Comparison between Three- and Four-Element Windkessel Models to Characterize Vascular Properties of Pulmonary Circulation

In 11 anaesthetised pigs the accuracy of the three-element (WK3) and the four-element (WK4) Windkessel models to describe hemodynamic properties of the pulmonary circulation was compared during six different experimental conditions increasing pulmonary arterial pressure: increase in left atrial pressure, increase in alveolar pressure, increase in pulmonary blood flow, endotoxin shock, mechanical obstruction of left pulmonary artery or histamine infusion. Our results showed that WK4 fitted better the data than did WK3 because values of 1-R2 decreased from 6 percent (WK3) to 1.4 percent (WK4) when WK4 was used (P < 0.0005). 1-R2 was an adequate marker of the accuracy of the linear regression used to solve equations of both models. Compliance values estimated by WK4 were decreased by 5% comparatively to WK3 (P = 0.008). However, this difference can be considered as not physiologically relevant. Values of characteristic resistance corresponding to R1 + (L/R2C) in WK4 and to R1 in WK3 were not different (P = 0.22). The relative changes in R1, R2, and C observed due to the different experimental conditions were comparable regardless of the model. In conclusion, the conversion of WK3 in WK4 by adding an inductance, whose physiological meaning is not clear, resulted in an increased statistical accuracy of the model, but did not seem to have relevant influence on parameters or their evolution during experimental conditions