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Of all the numbers on your sleep apnea test report, the AHI is the most important. On some sleep study reports, you will see the terms "RDI" or "REI" in place of AHI. But what does AHI mean and what's the difference between AHI, RDI, and REI?

Of the three, AHI as it is the most commonly used term and is an acronym for "apnea-hypopnea index." To understand the term, we first need to define what apneas and hypopneas are.


man having apnea in sleep

Apneas are a type of abnormal respiratory event that occur during sleep. The word originates from the Greek roots meaning "without" and "to breathe." The American Academy of Sleep Medicine (AASM) provides a scoring manual that delineates how all aspects of a sleep study should be interpreted, including defining what constitutes abnormal respiratory events such as apneas. To score an apnea on a sleep study there must be essentially no airflow (measured through a nasal pressure sensor or thermistor) for at least ten seconds.  A blood oxygen drop, known as an oxygen desaturation, is not part of the criteria for an apnea. There are three different types of apneas described in this manual:

Obstructive Apneas

obstructive apnea

Basically, there is TOTAL upper airway obstruction resulting in no airflow (the nasal pressure and/or thermistor sensors show no signal) but respiratory effort is sustained (the chest [RIP] belt signal is maintained), meaning that you are trying to breathe.

Central Apneas

central apnea

With a central apnea, there is complete absence of both airflow and respiratory effort. In essence, your brain has "forgotten to breathe."

Mixed Apneas

mixed apnea

Mixed apneas are a hybrid of central and obstructive apneas. In the initial part of a mixed apnea, there is neither airflow nor breathing effort, but effort begins in the latter part of the episode.

See (1) below for the AASM's full criteria for scoring apneas on a sleep study.



Hypopneas are the other major type of abnormal sleep breathing event scored on a sleep study. Overall, hypopneas are more common than apneas. The word "hypopnea" is derived from the Greek roots meaning "under" and "to breathe." Generally speaking, hypopneas are episodes of significantly reduced airflow (but they fall short of total airflow cessation) that are associated with either an oxygen desaturation or a brain awakening (arousal). There is a rift between the AASM and CMS (Medicare and Medicaid), the country's largest insurer, as to what constitutes a hyopnea. Based on the existing medical research, the AASM defines a hypopnea as requiring an oxygen desaturation of ≥3%, but CMS requires a ≥4% oxygen desaturation to score a hypopnea. Notably, CMS's definition is not based on scientific evidence. Under pressure, the AASM relented and changed their manual to allow either criterion to be used to score hypopneas.

While a seemingly small difference, in this author's experience, requiring at least a 4% oxygen desaturation for a hypopnea can eliminate up to 15-20% of patients from receiving a diagnosis of sleep apnea and getting CPAP or BPAP equipment paid for by Medicare or Medicaid!

Hypopneas can be classified as obstructive or central, but this is generally not done in clinical practice. See (2) below for the AASM's full hypopnea scoring criteria.

AHI Explained

Let's use a hypothetical patient to illustrate how the AHI is calculated. Jim received his sleep study test report back and it says that he slept for a total of 360 minutes (six hours). During the study, he had 60 apneas and 120 hypopneas. The AHI is calculated as follows:

AHI = (total apneas + total hypopneas) / total sleep time in hours

Therefore Jim's AHI is:

(60+120)/6 = 30

In other words, Jim averaged 30 abnormal breathing events per hour during the night of his sleep study. Essentially, he had a respiratory event every other minute of sleep!

AHI and Mild, Moderate, and Severe Sleep Apnea

Sleep apnea is divided into three categories by AHI:

mild sleep apnea: AHI ≥ 5 - < 15

moderate sleep apnea: AHI ≥ 15 - < 30

severe sleep apnea: AHI ≥ 30

The severity of sleep apnea is important to know because it influences risk for the consequences of sleep apnea  (such as the likelihood of developing high blood pressure), as well determining which sleep apnea treatments are available for the patient. Alternatives to CPAP, such as oral appliances for sleep apnea and Provent, can be considered for mild or moderate sleep apnea. The CPAP machine is the gold standard for all degrees of sleep apnea. A BPAP machine may be considered for more severe and complex cases of sleep apnea.

