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Obesity hypoventilation syndrome (OHS) is a condition in which severely overweight people fail to breathe rapidly or deeply enough, resulting in low oxygen levels and high blood carbon dioxide (CO<sub>2</sub>) levels. The syndrome is often associated with obstructive sleep apnea (OSA), which causes periods of absent or reduced breathing in sleep, resulting in many partial awakenings during the night and sleepiness during the day. The disease puts strain on the heart, which may lead to heart failure and leg swelling.
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Obesity hypoventilation syndrome is 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.
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The most effective treatment is weight loss, but this may require bariatric surgery to achieve. Weight loss of 25 to 30% is usually required to resolve the disorder. The disease was known initially in the 1950s, as "Pickwickian syndrome" in reference to The Pickwick Papers.
The low oxygen level leads to physiologic constriction of the pulmonary arteries to correct ventilation-perfusion mismatching, which puts excessive strain on the right side of the heart. When this leads to right-sided heart failure, it is known as cor pulmonale. Cor pulmonale occurs in about a third of all people with OHS. These factors together lead to sleep-disordered breathing and inadequate removal of carbon dioxide from the circulation and hence hypercapnia; given that carbon dioxide in aqueous solution combines with water to form an acid (CO<sub>2</sub>[g] + H<sub>2</sub>O[l] + excess H<sub>2</sub>O[l] --> H<sub>2</sub>CO<sub>3</sub>[aq]), this causes acidosis (increased acidity of the blood). Under normal circumstances, central chemoreceptors in the brain stem detect the acidity and respond by increasing the respiratory rate; in OHS, this "ventilatory response" is blunted.
The blunted ventilatory response is attributed to several factors. Obese people tend to have raised levels of the hormone leptin, which is secreted by adipose tissue and, under normal circumstances, increases ventilation. In OHS, this effect is reduced.
- Body mass index over 30 kg/m<sup>2</sup> (a measure of obesity, obtained by taking one's weight in kilograms and dividing it by one's height in meters squared)
- 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 (underactive thyroid), neuromuscular disease or congenital central hypoventilation syndrome
If OHS is suspected, various tests are required to confirm it. The most important initial test is the demonstration of elevated carbon dioxide in the blood. This requires an arterial blood gas determination, which involves taking a blood sample from an artery, usually the radial artery. Given that it would be complicated to perform this test on every patient with sleep-related breathing problems, some suggest that measuring bicarbonate levels in normal (venous) blood would be a reasonable screening test. If this is elevated (27 mmol/L or higher), blood gases should be measured.
Positive airway pressure
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 requires the use during sleep of a machine that delivers a continuous positive pressure to the airways and preventing the collapse of soft tissues in the throat during breathing; it is administered through a mask on either the mouth and nose together or if that is not tolerated, on the nose only (nasal CPAP). This relieves the features of obstructive sleep apnea and is often sufficient to remove the resultant accumulation of carbon dioxide. The pressure is increased until the obstructive symptoms (snoring and periods of apnea) have disappeared. CPAP alone is effective in more than 50% of people with OHS. Treatments without proven benefit, and concern for harm, include oxygen alone or respiratory stimulant medications. Medroxyprogesterone acetate, a progestin, and acetazolamide are both associated with an increased risk of thrombosis and are not recommended. This report, however, was preceded by other descriptions of hypoventilation in obesity. In the 1960s, various further discoveries were made that led to the distinction between obstructive sleep apnea and sleep hypoventilation.
The term "Pickwickian syndrome" has fallen out of favor because it does not distinguish obesity hypoventilation syndrome and sleep apnea as separate disorders (which may coexist).