Obesity Hypoventilation Syndrome (OHS)

What happens in OHS

In OHS, several factors combine to reduce breathing:

• Extra weight on the chest and belly makes breathing harder and lungs smaller
• The brain's breathing drive becomes less responsive to rising carbon dioxide
• Obstructive sleep apnea repeatedly blocks the airway at night, letting carbon dioxide build up
• Hormones such as leptin may be involved in reduced breathing drive

Carbon dioxide (CO2) rises (hypercapnia) and oxygen falls (hypoxemia). Over time, the kidneys retain bicarbonate to balance the acidity, and the heart is strained by low oxygen.

How common it is

OHS affects a significant proportion of people with severe obesity and obstructive sleep apnea. It is underdiagnosed: many people are first identified during a hospital admission for breathing failure.

Risk factors

• BMI of 30 or more, with risk rising sharply above 40
• obstructive sleep apnea
• weight concentrated around the waist, chest and neck
• sedatives, opioids and alcohol, which suppress breathing

Symptoms

• daytime sleepiness and falling asleep easily
• loud snoring, gasping and pauses in breathing at night
• morning headaches from high CO2
• shortness of breath on exertion
• fatigue and low energy
• poor concentration and memory, mood changes
• swelling of the legs and feet from heart strain
• bluish lips or fingertips in advanced disease

Diagnosis

OHS is diagnosed when:

1. BMI is 30 or more
2. arterial blood gas while awake shows CO2 above 45 mm Hg
3. other causes of hypoventilation are excluded, such as COPD, severe asthma, neuromuscular disease, chest wall deformity and hypothyroidism

Other tests:

• serum bicarbonate: a raised level is a useful screening test
• sleep study (polysomnography): confirms sleep apnea and nocturnal hypoventilation
• lung function tests and chest X-ray
• echocardiogram: for pulmonary hypertension and right heart strain
• blood count: for polycythemia (high red cell count)
• thyroid tests

OHS vs obstructive sleep apnea

Treatment

Positive airway pressure

• CPAP at night is the first-line treatment for most people with OHS and severe obstructive sleep apnea
• BiPAP (noninvasive ventilation) is used when CPAP is not enough, sleep apnea is mild, or CO2 remains high
• consistent nightly use, ideally more than 5 to 6 hours, improves CO2, oxygen, sleepiness and survival

Weight loss

Losing 25 to 30 percent of body weight can resolve OHS in many people. Options for treating obesity include:

• structured diet and exercise programs
• anti-obesity medicines such as GLP-1 receptor agonists
• bariatric surgery, which is often the most effective long-term option for severe obesity

Other measures

• supplemental oxygen in addition to PAP if oxygen remains low
• treating heart failure and fluid retention with diuretics
• avoiding alcohol, sedatives and opioids
• vaccination against flu and pneumonia
• tracheostomy in rare severe cases not responding to other treatment

Complications

• pulmonary hypertension and cor pulmonale (right-sided heart failure)
• acute respiratory failure needing intensive care
• high blood pressure and heart rhythm problems
• polycythemia
• higher risk of complications from surgery and anesthesia

Outlook

Untreated OHS has a high risk of hospitalization and death. With consistent PAP therapy and weight loss, breathing, sleepiness, heart function and quality of life improve and survival increases significantly.

Frequently asked questions

What is obesity hypoventilation syndrome?

OHS is a condition in which people with obesity breathe too shallowly or slowly, so carbon dioxide builds up in the blood and oxygen falls, even when awake. Most people with OHS also have obstructive sleep apnea.

What are the symptoms of obesity hypoventilation syndrome?

Common symptoms are daytime sleepiness, loud snoring, morning headaches, breathlessness, fatigue, poor concentration and leg swelling. Severe cases can cause confusion and bluish lips.

How is OHS different from sleep apnea?

Sleep apnea causes breathing pauses during sleep, but blood CO2 is normal while awake. In OHS, CO2 stays high even during the day, reflecting more severe breathing failure and higher risks for the heart.

How is obesity hypoventilation syndrome treated?

The main treatments are nightly CPAP or BiPAP and significant weight loss, which may include medicines or bariatric surgery. Oxygen, diuretics and avoiding sedatives may also be needed.

Can OHS be cured?

Substantial weight loss, often 25 to 30 percent of body weight, can resolve OHS in many people. Until then, consistent nightly PAP therapy controls the condition and reduces complications.