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Regional and directional compliance of the healthy aorta: an ex vivo study in a porcine model.

OBJECTIVES
To gain differential knowledge about the physiological compliance and wall strength of the different regions of the aorta, including the ascending aorta, arch and descending aorta in both the circumferential and longitudinal directions, and to generate a hypothesis on the pathophysiological mechanisms that lead to Type A aortic dissection.


METHODS
Fresh tissue specimens from 22 ex vivo porcine aortas were analysed on a tensile tester. Regional and directional compliance, failure stress and failure strain were recorded.


RESULTS
Aortic compliance appeared as a linear function of the natural logarithm (ln) of wall stress. Compliance significantly decreased along the length of the aorta. In the ascending aorta, longitudinal compliance significantly (P = 0.003) exceeded circumferential compliance, and the outer curvature was more compliant than the inner curvature (P = 0.03). In the descending aorta, this relationship is reversed: the circumferential compliance exceeded the longitudinal compliance, and the outer aspect was more compliant (P = 0.003). The median circumferential failure stress of all aortic segments was in the range of 2000-2750 kPa, whereas the longitudinal failure stress in the ascending aorta and the arch had values of 750-1000 kPa, which were significantly lower (P < 0.05). Surprisingly, the longitudinal failure stress of the inner aspect of the descending aorta was extraordinarily high (2000 kPa). Failure strain, similar to compliance, was highest in the ascending aorta and decreased along the aorta.


CONCLUSION
The aorta appears to be a complex organ with distinct regional and directional differences in compliance and wall strength that is designed to effectively absorb the kinetic energy of cardiac systole and to cushion the momentum of systolic impact. Under normotensive conditions and a preconditioned physiological morphology, the aortic wall works in the steep part of the logarithmic strain-stress function; under hypertensive conditions and pathological morphology, the wall reacts in an non-compliant manner. The high longitudinal compliance and low failure stress of the ascending aorta and subsequent pathological changes may be the main determinants of the recurrent patho-anatomy of Type A aortic dissection.