Does spontaneous respiration alter pulmonary artery input impedance?

The aim of this study was to determine whether spontaneous respiration influences pulmonary artery input impedance, a question that has received little attention in the literature. Impedance values were assessed during three different phases of the respiratory cycle, namely inspiration, expiration and postexpiration (i.e. the null respiratory flow period between expiration and the next inspiration) in five anaesthetized spontaneously breathing dogs. Firstly, impedance values during postexpiration were taken as the reference baseline, and compared with values obtained during inspiration and expiration. Then, differences between values in inspiration and in expiration were tested, taking impedance during inspiration as the baseline. Differences with respect to postexpiration were found for three parameters of input impedance: input resistance, characteristic impedance, and the frequency at the first zero-crossing of the impedance phase from negative to positive values (f(cross)). Input resistance was significantly lower in inspiration (85% of the baseline), characteristic impedance was significantly greater in inspiration and in expiration (112 and 119% respectively), and f(cross) was significantly lower in expiration (89%). By contrast, only input resistance differed significantly when inspiration was compared to expiration. Therefore, spontaneous respiration was shown to influence input impedance significantly. The observed changes in characteristic impedance and f(cross) might be explained by a stiffening of the pulmonary artery wall, due to neural and/or mechanical factors, during inspiration and expiration.