Milwaukee Journal Sentinel
(MCT)
MILWAUKEE, Wis. - People who stop breathing during sleep are more likely to lose their expected - and beneficial - drop in nighttime blood pressure, according to a new study by University of Wisconsin researchers.
This brings scientists one step closer to understanding how sleep apnea, characterized by brief pauses in breathing during sleep, contributes to the development of various cardiovascular diseases.
"We and other people have already found that sleep apnea is related to cardiovascular diseases such as hypertension, strokes and heart failure. We're always looking to see what are the mechanisms by which sleep apnea causes bad cardiovascular outcomes," said Khin Mae Hla, professor at the University of Wisconsin-Madison and lead author of the study, which is published in the June issue of Sleep.
Blood pressure usually drops by about 10 percent or 20 percent at nighttime when people sleep, which can help protect the body from cardiovascular disease. When blood pressure fails to drop at night, doctors call it "non-dipping."
Researchers from the UW School of Medicine and Public Health analyzed data from the ongoing Wisconsin Sleep Cohort Study, which included a subgroup of 328 volunteers who completed two or more 24-hour blood pressure monitoring studies and who were normal dippers at baseline. They found that sleep apnea patients who had normal dipping at the onset often went on to develop non-dipping when re-tested an average of 7.2 years later.
This is the first study to show an association over time between sleep apnea and non-dipping, Hla said.
After accounting for other factors that may affect results - age, sex, body mass index, hypertension, whether patients were on anti-hypertensive medication - researchers found that people who had between five to 15 sleep apnea events per hour were three times more likely to have non-dipping compared with people without sleep apnea. Sleepers with more than 15 sleep apnea events per hour were more than four times as likely to develop nocturnal blood pressure non-dipping compared with those without sleep apnea, Hla explained.
An apnea event includes sleep apnea, where a person stops breathing for at least 10 seconds, and hypopnea, where a person may not stop breathing but the airflow, and thus oxygen level, is reduced. The severity of sleep apnea is determined by the number of sleep apnea events that occur per hour.
This provides researchers with a sleep apnea index. In this study, five to 15 sleep apnea events is considered mild and more than 15 events per hour is considered moderate to severe.
Neurologist Joanna Galezowska, director of the Milwaukee Regional Sleep Disorders Center at Columbia St. Mary's, said sleep experts already knew that sleep apnea causes an elevation in blood pressure. When people stop breathing, she explained, blood pressure rises momentarily to compensate for the lack of oxygen. What Hla's study shows, however, is that blood pressure remains at elevated levels after the apnea event is over, she said.
"In a normal person at sleep, we are lowering the blood pressure as a natural physiological mechanism that helps us preserve our circulation system. In a sleep apnea patient, we are losing that mechanism, and more so, we are increasing the blood pressure," Galezowska said.
Sleep apnea is a common sleep disorder that affects more than 12 million Americans, according to the American Sleep Apnea Association. Risk factors include being overweight, male and older than 40, although it can affect anyone. In addition to the risk of cardiovascular disease, some of the other consequences of untreated sleep apnea include excessive daytime sleepiness, memory loss, impotency and headaches.
Physician Mary Klink, the clinic director of Wisconsin Sleep in Madison, said this study shows that people with even mild degrees of sleep apnea have a substantially increased risk of developing bad cardiovascular outcomes.
"Why treat cholesterol? If you don't, you're going to have trouble down the road. It's the same thing with sleep apnea," she said.
"I could fill a room with all the health risks of untreated sleep apnea."
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(c) 2008, Milwaukee Journal Sentinel. Distributed by Mclatchy-Tribune News Service.