Obesity-hypoventilation syndrome is characterized by daytime hypercapnia (arterial partial pressure of carbon dioxide of 45 mm Hg or more) in a patient with a body mass index of 30 kg/m2 or more, in the absence of alternative causes of hypoventilation. It should be suspected in patients with obesity who have daytime somnolence, dyspnea, fatigue, snoring, apnea, and sleep-related and daytime hypoxemia. The diagnosis is confirmed with overnight polysomnography. Additional tests such as measurement of arterial blood gases, serum bicarbonate, and arterial oxygen saturation can further support the diagnosis. Patients with obesity-hypoventilation syndrome have higher rates of intensive care unit admission and health risks; therefore, early diagnosis and specialist referral are crucial to reduce morbidity and mortality. Management includes weight loss (eg, through a comprehensive weight loss program or bariatric surgery) and positive airway pressure therapy (eg, continuous or bilevel positive airway pressure) to address hypercapnia and concurrent sleep-disordered breathing, if present.
Case 3. PG is a 45-year-old patient with severe obstructive sleep apnea and body mass index (BMI) of 41 kg/m2 (class 3 obesity) who presents to the emergency department for progressive daytime dyspnea and lethargy. Her oxygen saturation on room air while awake is low at 85%. An arterial blood gas measurement reveals a low pH of 7.24, an increased arterial partial pressure of carbon dioxide (Paco2) of 75 mm Hg, and a low arterial partial pressure of oxygen (Pao2) of 65 mm Hg. Her serum bicarbonate level is increased at 28 mEq/L (28 mmol/L). Chest radiography shows bilateral atelectasis without infiltrates or effusion. Following a dose of nebulized albuterol, she is placed on supplemental oxygen with bilevel positive airway pressure (BiPAP) and admitted to the intensive care unit.
Definition and Epidemiology
Obesity-hypoventilation syndrome, previously known as pickwickian syndrome, is defined by daytime hypercapnia (Paco2 of 45 mm Hg or more) in individuals with obesity (BMI of 30 kg/m2 or more) after excluding alternative causes of hypoventilation.1 This syndrome is classified into stages, as outlined in Table 1.2
Table 1 Stages of Obesity-Hypoventilation Syndrome
| Stage | Hypoventilation | OSA | Hypercapnia |
|---|---|---|---|
| 0 | At risk | OSA present | No hypercapnia |
| I | Obesity-associated hypoventilation | OSA or hypoventilation during sleep | Intermittent hypercapnia during sleep, full recovery when awake Serum bicarbonate level < 27 mEq/L (27 mmol/L) while awake |
| II | Obesity-associated hypoventilation | OSA or hypoventilation during sleep | Intermittent hypercapnia during sleep Serum bicarbonate level ≥ 27 mEq/L, increased during the day |
| III | Obesity-associated hypoventilation | OSA or hypoventilation during sleep | Sustained hypercapnia with Paco2 ≥ 45 mm Hg while awake |
| IV | OSA or hypoventilation during sleep | Sustained hypercapnia while awake with cardiovascular and metabolic comorbidities |
NOTE: Obesity is defined as body mass index ≥ 30 kg/m2.
OSA = obstructive sleep apnea. Paco2 = arterial partial pressure of carbon dioxide.
Adapted with permission from Randerath W, Verbraecken J, Andreas S, et al. Definition, discrimination, diagnosis and treatment of central breathing disturbances during sleep. Eur Respir J. 2017;49(1):1600959. © ERS 2025.
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