Static elastic wall properties of the abdominal porcine aorta in vitro and in vivo

OBJECTIVES: To characterise possible differences in vascular mechanics between in vitro and in vivo settings. DESIGN: Experimental animal study. MATERIAL: Segments of abdominal aorta from five 30kg pigs were studied in vitro and five porcine abdominal aortas were investigated in vivo. METHODS: Vessel luminal cross-sectional area (CSA) was assessed in vitro and in vivo by measuring the electrical impedance of fluid inside a distensible balloon mounted on a catheter. Wall thickness was measured in vitro by means of 20 MHz A-mode ultrasound. The obtained values in vitro were used to calculate wall thickness in vivo. RESULTS: The CSAs were larger in vitro than in vivo (p < 0.05). All segments showed stress-strain relations that accorded well with the exponential equation: Stress = a.exp(b. Strain). The y-axis intercepts (a) and the slope of the curves (b) were different in vitro and in vivo (p < 0.05). Differences were found between the aorta in vivo and in vitro regarding the circumferential stress-strain relations, i.e. the stiffest abdominal aorta was found in vivo. CONCLUSIONS: The differences in dimensions and wall stiffness may be attributed to the tethering to the surrounding tissues in vivo. Impedance planimetry combined with high frequency ultrasound is an accurate way of measuring segmental vascular elasticity in vivo.