More than 25 million Americans have obstructive sleep apnea, a dangerous disorder that causes sufferers to briefly stop breathing while they sleep, sometimes many times each night. The condition, caused by throat muscles that relax and block the airway, has been linked to heart disease, high blood pressure, strokes and Type 2 diabetes.

The standard treatment, the continuous positive airway pressure, or CPAP, machine, keeps the airway open by pumping a stream of air through a patient’s nostrils as he or she sleeps. The biggest problem with the therapy is non-compliance; many people find the air mask and hose uncomfortable and give up on the machine.

But a large new sleep study published Sunday raises a serious new issue: For people with existing cardiovascular disease and moderate to severe sleep apnea, CPAP doesn’t prevent heart attacks, strokes, hospitalizations or deaths any better than sleeping without the machine.

The finding surprised scientists from two Australian research institutes who conducted the research, because previous examinations have shown that CPAP seemed to have a positive impact on those problems.

In earlier studies, “the epidemiological data is very strong and the biomarker data is very strong,” said Craig S. Anderson, a professor of stroke neurology at the George Institute for Global Health in Sydney, and one of the leaders of the new study. So too is observational research on subjects who use CPAP machines, he said.

Virend Somers, a professor of medicine at the Mayo Clinic in Rochester, Minn., who was not involved in the research, called the new study “a very important paper” because it provides the first large, randomized, controlled look at CPAP benefits that were only previously assumed.

CPAP is far from worthless, according to the new research, which was published in the New England Journal of Medicine and presented at a cardiology conference in Rome on Sunday. The five-year study of people in seven countries confirmed that the pump reduced the daytime sleepiness that plagues sleep apnea patients, with corresponding improvements in mood, quality of life and work productivity. It also reduced sufferers’ anxiety and depression somewhat.

The study was by far the largest to look into obstructive sleep apnea, Mr. Anderson said. It involved 2,687 subjects between the ages of 45 and 75.

When the researchers divided the subjects into two roughly equal groups, they found no impact on heart attacks and strokes, as well as deaths from any cause or hospitalization for heart failure, unstable angina and other reasons.

It’s not clear why that might be, Mr. Anderson said. One possibility raised in both the study and an accompanying editorial is that the CPAP group was able to wear the masks only about 3.3 hours per night, a length of time that is consistent with CPAP users in the real world. That may not be enough time to affect the frequency of these cardiovascular events. Or it may be that cardiovascular disease is too difficult to modify in this way once it develops, Mr. Anderson said.

Dr. Somers, the Alice Sheets Marriott professor of medicine and a consultant in cardiovascular disease at the Mayo Clinic, said he would like to know whether better adherence to the CPAP regimen might convey more long-term benefits. It’s unknown whether the adrenaline and blood pressure surges that come with taking off the mask and returning to an apneac state in the middle of the night are as harmful as sleep apnea itself, he said.

Though the researchers didn’t look at it, Mr. Anderson said it is probably safe to assume that sleep apnea sufferers who don’t have pre-existing cardiovascular conditions would show similar results. But more research on that is needed, he said. Dr. Somers agreed.

However, he pointed out that patients who are most sleepy during the day were excluded from the study, because of the danger that they would be randomized to the group that wasn’t receiving CPAP and could hurt themselves or others in car accidents or in other ways. While it would be a difficult study to accomplish, Dr. Somers said, it could be important to know whether CPAP protects sleepy patients from cardiovascular events.

Sleep apnea is often, but not always, associated with increasing obesity, and many cases of sleep apnea remain undiagnosed. Common symptoms are loud snoring or gasping while sleeping.

First Published: August 29, 2016, 4:44 a.m.