The terminal rise of N2 concentration in the single breath nitrogen washout curve (SBNWC) has been interpreted to be due to airway closure, flow limitation, or changes of sequential emptying from each lung area. In our present study, we examined the effect of uneven pulmonary compliance in excised dog lungs on the pattern of SBNWC especially on phase IV. Uneven distribution of pulmonary compliance was made by connecting the normal lung lobe and the ventilated lung lobe which produces significantly elevated elastic recoil to the Y tube (connected to the lung). After excision of each lung, one lobe was used as control and another one lobe was ventilated for one hour with large tidal volume (50% TLC). Ten dog lungs were used for SBNWC study and three lungs were for multiple (M) BNWC study. All control lungs showed one compartment in MBNWC and demonstrated flat phase III and no phase IV in SBNWC. On the other hand, ventilated lungs showed the existence of fast and slow compartments in MBNWC. SBNWC revealed a steep phase III without phase IV in three cases and a flat phase III with remarkable phase IV in seven cases. In connected lungs MBNWC clearly demonstrated two compartments, and steep phase III without phase IV was observed in six cases and relatively flat phase III with apparent phase IV was noted in four cases in SBNWC. In the present study, we employed slow flow rate (20ml/sec) for SBNWC and the deflation pressure-volume curve showed no inflection point which has been regarded as the beginning of airway closure in all cases. Our results suggest that phase IV could be caused by the uneven distribution of pulmonary compliance.
