Dissecting aneurysms, or dissections, occur in large arteries by creating a false lumen in the media which is parallel to the true lumen. A tensile force is needed to tear the arterial wall in dissections. This tensile force could be produced in response to the pressure in the false lumen. In this study, tearing pressure was investigated. The studies were done in vitro on fresh isolated porcine thoracic aortas which were pressurized to 130 mmHg (physiological mean pressure). A volume of diluted India ink was injected into the media of the aortic wall to create a cavity called a "bleb". As the size of the bleb increased, the media was dissected. The difference between the pressure inside the bleb and that in the lumen was measured, and defined as tearing pressure. Ten pig thoracic aortas were studied. The results showed that tearing pressure decreased with the volume of blebs at the beginning when the blebs were first formed, and then reached a constant pressure after the shape of blebs did not change. The minimum tearing pressure required for the propagation of dissections is called propagation pressure which is 52/spl plusmn/10(SD) mmHg (n=10) in the range of physiological pressure, indicating that the propagation of dissection might be possible in vivo under physiological conditions. Based on the assumptions that each aortic wall has a constant Young's modulus at large strains and a maximum strain, and shear stress is not an important fact involved in dissections, a mathematical model is established for dissections of the aortic wall. The tearing pressure was inversely proportional to both the radii of blebs and the tangents of the angles at the leading edge of dissections before the blebs had a fixed shape, and then kept as a constant when the shape of blebs did not change. The model explained the change of tearing pressure with the geometry of blebs, and predicted tearing pressure would be lower in old human aortas than in young ones.
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