For people who have had a procedure to open blocked heart arteries, untreated sleep-breathing problems like snoring or apnea may raise the risk of a future heart attack or stroke, researchers say.

Patients in Japan who had the artery-clearing procedure after experiencing chest pain or a heart attack were more than twice as likely to have heart failure, a heart attack or a stroke in the next five years if they also had sleep-breathing problems.

Intermittent low-oxygen periods during sleep may increase stress or activate inflammatory responses that damage the heart, said lead author Dr. Toru Mazaki of the department of cardiology at Kobe Central Hospital in Japan.

Obstructive sleep apnea, the most common form of sleep apnea, is a condition where breathing stops for several seconds during sleep because tissue in the throat collapses, blocking the airway. This can happen dozens of times a night, interrupting sleep and leaving the person feeling tired during the day.

Snoring, usually the result of only partial blockage in the sinuses or other parts of the respiratory tract, also reduces the amount of oxygen a sleeper gets.

So-called sleep-disordered breathing, especially apnea, has been tied to increased risk of heart disease and stroke in past research.

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To see if it affects risk for people who have already been treated for blocked arteries, the researchers used heart and breathing monitors on 241 patients to track breathing issues over one night of sleep. All the participants had undergone angioplasty, the clearing of a blocked heart artery, including placement of a tube called a stent to keep the artery open.

The sleep measurements took place about one week after each person's angioplasty.

Just over half of the patients were found to have sleep-disordered breathing, meaning five or more disrupted breathing events per hour, according to the report in the Journal of the American Heart Association.

The researchers then tracked participants for an average of five and a half years. During that time, 10 people with sleep disordered breathing and three without sleep breathing issues died. Major adverse events like heart attack and stroke had occurred in more than 20 percent of those with sleep breathing issues, compared to 8 percent of those without breathing problems.

"There is limited awareness of sleep-disordered breathing among doctors who care for patients with heart attack," Mazaki told Reuters Health by email. "Doctors and patients should consider sleep studies following heart attack and angioplasty to rule out sleep-disordered breathing or take necessary precautions to restore healthy breathing during sleep."

Sleep-disordered breathing has been associated with cardiovascular risks and symptoms like high blood pressure, elevated glucose and abnormal heart rhythms, said Dr. Nieca Goldberg of NYU Langone Medical Center in New York who is also a spokesperson for the American Heart Association and was not part of the new study.

"Hopefully based on this study, doctors will pay more attention to their patient's symptoms of fatigue or awakening from sleep," Goldberg told Reuters Health by email.