Forced oscillation total respiratory resistance and spontaneous breathing lung resistance in COPD patients

Forced-oscillation total respiratory resistance (Rrs) has been shown to und erestimate spontaneous breathing lung resistance (RL,sb) in patients with a irway obstruction, probably owing to upper airway shunting. The present stu dy reinvestigates that relationship in seven severely obstructed chronic ob structive pulmonary disease patients using a technique that minimizes that artefact. Rrs at 8 and 16 Hz was computed for each successive forced oscillation cycl e. Inspiratory and expiratory RL,sb were obtained by analysing transpulmona ry pressure (Ptp),with a four-coefficient model, and compared to Rrs over t he same periods. "Instantaneous" values of RL,sb were also obtained by comp uting the dynamic component of Ptp, and compared to simultaneous values of Rrs. In both respiratory phases, good agreement between Rrs and RL,sb Was observ ed up to RL,sb values of approximately 15 hPa.s(-1).L-1 at 8 Hz and 10 hPa. s(-1).L-1 at 16 Hz, Instantaneous Rrs and RL,sb varied systematically durin g the respiratory cycle, exhibiting various amounts of flow- or volume-depe ndence in the seven patients; the amplitudes of their variations were signi ficantly correlated, but Rrs was much more flow-dependent than RL,sb in thr ee patients, Also, Rrs exceeded RL,sb at end-expiration in three instances, which could be related to expiratory flow limitation. In conclusion, total respiratory resistance is reliable up to much higher l evels of airway obstruction than previously thought, provided upper airway shunting is avoided.