(CNN) A new study might tempt some sleep apnea patients to unplug their machines. Continuous positive airway pressure, or CPAP, does not reduce the risk of heart attacks or other cardiovascular emergencies in sleep apnea patients with heart disease, the researchers say.

A common treatment for sleep apnea, CPAP therapy includes a small machine that supplies a sleeper with constant and steady air pressure through a mask or nose piece.

Despite the disappointing study results, CPAP is still "worthwhile," said Dr. Doug McEvoy, the principal investigator. He explained in a news release that patients using CPAP "are much less sleepy and depressed, and their productivity and quality of life is enhanced." The New England Journal of Medicine published the study results on Sunday.

Head-to-head comparison

Up to 60% of patients with cardiovascular disease also suffer from obstructive sleep apnea. Previous small-scale studies have showed that some CPAP patients are less likely to suffer a cardiovascular complication, and other studies have showed that nightly CPAP use lowers blood pressure and improves blood flow in patients.

One 2005 study including hundreds of men found three times as many fatal heart attacks and strokes over a 10-year period among those who did not use CPAP compared with those who did. Notably, only some of the participants had existing heart problems going into the study.

To test how effective CPAP was in reducing cardiovascular events among patients with cardiovascular disease, McEvoy and his colleagues designed the Sleep Apnea Cardiovascular Endpoints (SAVE) study.

An "event," as defined by the researchers, would include a heart attack, stroke, mini-stroke, hospitalization for heart failure or death from any cardiovascular cause.

The team recruited 2,717 patients with moderate to severe obstructive sleep apnea from 89 medical centers in seven countries. Most of the participants were older (about 61 years old), overweight, snorers and male; all had coronary artery or cerebrovascular disease. McEvoy, a professor at Flinders University in Adelaide, Australia, and his team randomly divided patients into two groups: One group received usual care alone, and the other received usual care plus CPAP.

"Usual care" consisted of advice on healthy sleep habits and lifestyle changes along with cardiovascular risk management.

The researchers discovered that 42% of the patients assigned to CPAP used the machines for an average of four or more hours each night, though the overall average duration was 3.3 hours per night. Importantly, their sleep apnea severity decreased from 29 breath pauses per hour to four.

However, in terms of cardiovascular emergencies, the patients using CPAP in addition to following a usual care plan showed no differences from the usual care-only patients after more than three years, on average. Specifically, 17% of patients in the CPAP group and 15.4% in the usual-care group had some kind of serious heart event.

"It's not clear why CPAP treatment did not improve cardiovascular outcomes," McEvoy said.

Some wins

An editorial accompanying the study suggested that since McEvoy and his colleagues recruited participants from a variety of geographic locations, limited resources in some places may have reduced some patients' ability to stick with the CPAP program.

Still, there were clear wins: CPAP significantly reduced snoring and daytime sleepiness and improved quality of life and mood. In fact, work attendance improved among patients using CPAP.

Past studies have shown that obstructive sleep apnea can negatively impact attention, memory, learning and overall intellectual function. A new, unrelated review of recent research found that CPAP improved verbal memory after just two to three months of use, while using CPAP for six hours a night offered even more intellectual function benefits. Those included included improvements in attention and visual memory, as well.

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"The evidence is strong," said Dr. Charles Davies, lead author of the review and a neurologist at Carle Neuroscience Institute who specializes in sleep medicine. Davies looked at scientific studies performed over the past few years, which used the latest and most validated tests. In one of the studies, the researchers compared CPAP with sham CPAP, in which participants used a machine that did not provide enough pressure to be effective, and found improvements in intellectual abilities after just two months of CPAP use for about four hours a night. In particular, participants sharpened their abilities to quickly shift their focus and concentrate.

Though the current study showed CPAP as ineffective with heart health risks, it still provides many benefits, including an uptick in mood and, some patients would say, a little white noise that can be helpful for many sleepers. Check with your doctor before unplugging.