Biomechanics in ascending aortic aneurysms correlate with tissue composition and strength.

Objective: This study correlates low strain tangential modulus (LTM) and transition zone onset (TZo) stress, biomechanical parameters that occur within the physiological range of stress seen in vivo, with tissue strength and histopathologic changes in aneurysmal ascending aortic tissue. Method: Ascending aortic aneurysm tissue samples were collected from 41 patients undergoing elective resection. Samples were subjected to planar biaxial testing to quantify LTM and TZo. These were then correlated with strength assessed from uniaxial testing and with histopathologic quantification of pathologic derangements in elastin, collagen, and proteoglycan (PG). Results: Decreased LTM and TZo were correlated with reduced strength (P < .05), PG content (P < .05), and elastin content (P < .05). Reduced TZo also was correlated with increased elastin fragmentation (P < .05). Conclusions: LTM and TZo are correlated with common biomechanical and histopathologic alterations in ascending aortic aneurysm tissue that are thought to relate to the risk of acute aortic syndromes. LTM and TZo are measured under conditions approximating in vivo physiology and have the potential to be obtained noninvasively using medical imaging techniques. Therefore, they represent parameters that warrant future study as potential contributors to our growing knowledge of pathophysiology, disease progression, and risk stratification of aortic disease.

The viscous behaviour of HES 130/0.4 (Voluven®) and HES 260/0.45 (Pentaspan®).

PURPOSE: Several fluids are available for volume therapy to address hypovolemia. We focus on two hydroxyethyl starches (HES) available for volume expansion in Canada, HES 130/0.4 (Voluven®) and HES 260/0.45 (Pentaspan®). Although information is available regarding their pharmacokinetic and risk/benefit profiles, this paper examines their viscous properties. METHODS: Dynamic viscosities of HES 130/0.4 and HES 260/0.45 were measured through capillary viscometry at 21°C and 37°C. The viscosities of the solutions were then measured through a closed flow loop at room temperature across physiologically relevant flow rates that maintained a laminar flow regime. RESULTS: Measured dynamic viscosity through capillary viscometry for HES 130/0.4 and HES 260/0.45 was 2.76 centipoises (cP) and 7.62 cP, respectively, at 21°C decreasing to 1.74 cP and 4.25 cP, respectively, at 37°C. Pipe flow analysis found that HES 130/0.4 (expiry 02/13) and HES 260/0.45 (expiry 10/10) displayed marginal variation in viscosity suggesting Newtonian behaviour. However, a sample of HES 130/0.4 (expiry 10/10) displayed an appreciable increase in viscosity (13%) at higher flow rates suggesting shear thickening behaviour. CONCLUSION: This study represents an innovative characterization of not only the viscosity of two commonly utilized HES solutions but also their viscous behaviour across physiologically relevant flow rates. The shear thickening behaviour of a sample of HES 130/0.40 (expiry 10/10) at high flow rates was not expected, and the effect this result may have on endothelial cell function is unknown.

The mechanical properties of endovascular stents: an in vitro assessment.

Endovascular stents are commonly used to manage arterial diseases such as Aortic Abdominal Aneurysm (AAA), aortic dissection and coarctation. The radial force the stent applies to the vessel must be large enough to resist stent migration, but not so large that the mechanical stimulus initiates adverse vessel remodeling. We employed two approaches to characterize the radial force of Gianturco stents: first, by applying an external pressure to the stent and, second, by measuring the force exerted by the stent when deployed. From the second approach, we determined the force exerted at various area reductions that correspond to clinically relevant diameter oversizings. In this study, stent stiffness was determined from the force-area reduction curves. Comparing similar stents of various diameters revealed that smaller diameter stent had greater radial force and stiffness than larger diameter stents. Comparing similar stents of various lengths revealed that stents with longer lengths (and greater number of wires) has greater force and stiffness. Overlapping two stents increased the force and stiffness to values greater than the sum of those parameters for the individual stents. These data may have important clinical implications for understanding the effect of oversized and overlapped stents on vessel mechanics.

Direct and series transmission of left atrial pressure perturbations to the pulmonary artery: a study using wave-intensity analysis.

Pressure waves are thought to travel from the left atrium (LA) to the pulmonary artery (PA) only retrogradely, via the vasculature. In seven anesthetized open-chest dogs, a balloon was placed in the LA, which was rapidly inflated and deflated during diastole, early systole, and late systole. High-fidelity pressures were measured within and around the heart. Measurements were made at low volume [LoV; left ventricular end-diastolic pressure (LVEDP) = 5-9 mmHg], high volume (HiV; LVEDP = 16-19 mmHg), and HiV with the pericardium removed. Wave-intensity analysis demonstrated that, except during late systole, balloon inflation created forward-going PA compression waves that were transmitted directly through the heart without measurable delay; backward PA compression waves were transmitted in-series through the pulmonary vasculature and arrived after delays of 90 +/- 3 ms (HiV) and 103 +/- 5 ms (LoV; P < 0.05). Direct transmission was greater during diastole, and both direct and series transmission increased with volume loading. Pressure waves from the LA arrive in the PA by two distinct routes: rapidly and directly through the heart and delayed and in-series through the pulmonary vasculature.