• non invasive ventilation
• sleep apnoea
• health economist

Over the past decade, increasing attention has been paid to the evaluation and management of obesity hypoventilation syndrome (OHS).1 This disorder is characterised by daytime hypercapnia and three main phenotypes of sleep disordered breathing, including severe obstructive sleep apnoea (OSA), combined OSA and OHS and isolated OHS.2 Rising rates of global obesity along with a greater awareness of the significant health and social costs of this disorder have been driving factors fuelling interest in how best to manage those with OHS. Although the cornerstone of treatment has been to address sleep breathing abnormalities using positive airway pressure (PAP) therapy, the mode of therapy which optimises outcomes in the most cost-effective manner has been less clear.3–6

In many centres, OHS has become a major indication for home ventilation, with most individuals prescribed bilevel therapy.7 However, OHS can present as chronic respiratory failure as a consequence of OSA, OSA and OHS or lone OHS, with the OSA and OSA-OHS phenotypes accounting for more than 90% of individuals diagnosed with OHS, 70% of whom will have apnoea-hypopnea indices>30 events/hour.4 Although continuous single level PAP therapy (CPAP) does not directly provide inspiratory assistance to increase tidal volumes, correction of upper airway obstruction in conjunction with increased resting lung volumes, resetting of the respiratory centres, reduced WOB and prevention of expiratory flow limitation8 can improve gas exchange, alleviate symptoms and improve quality of life. Several medium-term randomised studies3 5 6 and one long term randomised trial9 comparing CPAP to bilevel therapy have failed to find significant differences between these therapies in terms of resolving waking chronic respiratory failure, improving quality of life, therapy adherence, …