Obesity · Respiratory

Obesity hypoventilation: more than just big OSA

Written by Dr Ari Manuel, Consultant in Sleep & Respiratory Medicine

Obesity hypoventilation syndrome (OHS) — defined by obesity, sleep-disordered breathing, and daytime hypercapnia in the absence of an alternative cause — is substantially underdiagnosed in the UK. Many patients with OHS are managed as straightforward OSA for years before the ventilatory component is recognised. The consequences of under-treatment include pulmonary hypertension, right heart failure, and significantly increased mortality.

The role of adipose tissue hypoxia

Research conducted during my DPhil at the University of Oxford investigated adipose tissue hypoxia in severely obese patients and its relationship to ventilatory failure. We found that markers of adipose tissue hypoxia were elevated in severely obese patients with OHS compared to those with OSA alone — suggesting that metabolic dysfunction in obesity is at least partly driven by tissue-level hypoxia, and that OHS may represent a more advanced stage of obesity-related respiratory disease. This work has informed my approach to assessing the full metabolic context in patients presenting with respiratory symptoms.

Serum bicarbonate: the screening tool and its limits

Elevated serum bicarbonate (or total CO₂) has been used as a simple screening tool for OHS — the rationale being that chronic hypercapnia drives renal bicarbonate retention. In work published in CHEST (2015) and a more recent paper in Sleep (2025), I and colleagues have highlighted the limitations of this approach: bicarbonate can be elevated in the absence of frank daytime hypercapnia, and conversely can be suppressed by GLP-1 receptor agonists and SGLT2 inhibitors — both now in widespread use. Clinicians relying on bicarbonate alone to screen for OHS in patients taking these agents risk missing the diagnosis.

GLP-1 agents and OSA/OHS management

The approval of tirzepatide (Mounjaro) for weight management has changed the landscape significantly. Clinical trials including SURMOUNT-OSA have demonstrated meaningful reductions in AHI with tirzepatide in patients with OSA and obesity. For patients with OHS who achieve significant weight loss, ventilatory requirements may reduce over time, though device therapy should not be discontinued without formal reassessment. Where clinically appropriate and with specialist oversight, we can advise on referral pathways for GLP-1 agents as part of an integrated respiratory and weight management plan.