Obesity hypoventilation syndrome (OHS) is a condition that occurs in obese people, in which poor breathing leads to lower oxygen levels and higher carbon dioxide levels in the blood.

The exact cause of OHS in unknown. Most (but not all) patients with the syndrome have a form of sleep apnea.

OHS is believed to result from both a defect in the brain's control over breathing, and excessive weight (due to obesity) against the chest wall, which makes it hard for a person to take a deep breath. As a result, the blood has too much carbon dioxide and not enough oxygen. People with OHS are often tired due to sleep loss, poor sleep quality, and chronic hypoxia.

Excess (morbid) obesity is the main risk factor.

See also: Respiratory acidosis

The main symptoms of OHS are due to lack of sleep and include:

• Excessive daytime sleepiness
• Falling asleep during the day
• Increased risk for accidents or mistakes at work
• Depression

Symptoms of low blood oxygen level (chronic hypoxia) can also occur, such as shortness of breath or feeling tired after very little effort.

People with OHS are usually very overweight. Symptoms of OHS include:

• Bluish color in the lips, fingers, toes, or skin (cyanosis)
• Signs of right-side heart failure (cor pulmonale), such as swollen legs or feet, shortness of breath, or feeling tired after little effort
• Reddish complexion
• A short, thick neck and small airway passage in the mouth

Tests to confirm OHS include:

• Sleep study
• Lung (pulmonary) function
• Arterial blood gas

Doctors can tell OHS from obstructive sleep apnea by high carbon dioxide levels in the blood when a person is awake.