Obesity hypoventilation syndrome !

Severe obesity makes some people breathe too little, so carbon dioxide builds up in the blood while awake, not only during sleep.

Updated 2026-11-01

Structures involved

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Why it happens

What it is

The combination of obesity (body mass index of 30 or more) and daytime hypercapnia (arterial PaCO2 above 45 mmHg) during wakefulness, after other causes of hypoventilation have been excluded. Most patients also have obstructive sleep apnoea. It is also called Pickwickian syndrome.

Who gets it

Found in roughly one in ten to one in five people with obesity referred for sleep studies, and more often as body mass index rises above 40. It is underdiagnosed and often recognised only during an acute admission with hypercapnic respiratory failure. Rising obesity in Colombia and Latin America makes it increasingly relevant; living at high altitude, as in Bogotá, adds hypoxaemia.

Causes

Severe obesity reducing chest wall and lung compliance
Blunted central respiratory drive to carbon dioxide (leptin resistance is proposed)
Obstructive sleep apnoea with repeated nocturnal hypercapnia
Weak or mechanically disadvantaged respiratory muscles and diaphragm

Risk factors

Body mass index of 40 or more, central (abdominal) obesity
Severe obstructive sleep apnoea
Raised serum bicarbonate in a patient with obesity
Use of sedatives, opioids or alcohol
Coexisting hypothyroidism or chronic lung disease

Pathophysiology

Fat on the chest wall and abdomen pushes the diaphragm up, lowers lung volumes and increases the work of breathing. Upper airway obstruction during sleep causes repeated hypercapnia; the kidneys retain bicarbonate, which blunts the brainstem (medullary) response to CO2, so the hypoventilation persists into the day. Chronic hypoxaemia causes pulmonary vasoconstriction, pulmonary hypertension, right heart strain and secondary polycythaemia.

Classification

With obstructive sleep apnoea (about 90% of cases)
Sleep hypoventilation without significant apnoea
Severity graded by daytime PaCO2, hypoxaemia and presence of cor pulmonale

Symptoms and signs

Excessive daytime sleepiness and fatigue
Loud snoring, witnessed apnoeas and unrefreshing sleep
Morning headache (from nocturnal hypercapnia)
Breathlessness on exertion
Leg swelling, cyanosis and plethora when cor pulmonale and polycythaemia develop

Diagnosis

Clinical: Suspected in any person with obesity and daytime sleepiness, unexplained hypoxaemia, or hypercapnia found in hospital. Diagnosis requires obesity, awake hypercapnia and exclusion of other causes (COPD, neuromuscular disease, chest wall deformity, hypothyroidism, drugs).
Tests: Arterial blood gas while awake: PaCO2 above 45 mmHg with compensated respiratory acidosis. Serum bicarbonate of 27 mmol/L or more is a useful screen. Polysomnography confirms sleep apnoea and nocturnal hypoventilation. Spirometry (mild restriction), thyroid function, full blood count (polycythaemia), ECG.
Imaging: Chest X-ray to exclude lung or chest wall disease and to show cardiomegaly; echocardiography to look for pulmonary hypertension and right ventricular dysfunction.

Differential diagnosis

Isolated obstructive sleep apnoea without daytime hypercapnia
Chronic obstructive pulmonary disease with CO2 retention
Neuromuscular disease (amyotrophic lateral sclerosis, myasthenia gravis, diaphragm paralysis)
Kyphoscoliosis and other chest wall disorders
Severe hypothyroidism and drug-induced hypoventilation

Goal of treatment

Restore normal ventilation during sleep, normalise daytime CO2 and oxygen, prevent hypercapnic crises and cor pulmonale, and achieve sustained weight loss.

Treatment

Procedure: Positive airway pressure at night: CPAP when obstructive apnoea dominates, non-invasive ventilation (bilevel pressure) when hypoventilation persists or in severe disease.
Acute: In acute-on-chronic hypercapnic respiratory failure: non-invasive ventilation, controlled oxygen targeting moderate saturations, and intubation if consciousness falls or ventilation fails.
Lifestyle: Sustained weight loss through diet, physical activity and, when indicated, anti-obesity drugs; avoidance of alcohol and sedatives.
Surgical: Bariatric surgery for severe obesity can resolve or greatly improve the syndrome.
Follow-up: Nursing: upright positioning, continuous oximetry, monitoring of consciousness and repeated gas analysis, careful use of opioids and sedatives, and checking mask fit and adherence.

Complications

Acute-on-chronic hypercapnic respiratory failure
Pulmonary hypertension and cor pulmonale
Secondary polycythaemia
Heart failure and arrhythmias
Higher perioperative and anaesthetic risk

Prognosis

Untreated, mortality is high, with frequent admissions and early death from cardiorespiratory failure. Regular nocturnal positive airway pressure and weight loss improve gas exchange, symptoms and survival.

Prevention

Prevention and treatment of obesity
Screening of patients with obesity and sleep apnoea for raised bicarbonate or hypercapnia
Caution with sedatives and opioids in people with severe obesity

Test yourself

Think of the answer, then tap the question to see it.

What three criteria define obesity hypoventilation syndrome?

BMI of 30 or more, awake daytime hypercapnia (PaCO2 above 45 mmHg), and exclusion of other causes of hypoventilation.

Which simple blood test is a useful screen?

Serum bicarbonate; a raised value reflects metabolic compensation for chronic CO2 retention.

Why can uncontrolled high-flow oxygen be harmful in these patients?

It can worsen hypercapnia and acidosis; oxygen is titrated to moderate saturations together with ventilatory support.

What is the mainstay of long-term treatment?

Nocturnal positive airway pressure (CPAP or bilevel non-invasive ventilation) plus weight loss.

Which cardiac complication follows chronic hypoxaemia?

Pulmonary hypertension with right ventricular failure (cor pulmonale).

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Obesity hypoventilation syndrome (OHS) (MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine), 2024); Obesity Hypoventilation Syndrome (National Heart, Lung, and Blood Institute (NHLBI, NIH), 2024).

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Representative model, for educational purposes only.

Diaphragm highlighted on the Durgical 3D model