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 rece ived little attention in the literature. Impedance values were assesse d during three different phases of the respiratory cycle, namely inspi ration, expiration and postexpiration (i.e. the null respiratory flow period between expiration and the next inspiration) in five anaestheti zed spontaneously breathing dogs, Firstly, impedance values during pos texpiration were taken as the reference baseline, and compared with va lues obtained during inspiration and expiration, Then, differences bet ween values in inspiration and in expiration were tested, taking imped ance during inspiration as the baseline, Differences with respect to p ostexpiration were found for three parameters of input impedance: inpu t resistance, characteristic impedance, and the frequency at the first zero-crossing of the impedance phase from negative to positive values (fcross), 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 fcr oss was significantly lower in expiration (89%), By contrast, only inp ut 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 fcross might be explained by a stiffening of the pulmona ry artery wall, due to neural and/or mechanical factors, during inspir ation and expiration.