At first the researchers did not see a statistically significant difference between the groups. However, 36 percent of the CPAP group was using the machine less than four hours a night.
In a follow-up analysis, which the authors pointed out may be open to bias, the researchers found that patients using CPAP for at least four hours a night were 28 percent less likely than the control group to develop high blood pressure.
Dr. Stuart Quan, professor of sleep medicine at Harvard Medical School in Boston, wasn't surprised by the findings. "I already believe that sleep apnea impacts [high blood pressure] and treating with CPAP reduces the risk, so these studies do not affect my way of thinking about this," he said.
Quan added that he prescribes CPAP to patients with at least moderate obstructive sleep apnea or those with sleep apnea and symptoms such as daytime sleepiness or mood problems.
Medicare requires patients to use CPAP at least four hours a night for 70 percent of nights to cover the treatment. The insurance deductible for CPAP is usually between $100 and $500, Quan said.
While the study uncovered an association between CPAP use and reduced risk of developing high blood pressure, it did not prove a cause-and-effect relationship.
To learn more about sleep apnea, visit the U.S. National Heart, Lung, and Blood Institute.
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