Apnea vs. Hypopnea vs. RERA

Sleep-disordered breathing (SDB) is an all-inclusive term for a wide range of sleep related breathing problems from chronic snoring to obesity hypoventilation syndrome.

These conditions are sometimes referred to as sleep related breathing disorders (SRBDs) or sleep breathing disorders (SBDs) but the generally accepted nomenclature within the medical and scientific communities is sleep-disordered breathing or SDB.

SDB is a serious, pervasive problem in society today. Studies show that 1 in 5 adults worldwide have some degree of sleep apnea which, as shown in Figure 2, is only a subset on the spectrum of sleep-disordered breathing.

To complicate and confuse the issue, terms and acronyms are not always clear in the area of sleep medicine and methods for calculating the severity of sleep-breathing conditions can vary among health care providers and insurers. This article attempts to clarify some of this confusion starting with key definitions.

Three breathing abnormalities used to quantify the severity of sleep-disordered breathing are:

• Apnea
• Hypopnea
• RERA

What is Apnea?

A generally accepted definition of “apnea” is a temporary pause in breathing for 10 seconds or more. (A more detailed, technical definition used for scoring apneas in sleep studies is provided below.)

While the term apnea can refer to voluntary breath-holding as in aquatic activities such as free-diving and underwater sports, this article is focused on the involuntary suspension of breath that occurs with sleep apnea.

The type of sleep apnea (obstructive, central, or complex/mixed) depends on the cause of the breathing stoppage, i.e., whether it’s structural or neurological.

For the plural form, either apnea or apneas is acceptable with apnea generally being used in a broader context and apneas for more specific references.

The origin of the term apnea is the Greek word apnous, meaning “breathless.”

Interestingly, while apnea is by far the more common spelling, apnoea is a popular version of the term in Australia and the United Kingdom .

What is a Hypopnea?

A “hypopnea” is a reduction in breathing (as opposed to a complete stoppage as is the case with apnea) along with a specified drop in blood oxygen (oxygen desaturation). Some dictionaries define hypopnea simply as abnormally slow or shallow breathing. (A more technical definition used for scoring hypopneas in sleep tests is provided below.)

The word hypopnea is derived from hypo, meaning under or below normal, and the suffix -pnea meaning breathing.

Apneas and hypopneas are components of the apnea-hypopnea index or AHI which is one of two methods used to calculate the severity of one’s sleep apnea condition.

The Apnea-Hypopnea Index (AHI)

The apnea-hypopnea index (AHI) is the average number of apneas and hypopneas per hour of sleep with severity levels classified as follows:

• AHI of 5 to 14 = Mild Sleep Apnea
• AHI of 15 to 29 = Moderate sleep apnea
• AHI of 30 or greater = Severe sleep apnea

The AHI is used by many, if not most, health care providers and health insurance companies to diagnose and treat sleep apnea.

Similar to the apnea-hypopnea index is the respiratory disturbance index or RDI. The difference is the RDI includes a third class of sleep-breathing abnormalities called Respiratory Effort-Related Arousals or RERAs. (RERAs and RDI are discussed in more detail below.)

This is where much of the confusion (alluded to at the outset of this article) comes in. While many health care providers and insurers do not recognize RERAs for the purpose of diagnosing sleep apnea, they sometimes use the acronyms AHI and RDI interchangeably in their policies and guidelines. They then provide a side note explaining that only apneas and hypopneas are considered for AHI and/or RDI. Hence, a recipe for confusion. Here is an example from Aetna. (The term polysomnographic facilities in the Aetna quotation refers to sleep centers where sleep testing takes place.)

Leg movement, snoring, respiratory effort related arousals (RERAs), and other sleep disturbances that may be included by some polysomnographic facilities are not considered to meet the AHI and/or RDI definition in this policy. Although AHI and RDI have been used interchangeably, some facilities use the term RDI to describe a calculation that includes these other sleep disturbances. Requests for positive airway pressure devices will be considered not medically necessary if based upon an index that does not score apneas and hypopneas separately from other sleep disturbance events. Only persons with an AHI and/or RDI, as defined in this policy that meets medical necessity criteria may qualify for a positive airway pressure device.

Source: Obstructive Sleep Apnea in Adults, Aetna Clinical Policy Bulletins, 3/15/2019

To fully appreciate the potential for confusion between AHI and RDI, a clear understanding of RERAs is required.

What is a RERA?

As previously explained, RERA stands for respiratory effort-related arousal. However, it is sometimes translated to “respiratory event related arousal.” While they both refer to the same sleep-breathing condition, the former is a more accurate designation since a distinguishing feature of RERAs is an instinctive increase in respiratory “effort” as a result of a decrease in airflow.

The following comprehensive definition of RERA comes from the Mayo Foundation for Medical Education and Research.

