Derivation of the ascending aortic-carotid pressure transfer function with an arterial model

To devise a method of deriving the ascending aortic pressure waveform from the noninvasively determined carotid arterial waveform, ascending aortic and carotid arterial pressures were recorded in 13 patients aged 58.5 +/- 10.0 (SD) yr. A single viscoelastic tube terminated with a modified windkessel was used to model the carotid arterial system. For each patient the model parameters, characteristic impedance of the tube (Z(0)), reflection coefficient at the termination (Gamma), and time constant of the windkessel (tau), were estimated by minimizing the root-mean square error between the measured and predicted carotid waveforms, with the ascending aortic pressure waveform as input. The resulting arterial parameters were realistic: Z(0) = 729.5 +/- 246.8 dyn . s . cm(-3), Gamma = 0.75 +/- 0.19, and tau = 0.16 +/- 0.17 s. A generalized model constructed with these mean parameters yielded a smaller error between predicted End measured carotid arterial pressures (3.4 +/- 1.3 mmHg) than between ascending aortic pressure and measured carotid arterial pressure (4.4 +/- 1.6 mmHg, P < 0.01) and also reproduced the carotid wave contour indexed by the ratio of late systolic to early systolic peak amplitude: predicted = 1.26 +/- 0.05 and measured = 1.24 +/- 0.16 vs. aortic = 1.55 +/- 0.19.