Obesity hypoventilation syndrome

Obesity hypoventilation syndrome (OHS)- a condition in which severely overweight people fail to breathe rapidly or deeply enough.

It results  in low oxygen levels and high blood carbon dioxide (CO2) levels in such obese persons.

About a third of all people with morbid obesity have elevated carbon dioxide levels in the blood.

In people with obstructive sleep apnea, 10–20% meet the criteria for OHS as well. 

The risk of OHS is much higher in those with more severe obesity, i.e. a body mass index (BMI) of 40 kg/m2 or higher. 

It is twice as common in men compared to women. 

Defined as the combination of obesity and an increased blood carbon dioxide level during the day that is not attributable to another cause of excessively slow or shallow breathing.

Obesity hypoventilation syndrome is a form of sleep disordered breathing. 

OHS is often associated with obstructive sleep apnea (OSA).

OHS strains the heart, and may lead to heart failure and leg swelling.

Also known as the Pickwickian syndrome.

It often improves with positive airway pressure treatment.

Defined as the combination of obesity and an increased blood carbon dioxide level during the day that is not attributable to another cause of excessively slow or shallow breathing.

The most effective treatment is weight loss.

Weight loss of 25 to 30% is usually required to resolve the disorder.

Non-invasive positive airway pressure (PAP), usually in the form of continuous positive airway pressure (CPAP) at night, is the primary therapy

Sleepiness may be worsened by elevated blood levels of carbon dioxide, which causes drowsiness.

Associated with depression, and hypertension.

Carbon dioxide elevation can also cause headaches, which tend to be worse in the morning.

Its associated low oxygen level leads to constriction of the pulmonary arteries to correct ventilation-perfusion mismatching, adding excessive strain on the right side of the heart, leading to right sided heart failure.

Cor pulmonale occurs because the heart has difficulty pumping blood through the lungs. 

Ch\haracteristic findings are the presence of a raised jugular venous pressure, a palpable parasternal heave, a heart murmur due to blood leaking through the tricuspid valve, hepatomegaly, ascites and leg edema.

Cor pulmonale occurs in about a third of all people with OHS.

With OHS the work of breathing is increased as adipose tissue restricts the normal movement of the chest muscles,  makes the chest wall less compliant, the diaphragm moves less effectively, respiratory muscles are fatigued more easily, and airflow in and out of the lung is impaired by excessive tissue in the head and neck area. 

People with obesity need to expend more energy to breathe effectively.

Their sleep-disordered breathing and inadequate removal of carbon dioxide from the circulation and hence hypercapnia, causing acidosis.

The central chemoreceptors in the brain stem detect the acidity, and respond by increasing the respiratory rate.

However, ventilatory response to CO2 is blunted.

Obese people tend to have raised levels of the hormone leptin, which is secreted by adipose tissue and under normal circumstances increases ventilation, but this effect is reduced in OHS.

Low oxygen levels lead to hypoxic pulmonary vasoconstriction, causing 

chronic vasoconstriction leading to pulmonary hypertension.

This in turn puts strain on the right ventricle that  undergoes remodeling,and leads to corbpulmonale.

The chronically low oxygen levels in the blood also leads to increased release of erythropoietin activity and erythropoeisis , resulting  in polycythemia.


Body mass index over 30 kg/m2.

Arterial carbon dioxide level over 45 mmHg or 6.0 kPa as determined by arterial blood gas measurement.

No alternative explanation for hypoventilation, such as use of narcotics, severe obstructive or interstitial lung disease, severe chest wall disorders such as kyphoscoliosis, severe hypothyroidism, neuromuscular disease or congenital central hypoventilation syndrome.

The most important initial test is the demonstration of elevated carbon dioxide in the blood, requiring an arterial blood gas determination.

Measuring bicarbonate levels in venous blood would be a reasonable screening test: if elevated (27 mmol/l or higher), blood gasses should be measured.

To distinguish various subtypes, polysomnography is required. 

Echo- and electrocardiography may also show strain on the right side of the heart caused by OHS, and spirometry may show a restrictive pattern related to obesity.

Two subtypes are recognized:

The first is OHS in the context of obstructive sleep apnea; this is confirmed by the occurrence of 5 or more episodes of apnea, hypopnea or respiratory-related arousals per hour a high apnea-hypopnea index, during sleep. 

The second is OHS primarily due to sleep hypoventilation syndrome:

a rise of CO2 levels by 10 mmHg after sleep compared to awake measurements and overnight drops in oxygen levels without simultaneous apnea or hypopnea.

90% of all people with OHS fall into the first category, and 10% in the second.


THE most important treatment is weight loss—diet, exercise, medications, or bariatric surgery.

Weight loss is not always successful.

If the symptoms are significant, nighttime positive airway pressure (PAP) treatment is employed.

Positive airway pressure, initially in the form of continuous positive airway pressure (CPAP), is a useful treatment for obesity hypoventilation syndrome, particularly when obstructive sleep apnea coexists. 

CPAP alone is effective in more than 50% of people with OHS.

If the oxygen levels are persistently too low, the hypoventilation itself may be improved by switching from CPAP treatment to bi-level positive pressure: higher pressure during inspiration and a lower pressure during expiration.

The  addition of oxygen therapy may be necessary. 

Tracheostomy combined with mechanical ventilation with an assisted breathing device may be a last resort approach.

Medroxyprogesterone, acetazolamide are both associated with an increased risk of thrombosis and are not recommended.

OHS is associated with a reduced quality of life.

OHS often occurs with several other asthma (in 18–24%), type 2 diabetes (in 30–32%). 

Its main complication is heart failure, which affects 21–32% of patients.

Severe OHS have an increased risk of death reported to be 23% over 18 months and 46% over 50 months. 

This risk is reduced to less than 10% in those receiving treatment with PAP, which also reduces the need for hospital admissions and reduces healthcare costs.

The average age at diagnosis is 52. 

American Blacks are more likely to develop OHS.

Obese Asians are more likely than people of other ethnicities to have OHS at a lower BMI as a result of physical characteristics.

The rates of OHS will rise as the prevalence of obesity rises.

Leave a Reply

Your email address will not be published. Required fields are marked *