Is the Apnea/Hypopnea Index the Best Measure of Obstructive Sleep Apnea?

Obstructive sleep apnea (OSA) continues to challenge otolaryngologists and patients alike, with estimates of the condition affecting between 2% and 4% of the adult population in the United States. Gold standard OSA diagnosis is made through a polysomnogram (PSG) test, which uses the apnea/hypopnea index (AHI) as its main defining measure.

The AHI, which quantifies the number of times each hour a patient has a total (apnea) or partial (hypopnea) blockage of breathing during sleep, has been the most-used measure, not only of how OSA is diagnosed, but also of how well treatment modalities, including continuous positive airway pressure (CPAP) and surgery, improve breathing patterns.

Recently, however, some otolaryngologists have been questioning whether the AHI should be the main—and sometimes only—determining factor of treatment effectiveness, or whether other measures such as sleepiness scales, quality of life (QOL) measurements, and physiological measurements such as blood pressure should play a more prominent role.

The Challenges of AHI

Much of the focus around this questioning has arisen not only because of AHI’s value as a measurement index, but also because of its changing definition.

“As an index, the AHI can vary a lot between sleep centers and even within the same sleep center. You’ll get a different number depending on which definition and sensors you use,” said

Ofer Jacobowitz, MD, PhD, assistant clinical professor of otolaryngology at Mount Sinai Hospital in New York City. “Hypopnea can be defined based on either a 30% or 50% decrease in inflow and associated with either a 3% or 4% oxygen desaturation.or even an arousal. The recommended definition of hypopnea has changed multiple times over the years.”

The effects of this shifting definition have been noted in research. In a 2012 study published in The Laryngoscope that examined the effects of different PSG scoring systems on outcome measurement following OSA surgery, the researchers noted that interpretation of OSA surgical treatment literature remains problematic, because the study authors continue to use different AHI criteria for investigation and different AHI thresholds for defining surgical success (Laryngoscope. 2012;122:1878-1881). They found that the success rate for OSA surgical treatment ranged from 38.9% to 91.7%, depending on the criteria and metric used to define a successful outcome.

Another issue is that, even with a stable definition, the AHI number may not represent an accurate picture of an individual patient’s experience with the disease. “The AHI tells you about the sum of apneas and hypopneas, but two patients with the same AHI number may have completely different scenarios—one with mostly apneas and longer or more severe desaturations and one with mostly hypopneas with minimal desaturations,” said Dr. Jacobowitz.

As an index, the AHI varies a lot, and there are multiple ways to define it. You’ll get a different number depending on which definition you use.
—Ofer Jacobowitz, MD, PhD

“It used to be thought that the more severe a patient’s sleep apnea, the more sleepy he or she would be, but that turns out not to be the case,” said Eric Kezirian, MD, MPH, professor of clinical medicine in the department of otolaryngology-head and neck surgery at the Keck School of Medicine of the University of Southern California in Los Angeles. “Sleep apnea can reduce the sleep quality for patients, resulting in sleepiness, fatigue, and decreased quality of life. It turns out the AHI doesn’t capture that.”

Part of the issue is that people can be fatigued for a number of reasons, Dr. Kezirian added. They may not be sleeping enough, they may have insomnia, or there may be other sleep issues. “You want to have some objective way to measure how well you’re treating OSA,” he said. “The AHI is certainly part of that; it’s a single number that allows you to get a sense of what a patient’s breathing patterns are like. But we don’t treat numbers, we treat patients, and so we care about the broader implications of the treatment.”

Those broader implications are important to the overall health of OSA patients. Another recent study, published in The Laryngoscope, focused on outcome measures in OSA and noted a disconnect between the AHI levels used to denote therapy outcomes and real-world clinical outcomes, such as QOL, patient perception of disease, cardiovascular measures, and overall survival (2014;124:337-343).

These researchers looked at 21 studies on outcome measures in addition to the AHI that were published between 1997 and 2012. The authors found that patients with OSA scored differently in measurement tools in all categories when compared with control populations or after treatment and that, in general, there was a poor correlation with AHI.

“The issue with AHI is that it’s only part of the definition of OSA—it is a marker of sleep apnea, a surrogate variable of the disease,” said Dr. Jacobowitz. “AHI will remain important because there is reasonable evidence that when a patient’s AHI is over 30, it is associated with increased mortality. But it’s an indirect measure of only the respiratory component of sleep apnea and does not measure sleepiness. For example, if the AHI is less than 15, you can’t make an OSA diagnosis unless the patient has associated symptoms, and that’s exactly what we’re talking about: sleepiness, quality of life, and more.”

