Nonetheless, individual patients with OSA who are intolerant or unresponsive to initial CPAP therapy can respond favorably to bilevel PAP.21–22 OSA patients with comorbidities such as COPD, obesity-hypoventilation syndrome (OHS) or morbid obesity, CO2 retention, and prolonged nocturnal oxygen desaturations (reflecting nocturnal hypoventilation) are associated with better success rates.,21–22 APAP and expiratory phase pressure relief (eg, Cflex or Biflex; Respironics; Murrysville, PA) are alternatives that may also improve initial acceptance and subsequent compliance. By responding to changes in signals including flow, snoring, or impedance, APAP seeks the minimum effective level of PAP, reducing mean airway pressure. Improved adherence with APAP compared to CPAP has been shown in some studies,26 but not in two metaanalyses and a subsequent randomized crossover study.27–29 These conflicting findings might be due to differences in patient populations, or in APAP devices used in the various studies.30In another randomized crossover study31in patients with primary CPAP intolerance, CPAP ≥ 12 cm H2O, or mixed sleep apnea syndromes with ≥ 10% central respiratory events, APAP was as effective as bilevel PAP at reducing respiratory disturbances and was subjectively preferred over bilevel PAP by most patients. Expiratory-phase pressure relief varies pressure on a breath-by-breath basis, lowering pressure early during exhalation to enhance comfort, but the pressure returns to the critical pressure needed for upper airway patency before the next inspiratory cycle occurs. Although the efficacy of flexible PAP is comparable to that of CPAP for OSA, as with APAP, there are conflicting data about whether it enhances adherence compared to CPAP.33 However, neither APAP nor flexible PAP increase pressure during inhalation and, therefore, both may be less effective than bilevel PAP for management of hypoventilation associated with OSA.