Armed with the strongest evidence to date, researchers at the University of Wisconsin Medical School have established that sleep apnea -- episodes of breathing pauses during sleep -- is likely to be an important cause of hypertension. Results of their eight-year study involving more than 700 people appear in this week's (May 11) New England Journal of Medicine.
The UW researchers found that even mild to moderate levels of sleep apnea produced a substantial risk of hypertension. And the graver the sleep apnea, the greater the likelihood of developing hypertension, they observed. The new data come form the Wisconsin Sleep Cohort Study (WSCS), an on-going, population-based investigation of cardiopulmonary problems linked to sleep disorders. Funded by the National Heart, Lung and Blood Institute (NHLBI), the WSCS is directed by Dr. Terry Young, UW Medical School professor of preventive medicine.
According to NHLBI Director Dr. Claude Lenfant, "This study dramatically enhances our understanding of the role of sleep apnea in hypertension and provides additional evidence that it may be an independent risk factor for hypertension. Once we have additional follow-up data from the large NHLBI Sleep Heart Health Study, we should be able to make a more reliable determination of the precise role of sleep apnea as a risk factor for cardiovascular disease."
Most previous studies of sleep apnea and hypertension have provided only a cross sectional, or a one-glimpse-in-time, look at the simultaneous occurrence of the two disorders, according to lead author Dr. Paul Peppard of the UW Medical School department of preventive medicine. Some studies have been based on participants' individual reports of snoring, but not on objectively assessed sleep apnea.
"Our findings indicate that sleep apnea might be an important factor in the development of hypertension over time," said Peppard.
In the study, 709 WSCS participants, all randomly selected state employes, were monitored in an overnight stay in a special sleep laboratory at University of Wisconsin Hospital and Clinics' General Clinical Research Center. Researchers measured the number of episodes of breathing pauses each participant had during sleep to determine the presence and severity of sleep apnea. Five to fifteen pauses per hour represented mild to moderate sleep apnea; 15 or more pauses indicated moderate to severe sleep apnea. Participant health was reassessed four years later and in some cases eight years later as well.
The Wisconsin team statistically accounted for other factors related to hypertension -- age, gender, weight, menopause, smoking, alcohol use and education -- but still found that sleep apnea played a significant and independent role in hypertension risk. People with mild to moderate sleep apnea were twice as likely to become hypertensive; people with moderate to severe sleep apnea were almost three times as likely to become hypertensive.
"These findings tell us that clinicians should take seriously any symptoms or signs of sleep apnea in their patients," said Peppard. "New public health programs may also be important in helping people make lifestyle changes that offer protection against sleep apnea and its consequences."
"We are now studying whether sleep apnea aggravates existing cardiovascular disease and whether modifying the severity of sleep apnea may reduce the risk of hypertension and other health problems," noted Young.
Earlier findings from the Wisconsin Sleep Cohort Study include: sleep apnea is much more common than previously thought (New England Journal of Medicine, April 27, 1993); smoking puts people at high risk for sleep apnea (Archives of Internal Medicine, Oct. 10, 1994); doctors fail to recognize sleep apnea in women (Archives of Internal Medicine, Dec., 1996); people with undiagnosed sleep apnea may be at greater risk of automobile accidents (Sleep, Aug., 1997).
Dian Land
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