Cross-sectional compliance overestimates arterial compliance because it neglects the axial strain

A high resolution echo-tracking system permits the calculation of cross-sectional compliance considering vessel diameter variations alone, and assumes that longitudinal movement of the vessel wall due to pulse pressure is negligible. However, using piezoelectric crystals sutured on the adventitia of the vessel wall we demonstrated that arterial length changes up to 5% (mean 2.7%) as a function of pulse pressure. Therefore, cross-sectional compliance seems to provide a limited approximation of the real phenomenon because it neglects axial vessel movement. Axial vessel movement is taken into account when the vessel compliance is calculated according to the principle of continuity of the mass: [equation: see text]. To verify this hypothesis we measured the blood flow gradient through 10 cm long segments of 10 pig carotid arteries (Qin - Qout) and divided it for the derivative of blood pressure over a given time (deltaP/deltat). For the same vessels, we calculated the cross-sectional compliance (CC) using the echo-tracking system (NIUS 02). We found a CC of (5.91 +/- 0.4) x 10(-7) micro m(2)/mm Hg and a segmental carotid compliance or dynamic compliance (C(d)) of (6.21 +/- 0.2) x 10(-8) micro m(3)/mm Hg. The impact of axial strain in calculations of compliance results in a dynamic compliance, which is one order of magnitude smaller than traditionally calculated arterial compliance.