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Biomedical subjects

T L Yearwood

Publications and source records attributed to T L Yearwood.

7 recordsLinked to original sources
PubMed1989

Prediction of stenotic valve orifice area: an in vitro study on a bioprosthesis.

The Gorlin equation for the hemodynamic assessment of valve area is commonly used in cardiac catheterization laboratories. A study was performed to test the prediction capabilities of the Gorlin formula, as well as those of the Aaslid and Gabbay formulas for the effective orifice area of a porcine valve of varying degrees of stenosis. Pressure gradient, flow, and valve opening area measurements were performed on Carpentier-Edwards porcine valve prostheses (made stenotic by suturing at the commissures) mounted in the aortic position of an in vitro pulse duplicator. With the known valve orifice area, a discharge coefficient was computed for each of the three orifice area formulas. After some theoretical considerations, it was proposed that the discharge coefficient would be a function of the flow rate through the valve. The discharge coefficient was observed to increase with increasing systolic flow rate. An empirical relationship of the discharge coefficient as a linear function of the systolic flow rate was determined through a regression analysis, with a different relationship for each orifice area formula. Using this relationship in the orifice area formulas improved the accuracy of the prediction of the effective orifice area with all three formulas performing equally well.

Aortic Valve Stenosis↗
PubMed1989

Experimental fluid dynamics of aortic stenosis in a model of the human aorta.

Aortic stenosis has been modelled in an in vitro, pulsatile mock circulatory system (MCS) using a porcine valvular prosthesis, and studied with a laser Doppler anemometer (LDA). The MCS incorporated an acrylic model of the human aorta made from a cadaveric casting in situ. A Carpentier-Edwards aortic valve prosthesis was placed in the MCS after being rendered stenotic by suturing of the valve cusps. Flow velocity profiles across the lumen of the aorta in the presence of aortic stenosis were determined using LDA at two preselected sites in the ascending aorta, and at one preselected site in the brachiocephalic artery. Results indicate that a strong systolic jet bordered by transient vortices with intensely reversed flows is produced distal to severely stenotic aortic valves, becoming less intense with a lesser degree of stenosis. Peak fluid velocities in the systolic jet were determined by LDA at distances of 2.6 and 5.6 cm from the valve inlet for a mean flow rate of 5.2 l min-1. Peak systolic pressure gradients and peak turbulent axial stresses were also determined and found to increase dramatically with stenosis. Furthermore, increasing degrees of stenosis also resulted in more severely disturbed flows in the brachiocephalic artery. Peak fluid velocities and their associated turbulent axial stresses in the systolic jets produced by aortic valvular stenosis are remarkably sensitive to even small changes in the calculated valve orifice areas, and can therefore be very useful in assessing the severity and progression of valvular disease. In addition, increasing degrees of aortic stenosis cause more turbulence to be transported into the brachiocephalic artery.

Aorta↗
PubMed1983

Effect of valve orientation on flow development past aortic valve prostheses in a model human aorta.

The effect of valve orientation on flow development in a model human aorta was studied by means of a qualitative flow visualization technique. The model replicated the geometry of the human aorta and the experiment simulated a physiologically realistic pulsatile flow. The following valves were studied: Starr-Edwards Stellite, Starr-Edwards silicone, Björk-Shiley spherical disc, Björk-Shiley convexo-concave disc, and Hall-Kaster tilting disc. All the valves had a tissue anulus diameter of 27 mm. With the ball-in-cage valves, the flow in the ascending aorta was predominantly axial and uniform throughout systole, while vortex formation was observed downstream from the ball. With the tilting disc valves, the flow development in the aorta was a function of the orientation of the valves. With the major flow orifice directed toward the commissure between the right and noncoronary cusps, the fluid motion was predominantly in the axial direction through early systole. A vortex developed along the wall of lesser curvature of the aorta with the progression of systole. In early diastole, a well-defined flow reversal was observed along the lesser curvature of the aorta. With the major flow orifice directed toward the left coronary cusp, the fluid motion, although predominantly axial, was not uniform in the ascending aorta. Regions of relative stasis present near the wall of greater curvature subsequently developed into a trapped vortex throughout the cardiac cycle. With the major flow orifice directed more posteriorly, an improved fluid dynamic characteristic was observed, and there was no trapped vortex present near the wall of greater curvature. The flow visualization study in the model human aorta suggests that, from a fluid dynamic point of view, orientation of the major flow orifice of the tilting disc valve toward the wall of lesser curvature is not advisable.

Aortic Valve↗
PubMed1982

Physiological pulsatile flow experiments in a model of the human aortic arch.

An experimental investigation of physiologically relevant pulsatile flow in a model of the human aortic arch has been conducted. The model aortic arch flow chamber was fabricated in clear acrylic from an in situ casting of the human aorta and was incorporated in a mock-circulatory system. The model excluded the coronary sinuses and the three major branching arteries of the mid-arch region in order to concentrate only upon the effects of the multi-dimensional curvatures and tapering in the aorta. Furthermore, a flow straightening section was placed upstream to the flow chamber to eliminate any fluid disturbances created by the prosthetic aortic valve used in these studies. The qualitative flow visualization studies in the model aorta revealed the presence of strong secondary fluid motions near the inner wall. These helical flows dissipated during diastole, being greatly affected by the dramatic flow reversals which occurred along the inner wall at the onset of diastole. Quantitative studies were conducted using a three-sensor hot-film velocity probe to determine the axial, radial and tangential velocity components at various cross-sections in the aorta. The results showed rapid reversal of axial velocity near the inner wall at the onset of diastole.

Aorta, Thoracic↗