What do AHI, RERA, Arousal and RDI mean?

What do these terms mean in a sleep study report?

Which one is used by insurances to determine approval for CPAP therapy?

Learn about the different type of respiratory events, how they are scored and what they mean for treatment options and insurance coverage of CPAP therapy.

What is AHI?

Apnea Hypopnea Index. 

It’s the average number of combined apneas and hypopneas per hour. It is most often used to determine the severity of a person’s sleep apnea.

What is an apnea?

An apnea is when your breathing stops for at least ten seconds while sleeping. The figure below shows how the sensors used during a sleep study measure your breathing. This criteria is used for scoring a sleep study. This is the part of the process where a technician reviews the data that is recorded from your overnight sleep study.

Apnea scoring criteria

Different types of apneas:

  • Apnea: There is a drop in the signals shown above of at least 90% for at least 10 seconds.
  • Obstructive apnea: Meets the criteria of an apnea (above) and continued or increased effort to breathe is noted (the effort is due to an obstruction of the airway, learn more here).
  • Central apnea: An apnea with no effort to breathe made during the entire period (this is a neurological issue, learn about central sleep apnea here).
  • Mixed apnea: An apnea with no effort to breathe in the beginning of the event followed by effort to breathe in the second part of the event.

What is a hypopnea?

A hypopnea is a significant reduction in airflow while sleeping (not a complete stop) that causes a drop in blood oxygen saturation and then an arousal.

Two ways to calculate hypopneas (based on Medicare, see link in sources):

(1) 50% reduction in airflow with 3% blood oxygen desaturation

(2) 30% reduction in airflow with 4% blood oxygen desaturation

Hypopnea Scoring Criteria Image

What is a RERA?

Respiratory Effort Related Arousal.

An event that causes an arousal or a decrease in oxygen saturation, without qualifying as an apnea or hypopnea. 

An arousal is when the sensors measuring your brain activity, called EEG, show that your brain waves change to alpha wave form. This means you’ve woken up even if you don’t realize it. It is the repeated waking of your brain as a result of a respiratory disturbance lasting at least 10 seconds that causes the damage from sleep apnea, regardless of the type of event that is occurring or whether the breathing pause is a full pause or a partial pause. Therefore, many sleep centers and insurance companies are starting to recognize RERA as an indicator of the severity of disease.

Rera Scoring Criteria Image

What is RDI?

Respiratory Disturbance Index.

This is your combined number of apneas, hypopneas, and RERAs per hour of sleep. Physicians who use this scale typically follow the same guidelines that are used for the apnea/hypopnea index (AHI).

Medicare defines RDI as the average number of apneas and hypopneas (so it’s really the same as AHI), they do not include RERAs.

Most commercial insurances refer to the AASM standards which includes apneas, hypopneas or RERAs. 

What is mild, moderate or severe sleep apnea?

As detailed above, AHI has been the traditional measure used to determine the severity of sleep apnea; however, many institutions and insurances are looking at RERAs because any respiratory event that lasts at least 10 seconds and causes an arousal can cause damage and should be treated. The chart below lists AHI criteria but RERA should also be considered.

None/Normal: Almost everyone experiences the occaisional apnea, so having less than 5 of these events per hour is considered normal and no treatment is recommended.

Mild Sleep Apnea: CPAP is not usually recommended as the first option for treating mild sleep apnea. Other interventions, such as weight loss, use of positional tricks (if sleep apnea occurs primarily while sleeping supine, or on your back), improvements in sleep hygiene and oral appliances are typically recommended (learn more about CPAP alternatives here).

Moderate and Severe Sleep Apnea: CPAP therapy is usually recommended. Most insurance companies will authorize payment for CPAP if your AHI is moderate to severe (learn about how CPAP therapy works to treat sleep apnea here). 

AHI and Severity of Sleep Image

For a more in-depth look at the terms and measurements found in sleep studies, download our Guide to Understanding Sleep Study Results.

