A new “Adherence Index” provides surprising insights into CPAP use 12 months after obstructive sleep apnea therapy initiation.

By Sree Roy

When a patient who has obstructive sleep apnea but seldom uses their CPAP continues to complain of daytime sleepiness or morning headaches, they are typically told to try harder. Perhaps a new CPAP mask? Or maybe a pressure adjustment? Sleep specialists fling resources at lowering the patient’s apnea-hypopnea index (AHI) to below 5—with the assumption that if only that goal can be reached, the patient’s daytime symptoms will resolve.

But could it be that the patient’s poor sleep is…just a coincidence? 

“If there is a big association between two common things, people assume they always go together,” says Magdy Younes, MD, FRCPC, PhD. “But some patients don’t sleep enough or they have other problems that make them tired, like thyroid problems or heart disease….Poor sleep is universal.” For people who have poor sleep and have an AHI below 5, it’s obviously not sleep apnea that’s causing their symptoms. For people who have obstructive sleep apnea and symptoms that also look like OSA, well, it could certainly be the OSA, but it might still be something else.

Younes, a distinguished professor emeritus at the University of Manitoba in Canada, does not practice respiratory medicine anymore. But he does still “have fun trying to figure things out.” And two recent papers on which he is first author shed light into CPAP adherence—specifically using sleep depth measurements from the diagnostic polysomonogram to quite accurately predict CPAP use 12 months later. This “Adherence Index” offers new precision into understanding the relevance of a person’s obstructive sleep apnea to their sleep symptoms. In the future, it could be used to give sleep specialists insight into whether a new mask or pressure change would truly help, or whether the patient should instead be re-evaluated to get to the bottom of their sleep symptoms.

How does the Adherence Index correlate so well with long-term CPAP use? There is a surprisingly simple answer to that question: Most of the people who use CPAP 12 months later are those for whom CPAP notably improved their sleep.

“CPAP is a double-edged sword,” Younes says. “One side of the sword fixes sleep, but CPAP is also a major disruptor of sleep.” Discomforts such as air leaks or difficulty with exhalation can truly make some people’s sleep depth worse, his research has found. “It is fixing one problem but creating another problem,” he says.

The crux of this is in a paper published earlier this year in SLEEP, in which Younes and co-investigators detail nine base Odds Ratio Product (ORP) sleep architecture profiles. ORP is an EEG metric developed by Younes that assigns a number between 0 (deep sleep) and 2.5 (full wakefulness) to a person’s state and is directly related to sleep depth/arousability. The nine profiles remind me of a personality test. Instead of being identified as ENTJ, a person may be a “2,1”—with a suggested underlying physiological mechanism of “average amounts of deep sleep and sleep depth/little time in full wakefulness and lower ORPwake suggest insufficient sleep.” The suggested clinical interpretation of a “2,1” is: “Likely normal but may benefit from increasing time in bed if excessively sleepy.”

What does all of this have to do with OSA and the Adherence Index?

“I noticed that with the ORP measurement, many patients with sleep apnea, even some of the very severe ones, have perfectly normal sleep depth and sleep time and other parameters,” Younes says. These patients aren’t going to notice benefits of CPAP use (barring the placebo effect) because their sleep quality is fine to begin with. But three of the profiles of poor sleep in people with obstructive sleep apnea improve dramatically with CPAP use—and these are the patients who are likely to stick with CPAP therapy long-term.

Indeed, the Adherence Index incorporates only three variables—ORP in non-REM, AHI, and hypoventilation—as those were the ones that the multiple linear regression analysis identified as significant.

Now, this information does not imply whatsoever that a person with a low Adherence Index should quit CPAP. First, the Adherence Index could be flawed, such as by missing other important variables. Second, because so much of the trouble with obstructive sleep apnea is its long-term sequelae, it would be premature for anyone to use this as an excuse to give up on CPAP. We first need the science to convincingly link (or not) CPAP use with long-term health outcomes. “We should do a clinical trial,” Younes says, envisioning one in which everyone with OSA tries CPAP, and if they like it, they stay on it. If they don’t like it—and it’s objectively not improving their sleep or other short-term health outcomes, per the ORP metric—they can quit (as many people do today anyway). “And then after the results of the clinical trial, we’d see if CPAP did anything—improve their health, or lower their risk of hypertension or diabetes,” Younes says. “For the people who were predicted not to benefit but who continued to use the machine, can we document a benefit in any other area?”

Kari Lambing, PhD, a sleep scientist at Cerebra, which exclusively licenses ORP, says the Adherence Index could help set expectations for obstructive sleep apnea patients. “If their sleep is already pretty good, you can have a conversation with them at the beginning,” she says. For example, she says a sleep specialist might say, “Your sleep may not improve a lot with CPAP, but it is still important for your health, including for medical comorbidities.”

The potential applications for patients who are predicted to improve on CPAP are perhaps even more exciting. (And this group does include most, though not all, of people with a very high AHI.) These sleep apnea patients should, in fact, try harder. Here, sleep specialists could be confident that the time and money to achieve maximum CPAP usage would be resources well spent.

Sree Roy is editor of Sleep Review.

References

Younes MK, Beaudin AE, Raneri JK, et al. Adherence Index: sleep depth and nocturnal hypoventilation predict long-term adherence with positive airway pressure therapy in severe obstructive sleep apnea. J Clin Sleep Med. 2022 Aug 1;18(8):1933-44.

Younes M, Gerardy B, Pack AI, et al. Sleep architecture based on sleep depth and propensity: patterns in different demographics and sleep disorders and association with health outcomes. Sleep. 2022 Jun 13;45(6):zsac059.

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