Abstract: Objective To study the use of oscillatory resistance (Rrs) and reactance (Xrs) to guide the settings of pressure level of ventilators in patients with severe chronic obstructive pulmonary disease (COPD) during noninvasive positive pressure ventilation (NPPV). Methods Five Hz oscillatory inspiratory Rrs (Rrs,in) at different NPPV pressure levels in 8 patients were calculated and compared to inspiratory pulmonary rwsistance (RL,in) measured by means of oesophageal manometry. The difference between inspiratory and expiratory Xrs (△Xrs) at different CPAP levels were also measured for each breathing cycle, which were subsequently analyzed and classified as flow-limited (EFL) or non-EFL by means of Mead-Whittenberger method. Then, the relationship among CPAP levels, the percentage of EFL breathing cycles at different CPAP levels and △Xrs were analyzed and the threshold value of △Xrs with maximum sensitivity and specificity to detect EFL were calculated. Results The mean values of Rrs,in and 0.64, P<0.01, but the agreement between these two resistances were poor and with a median of -2.7 F = 109.6, P<0.01. Increase in CPAP level resulted in decrease in AXrs and in numbers of EFL breathing cycles. The mean value of △Xrs in EFL breathing cycles was significantly higher than that in NFL, and the specificity of 97% respectively. Conclusions Although slightly underestimated, Rrs,in is helpful to guide the settings of inspiratory positive airway pressure level, which will appropriately overcome the elevated pulmonary resistance, and the threshold value of △Xrs, of end-expiratory pressure, which will eliminate the majority of EFL breathing cycles.