Pulmonary resistance and compliance by auto- and cross-correlation functions of transpulmonary pressure and flow.

A technique is described on how to calculate the pulmonary resistance and compliance from the higher order harmonics present in mouth flow and transpulmonary pressure signals during spontaneous breathing. The estimates of resistance and compliance (or rather reactance) obtained from these harmonics following a conventional Fourier transform are not reliable because of a lack of reproducibility. This is obviated by a preliminary smoothing of the signals by means of auto- and cross-correlation functions. Both the conventional and the modified technique yield identical results for the fundamental component of breathing.