Single breath N2 washout in papain-induced pulmonary emphysema

Single breath nitrogen washout tests were analyzed in dogs (n=8) with healthy lungs and after development of emphysema. The animals were in the supine position and studied during anaesthesia and mechanical ventilation (F_iO₂=0.4, F_iN₂=0.6). During controlled expiration with constant flow (V_E=0.15 1/s) onset of phase IV of the alveolar plateau was related to airway closure of dependent lung regions (closing volume CV). In the control state, CV accounted for 6.2±1.5% VC, and closing capacity (CC) was lower than functional residual capacity (FRC). Likewise, gas exchange was normal in all animals (PaO₂=24.7±3.32 kPa, PaCO₂=5.18±0.53 kPa, P_A-aO₂=2.6±0.3 kPa). Panlobular emphysema (PLE) was induced by inhalation of papain (100 mg/kg). After three weeks development of PLE was documented by messurements of lung volumes (functional residual capacity (FRC), expired vital capacity (EVC), total lung capacity (TLC), residual volume (RV)), pulmonary mechanics (dynamic and static compliance (C_dyn, C_stat), mean airway resistance (R_aw)), gas exchange (PaO₂, PaCO₂, P_A-aO₂), and by radiomorphological analysis. In the PLE-group, FRC and RV (p0.05), and C_stat (p0.01) were significantly elevated. CV increased to 16.2±2.7% VC (p0.01) and CC exceeded FRC by 80 ml, indicating that tidal volume breathing took place within the range of closing volume. Oxygenation was significantly impaired (PaO₂=18.6±3.72 kPa, P_A-aO₂-6.5±1.1 kPa, p0.05), but not CO₂-elimination. Pathological analysis by radiomorphological means showed dissiminate parenchymal lesions compatible with emphysema of grade II severity located predominantly in subpleural areas. In dogs with papain-induced PLE, premature closure of dependent airways is enhanced, which is due to structural changes and a loss of elastic recoil in the lungs.