Resistance measurements with the airflow perturbation device (APD) were compared to directly measured pulmonary resistances with an esophageal balloon to validate the APD. The APD perturbs the flow and the mouth pressure during regular breathing. The ratio of mouth pressure perturbations to the flow perturbations was used to calculate the inspiratory, expiratory and average respiratory resistance. Six healthy subjects were tested during tidal breathing when known external resistances were added during inspiration, during expiration, and during both inspiration and expiration. The difference between the averaged APD measured and directly measured pulmonary resistances was 0.59 &plusmn 1.25 (mean &plusmn SD) cmH₂O/L/s. Compared to the magnitude of the known increase in added resistance, the APD measured resistance increased by 79 %, while the directly measured pulmonary resistance increased only by 56%. During addition of external resistances to both inspiration and expiration, the changes in inspiratory and expiratory pulmonary resistance were only 36 % and 62 % of the added resistance, respectively. On the other hand, the APD inhalation and exhalation resistance measured between 82 % and 76 % of added resistance change. It was concluded that the APD detects changes in external resistance at least as well and probably better than classical measurements of pulmonary resistance.

Additionally, expiratory isovolume pressure - flow (IVPF) curves, which show the pressure at which the flow becomes limited during forced expiration, were constructed in six healthy subjects with the classical invasive method of esophageal balloon (EB) and the alternative noninvasive method of stop - flow (SF) at 25, 50, and 75 % vital capacity (VC). The difference between the pressures (P_max) and flow (Q_max) at which flow limitation first occurs and correlation with the stop - flow and esophageal balloon methods were studied. Additionally, the resistance at flow limitation was compared to the APD resistance during forced breathing. On average, P_SF,max was 5.6 and 4.4 times P_EB,max at 25 %VC and 50 %VC, respectively. Q_SF,max was 0.68 and 0.59 times Q_EB,max at 25 %VC and 50 %VC, respectively. No correlation was found between the stop - flow and esophageal balloon methods as well as between the resistances at flow limitation.