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What Causes Obesity Hypoventilation Syndrome?

Breathing Disruption Has Multiple Causes

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Updated July 12, 2012

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Obesity hypoventilation syndrome consists of significant difficulty breathing among people who are obese, but what causes it? By better understanding why it occurs, you may be able to seek the proper treatments that can put things right. It is also important to appreciate the connection to obstructive sleep apnea, a common condition with overlapping symptoms.

How Obesity Hypoventilation Syndrome Occurs

Obesity hypoventilation syndrome (OHS) occurs when breathing is inadequate to rid the body of carbon dioxide in someone who is obese. There may be a handful of underlying reasons that contribute to this outcome. Ultimately, the result is the same, and these problems breathing may lead to complete respiratory failure. This can be identified by measuring the levels of carbon dioxide in the blood, which become elevated during wakefulness in people with obesity hypoventilation syndrome.

Carbon dioxide is a waste product that is normally blown off from our lungs in exchange for oxygen. When breathing becomes inadequate, due to a variety of causes, this cannot occur. Instead, the carbon dioxide remains in our circulation and slowly builds up. It becomes a poison with toxic effects, leading to sleepiness and (eventually) unconsciousness or even death.

The term hypoventilation refers to inadequate breathing. It may result when the breaths are not of sufficient volume or when they do not occur frequently enough. Imagine only being able to fill your lungs half full. These shallow breaths would make it difficult to eliminate the carbon dioxide and take in the oxygen that you need to live. Moreover, taking a breath less often than you need will quickly leave you feeling short of breath. The hypoventilation that characterizes this condition may be due to a combination of these factors. Unfortunately, those who are afflicted find these limitations are beyond their conscious control to overcome.

The Important Role of Obstructive Sleep Apnea

It cannot be overstated how central a role obstructive sleep apnea has in this condition. In fact, sleep apnea occurs in 85 to 92% of people with obesity hypoventilation syndrome. This overlap may be due to a similar underlying mechanism. It is also possible that OHS represents an extreme form of sleep apnea in which breathing becomes so compromised that it begins to have other daytime consequences, specifically shortness of breath (or dyspnea) with exertion.

As a reminder, sleep apnea occurs when the upper airway becomes partially or completely blocked during sleep. This obstruction leads to audible pauses in breathing. This disruption has two consequences: Oxygen levels drop while carbon dioxide levels rise. If these events of apnea are infrequent, your body is able to recover and there may be no appreciable consequences. However, when apnea occurs more often, there is no time to set things right. The processes that would normally compensate, including changes to correct the chemical balance of your blood, cannot occur.

Breathing Becomes More Difficult in Obesity

In general, the effort to breathe becomes more difficult among people who are obese. It is difficult to expand lungs against the added pressure that the excessive weight imposes. Picture yourself trying to inflate a balloon with a straw. It is hard work. Now put a heavy book on top of the balloon and try the same thing. It becomes a real chore. In the same way, the extra weight on an obese person makes it challenging for the lungs to fill up.

The lungs are normally filled with the help of the diaphragm and respiratory muscles along the rib cage. When these muscles pull, the lungs fill like a bellows. Obese people have a modest reduction in the strength of the muscles. Not only are they fighting against the resistance described above, but the muscles used aren’t as strong as they should be either.

These factors in combination lead to an increased work of breathing. This will tire a person out, so that eventually shallower or less frequent breaths are taken. This results in the hypoventilation that so characterizes this syndrome.

The Body’s Adaptation Worsens Hypoventilation

As a result of the difficulty breathing, the body attempts to adapt to the situation. Unfortunately, some of these changes actually make the hypoventilation worse.

The brain begins to ignore the signals of low oxygen levels and high carbon dioxide in the blood. These signals would normally trigger the brain to prompt the body to breathe more rapidly in an attempt to correct the abnormalities. When the condition becomes chronic, the alarm gets ignored. Fortunately, treatment quickly corrects this built-in response system.

It is also well-known that obese people have abnormal levels of a hormone called leptin. It is not clear what role leptin may play in changing breathing patterns, however. The research on this has led to conflicting evidence to this point.

Finally, because the lungs are not fully inflated, the lower lobes may remain collapsed. This makes it difficult to aerate the blood that circulates to these parts of the lungs. As a result, the problems with oxygen and carbon dioxide exchange are exacerbated.

The underlying causes of obesity hypoventilation syndrome are multifactorial. Ultimately it occurs when there is inadequate exchange of oxygen and carbon dioxide. This may be due in part to the physical limitations imposed on the lungs by obesity. There is also clearly a role for obstructive sleep apnea, as this disrupted nighttime breathing makes things worse. Even the body's natural adaptions begin to fail. Fortunately, there are effective treatment options available that can correct this situation.

Sources:

Bickelmann, AG et al. "Extreme obesity associated with alveolar hypoventilation; a Pickwickian syndrome." Am J Med 1956;21:811.

Martin, TJ et al. "Alveolar hypoventilation: A review for clinicians." Sleep 1995;18:617.

Mokhlesi, B et al. "Obesity hypoventilation syndrome: prevalence and predictors in patients with obstructive sleep apnea." Sleep Breath 2007;11:117.

Mokhlesi, B et al. "Assessment and management of patients with obesity hypoventilation syndrome." Proc Am Thorac Soc 2008;5:218.

Piper, AJ et al. "Current perspectives on the obesity hypoventilation syndrome." Curr Opin Pulm Med 2007;13:490.

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