Aneurysmal Hypertension and its Relationship to Sac Thrombus: A Semi-qualitative Analysis by Experimental Fluid Mechanics

Abstract

Objectives. To ascertain the effect of aneurysm thrombus and luminal diameter on arterial blood pressure within the abdominal aortic aneurysm lumen and at the sac wall. Methods. A lifelike abdominal aortic aneurysm was incorporated in a pulsatile flow unit, using systemic blood pressure settings of 140/100 mmHg and 130/90 mmHg (denoted the high and low settings, respectively). Aneurysm sac pressure was measured in the absence of thrombus within the sac. This was repeated after a thrombus analogue (gelatine) was introduced into the aneurysm model in an asymmetric fashion. Luminal and sac wall pressures were compared to the systemic pressure, and to each other, in both blood pressure settings. Statistical analysis was performed using ANOVA in Minitab 13. Results. In the empty sac, the luminal and sac wall pressures were identical to the systemic pressures at the high and low settings. After introduction of thrombus, pressure was transmitted in a monophasic pulsatile fashion, measuring 166/142/151 mmHg (SP/DP/MP) at the sac wall, while the corresponding intraluminal pressure was 164/136/145 mmHg (p , 0.001, high setting). By contrast, in the low setting, these readings were 157/133/141 (sac wall) and 160/128/138 mmHg (lumen; p , 0.001). The sac wall pressures were significantly higher than the luminal pressures for both high and low settings (p , 0.001). Conclusions. Thrombus has a significant effect on the intraaneurysmal lumen itself and causes localised hypertension with high intraluminal pressures. The differences between the sac wall/luminal pressures may affect regional aneurysm wall biomechanics, but needs further study.

Engineering Silicone Rubbers for In Vitro Studies: Creating AAA Models and ILT Analogues With Physiological Properties

“…We also considered the ILT as a single layer due to these constraints. This approach is consistent with previously published experimental modelling of AAA [11,12,19,20]. From the uniaxial tensile testing we determined that our ILT analogues had Young’s modulus of 0.24Mpa (ILT1) and 0.33MPa (ILT2) which is similar to values for the medial layer found by Wang et al [38].…”

“…The primary limitation of this study is the idealisation of considering the ILT and the artery wall as single layers. While this is a gross assumption, it is less significant than using ILT analogues without investigating their mechanical properties, as previously reported [11,12,41]. We believe that our simplified one layer ILT analogue is more representative than materials such as dough or gelatin.…”

“…In vivo the arteries are in a state of constant longitudinal tension [14]. As this is difficult to model in vitro it is usually not considered [4–12]. The abdominal aorta is surrounded by visceral organs, abdominal muscle and the spine which undoubtedly influence its deformation under pressure [15].…”

“…While many thrombus analogues ranging from gelatin to dough have been used in in vitro studies, a repeatable manufacturing method for creating physiological AAA models with integrated physiological thrombus analogues has not been fully described [11,12,25]. Here we describe an injected silicone rubber based AAA model of ideal geometry with realistic vessel stiffness with a built in silicone rubber ILT analogue with realistic Young’s modulus.…”

Computational Evaluation of Aortic Aneurysm Rupture Risk: What Have We Learned So Far?

“…On the other hand, there is still some concern regarding the protective role of ILT, since many authors who evaluated in vivo the influence of ILT on intraluminal pressure transmission reported that the presence of ILT fails to reduce the transmission of intraluminal pressure on the AAA wall, consequently leaving the risk of AAA rupture unaffected. 18,19 The above has been reported to hold true even in thrombosed aneurysms. 20–26…”