An RERA is a series of breaths occurring for at least 10 seconds associated with an ever-increasing respiratory effort against a narrowed upper airway that terminates with arousal from sleep before criteria for a true apnea or hypopnea event are met. With esophageal pressure monitoring, RERAs are marked by progressively negative esophageal pressure deflections (reflecting increasing work of breathing) during the breaths immediately preceding an arousal. Upper airway resistance syndrome is the condition of excessive sleepiness associated with 10 or more RERAs per hour.

Source: Olson, Eric J. et al., “Obstructive Sleep Apnea-Hypopnea Syndrome,” Mayo Clinic Proceedings, Volume 78, Issue 12, 1545 – 1552

In other words, a RERA is an increasing effort in breathing lasting 10 seconds or more due to an airflow restriction that does not meet the criteria for an apnea or hypopnea (i.e., less significant reduction in airflow; minimal or no drop in blood oxygen) but does result in an arousal from sleep.

Along with FLEs (flow limitation events) and some chronic snoring levels, RERAs fall into a category of sleep-breathing dysfunction called UARS or upper airway resistance syndrome.

UARS is caused by a narrowing of the upper airway resulting in sleep disruptions as the respiratory system works harder to take in oxygen and expel carbon dioxide. These types of abnormal sleep-breathing events are more subtle and do not meet the criteria for apneas or hypopneas. If this sounds like the definition for RERA, it is, except for the fact that RERAs have a duration of at least 10 seconds.

So, RERAs are a subset of the larger class of abnormal breathing episodes called UARS. An important aspect of UARS (and RERAs) is the ongoing arousals from sleep they cause. Even through they do not qualify for a diagnosis of sleep apnea according to the apnea-hypopnea index (AHI, used by most insurance providers) the resulting sleep fragmentation keeps one from achieving the all-important restorative stages of sleep, i.e., deep sleep and REM. Combined with the airflow limitations in breathing, this can have serious negative health consequences as described in this excerpt from a 2015 article published on the National Institutes of Health website.

Untreated UARS individuals can present low quality of life and cardiovascular consequences. Sleep and daytime symptoms, such as fatigue, insomnia and depressive mood, in untreated UARS usually increase over time. The syndrome׳s characteristic esophagic pressure (Pes) negativity can cause a diastolic leftward shift of the interventricular heart septum and a consequent ventricular “collapse”. The longlasting flow limitation episodes can induce a small increase in end-tidal carbon dioxide (PetCO₂) that can stimulate the sympathetic nervous system activity. This could cause hypertension, and cardiovascular and metabolic consequences. Even an increase in inflammatory markers can happen in non-treated UARS individuals.

Source: de Godoy, Luciana B M et al. “Treatment of upper airway resistance syndrome in adults: Where do we stand?.” Sleep science (Sao Paulo, Brazil) vol. 8,1 (2015): 42-8. doi:10.1016/j.slsci.2015.03.001

The confusing and concerning thing about RERAs is, while they have been scientifically validated as a sleep-breathing abnormality that can negatively impact health, they may or may not be recognized and considered for treatment depending on the health care provider and insurance carrier involved.

Respiratory Disturbance Index (RDI)

That brings us to the respiratory disturbance index or RDI. To add to the confusion perpetuated by insurance carriers that refer to RDI interchangeably with AHI, the definition of RDI can also vary when referencing reliable health information sources online.

My research has shown that older studies reflect the antiquated definition of RDI in which it is simply another label for the AHI. (Average number of combined apneas and hypopneas per hour of sleep.)

A turning point seems to be when the American Academy of Sleep Medicine (AASM), which sets standards in sleep medicine, assembled the Sleep Apnea Definitions Task Force to review the rules for scoring respiratory events in the 2007 AASM Manual for the Scoring of Sleep and Associated Events. Among the revisions, the task force recommended that the RDI include RERAs.

RDI = AHI + RERA index (Recommended by the AASM Sleep Definitions Task Force)

Nevertheless, to this day in 2019 at the time of this writing, a debate about whether the RDI should include RERAs drags on. In a recent article published on Medscape.com, a distinguished group of medical professionals specializing in pulmonary and sleep medicine explain it this way.

No universal consensus exists on whether the AHI or the RDI should be the standard index used to determine treatment by specialists and insurance carriers, with Medicare being the most confusing as it varies by region as to whether AHI and RDI can be used. This needs to be resolved as soon as possible. One study found that 30% of symptomatic patients would have been left untreated if the AHI were used rather the RDI.

Source: Himanshu Wickramasinghe, et al., “Obstructive Sleep Apnea (OSA) Differential Diagnoses – Diagnostic Considerations” medscape.com, Updated 3/22/2019.

Unfortunately, given the economic impact, it doesn’t seem likely that all health insurance providers (or even a majority) are going to heed the AASM recommendation regarding RDI if they haven’t already.

Here is an excerpt from CMS.gov clarifying the Medicare position on the RDI definition. (Medicare coverage can vary by region.)

The respiratory disturbance index (RDI) is defined as the average number of apneas plus hypopneas per hour of recording without the use of a positive airway pressure device. For purposes of this policy, respiratory effort related arousals (RERAs) are not included in the calculation of the RDI.

