Compliance of the main pulmonary artery during the ventilatory cycle

Transmural pulmonary arterial pressure (Ppa), diameter (D), and length (L) of a segment of the main pulmonary artery (MPA) were measured simultaneously in anesthetized open-chest dogs. The instantaneous volume was calculated from D and L. Pulmonary arterial elasticity for diameter (Ep(D)) was calculated as the ratio of the amplitude of Ppa to D oscillations normalized by the mean D. Similar indexes were calculated for L (Ep(L)) and V (Ep(V)). Compliance per unit length was calculated from the dimensions and elasticity of the MPA. Under control conditions with 5 cmH₂O positive end-expiratory pressure, Ep(D), Ep(L), and Ep(V) at cardiac frequency were 175 ± 27, 147 ± 27, and 55 ± 7 cmH₂O, respectively. Ep(D) increased with positive end-expiratory pressure, but Ep(L) decreased and Ep(V) was unaffected. Ep(D), Ep(L), Ep(V), and compliance per unit length were not significantly different between the start of inspiration and the start of expiration. In addition, there were no significant phase differences between the oscillations of Ppa and V at respiratory frequency. We conclude that the previously reported time variation of pulmonary arterial compliance during the ventilatory cycle is not due to time-varying properties of the MPA.