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Obesity Hypoventilation Syndrome
The following article reviewed information published on the obesity hypoventilation syndrome (OHS) up to 2008. Patients with OHS usually also have obstructive sleep apnea (OSA) but often require treatment other than, or in addition to, continuous positive airway pressure (CPAP).
SUMMARY OF:
Assessment and Management of Patients with Obesity Hypoventilation Syndrome
http://pats.atsjournals.org/cgi/content/short/5/2/218
Authors: Mokhlesi B, Kryger MH, Grunstein RR. Published in: Proceedings of the American Thoracic Society; Year: 2008; Volume: 5; Pages: 208-225.
This syndrome is often diagnosed late in its course. Patients may have symptoms of OSA such as: snoring, daytime sleepiness, morning headaches, and choking sensations at night. They may also have significant shortness of breath. The diagnosis of OHS requires a blood test taken from an artery, called an arterial blood gas, which shows a high level of carbon dioxide and low level of oxygen. However, there are many other things that can cause this abnormality as well, so it needs to be interpreted by a doctor. Patients with the OHS may also have abnormal breathing tests, showing small lungs (this is due to the excess body weight limiting chest wall movement and expansion of the lungs) or reduced sensitivity of the breathing control system to abnormal carbon dioxide and oxygen levels. The diagnosis also requires a sleep study to see if the person has OSA, because this is commonly associated with, and may also play a role in causing OHS.
Although the causes of OHS are not completely clear, the authors of this paper review some of the most likely theories. The first is that excess body weight places a strain on the breathing system, increasing the work of breathing to such an extent that the respiratory muscles (diaphragm and chest wall muscles) can no longer work hard enough to maintain a normal depth of breathing. This causes the person to reduce the depth of their breathing (hypoventilation) which causes carbon dioxide levels to rise, and oxygen to fall. The second cause is related to a reduced response to elevated carbon dioxide levels and low oxygen levels in these patients. This can be related to OSA, which is present in 90% of patients with OHS. In OSA, recurrent breathing pauses (apneas) caused by collapse of the throat during sleep cause blood levels of carbon dioxide to rise, and oxygen to fall. Over time, the exposure of the brain to these elevated carbon dioxide and reduced oxygen levels may reduce its drive to breathe more deeply in order to lower carbon dioxide and raise oxygen to the normal range. For reasons not well understood, these abnormalities of the breathing control system that develop during sleep carry over into the daytime so that carbon dioxide becomes elevated and oxygen becomes reduced not only at night, but around the clock.
The article outlines the importance of identifying people with OHS since they can develop life-threatening complications including: respiratory failure (dangerously high carbon dioxide and low oxygen), elevated pressure in the blood vessels of the lungs (pulmonary hypertension), and heart failure.
Finally, this paper reviews treatments available for OHS. In those with OSA, the OSA must be treated. Many years ago this used to be done with a tracheostomy, a hole made in the windpipe just below the voice box, to bypass the collapsed throat during sleep. However, this has largely been replaced with continuous positive airway pressure (CPAP), applied through a mask over the nose or mouth, that props the throat open and prevents apneas. In some people with OHS however, CPAP may not work because it does not provide enough assistance to increase the depth of breathing, either because the patient is too obese or because their brain fails to send a signal strong enough to stimulate the muscles of breathing to work harder. In such individuals a breathing machine called bi-level positive airway pressure (BiPAP) or non-invasive ventilation (NIV) provides much greater assistance to increase the depth of breathing on inspiration, while at the same time propping open the throat to prevent OSA. Other treatment options include oxygen therapy, needed in about half of the patients with OHS, in combination with CPAP or BiPAP. Medications that stimulate breathing have been studied but, in most cases, have not been shown to work. Weight loss is recommended for all patients with OHS. The required weight loss is often difficult to achieve by diet or exercise alone, and may require a surgical procedure that reduces the size of the stomach so that patients cannot eat as much. This is called bariatric surgery, which is often successful in causing major weight loss, improving OSA, and unloading the respiratory muscles allowing them to increase the depth of breathing and to reduce carbon dioxide and raise oxygen levels into the normal range.
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