Brett L. Fowler

Mechanical properties of abdominal aortic aneurysm wall.

There is a need to understand why and where the abdominal aortic aneurysm may rupture. Our goal therefore is to investigate whether the mechanical properties are different in different regions of the aneurysm. Aorta samples from five freshly excised whole aneurysms, U 5 cm in diameter, from five patients, average age 71 - 10 years, were subjected to uniaxial testing. We report the wall thickness, yield stress and strain, and parameters that describe nonlinear stress-strain curves for the anterior, lateral and posterior regions of the aneurysm. The posterior region was thicker than the anterior region ( 2.73 - 0.46 mm versus 2.09 - 0.51 mm). The stress-strain curves were described by † = a k b, where † is true stress and k is engineering strain. In the circumferential direction, the wall stiffness increased from posterior to anterior to lateral. In the longitudinal direction, the lateral and anterior regions showed greater wall stiffness than the posterior region. The wall stiffness was greater in the circumferential than longitudinal direction. The anterior region was the weakest, especially in the longitudinal direction (yield stress † Y = 0.38 - 0.18 N mm -2 ). For a less complex model the aneurysmal wall could be considered orthotropic with † = 12.89 k 2.92 and 4.95 k 2.84 in the circumferential and longitudinal directions. For the isotropic model, † = 7.89 k 2.88. In conclusion, different regions of the aneurysm have different yield stress, yield strains, and other mechanical properties, and this must be considered in understanding where the rupture might occur.There is a need to understand why and where the abdominal aortic aneurysm may rupture. Our goal therefore is to investigate whether the mechanical properties are different in different regions of the aneurysm. Aorta samples from five freshly excised whole aneurysms, > or = 5 cm in diameter, from five patients, average age 71 +/- 10 years, were subjected to uniaxial testing. We report the wall thickness, yield stress and strain, and parameters that describe nonlinear stress-strain curves for the anterior, lateral and posterior regions of the aneurysm. The posterior region was thicker than the anterior region (2.73 +/- 0.46 mm versus 2.09 +/- 0.51 mm). The stress-strain curves were described by sigma = a epsilon(b), where sigma is true stress and epsilon is engineering strain. In the circumferential direction, the wall stiffness increased from posterior to anterior to lateral. In the longitudinal direction, the lateral and anterior regions showed greater wall stiffness than the posterior region. The wall stiffness was greater in the circumferential than longitudinal direction. The anterior region was the weakest, especially in the longitudinal direction (yield stress sigmaY = 0.38 +/- 0.18 N mm(-2)). For a less complex model the aneurysmal wall could be considered orthotropic with sigma = 12.89epsilon(2.92) and 4.95epsilon(2.84) in the circumferential and longitudinal directions. For the isotropic model, sigma =7.89epsilon(2.88). In conclusion, different regions of the aneurysm have different yield stress, yield strains, and other mechanical properties, and this must be considered in understanding where the rupture might occur.

A new aortic root prosthesis with compliant sinuses for valve-sparing operations ☆

Abstract Background . We designed and tested a novel aortic root prosthesis with compliant sinuses for valve-sparing operations. Methods . In eight human aortic roots, the aorta was trimmed 2 mm above the leaflet attachment. The aortic portion of the graft was made by scalloping the Dacron tube. Three sinuses were made individually after turning z-folds in the fabric 90 degrees. Three rectangular pieces were cut and purse strings sewn in each to form the sinuses. The graft was sutured to the aortic root and studied in a left heart simulator. The leaflet motion was recorded (500 frames/second), commissural movement was measured with ultrasound, and the shape of the root was determined from a mold. Seven intact aortic roots were also studied. Results . In the aortic graft roots, the valves were competent and leaflets opened rapidly into a circular orifice, not touching the sinus wall. Commissural diameter increased by 22% when pressure increased from 0 to 80 mm Hg, and increased by a further 6.6% when pressure increased to 120 mm Hg. The sinuses had a teardrop shape. Conclusions . The dynamics of the aortic graft root and the leaflets were comparable to that of the intact aortic root. This prosthesis is being introduced in clinical practice.

A method for the measurement of left ventricular overload for aortic valve insufficiency

BACKGROUND The degree of left ventricular overload in patients with aortic valve insufficiency (AI) plays an important role in determining the need and timing of surgical intervention. Because hemodynamic evaluation of AI may potentially predict the effects of an insufficient valve on the ventricle before they occur, it would be useful to guide valve surgery with such a diagnostic tool. The purpose of this study was to test the performance of a new hemodynamic index based on mechanical energy loss for the measurement of the effects of insufficiency on ventricular workload. METHODS AND RESULTS An intact and subsequently perforated aortic bioprosthesis was tested within an in vitro model of the left heart, varying cardiac output, diastolic aortic pressure, and the size of perforation. Regurgitant orifice area (ROA), regurgitant volume (RV), regurgitant fraction (RF), and energy loss index (ELI) were measured for each experimental condition and plotted against the increase in workload per unit volume net forward flow (DeltaWPV) due to perforation. ROA, RV, and RF showed good correlations with DeltaWPV, but the relationship between these variables and DeltaWPV became ambiguous as their magnitudes increased. ELI had a near perfect linear relationship with DeltaWPV (slope=1.00, r(2)=0.98) independent of the experimental condition. CONCLUSIONS RV, RF, and ROA do not by themselves fully describe the increase in difficulty the ventricle has in moving the blood across an insufficient valve. ELI, in contrast, was found to be a very good measure of the decrease in pump efficiency due to aortic valve insufficiency.