Specifically they say that sleep apnea, a disorder marked by breathing interruptions during sleep, may contribute to the development of heart failure, rather than the other way around.
"This has been an area of great interest," says Dr. Virend Somers, one of the study's authors and a professor of medicine and cardiology at the Mayo Clinic in Rochester, Minn. The work appears in the Feb. 11 issue of Circulation: Journal of the American Heart Association.
Both obstructive sleep apnea (which involves an actual obstruction of the airways) and central sleep apnea (in which breathing reflexes just stop for brief periods of time) exist in people with heart failure. Central sleep apnea is the more common of the two among heart patients. "We don't know why. We've always assumed it was a consequence of heart failure," Somers says.
In fact, the opposite seems to be the case. "This study raises suspicion that central apnea is doing bad things to vulnerable people," Somers says. "Certainly, it seems to come before overt heart failure."
The study team, which was led by Dr. Paola Lanfranchi of Fondazione Salvatore Maugeri in Veruno, Italy, looked at 47 people who were considered at high risk for heart failure because they had problems with their left ventricular heart pumping function. None of the participants had actually developed full-blown failure yet, making this study somewhat unusual. More than half (55 percent) of the study subjects also had central sleep apnea; 36 percent had severe apnea. Some 11 percent had obstructive sleep apnea.
"Two main things came out of the study," Somers says. "Yes, there's a fairly high likelihood that someone with dysfunction of the heart will have central sleep apnea, and it turns out that those with central sleep apnea and heart dysfunction have an increased likelihood of heart arrhythmias even during the day."
It's not clear what the underlying biological mechanisms may be. "It may be related to activation of the sympathetic [adrenaline-producing] nerves that is known to occur during central sleep apnea," Somers postulates. "Adrenaline-like substances have detrimental effects in heart failure patients."
That would also be the guess of Dr. Thomas Kilkenny, director of the Institute of Sleep Medicine at Staten Island University Hospital. Sleep apnea -- whether central or obstructive -- can begin early in life, although it doesn't usually manifest until a person reaches adulthood. "This is a slowly progressive disorder, and a significant amount of damage occurs to the heart," he says.
A person with sleep apnea suffers little or not-so-little interruptions to their breathing. "You have some people that choke for 45 seconds or a minute and oxygen levels can dip to almost half of what they should be," Kilkenny says. "At the end, the body gets upset with the fact that it's not breathing correctly, and then there's the awakening."
And it's the awakening that may be at the heart of the problem, so to speak.
"That awakening causes damage to the heart because it releases a whole cascade of the fight-or-flight chemicals, such as norepinephrine and epinephrine," Kilkenny says. "The body thinks it's suffocating. All the chemicals rev up. Blood pressure goes sky high and, when the body figures out that you're taking breaths, the chemicals go back down," he says. But then the cycle repeats itself.
Kilkenny says he has seen people go through this cycle 110 times an hour. The worst was 220 times an hour. Multiply that by 30 years and you can see how the heart might suffer.
Answering this "why" question will be crucial in formulating prevention and treatment strategies. The current research points to which direction to go in.
"This doesn't provide a clear-cut next step to diagnosis and treating, but it suggests that we should be more careful in evaluating people at risk for heart failure for an apnea," Somers says. "We don't know for sure if people with central apnea are more likely to go on for bad heart failure, but our findings suggest that, yes, they are going to be at higher risk."