One of these days, it might pay to locate sleep laboratories next to heart catheterization laboratories, some medical researchers say.
The reason is that sleep apnea, a common yet rarely diagnosed problem, is turning out to play a major, insidious role in heart disease, including congestive heart failure, high blood pressure and chest pain. Yet many cardiologists and heart disease patients are not aware of the problem, researchers into apnea have found.
A handful of "small but convincing" studies, all reported within the last five years, point to a strong link between heart disease and apnea, said Dr. James Kiley, director of the National Center of Sleep Disorders Research at the National Institutes of Health in Bethesda.
And at least one study suggests the value of a form of treatment for apnea that involves sleeping with a mask over the nose and mouth.
Because the studies are small, involving no more than a few hundred patients, "the link has not yet been proven in a definitive fashion," Dr. Kiley said. But the stakes are high -- millions of people could be affected by both problems.
So the institutes have begun a five-year, multicenter trial to determine if apnea is a direct cause of heart disease or merely an associated symptom.
Apnea, which comes from a Greek word meaning want of breath, first came to medical attention in 1965 when researchers began putting people into sleep laboratories, measuring their respiration and brain activity throughout the night. It results in brief interruptions of breathing during sleep, which in turn put a strain on the heart.
For example, as people stop breathing, the brain initiates a so-called fight or flight response up to 60 times each hour, giving the heart repeated jolts throughout the night. Instead of being restful and restorative, sleep becomes disruptive and dangerous.
This strain on the heart has especially important implications for the 2.5 million Americans with congestive heart failure, said Dr. Douglas Bradley, director of the Research Sleep Laboratory at the University of Toronto.
Most of these patients take a large assortment of drugs, yet their condition continues to deteriorate, he said. Many are severely short of breath and cannot walk from the couch to the kitchen without feeling exhausted.
Some studies have shown that a nondrug therapy, using a machine that delivers air to the lungs during the night, can work miracles in some of these patients, Dr. Bradley said. Their blood pressure falls, their fatigue goes away and they can cut back on medications.
A tough call
One difficulty with making patients aware of apnea, said Dr. John Remmers, a professor of medicine at the University of Calgary, is that most people who have sleep apnea do not realize it. The onset is slow and insidious, and people do not suddenly find themselves in pain, Dr. Remmers said.
Doctors also may not connect sleep apnea with the kinds of symptoms seen in their patients, including fatigue, high blood pressure, depression and impotence.
In addition, the method of diagnosis is cumbersome and costly. People need to spend a couple of nights in a sleep laboratory, hooked up to machines, at a cost of $1,000 to $2,000.
But the problem is huge, Dr. Remmers said. Estimates are that 10 percent of American men and 5 percent of women have sleep apnea. The problem may be less common in women because the hormone progesterone promotes breathing.
There are two kinds of sleep apnea and both are involved in heart disease, Dr. Kiley said.
The first, called central apnea, is more common in patients with ++ congestive heart failure. Such patients tend not to snore and many are not overweight, which is a risk factor for apnea. In fact, these patients may develop sleep apnea after having had a heart attack or other cardiac injury, Dr. Kiley said. But once the apnea sets in, it starts a vicious cycle that further damages the heart.
A damaged heart does not pump enough blood to the lungs for oxygenation, Dr. Kiley said. The lungs may fill with fluid and carbon dioxide builds up. During the day, receptors in a neck artery sense the low blood oxygen and send signals to the brain stem, telling the person to breathe. Most heart failure patients hyperventilate all day to get enough air, he said.
But when a person falls asleep, Dr. Kiley explained, the brain enters a state that is neurochemically different from wakefulness. When these same receptors detect low oxygen levels during sleep, they delay sending a message to the brain stem and the heart patient stops breathing.
After many seconds, an alarm is set off and breathing is resumed -- but only after causing some serious trouble.
The alarm both stimulates the brain stem and the higher cortex, which wakes up (usually without awareness) and it overreacts, Dr. Kiley said. In a startle response, it releases a shot of adrenalin that causes the heart to race. At the same time, the low oxygen level causes blood pressure to shoot up.
This chronic elevation of adrenalin and blood pressure causes irregular heartbeats and with elevated blood pressure over time reduces the heart's pumping ability. In such cases sleeping makes heart disease worse.
Doctors give drugs to lower blood pressure but they are treating patients based on daytime blood pressure, not realizing that it may hit peaks at night, Dr. Kiley said. Moreover, some of the drugs interfere with sleep, worsening the apnea and putting a further load on the heart, he said.
The second type of apnea is obstructive. It is most common in people who are overweight with fat necks, said Dr. Shahrokh Javaheri, a professor of medicine at the University of Cincinnati. Muscle tone keeps the throat open during the day, he said, but at night the weight of the oversize neck narrows the airway and the tongue falls back and closes it. The person struggles to breathe.
When this happens, arteries in the lung detect low oxygen and constrict. The heart's right chamber, which sends blood to the lungs, has to pump harder to overcome increased pressure, Dr. Javaheri said. With time, the right side of the heart suffers damage.
Meanwhile, the brain's oxygen receptors are arousing the cortex, creating havoc with adrenalin.
Virtually all people with obstructive sleep apnea snore, but the reverse is not true, Dr. Remmers said.
RTC Among the small studies that have revealed a link between heart disease and apnea is one in which Dr. Javaheri asked 42 patients with heart failure to spend a night in a sleep laboratory. Nearly half had severe, undiagnosed apnea.
One study that suggests a possible treatment was conducted )) by Dr. Bradley in Toronto. He asked eight obese men with obstructive sleep apnea and heart failure of no known cause to use a special breathing device called CPAP, for continuous pneumatic airway pressure. The device uses a face mask to deliver gentle air pressure to the lungs.
"When you apply pressure to the lungs, you also increase pressure to the heart," Dr. Bradley said. "It's like CPR, but it uses the lungs instead of hands. It really reduces the heart's workload, allowing it to rest and recover function."
After one month on CPAP, the eight patients showed striking improvement, far greater than any drugs could do, Dr. Bradley said. Then CPAP was withdrawn. After a week, their former condition returned, he said.
In similar studies, Dr. Bradley found that with CPAP the patients' quality of life improved greatly. In some cases, blood pressure returned to normal levels.
Encouraged by these results, the national institutes have begun a large study of 6,000 people at risk for heart disease, including factors like family history, genetic predisposition and high blood pressure. Subjects will be divided into those who have existing heart disease and those who do not.
Each person will have his or her sleep monitored at home, using a special vest worn over the chest. One finger and the forehead are also fitted with electrodes. The equipment records sleep patterns that are later analyzed by computers, looking for central and obstructive apnea.
"If those with apnea have more heart attacks and more severe disease over time, it would be a strong indication that apnea is an independent risk factor for heart disease," Dr. Kiley said.
Meanwhile, federal researchers are trying to promote awareness the problem. "Our message is that if you are a heavy snorer, if you've been told you have episodes of stopping breathing, you should not treat this as benign," Dr. Kiley said. "You could be putting your heart at increased risk." Individuals can ask their doctors for referral to a sleep laboratory.