BEAT-BY-BEAT CHANGES OF VISCOELASTIC AND INERTIAL PROPERTIES OF THE PULMONARY-ARTERIES

We tested the hypothesis that pulmonary arterial input impedance varie s during the ventilatory cycle due to alterations not only of the visc oelastic components of the pulmonary vasculature but also due to chang es of the inertial components. A four-element lumped-parameter model w as used to fit the pulmonary arterial pressure-flow recordings in the time domain in 10 anesthetized dogs. The four elements consisted of a resistor (R) that represents input resistance, a second resistor (R1) and a capacitor (C1) that represent the viscoelastic properties of the pulmonary vasculature, and an inductor (L1) that represents inertial properties of blood within the pulmonary vasculature. The parameters w ere evaluated at each heartbeat throughout the ventilatory cycle at th ree levels of positive end-expiratory pressure. All four parameters va ried significantly during the ventilatory cycle. R, C1, L1, and R1 var ied by up to 97, 33, 13, and 17%, respectively. Changes in parameter v alues were most apparent at the start of expiration when the most rapi d changes of lung volume occur. This pattern of the results is consist ent with the hypothesis that the time variation of pulmonary arterial impedance is due to dynamic shifts of blood volume between the extra-a lveolar and alveolar arteries.