Respiratory Disturbance Index (RDI)

In some sleep centers, RDI is used interchangeably with AHI. But most of the time, RDI is used to indicate a different index that not only includes apneas and hypopneas but also abnormal respiratory events called respiratory effort-related arousals (RERAs).

What is a RERA?

A RERA is defined as a small reduction in airflow (less than a hypopnea) that leads to an arousal but does not result in a significant oxygen desaturation. Often, the respiratory effort will increase during the breathing event.

respiratory effort-related arousal (RERA)

The AASM's complete criteria for scoring RERAs are described below in (3).

To recap, RDI can be defined as:

RDI = (total apneas + total hypopneas + total RERAs) / total sleep time in hours

Respiratory Event Index (REI)

Home sleep test kits are being more frequently used to diagnose sleep apnea. Most of the available home sleep apnea testing devices do not directly measure sleep as they don't have EEG electrodes to monitor brain waves; it's assumed that people are sleeping while using them. Therefore, some experts in the field argue that "AHI" is not the best term to use on a home sleep study report because we don't actually measure the denominator. Thus, the AASM created a new term specific to home sleep apnea tests:  the "respiratory event index" or REI. The denominator of the REI is based on total monitoring time:

REI = (total apneas + total hypopneas)/ total monitoring time in hours

Now you're a more educated patient. Future articles will explore and explain other aspects of sleep study reports. Stay tuned! 

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sleep apnea test home sleep study Joseph Krainin Joseph Krainin, M.D., FAASM is the founder of Singular Sleep, the world's first online sleep center.  He is a Fellow of the  American Academy of Sleep Medicine and board certified in both sleep  medicine and neurology. He has been practicing medicine  for over 10 years.

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    (1) AASM scoring rule for apneas:

    Scoring of Apneas

    1. Score a respiratory event as an apnea when BOTH of the following criteria are met:  

    a. There is a drop in the peak signal excursion by ≥90% of pre-event baseline using an oronasal thermal sensor (diagnostic study), PAP device flow (titration study) or an alternative apnea sensor (diagnostic study).

    b. The duration of the ≥90% drop in sensor signal is ≥10 seconds.

    2. Score an apnea as obstructive if it meets apnea criteria and is associated with continued or increased inspiratory effort throughout the entire period of absent airflow.  

    3. Score an apnea as central if it meets apnea criteria and is associated with absent inspiratory effort throughout the entire period of absent airflow.  

    4. Score an apnea as mixed if it meets apnea criteria and is associated with absent inspiratory effort in the initial portion of the event, followed by resumption of inspiratory effort in the second portion of the event.  

     (2) AASM scoring rule for hypopneas:

    Scoring of Hypopneas

    1A. Score a respiratory event as a hypopnea if ALL of the following criteria are met: 

    1. The peak signal excursions drop by ≥30% of pre-event baseline using nasal pressure (diagnostic study), PAP device flow (titration study), or an alternative hypopnea sensor (diagnostic study).

    2. The duration of the ≥30% drop in signal excursion is ≥10 seconds.

    3. There is a ≥3% oxygen desaturation from pre-event baseline or the event is associated with an arousal.

    1B. Score a respiratory event as a hypopnea if ALL of the following criteria are met:  

    1. The peak signal excursions drop by ≥30% of pre-event baseline using nasal pressure (diagnostic study), PAP device flow (titration study), or an alternative hypopnea sensor (diagnostic study).

    2. The duration of the ≥30% drop in signal excursion is ≥10 seconds.

    3. There is a ≥4% oxygen desaturation from pre-event baseline.

     (3) AASM scoring rule for respiratory effort-related arousals:

    If  electing to score respiratory effort-related arousals, score a respiratory event as a respiratory effort-related arousal (RERA) if there is a sequence of breaths lasting ≥10 seconds characterized by increasing respiratory effort or by flattening of the inspiratory portion of the nasal pressure (diagnostic study) or PAP device flow (titration study) waveform leading to arousal from sleep when the sequence of breaths does not meet criteria for an apnea or hypopnea. 

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