Total respiratory impedance and early emphysema.

It can be postulated that patients in early stages of pulmonary emphysema have normal values of total respiratory resistance and reactance. The purpose of this study was to investigate whether pulmonary emphysema, detected functionally by a decrease of the single breath diffusing capacity (DLCO) by at least 25% of predicted, and an increase of the static lung compliance (CLst) by at least 50% of predicted, can be accompanied by normal values of respiratory resistance (Rrs) and reactance (Xrs), measured between 2 and 24 Hz by the forced oscillation technique. In a prospective study, we determined CLst in 26 patients, who had been selected on the basis of normal values of Rrs and Xrs, and a DLCO of less than 75% of predicted. In 17 of these patients, CLst was more than 150% of predicted. Since there were only minor abnormalities on routine lung function tests and chest X-ray, it is likely that these patients presented early emphysema. In the nine other patients, CLst was within normal limits: four suffered from interstitial lung disease; the remaining five were probably in a preliminary stage of early emphysema. In conclusion, early emphysema should systematically be considered as the first diagnosis in patients with normal values of Rrs and Xrs, and a decrease of DLCO. Onset of interstitial lung disease is a possible alternative.