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Variation in Apnea Hypopnea Index (AHI) Methods Interferes with Diagnosis, Treatment of Obstructive Sleep Apnea

by Maybelle Cowan-Lincoln •

Scoring a patient’s Apnea Hypopnea Index (AHI) has been calculated using various methods over time. Currently, as many as three hypopneas definitions are used:

  • AHI Recommended: Abnormal respiratory event lasting >10 seconds, with >30 percent reduction in chest movements, plus oxygen desaturation of >4 percent.
  • AHI Alternative:  A reduction of nasal airflow of >50 percent, plus oxygen desaturation of >3 percent or an arousal.
  • AHI Chicago: >50 percent decrease in airflow, or <50 percent decrease in airflow accompanied by oxygen desaturation >3 percent or an arousal.

Incomplete scoring definitions have a particularly negative impact for a complicated disorder when diagnosis or treatment success is based on one number: AHI. Different sleep labs use different diagnostic criteria, so scores vary from one setting to another. The Chicago version, which was used prior to 2007, tends to increase the number of hypopneas, while the recommended scale, implemented by the American Academy of Sleep Medicine (AASM) in 2007, decreases them.

The current recommended definition focuses only on desaturations, with no importance given to arousals from sleep. Pell Ann Wardrop, MD, medical director of the St. Joseph Sleep Wellness Center in Lexington, Ky., explained that this omission causes problems for women and some younger men during the initial diagnosis process. These patients often experience more arousals, with fewer instances of oxygen desaturation. When arousals re not scored, these individuals are found not to have sleep apnea and receive no therapy—and no relief.

“The existing scoring actually discriminated against women,” said Dr. Wardrop. “Many times, I would tell my lab tech to go back and rescore a patient’s polysomnography results according to the alternative scale so they could get the necessary diagnosis and treatment.”

In an effort to identify and treat individuals with obstructive sleep apnea (OSA) missed by the current diagnostic criteria the AASM has developed a new scoring rule for hypopneas. According to the new criteria a respiratory event is a hypopnea if all of the following criteria are met:

  • The peak signal excursions drop by 30 percent or more of pre-event baseline using nasal pressure, PAP device or an alternative hypoponea sensor;
  • The duration of the 30 percent (or more) signal excursion lasts at least 10 seconds; and
  • There is >3 percent oxygen desaturation from pre-event baseline OR the event is associated with an arousal.

“That ‘or an arousal’ is really important,” said Dr. Wardrop. “The AASM has recognized that not scoring arousals was an issue and fixed it.” Overall, the AHI is poorly predictive of many OSA complications, highlighting the need for development of new outcome measures.