Apnoea Hypopnoea Index as a Digital Biomarker

The apnoea hypopnoea index is the number of apnoeas and hypopnoeas, meaning complete and partial interruptions of breathing, divided by hours of sleep. It is the accepted definition of obstructive sleep apnoea severity, with conventional thresholds separating mild, moderate and severe disease, and it is the number that determines whether a person is diagnosed and offered treatment.

Its position is worth stating precisely. The index is not a physiological quantity that exists independently of how it is scored. An apnoea and a hypopnoea are defined by scoring rules, and those rules have changed over time and still differ between the criteria laboratories apply. Two technicians scoring the same night under different hypopnoea definitions can produce indices that differ enough to change a diagnosis, which is a known and documented problem rather than an edge case.

The reference measurement is attended polysomnography, which records airflow, respiratory effort, oxygen saturation, electroencephalography and more, allowing events to be scored against actual sleep time. Home sleep apnoea testing records fewer channels and often estimates recording time rather than measuring sleep, which typically produces a lower index than a laboratory study on the same person.

Consumer wearables approach the problem differently. Rather than scoring individual events, sleep apnoea notification features look for patterns of breathing disturbance across many nights and flag people who should seek assessment. Some of these features have received regulatory clearance, which places them closer to the clinical pathway than most digital measures, but they estimate the presence of a pattern rather than producing a scored index.

The index is used to diagnose and grade sleep apnoea, to determine treatment eligibility, and as the primary endpoint in trials of positive airway pressure, positional therapy, mandibular devices, surgery, weight loss and pharmacological approaches. In the public registry of digital endpoints it is used as a primary endpoint by every registered trial that has adopted it, which is true of almost no other measure in this library.

Given how much of the population is undiagnosed, case finding is the other major use. A wearable that identifies possible sleep apnoea and routes the person to a diagnostic test addresses that gap directly. Trials pair the index with daytime sleepiness measures such as the Epworth Sleepiness Scale, since reducing breathing events without relieving daytime symptoms is an incomplete result.

The index counts events without weighting how long they last or how much oxygen desaturation they cause, so two people with the same value can carry different physiological burdens. It correlates only moderately with symptoms, which is why a person with a modest index can be severely sleepy and someone with a high index can feel well.

Scoring criteria remain a live source of variability, and comparing indices across studies that used different hypopnea definitions is unreliable. Home testing and laboratory testing are not interchangeable numbers. Consumer notification features are cleared to prompt assessment, not to diagnose, grade severity or confirm that treatment is working, and treating a watch notification as a substitute for a sleep study is a misuse of the technology.

• Kapur VK, et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017. pubmed.ncbi.nlm.nih.gov
• Benjafield AV, et al. Estimation of the global prevalence and burden of obstructive sleep apnoea: a literature-based analysis. Lancet Respir Med. 2019. pubmed.ncbi.nlm.nih.gov
• Patil SP, et al. Treatment of adult obstructive sleep apnea with positive airway pressure: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2019. pubmed.ncbi.nlm.nih.gov
• Kapoor M. Hypopnea scoring criteria: time to move toward a single standardized definition. J Clin Sleep Med. 2018. pubmed.ncbi.nlm.nih.gov
• Punjabi NM. The epidemiology of adult obstructive sleep apnea. Proc Am Thorac Soc. 2008. pubmed.ncbi.nlm.nih.gov