Noninvasive determination of the pulmonary artery input impedance

A reliable noninvasive method for the estimation of pulmonary function in healthy and diseased subjects should be of great importance in the prognosis, diagnosis, and treatment of pulmonary hypertension. Here we propose such a method, which is based on the parameter identification of the five-element Windkessel model of pulmonary circulation. The method requires the following input variables: the heart rate, the stroke volume, the Doppler echocardiographic measurements of the tricuspid regurgitation and the pulmonary valve velocity profiles, and estimations of the right atrium and the pulmonary vein pressure. The stroke volume is calculated as a product of the left ventricle outflow tract area and the velocity-time integral measured at the same place. The model parameter identification procedure is based on minimization of the root mean square error between the pulmonary artery root pressure calculated from the aforementioned Doppler velocity profiles (from the Bernoulli equation applied during the ejection phase) and the pressure from the Windkessel model during the same period. The output from the model contains the calculated pulmonary artery input impedance (i.e. the model parameters: pulmonary vascular resistance, pulmonary artery proximal and distal compliances, inertance, and characteristic impedance) and the pulmonary artery pressure profile during the whole heart period. Our method is applied to a subject with pulmonary hypertension. The right heart Swan-Ganz catheterization has been performed in this subject. The results obtained by using this method show that the five-element Windkessel model reconstructs the main features of the pulmonary artery input impedance very well: its modulus shows the minimum where the phase angle changes its sign. The pulmonary vascular resistance, the systolic, diastolic and mean pulmonary artery pressures obtained from the method are in good agreement with the values obtained invasively from the catheterization. Sensitivity analysis shows that the mean pulmonary pressure is fairly insensitive to slight overestimation/ underestimation of all input parameters, except for the right atrium pressure. The absolute error in the mean pulmonary artery pressure is nearly the same as the error in the right atrium pressure. Since the proposed method offers a deeper insight into the pulmonary circulation than the catheterization itself because it provides the proximal and distal compliance, the inertance and the characteristic impedance, it seems that it can serve in clinical practice as a good substitute for catheterization.