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Research Papers

Differences in Transmural Pressure and Axial Loading Ex Vivo Affect Arterial Remodeling and Material Properties

[+] Author and Article Information
Amanda R. Lawrence

Department of Bioengineering and Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, PA 19104

Keith J. Gooch1

Department of Bioengineering and Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, PA 19104; Department of Biomedical Engineering and Davis Heart Lung Research Institute, The Ohio State University, Columbus, OH 43210gooch.20@osu.edu

1

Corresponding author.

J Biomech Eng 131(10), 101009 (Sep 04, 2009) (8 pages) doi:10.1115/1.3200910 History: Received November 17, 2008; Revised July 05, 2009; Published September 04, 2009

Arterial axial strains, present in the in vivo environment, often become reduced due to either bypass grafting or the normal aging process. Since the prevalence of hypertension increases with aging, arteries are often exposed to both decreased axial stretch and increased transmural pressure. The combined effects of these mechanical stimuli on the mechanical properties of vessels have not previously been determined. Porcine carotid arteries were cultured for 9 days at normal and reduced axial stretch ratios in the presence of normotensive and hypertensive transmural pressures using ex vivo perfusion techniques. Measurements of the amount of axial stress were obtained through longitudinal tension tests while inflation-deflation test results were used to determine circumferential stresses and incremental moduli. Macroscopic changes in artery geometry and zero-stress state opening angles were measured. Arteries cultured ex vivo remodeled in response to the mechanical environment, resulting in changes in arterial dimensions of up to 25% and changes in zero-stress opening angles of up to 55°. While pressure primarily affected circumferential remodeling and axial stretch primarily affected axial remodeling, there were clear examples of interactions between these mechanical stimuli. Culture with hypertensive pressure, especially when coupled with reduced axial loading, resulted in a rightward shift in the pressure-diameter relationship relative to arteries cultured with normotensive pressure. The observed differences in the pressure-diameter curves for cultured arteries were due to changes in artery geometry and, in some cases, changes in the arteries’ intrinsic mechanical properties. Relative to freshly isolated arteries, arteries cultured under mechanical conditions similar to in vivo conditions were stiffer, suggesting that aspects of the ex vivo culture other than the mechanical environment also influenced changes in the arteries’ mechanical properties. These results confirm the well-known importance of transmural pressure with regard to arterial wall mechanics while highlighting additional roles for axial stretch in determining mechanical behavior.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 2

Incremental modulus (Einc) versus normalized outer diameter under both active ((a) and (b)) and passive ((c) and (d)) SMC conditions. For cultured artery groups: normotensive/λ=1.5 (○) (n=5), hypertensive/λ=1.5 (●) (n=5), normotensive/λ=1.3 (◻) (n=4), and hypertensive/λ=1.3 (◼) (n=3). For freshly isolated arteries at each axial stretch ratio: λ=1.3(−−) and λ=1.5(—)(n=9). Diameters were normalized to the first values of the passive Einc-D curves. Average Einc SE: freshly isolated/λ=1.3:99.65 kPa and freshly isolated/λ=1.5:167.32 kPa; average normalized diameter SE: freshly isolated/λ=1.3:0.03 and freshly isolated/λ=1.5:0.02.

Grahic Jump Location
Figure 3

Stress versus strain under both active ((a) and (b)) and passive ((c) and (d)) SMC conditions. For cultured artery groups: normotensive/λ=1.5 (○) (n=5), hypertensive/λ=1.5 (●) (n=6), normotensive/λ=1.3 (◻) (n=4), and hypertensive/λ=1.3 (◼) (n=3). For freshly isolated arteries at each axial stretch ratio: λ=1.3(−−) and λ=1.5(—)(n=9). Average stress SE: freshly isolated/λ=1.3:6.46 kPa and freshly isolated/λ=1.5:10.00 kPa; average strain SE: freshly isolated/λ=1.3:0.03 and freshly isolated/λ=1.5:0.02.

Grahic Jump Location
Figure 4

Arteries cultured with pulsatile flow (▲) and with the MMP inhibitor GM6001 (×) compared with the control steady flow group (◻), all at normotensive pressure/λ=1.3. Pressure versus normalized outer diameter ((a) and (b)); stress versus strain ((c) and (d)); incremental modulus versus normalized outer diameter (e) and (f). For cultured artery groups: steady flow (n=4), pulsatile flow (n=5), steady flow with GM6001 (n=4). For fresh arteries at λ=1.3 (denoted as – –; passive) for (b), (d), and (f) (n=9). (b) Freshly isolated/λ=1.3: average pressure SE: 0.15 mmHg and average normalized diameter SE: 0.03; (d) freshly isolated/λ=1.3: average stress SE: 6.46 kPa and average strain SE: 0.03; (f) freshly isolated/λ=1.3: average Einc SE: 99.65 kPa and average normalized diameter SE: 0.03.

Grahic Jump Location
Figure 1

Pressure versus normalized outer diameter under both active ((a) and (b)) and passive ((c) and (d)) SMC conditions. For cultured artery groups: normotensive/λ=1.5 (○) (n=5), hypertensive/λ=1.5 (●) (n=6), normotensive/λ=1.3 (◻) (n=4), and hypertensive/λ=1.3 (◼) (n=3). For freshly isolated arteries at each axial stretch ratio: λ=1.3(−−) and λ=1.5(—)(n=9). Diameters were normalized to the first values of the passive P-D curves. Average pressure SE: freshly isolated/λ=1.3:0.15 mmHg and freshly isolated/λ=1.5:0.19 mmHg; average normalized diameter SE: freshly isolated/λ=1.3:0.03 and freshly isolated/λ=1.5:0.02.

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