Peripheral "oscillatory" compliance is associated with aortic augmentation index

The augmentation index (AIx) and "oscillatory" compliance (C-2) are wave contour analysis parameters for the central aorta (P-ao) and radial artery pressure wave (P-rad)(x) 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 C-2 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 C-2, respectively. Transfer function was used to calculate reconstructed aortic pressure and AIx(rec). AIx(rec) underestimates AIx(meas) by 0.03 +/-0.16, but both values correlate well (r=0.64; P<0.001). C-2 and AIx were inversely correlated (r=-0.36; P<0.001 for AIx(meas); r=-0.30; P <0.01 for AIx(rec)). Both AIx(meas) (0.06 +/-0.17 versus 0.20 +/-0.21; P <0.01) and AIx(rec) (0.04 +/-0.12 versus 0.16 +/-0.16; P<0.001) were lower after nitroglycerin, whereas C-2 increased only nonsignificantly (0.080<plus/minus>0.036 versus 0.071 +/-0.042). C-2 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 C-2, AIX could be a more appropriate parameter to use in the clinical setting because it is determined directly from the pressure wave contour.