The aim of this work has been to propose a new method for measuring respiratory resistances. This method is based on differences between Rib Cage and Abdomen surface area changes measured by inductance plethysmography. These differences were assessed by various calculation of a Rib Cage-Abdomen distance. Distance between asters (aster = vectorial representation of first four harmonics of the Fourier analysis performed on each breath) appeared to be the most sensitive and reproducible and the less variable. This distance increases significantly in healthy subjects with resistive loads (3 à 13 cmH2O.l-1.s) added during the whole breath, both seated and supine. The distance-resistance regressions lines are parallel for all subjects. This result suggests the possibility for evaluating changes in respiratory resistance by using a pooled slope. An assessment of the method was carried out using recordings performed during methacholine-induced changes in airway resistances. It was found that the Rib Cage-Abdomen distance did not increase with the airway resistance as measured by whole body plethysmography for the whole group of subjects. The resistive loading in healthy subjects resulted in (1) a significant increase in inspiratory and expiratory duration, in tidal volume, a slight decrease in minute ventilation as assessed by slight changes in end tidal CO2, whereas there were no changes in the airflow profile and inspiratory /total duration ratio, and (2) no changes in the mean heart rate but an increase in heart rate variability, due mainly to an increase in respiratory sinus arrhythmia. No significant changes were observed in the specific indicies of respiratory sinus arrhythmia, when comparing breathing at a respiratory frequency obtained by adding a resistive load to breathing at the same imposed respiratory frequency.
