Time constant histograms from the forced expired volume signal: a clinical evaluation

We evaluated a multicompartment analysis of forced expiration, based on modelling the lung as a set of twenty parallel compartments emptying exponentially with time constants ranging from 0.1-10s; the forced expired volume signal was represented ba a histogram showing the fraction of forced vital capacity as a funtion of compartmental time constants. We applied this technique to 80 healthy and 12 asthmatic subjets. The histograms computed from three consecutive forced expirations were poorly reproducible in 18 of the 80 healthy and 2 of the asthmatic subjets. In the asthmatics, the time constant histograms conveyed no additional information on bronchial obstruction, beyond that already present in standard spirometric indices. A simulation study showed a high sensitivity of the histograms to the truncation of the terminal part of forced expiration. We conclude that the usefulness of the time constant histogram technique appears doubtful.