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PHILADELPHIA, May 29 -- The most obvious clinical feature of obstructive sleep apnea may be weariness of mind, but this breathing disorder also slowly and silently wears down the heart, according to researchers.

Recent evidence suggests that 60% of people hospitalized for myocardial infarction also have obstructive sleep apnea. Yet occurrence among the general population is only about 9%, said Jose R. Marquina, M.D., of the Collier County Medical Society in Naples, Fla., at the American Academy of Physician Assistants meeting here.

In addition, recent research also suggests that as many as 70% of patients hospitalized for stroke have obstructive sleep apnea as a comorbid condition, Dr. Marquina said.

During the night, severe sleep apnea sufferers stop breathing anywhere from 30 to 120 times per hour, and the resulting lack of oxygen in the body strains and stupefies the cardiovascular system, Dr. Marquina said.

Over the years, hypoxemia in patients with obstructive sleep apnea can damage the sinus node, the group of specialized heart cells that govern cardiac rhythm, he said. This hypoxic damage causes a variety of cardiac arrhythmias including bradycardia, atrial fibrillation, and sinus arrest.

In addition, he said, the stress of hypoxemia causes the body to release catecholines, a group of hormones linked to hypertension.

Oxygen levels in the blood are also involved in controlling pulmonary artery pressure, Dr. Marquina said, and chronic hypoxemia can, therefore, also lead to pulmonary hypertension. In fact, recent evidence suggests that 12% to 17% of patients with obstructive sleep apnea also have pulmonary hypertension, he said.

These cardiovascular effects of obstructive sleep apnea occur in addition to the more well known neurological effects, which include cerebral anoxia, cerebrovascular disease, and impaired memory and concentration, Dr. Marquina said.

Obstructive sleep apnea can also severely reduce a person's overall quality of life. The condition has been associated with poor performance at work or school, depression, and marital problems, he said.

Men are affected twice as often as women: occurrence is 4% among men versus 2% among women, Dr. Marquina said. The risk for obstructive sleep apnea also increases among those who are overweight. Among overweight men the occurrence rises to 24%, and it reaches 9% for overweight women, he said.

As many as 90% of cases of obstructive sleep apnea go undiagnosed, Dr. Marquina said. Clinicians should be more aware of the symptoms, which include excessive daytime sleepiness or tiredness upon waking. "The typical patient will say they slept for eight hours but woke up still feeling tired," he said.

Another symptom is loud snoring, although snoring also occurs normally in about 70% of individuals age 35 and older, Dr. Marquina said. Patients will often deny snoring, so it may be more useful to ask a bed partner about this condition, he added.

Up to five episodes of apnea per hour while sleeping is also normal for healthy individuals, Dr. Marquina said. But five to 15 episodes identified by polysomnography indicate mild obstructive sleep apnea, 15 to 30 are defined as a moderate condition, and 30 or more are defined as severe obstructive sleep apnea, he said.

In addition, obstructive sleep apnea tends to be associated with a neck circumference of 18 inches or more, he said.

"Obstructive sleep apnea must be treated not only to improve the symptoms of fatigue and overall quality of life but to prevent the development of cardiovascular problems," Dr. Marquina concluded.

Dr. Marquina is a speaker for Cephalon.

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