Background: Compliance with CPAP treatment for OSA is not reliably predicted by the severity of symptoms or physiological variables. I conducted a series of studies to examine a range of factors that may affect compliance with CPAP. Methods: I performed a retrospective study examining association of demographic factors and OSA severity with long-term CPAP compliance. In a prospective study, I looked at the correlation of short and long-term CPAP compliance with socio-economic status, education, type D personality, demographics, disease severity, mood and clinician's prediction. I undertook a prospective, cross-over trial comparing the impact of Bi-level PAP therapy in individuals with low tolerance of CPAP. Results: In a retrospective analysis, an improvement in subjective daytime somnolence was correlated with optimal compliance. In the prospective study, median compliance with CPAP at 6 months was 5.6 (3.4- 7.1) hours/night with 73% of subjects using CPAP ≥ 4 hours/night. Compliance with CPAP was not found to be associated with socio-economic class for people in work, type D personality, education, sex, age, baseline sleepiness (ESS score) or disease severity (ODI). The clinician's initial impression had no predictive value for individual patients. Subjects who were long-term unemployed or reporting mood disorders (High Beck's Depression Index scores) were likely to have poor compliance and sub-optimal CPAP usage (OR 4.6, p = 0.011 and OR 1.4. p=0.04 respectively). Subjects experiencing side effects after the first night on treatment showed lower acceptance and subsequent compliance. In the cross-over trial, changing to Bi-level PAP in individuals with suboptimal compliance due to pressure related intolerance, did not lead to an improvement in CPAP compliance. In post-hoc analysis, compliance and comfort were better in the subgroup that complained of difficulty with exhalation on CPAP. Conclusion: My research as presented in this thesis, did not find an association between disease severity (ODI), socio-economic status (for people in employment), education or personality type and CPAP compliance. My research demonstrated that subjects with long-term unemployment, mood disorders and those experiencing side effects on the first night of treatment were likely to have sub-optimal compliance. Changing to Bi-level PAP is only likely to be useful for a sub-group of subjects experiencing pressure related intolerance. More research is needed to explore whether intensive support to individuals with OSA and long term unemployment, as well as mood disorders, may improve compliance.

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