CFlex (Respironics) Bi-Level device lowers expiratory PAP (EPAP) early in expiration returning EPAP to the set inspiratory PAP (IPAP) before the start of the next inspiration. Most patients with OSAS will express a preference for either CPAP or CFlex at the time of treatment initiation. We investigated the possibility that choice of CFlex was related to interaction of breathing pattern and the CFlex device in 43 newly diagnosed patients with OSAS in a single-blind study. Inspiratory and expiratory time (Ti, Te) were measured by rib-cage and abdominal inductance belts. IPAP and EPAP were measured at the mask by pressure transducer. In 23 patients we recorded simultaneous flow in the CPAP circuit and by using mask specific leak rates derived values for inspiratory tidal volume (Vt insp): (Vt insp = (Mean Inspiratory Circuit Flow x Ti) - (Mask leak x Ti)) Patients tried CPAP or CFlex in random order for periods of 10 minutes or until breathing was stable. One minute epochs of stable breathing were used to calculate mean values for Ti, Te, IPAP, EPAP and Vt insp
Results 19 patients stated a preference for CFlex (CFlexpref), 20 for CPAP (CPAPpref). 4 had no preference and received CPAP . We included them in the CPAP . pref group for analysis. For the whole group (n=43) there was a small but significant fall in mean EPAP on CFlex compared with mean EPAP on CPAP (10.4 vs 11.5 cm H2O). Comparing CPAP . pref with CFlexpref we found no significant difference between mean EPAP on CPAP and mean EPAP on CFlex making it unlikely that preference for CFlex was based only on EPAP reduction. In the 23 patients (CPAPpref=13, CFlexpref=10) who had Vt insp measured we derived an index of the flow resistive load to expiration (Table). The CFlexpref group demonstrated a significant fall in this index of expiratory load. The changes in the CPAPref group were more variable. The findings are consistent with CFlex preferers generating and sensing a larger reduction in expiratory load on CFlex.