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.”
Explore this issue:December 2014
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.”