Introduction 1 2 5 6 11 A typical complaint by those using conventional CPAP is that it is difficult to exhale against positive pressure. Therefore, reducing pressure during exhalation may improve tolerance with therapy. C-Flex (Respironics, Murrysville, PA, USA) is a novel algorithm designed to provide pressure relief during expiration, while maintaining optimal pneumatic splinting for effective therapy. 12 13 15 This is the first study to evaluate the REMStar Auto with C-Flex (RSA C-Flex) as a therapy for patients with OSA. The major purpose of the study was to assess the ability of RSA C-Flex to effectively treat the sleep apnea events. Satisfaction and preference from the patients’ perspective, and the number of required interactions per night by technicians were also determined. Materials and methods Study subjects 16 The study was approved by the Ethics Board at the University of British Columbia. Procedures and techniques Sleep laboratory rooms were configured with a device that could deliver either conventional CPAP or RSA C-Flex. The study occurred on two separate nights, within 2 weeks of each other. For the first night, patients were randomized to either conventional CPAP at the previously determined effective pressure or RSA C-Flex. The patient was not aware of which of the therapies they would be receiving. However, the technician, by necessity, was aware of the therapy choice. On the second experimental night, subjects received the alternative therapy (crossover design). 17 16 18 Preference for device was assessed on the morning after the second experimental night. Subjects were asked “In comparison to the first night of the study, please state a preference for the CPAP machine set-up”. Subjects were asked to place a mark on a line to indicate the degree of preference for the first night (0–10) or the second night (0–10). Mean score was calculated for both nights, with a score of less than 0 indicating preference for the other night. The technicians recorded any required interactions with the subject, during the night. Conventional CPAP without C-Flex CPAP pressure was set to the therapy pressure determined during a previous titration study. A 20-min ramp was used with the initial pressure being set as 66% of the previously titrated pressure. RSA with C-Flex 1 Fig. 1 p delivered p base p relief p relief  2 Statistical analysis t Results 2 2 1 p 2 2 Table 1 Outcomes with RSA C-Flex vs standard CPAP   RSA C-Flex CPAP P AHI (events/h) 4.2 ±  2.4 ± 0.7 0.1 Sleep latency (min) 17.0 ± 5 12.3 ± 3 0.4 Sleep efficiency (%) 82 ± 2 83 ± 2 0.6 WASO (min) 67 ± 14 65 ± 12 0.9 REM % 25 ± 2 24 ± 2 0.7 Stage 3/4 (%) 4 ± 1.5 1.6 ± 0.5 0.13 Mean oxyhemoglobin saturation (%) 97.3 ± 0.3 97.2 ± 0.3 0.93 PLMI (events/h) 2.8 ± 1.0 3.8 ± 1.5 0.59 Values given as mean±SEM AHI WASO PLMI Fig. 2 p p  3 t p 4 Fig. 3 t p  Fig. 4 a t p b p  t p 4 p Discussion In our study, RSA C-Flex was as effective as standard CPAP in treating sleep-disordered breathing. There were no differences in sleep-related breathing or architecture variables between the arms. However, the RSA C-Flex arm was associated with a trend in improvement in patient satisfaction as assessed by VAS scores. Patients preferred RSA C-Flex over standard CPAP. 19 p 12 20 In this two-night study, we showed that RSA C-Flex is as effective as standard CPAP in reducing AHI. We noted that one patient with a treatment AHI of >20 had a high proportion of central sleep apnea. Review of this patient’s diagnostic study revealed a very high baseline AHI (>100/h) with the presence of central apneas (index>20/h). As such we feel that residual central apneas in this patient reflect baseline disease rather than being caused by RSA C-Flex. However, the possibility that the RSA C-Flex may predispose to central events in a subgroup of patients cannot be excluded. Patients preferred RSA C-Flex over standard CPAP, and may have found it more comfortable. As such, we speculate that RSA C-Flex may improve compliance if used as chronic therapy. Furthermore, one of the major determinants of long-term compliance with CPAP is the early experience with the therapy. For many patients, in-laboratory titration represents the first exposure of any duration to CPAP therapy (Appendix 2). Any positive impact on this initial experience has the potential to improve compliance in the longer term. Use of RSA C-flex may thus be useful for initial titration of CPAP in the laboratory. For similar reasons, RSA C-flex may be useful in patients who are being titrated at home with APAP. The impact of RSA C-Flex on long-term compliance with chronic use, as “salvage” therapy of patients intolerant of standard CPAP, or its utility as part of a laboratory/ambulatory CPAP titration need to be studied in larger clinical trials. We acknowledge that there are a number of limitations to our study. First, although the patient was not told of the nightly treatment assignment, the technician was not blinded. For our study, it was not possible for the technician to be unaware of the treatment. Second, we have studied a relatively small number of patients. Third, we have only studied patients for two nights in the laboratory and as such, we cannot extrapolate our study to chronic treatment. We have not tested CPAP adherence in this study; our results merely suggest a reasonable hypothesis for future study—that use of the RSA C-Flex may have a favorable effect on compliance. Finally, this study included patients with predominately severe disease who were compliant with CPAP. We may not be able to extrapolate our results to patients with less severe sleep apnea, or to patients with other concomitant respiratory diseases (such as COPD). Conclusion In this small short-term study, both RSA C-Flex and standard CPAP (with pressure defined by an in-laboratory CPAP titration) were effective in treating sleep apnea. Most patients preferred RSA C-flex. Future studies with this technology are warranted to assess its effects on long-term compliance and its potential as rescue therapy for those initially noncompliant with standard CPAP therapy.