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Obese patients may experience more severe blood oxygen desaturation during apnea and hypopnea events, researchers say.

Body mass index (BMI) was positively associated with the severity of oxygen desaturation, independently of confounding variables that included age, gender, sleeping position, baseline oxygen saturation, according to Paul E. Peppard, PhD, of the University of Wisconsin-Madison.

Researchers reported their findings online in the American Journal of Respiratory and Critical Care Medicine.

"We knew that excess body weight is strongly related to more frequent breathing events in persons with sleep-disordered breathing," Peppard said in a statement. "In this study, we wanted to go a step further and measure how much the excess weight contributes to the severity of individual breathing events."

It's well-established that obesity increases the risk and severity of sleep-disordered breathing, and some studies have shown that weight loss is effective in reducing apneas and hypopneas.

But the specific extent to which body weight contributes to blood oxygen desaturation during these events had not been determined.

So the researchers assessed 750 patients in the Wisconsin Sleep Cohort Study, ages 30 to 62, who were monitored for oxygen saturation, duration, and other characteristics. A total of 37,473 breathing events were observed during polysomnography studies.

Obese patients accounted for 40% of the cohort but contributed 62% of breathing events, and mean oxygen desaturation was 4.8%.

The researchers found that BMI was positively associated with oxygen desaturation severity independent of age, gender, sleeping position, baseline oxygen saturation, and event duration.

For example, they said, a 50-year-old nonsmoking male with a BMI of 30 kg/m2 and a baseline saturation of 100% who has a 30-second hypopnea with an 80% tidal volume while on his side in non-rapid eye movement (NREM) sleep would likely have a mean change in oxygen saturation of 5.9% (95% CI 5.5% to 6.2%).

If that BMI were increased to 40 kg/m2, the researchers said, and all other variables were unchanged, that patient would have desaturation of about 6.4%.

BMI also predicted greater desaturation in rapid eye movement (REM) sleep than in NREM sleep.

"The impact of BMI is significantly modified by sleep state, thus, changes in oxygen saturation during apnea or hypopnea are predicted to be greater in REM compared with NREM sleep, but are especially so in persons with higher BMI," the researchers said.

They also noted that each 10 kg/m2-increase in BMI predicted a 1% greater mean blood oxygen desaturation for those in REM sleep experiencing hypopnea events.

The researchers said that other variables were important in predicting change in oxygen saturation. The supine position was associated with a 0.54% greater change in saturation than the lateral position.

Increasing age and smoking also independently predicted greater oxygen desaturation.

As for the potential mechanisms linking obesity with desaturation, the researchers said that excess body weight causes a significant reduction in lung volume and affects ventilation-perfusion ratio.

The study was limited by lack of detailed pulmonary function testing, and Peppard called for further research into how repeated oxygen desaturations affect clinical outcomes.

"These results reinforce the importance of excess weight as a risk factor for the development, progression, and severity of sleep-disordered breathing," Peppard said.

"Clinicians should consider the possibility that particularly overweight patients might be experiencing severer consequences of [the disease] even if they have the same number of breathing events as less overweight patients."