Although in-vitro biomechanical behaviour of arteries is commonly studied under quasi-static conditions, dynamic (pulsatile, in-vivo) behaviour may differ substantially. We developed a set-up to characterize mouse carotid artery biaxial mechanics under quasi-static and dynamic conditions, using high-frequency ultrasound to track diameter and two-photon laser scanning microscopy (TPLSM) to capture the microstructure. We aimed to (1) quantify reproducibility and (2) compare quasi-static and dynamic elastic behaviour.
Design and method:
After euthanasia, eight carotid arteries from four male surplus mice were mounted between glass micropipettes. Four carotids were tested directly after euthanasia, four on the day thereafter. Pressure (P) was recorded at the distal pipette; axial force (F) was recorded by a load cell. First, arteries were stretched to in-vivo length and exposed to quasi-static pressure inflation from 0–200 mmHg. Second, axial stretch (λz) was varied for constant pressures of 60/100/140 mmHg to determine an axial stiffness coefficient. Third, vessels were exposed to pulsatile pressures (systolic pressures of 80/120/160 mmHg, at 5 Hz) and at frequencies (f) of 2.5/5/10 Hz at 120 mmHg. Single-point pulse wave velocity (PWV; Bramwell-Hill) was determined and compared with corresponding PWVs calculated from the quasi-static pressure-diameter curve. The axial stiffness coefficient was obtained as the local slope of the F-λz curve. Fourth, information on adventitial collagen-structure deformation was obtained using second-harmonic generation TPLSM. The first three protocol steps were performed in duplicate to determine coefficients of variation (CVs).
CVs for PWV were ∼9% for low/medium and 27% for high pressure. For f2.5 and f10 Hz these were 16% and 26%. Dynamic PWVs were higher than quasi-static PWVs for all test conditions (p < 0.012; mean ± SD, at f5 Hz: PWVP80 2.4 ± 0.2 vs. 2.1 ± 0.1m/s, PWVP120 6.3 ± 0.6 vs. 5.0 ± 0.5m/s, PWVP160 13.3 ± 1.9 vs. 10.3 ± 2.2m/s, and at P120 mmHg: PWVf2.5 3.9 ± 0.7 vs. 3.5 ± 0.3m/s, PWVf10 3.1 ± 0.9 vs 2.5 ± 0.2m/s). Axial stiffness coefficients increased with pressure 60–140 mmHg (1.3 ± 0.4, 2.4 ± 1.0, 4.9 ± 2.8 grams). Measurements from fresh and non-fresh vessels were not significantly different (p > 0.099). From 60–100 mmHg undulation of adventitial collagen strands disappeared, whereas from 100–140 mmHg orientation changed (Figure).
Our innovative set-up shows well-acceptable reproducibility and demonstrates the importance of quasi-static and dynamic conditions when studying arterial mechanics.