice area (EOA) of an aortic heart valve is an important step in diagnosing aortic stenosis, or the obstruction of blood flow through the aortic valve. Different methods for calculating effective orifice area have been developed. In the present paper, a relationship between the time profile of the orifice area of the aortic valve and the velocity upstream and downstream of the valve is investigated. The following ???? literature reviews attempt to discuss this problem. In a research article by Blais, Pibarot, Dumesnil, Garcia, Chen and Durand (2001), valve resistance has been determined to be value of EOA based on a variant of the Gorlin equation:

. They also give the . A low

ejection fraction is defined as <40%. They found that fixed stenoses and mechanical prostheses did not react to increased flow by increasing their EOA and that bioprostheses increased their EOA only moderately in response to an increase in flow. They conclude that in determining the severity of stenosis, using EOA or resistance is insufficient and a holistic approach which uses both parameters is preferable. In an article by Beauchesne, deKemp, et al. (2003), a continuity equation approach was taken to measure the effective orifice area (EOA) of the aortic valve. Doppler cardiography was used to determine the velocity in the left ventricular outflow tract (VLVOT) and the diameter of the LVOT (LVOT diam). They also took a measurement of the transvalvular velocity (VAS). Using these measurements, they were able to calculate the EOA as (EOA = (VLVOT X LVOTarea)/VAS). They calculated EOA at 1% time intervals of the total ejection period (EP) between 10 and 90% of the total EP. They assumed that the LVOTarea remained constant throughout the ejection period and that the LVOT had a perfectly circular profile. They discovered that the time to maximum VLVOT and the EOA opening rate had the strongest relationship with mean transvalvular pressure gradient. This means that for people with severe aortic stenosis, the aortic valve takes considerably longer to fully open and to provide the maximum blood velocity. A study was conducted by Bellhouse (1969) in which he investigated the pressure distribution in aortic valves by conducting practical experiments using a rigid-walled model placed in a pulsatile water tunnel. He discovered through both practical measurements and through theoretical calculations that the vortices which form in the sinuses behind the valve cusps in systole aid in advantageous fluid behaviour. Specifically, he determined that the vortices help to position the cusps in peak

systole and also assist in the prevention of jet formation during valve closure. He also cites a study (Bellhouse and Bellhouse, 1968) which finds that if the sinuses were removed from the valve geometry, valve efficiency drops by about 20%. A study by Gilon, Cape, et al. Investigated the impact on valve efficiency of not just physical orifice area but also of the three dimensional valve geometry. They found that flat-profile valves had increased pressure losses of more than 40% over funnelshaped valves. The study was conducted experimentally by generating a 3D model of the heart valves of patients suffering from varying degrees of aortic stenosis. This model was used to generate a polymer prototype of the valve. Each prototype was then subjected to a steady flow of a fluid with physiologic viscosity and density.