Other Measures of OSA

While researchers commonly use AHI, other metrics have been used alongside it to give a broader sense of treatment, according to Dr. Kezirian, including the Epworth Sleepiness Scale and QOL measurement questionnaires. Additional measures also include blood pressure, oxygen desaturation index, psychomotor vigilance tasks, and, over the long term, serious cardiovascular events and mortality.

In clinical application, these other measurements can give a clearer picture of the patient’s reason for seeking treatment, particularly where OSA surgery is concerned. “A sleep study comes from a single night, either in a sleep laboratory where patients are hooked up to many different monitors, or at home where, although there are fewer monitors, it can still be disruptive,” said Dr. Kezirian. “The study may not capture the general pattern of a patient’s sleep over longer periods of time. This single snapshot of one night may not represent what’s typically happening for a particular patient for a number of reasons: Many patients tend to sleep more on their backs during studies and may give an artificially worse picture of their sleep apnea, and there is some disruption of sleep by the monitors, to name just a couple of those reasons. For patients and sleep surgeons considering surgery, there are many gradations of sleep apnea and a number of reasons why the AHI might not capture the effects of treatment, good and bad. That’s why other measures are helpful.”

They are not, however, without their problems, including the fact that the questionnaire measurements are highly subjective and can have a placebo effect. “If a patient undergoes surgery and wants to feel better, they sometimes will,” said Dr. Kezirian. “A better assessment would include a combination of metrics. An otolaryngologist could look at the sleep study result, but also at how that patient is doing overall.”

Looking Forward

“The goals of surgical OSA treatment are the reduction of cardiovascular risk, increased survival, reduced sleepiness, improved quality of life, and, of course, reduced snoring,” said Dr. Jacobowitz. “These can only be captured by using the AHI in conjunction with other quality of life, physiological, and clinical measurements.

So why haven’t alternate metrics been used more often in the clinical assessment of OSA treatment? Dr. Jacobowitz believes it’s a matter of familiarity and ease with using a single quantifiable parameter—the AHI. “The traditional gold-standard treatment of OSA is CPAP [continuous positive airway pressure], and CPAP was designed to improve AHI,” he added.

There is some evidence validating the use of a variety of metrics in outcome measurements. In the 2012 Laryngoscope study, outcomes not only showed a reduction in AHI (in all indices) but also a reduction in patient-reported symptoms. “OSA is not defined solely by a metric; the diagnosis and management of this condition takes into account patient symptomatology as well as disease severity…. Polysomnographic parameters as outcome measures are important surrogates of some clinical outcomes, such as cardiovascular risk, but they should not be mistaken for clinical outcomes themselves,” said the authors. “Similarly, the definition of surgical success should be by more than just the AHI reduction alone, and other outcomes should be included in assessment of postoperative consideration.”

“For CPAP, although you can normalize the AHI in the sleep lab, often there is residual elevated AHI at home and many patients do not use CPAP for the entire night at home,” added Dr. Jacobowitz. “When you look at this AHI variable with regard to sleep surgery outcome, typically the AHI is reduced significantly but doesn’t normalize completely. At the same time, with respect to meaningful primary clinical outcomes, CPAP and surgery can reduce cardiovascular morbidity and decrease the rate of car accidents despite that imperfect AHI reduction.”

For the future, Dr. Kezirian sees more otolaryngologists adopting broader assessments of patients. “These questionnaires have been around for a while, but they are now being used more often in routine clinical practice. They go beyond just asking, ‘How are you doing?’” he said. “Using the questionnaires helps us determine the benefits of treatment if outcomes are not perfect, so we can tell if someone is making progress. The AHI alone is too simplistic. Patients may have no or little change in their AHI but still feel better, but they can also show major improvement in the AHI but still feel awful, which isn’t good enough either.”

Dr. Jacobowitz believes that widespread adoption will come with greater emphasis on alternative measurements in any clinical trial for OSA. “This isn’t difficult for quality-of-life measures, but it will present a challenge for some other variables such as cardiovascular incidents because they must be measured over a very long time,” he said. “But we have to remember what’s important to the patient and for our health system: how the patient is functioning, and the overall status of their health.”

Amy Hamaker is a freelance medical writer based in California.