Download the Guide


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  1. Jean Kruck Reply

    I have recently commenced on CPAP–( 5 weeks ) I have been reading the meaning of terms and abbreviations used I am interested in getting a copy of -Guide to Understanding Sleep study Results – 2 pages but cannot download it. I would be grateful if you could email me a copy Regards Jean

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  3. Molly-Min Mao Reply

    Could you provide a current link of Medicare rule “50% reduction in airflow with 3% blood oxygen desaturation”? Because I only knew the rule of 30% reduction in airflow with 4% desats. thanks very much

  4. Hiemstra Reply

    Thanks for saving my life

  5. Lynne Jaffe Reply

    I found this information very helpful; however, there is no hyperlink to download the guide, so it is not available.

  6. Carol Jackson Reply

    I did an in-home sleep study and was diagnosed with Mild Sleep Apnea. (AHI of 13.4). The study recommends Therapeutic CPAP tritation. Does Medicare require this second study? I thought it was separate from the diagnosis and only needed to determine pressure settings on my machine.
    Thank you.

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  11. Mohamed Ben el achhab Reply

    estoy interesado en obtener informacion de la Apnea

  12. DMZABO Reply

    First off a very informative blog/post. Thank you for that. I’ve been on a Bi-Pap machine for 3+ yrs and it’s going well. But if you want to take your sleep apnea knowledge a giant step further than I suggest you go to SLEEPFILES.COM
    This is where you download your nightly sleep information off of your SD card that’s in your sleep machine/device and it will show you charts of your last nights sleep that will help you be able to talk with your doctor about. It’s known as OSCAR and it’s absolutely SAFE TO DOWNLOAD ON YOUR COMPUTER.
    AND ITS ALSO FREE OF CHARGE TO DOWNLOAD. This is such helpful information that you can use to lower your AHI numbers possibly and to help you stay in compliance with the insurance companies too.
    It’s been helpful to me and I’ve been using this for about 2 weeks and I really enjoy gaining knowledge on how to get better sleep with sleep apnea. Most of the time we go through the sleep tests and you find out whether your a candidate for sleep apnea therapy and the doctor only sees you once a year if that. That’s after the person who gives you your machine shows you how to hit on and that’s basically it. Well using this app gives you the tools to help you get the right setting and the right equipment (leaks are a real problem) . In other words you’ll have a good chance at really knowing how to fight Apneas and it’s side effects.

  13. Aster Perez Reply

    It’s the average number of combined apneas and hypopneas per hour. It is most often used to determine the severity of a person’s sleep apnea.

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  17. Richard William Reply

    Thanks for telling me about Apnea and Hypopnea. I don’t know about these before. I learn from your article that a good night sleep is very important for human health.

  18. devid Reply

    AHI stands for Apnea-Hypopnea Index, which measures the severity of sleep apnea by counting the number of apneas and hypopneas per hour. RERA stands for Respiratory Effort-Related Arousal and indicates partial airway obstructions during sleep. Arousal refers to brief awakenings from sleep. RDI stands for Respiratory Disturbance Index, encompassing all respiratory events during sleep, including apneas, hypopneas, and arousals.

  19. Arslan Feroz Reply

    Thanks for breaking down these sleep-related terms! It’s eye-opening to learn about AHI, RERA, Arousal, and RDI and how they play a crucial role in assessing sleep quality and identifying potential sleep disorders. The intricacies of our respiratory patterns during sleep are fascinating, and it’s clear that these metrics provide valuable insights for both individuals and healthcare professionals. If anyone has personal experiences with sleep studies or tips on maintaining good sleep health, I’d love to hear about it. Here’s to a better understanding of our sleep patterns!

  20. moto x3m Reply

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  21. www.concretedrivewaysmiami.com Reply

    This is the total number of apneas, hypopneas, and RERAs you have in a single sleep hour.

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