Objective To assess the effect of continuous positive airway pressure ( CPAP) on expiratory flow limitation in severe chronic obstructive pulmonary disease ( COPD) patients during noninvasive ventilation by oscillatory reactance ( Xrs ) . Methods Eight patients with stable COPD and chronic hypercapnic respiratory failure( type II) received noninvasive ventilation with a traditional CPAP ventilator through a nasal mask were enrolled. The CPAP were successively set as 4, 8 and 12 cm H2O respectively. The forecd oscillation( 5 Hz, 2 cm H2O) was imposed into the mask and the flow and nasal pressure were measured at the airway opening. The difference between inspiratory and expiratory Xrs( ΔXrs)were calculated for each breathing cycle and average ΔXrs was calculated at different pressure levels according to the established algorithm. Meanwhile, the oesophageal pressure was also measured by a balloontipped catheter and transpulmonary pressure was calculated. The breathing cycles above were analyzed subsequently and classified as expiratory flow-limited( EFL) and non-EFL breath. In addition, flow and nasal pressure when breathing naturally( CPAP = 0 cmH2O) was also collected for each patient and the EFL breath cycles was identified as baseline. Then, the percentage of EFL breathing cycles and ΔXrs were calculated for each CPAP level and their relationship was analyzed. The threshold value of ΔXrs with maximum sensitivity and specificity to detect EFL and the optimal CPAP to suppress the development of EFL were computed. Results ①CPAP increased from4 to 8 and 12 cm H2O resulted in fall of mean values of ΔXrs from2. 67 to 1. 62 and 1. 31 cm H2O· s- 1 · L- 1 , respectively( ΔXrs at CPAP 0 cm H2O was not detected) , and the decrease of ΔXrs when CPAP up to 8 cm H2O from 4 cm H2 O was significant ( Z = - 2. 68, P = 0. 01) . ②CPAP significantly suppressed the development of EFL, when CPAP increased from0 cm H2O to 4,8 and12 cmH2O resulted in decrease in the percentage of breathing cycle from 29. 8% to 9. 9% , 8. 1% and 4. 4%, respectively( 2 = 15. 6, P = 0. 01) . ③ ΔXrs was related to the degree of EFL and the mean value of ΔXrs in EFL breathing cycles was significantly higher than that in non-EFL’s. When ΔXrs decreased to 1. 83 cm H2O· s- 1 · L- 1, the majority of breath showed non-EFL, with a sensitivity of 94% and specificity of 97% for detecting EFL, respectively. Conclusions ΔXrs is an indicator of the occurrence of EFL. Appropriate CPAP to render the value of ΔXrs equal to or slightly less than 1. 83 cm H2O·s - 1 ·L- 1 may effectively suppress the development of EFL in severe COPD patients during noninvasive ventilation.