Both obesity and obstructive sleep apnea are increasingly common, and the overlap of the two may cause a condition called obesity hypoventilation syndrome. Difficulty breathing and the retention of carbon dioxide characterize this syndrome. How do you know if you have this disorder? What are the key features for the diagnosis of obesity hypoventilation syndrome? Are there additional tests that your doctor should do to rule out other potential causes of your difficulty breathing? Learn more about the diagnosis of obesity hypoventilation syndrome.
Key Features of Obesity Hypoventilation Syndrome
There are several key elements that are used to diagnose someone with obesity hypoventilation syndrome. The diagnosis relies on both a careful physical examination as well as additional testing.
First, by definition, you must be obese, with a body mass index (BMI) greater than 30. Frequently the BMI is quite elevated, often greater than 50. People with a thick neck and narrow upper airway are also predisposed to the condition. Your doctor might notice changes in the color of your skin (either red or blue discoloration). There may be swelling in your feet. In listening to your heart and lungs, it might also become clear that you have signs of heart failure.
Laboratory studies will be arranged to further investigate your condition. Basic studies of the chemicals or electrolytes in your blood, including magnesium and phosphorus, should occur. A key measurement is the level of carbon dioxide during wakefulness in your arterial blood (the blood that flows through your oxygenated arteries, not your veins). This may be assessed with an arterial blood gas (ABG). In addition, your oxygen and bicarbonate levels are measured. It might also be useful to measure your blood count (called a hematocrit) and the function of your thyroid with the thyroid stimulating hormone (TSH) test.
Further Testing May Prove Necessary
It is important to rule out other possible causes of your difficulty breathing. This requires a careful assessment of your heart and lungs. A simple electrocardiogram (EKG) may be arranged in the office. Formal testing of your heart function with an echocardiogram may also be warranted.
In some cases, heart failure may be investigated with cardiac catheterization, in which the pressure within the blood vessels, function of the valves, and movements of the heart muscle are measured. People with obesity hypoventilation syndrome may have right atrial and ventricular hypertrophy and pulmonary hypertension.
Pulmonary function testing (PFTs) should reveal no other cause of the shortness of breath aside from obesity. People who are overweight or obese have a reduced ability to fill their lungs (resulting in a lowered vital capacity). The ability to move the air is not compromised, however. At the end of exhaling a breath, there is a lower amount of air remaining in the lungs of obese people (because there was less air to begin with).
A routine chest x-ray may show enlargement of the heart and the diaphragm may be higher on each side due to the obesity (the large abdomen collapses the lungs). Rarely, a specialized scan called a computerized tomography (CT) pulmonary angiogram of the chest may be done.
Finally, one of the most definitive tests is a routine sleep study called a polysomnogram (PSG). During the PSG of someone with obesity hypoventilation syndrome, the oxygen levels will fall during sleep. There will be evidence of obstructive sleep apnea with prolonged pauses in breathing. It will be clear that the affected person is not able to recover as quickly from the events of apnea, confirming an underlying difficulty breathing. Most people with obesity hypoventilation syndrome will be started on continuous positive airway pressure (CPAP) or bilevel treatment the night of their sleep study.
Ruling Out Other Causes
The diagnosis of obesity hypoventilation syndrome cannot occur if there is another underlying cause of the breathing difficulties. Many of the tests described above are attempting to rule out these other causes of respiratory failure. When high levels of carbon dioxide are present, the following conditions should be ruled out:
- Lung problems
- Chronic obstructive pulmonary disease (COPD)
- Restrictive lung disease (diseases that affect the muscles of breathing, the chest wall, and the tissue surrounding the lungs)
- Left-sided ventricular heart failure
- Hypothyroidism (an underactive thyroid gland)
- Paralysis of the diaphragm
When these conditions have been ruled out through the appropriate testing, the diagnosis of obesity hypoventilation syndrome can be made in the setting of obesity with increased levels of carbon dioxide within the blood. Once the diagnosis has been made, you can turn your attention to effective treatment.
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.
