Peripheral "oscillatory" compliance is associated with aortic augmentation index

The augmentation index (AIx) and "oscillatory" compliance (C₂) are wave contour analysis parameters for the central aorta (P_ao) and radial artery pressure wave (P_rad), respectively. Both are sensitive to cardiovascular risk factors such as aging, hypertension, and diabetes and have been proposed as prognostic markers for cardiovascular disease. In this work, we studied the relation between both. We first calculated P_rad corresponding to a typical aortic A-type (AIx >0.15) and C-type wave (AIx <0), taken from the literature, by using a generalized aorta-radial pressure transfer function. P_rad corresponding to C-type waves yielded the highest C₂ value. We further used simultaneously measured aortic and radial artery pressure in 45 human subjects age 34 to 84 years (63±12 [SD]) at baseline and after administration of nitroglycerin to calculate AIx_meas and C₂, respectively. Transfer function was used to calculate reconstructed aortic pressure and AIx_rec. AIxrec underestimates AIx_meas by 0.03±0.16, but both values correlate well (r=0.64; P<0.001). C₂ and AIx were inversely correlated (r=-0.36; P<0.001 for AIx_meas; r=-0.30; P<0.01 for Alx_rec). Both AIx_meas (0.06±0.17 versus 0.20±0.21; P<0.01) and Alx_rec (0.04±0.12 versus 0.16±0.16; P<0.001) were lower after nitroglycerin, whereas C₂ increased only nonsignificantly (0.080±0.036 versus 0.071±0.042). C₂ is related to AIx and reflects, at least in part, hemodynamic changes affecting central aortic pressure. Nevertheless, given the model assumptions and computational steps associated with calculating C₂, AIx could be a more appropriate parameter to use in the clinical setting because it is determined directly from the pressure wave contour.