Respiratory and upper airways impedance responses to methacholine inhalation in spontaneously breathing cats

The upper airways may contribute to the increase in respiratory resistance induced by methacholine (Mch). The aim of this study was to simultaneously assess the Mch response of upper airways and lower respiratory resistances (Rua, Rrs,lo) and reactances (Xua, Xrs,lo), and to test whether the change of total respiratory resistance and reactance after Mch were affected by up per airways mechanisms. Seven cats breathing spontaneously were studied under chloralose, urethane anaesthesia. Forced oscillations were generated at 20 Hz by a loud-speaker connected to the pharyngeal cavity. A pneumotachograph was placed between r ostral and caudal extremities of the severed cervical trachea. Pressure dro ps were measured across the upper airways and across the lower respiratory system. Rua, Xua, Rrs,lo and Xrs,lo were obtained after nebulized normal sa line and Mch administered directly through the tracheostomy, The analysis f ocused on Mch tests showing clear positive upper airways response. Volume a nd flow dependence of Rrs,lo and Rua were assessed during tidal inspiration using multiple linear regression analysis. After Mch, Rrs,lo increased and became negatively volume dependent, while t he increase in Rua was associated with no significant change in volume depe ndence; Xrs,lo became negative while Xua did not change. The upper airways response to methacholine may thus contribute to the incre ase in total respiratory resistance but may not account for either its nega tive volume dependence or the decrease in total resistance. It is surmised that these features more specifically reflect alterations in respiratory me chanics occurring at the level of the intrathoracic airways.