BACKGROUND: Maximal expiratory flow rate is determined by the size of airway, the elastic recoil pressure and the collapsibility of airway in the lung, and one of major functional impairments of emphysema, which represents COPD, is the obstruction of expiratory flow. Neverthless, expiratory narrowing of upper airway may be recruited as a mechanism for minimizing airway collapse, and maintaining lung volume and hyperinflation by an endogenous positive end-expiratory pressure in patients with airflow obstruction. We investigated the physiologic role of trachea in respiration in emphysema. METHOD: We collected 20 patients with emphysema (which was diagnosed by radiologic and physio logic criteria) from January to August in 1997 at Seoul Municipal Boramae Hospital, and chest roentgenogram, high resolution computed tomography(HRCT), and pulmonary function tests including arterial blood gas analysis and body plethysmography were done from each patient. Cross-sectional area of trachea was measured according to the respiratory cycle on the level of aortic arch by HRCT and calibrated with body surface area. We compared this calibrated area with such parameters of pulmonary function tests as PaCO2, PaO2, airway resistance, lung compliance and so on. RESULTS: Expiratory cross-sectional area of trachea has significant correlation with PaCO2 (r=-0.61, p<0.05), PaO2 (r=0.6, p<0.05), and minute ventilation (r=0.73, p<0.05), but inspiratory cross-sectional area doesn't (r=-0.22, p>0.05 with PaCO2, r=0.26, p>0.05 with PaO2, and r=0.44, p>0.05 with minute ventilation). Minute ventilation has significant correlation with tidal volume (r=0.45, p<0.05), but it doesn't have significant correlation with respiratory frequency (r=-0.31, p>0.05). Cross-sectional area of trachea doesn't have any significant correlation with other parameters of pulmonary function such as FEV1, FVC, FEV1/FVC, peak expiratory flow, residual volume, diffusing capacity, airway resistance, and lung compliance, whether the area is expiratory or inspiratory. CONCLUSION: Cross-sectional area of trachea narrowed during expiration in emphysema and its expiratory area has significant correlation with PaCO2, PaO2, and minute ventilation.