Estimation of cardiovascular system parameters using noninvasive measurements

Rangarajan, Nagarajan
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Engineering Science and Mechanics

Mathematical models have been widely used to simulate the pulsatile flow of blood in a segment of an artery. These models contain several physiologically significant parameters, such as arterial diameter and arterial compliance, and for diseased arteries, parameters pertaining to arterial constrictions (stenoses). Generally these parameters are difficult to measure directly. The aim of the present study was to develop a suitable parameter estimation scheme, based on noninvasive measurements of flow and pressure in the arterial segment, to predict the stenosis parameters in the model. The mathematical model used in this study for pulsatile flow in a tube containing a simulated stenosis was solved with the finite-element method;In the first part of the study, a hydraulic model simulated pulsatile flow in a segment of an artery. Pulsatile pressure and flow waveforms were measured noninvasively by a phase-locked, ultrasonic echo-tracker, and a continuous-wave Doppler flowmeter, respectively. These waveforms compared well with corresponding directly measured waveforms;The method of ordinary least squares, incorporating the Gauss-Newton linearization scheme, was used to estimate the location and severity of a simulated stenosis introduced in the tube. Measured proximal flow and a lumped distal resistance were used as boundary conditions for the model, with a measured proximal pressure used for parameter optimization. The estimation scheme was first validated by model-to-model tests utilizing computer generated waveforms as input data. Subsequently, experimental data from the hydraulic model, which incorporated stenoses ranging in severity from 94.3% to 74.8%, were used as input to the estimation scheme. Estimates for the stenosis severity and location, based on noninvasive measurements of pressure and flow in the tube, compared well with the corresponding directly measured values, and also with estimates obtained with invasively measured data;Limited animal experiments were carried out in the second part of the study. Reasonable estimates of the severity of stenoses, artificially induced in the femoral arteries of dogs, were obtained for stenoses ranging in severity from 90% to 60%. These estimates were based on invasively measured flow and pressure waveforms in the femoral artery.