Source: Local Coverage Determination (LCD): Positive Airway Pressure (PAP) Devices for the Treatment of Obstructive Sleep Apnea (L33718), CMS.gov, U.S. Centers for Medicare & Medicaid Services (Note: to view this source you must scroll down on the initial page rendered and accept/click the terms and conditions.)

Digging Deeper into RDI

In digging deeper, it appears that a contributing source of confusion is the fact that the American Academy of Sleep Medicine decided to classify RERAs as “optional.” That’s according to a 2012 article in the Journal of Clinical Sleep medicine. Here is an excerpt.

The task force reached consensus on the definition of RDI as the sum of the AHI and RERA index. However, reporting of the RDI metric should be considered [Optional] (Consensus) as scoring RERAs is also considered [Optional].

Source: Berry, Richard B et al. “Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual … ” Journal of clinical sleep medicine : vol. 8,5 597-619. 15 Oct. 2012, doi:10.5664/jcsm.2172

Using the AASM recommended version of RDI (apneas + hypopneas + RERAs), Figure 4, below, illustrates the difference between AHI and RDI.

The severity scale for the respiratory disturbance index (RDI) is identical to the apnea-hypopnea index. One must simply keep in mind that the RDI includes RERAs (if using the AASM recommended version of RDI).

• RDI of 5 to 14 = Mild Sleep Apnea
• RDI of 15 to 29 = Moderate sleep apnea
• RDI of 30 or greater = Severe sleep apnea

It seems to me that the confusion between AHI and RDI can be easily minimized, if not remedied. If RERAs are not recognized for a sleep study authorized by a particular insurance carrier, simply omit the RDI altogether on the PSG (polysomnography, sleep test) report? Or, better yet, in addition to the AHI, add the true, AASM recommended version of RDI as an informational side-note on the report.

I suspect that health insurance carriers would not be keen on the second option since that might be interpreted by some as an indication that the diagnosis is understated and the treatment is less than appropriate. (And based on scientific evidence, it seems to me, they would be right.)

Technical Definition of Apnea in Adults

For purposes of clinically diagnosing sleep apnea and determining treatment and insurance eligibility, a more precise definition of an apnea is required. The American Academy of Sleep Medicine (AASM) states that in order to score an abnormal breathing event as an apnea, the following criteria must be met:

1) There is a drop in peak signal excursion (air flow in breathing) by at least 90% of pre-event baseline

2) The duration of the 90% airflow drop is at least 10 seconds.

Source: Berry, Richard B et al. “Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual … ” Journal of clinical sleep medicine : vol. 8,5 597-619. 15 Oct. 2012, doi:10.5664/jcsm.2172

This level of detail is required for a sleep study (i.e., polysomnography or sleep test) which measures the quality of one’s sleep and provides results (i.e., polysomnogram) which can assist health care providers in identifying and treating sleep disorders. Sleep study results also enable insurance providers to determine coverage.

Health care and insurance providers go even further when defining apneas, hypopneas, and RERAs by specifying the types of sensors that can be used by sleep clinics when administering sleep studies. That level of detail is outside the scope of this article.

Technical Definition of Hypopnea – another Area for Confusion

The technical definition of hypopnea introduces yet another area of confusion driven chiefly, again, by health insurance providers and accommodated by the AASM. In this case, there is a double standard actually published in the AASM scoring manual for hypopneas in adults.

The first standard is the AASM’s Recommended criteria for hypopnea as follows.

a. 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).

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

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

Source: American Academy of Sleep Medicine, AASM clarifies hypopnea scoring criteria, September 2013

An important aspect of the above standard is the second part of item “c” … and/or the event is associated with an arousal. Since insurance carriers like Aetna and Medicare do not recognize RERAs (arousal-based events) for the computation of true RDI, it seems logical that they would not recognize other arousal-based events that would increase the quantity of hypopneas on sleep study reports. That would result in more SDB (sleep disordered breathing) sufferers qualifying for treatment which would impact their bottom line.

So, the second standard for scoring hypopneas in a sleep study is for health insurance providers like Aetna and Medicare that uphold a more conservative policy. It’s referred to as the AASM’s Acceptable option as follows.

a. 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).

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

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

Source: American Academy of Sleep Medicine, AASM clarifies hypopnea scoring criteria, September 2013

This next definition of hypopnea is used by Medicare and can be found on CMS.gov.

Hypopnea is defined as an abnormal respiratory event lasting at least 10 seconds with at least a 30% reduction in thoracoabdominal movement or airflow as compared to baseline, and with at least a 4% oxygen desaturation.

Centers for Medicare & Medicaid Services, CMS.gov, CPAP for Obstructive Sleep Apnea, 2/25/2016

In addition to the elimination of arousal-based events in the AASM Acceptable option (a.k.a. payer standard), blood-oxygen levels must drop by at least 4%. That, of course, reduces the number of SDB sufferers that qualify for treatment.

In March of 2013, the American Academy of Sleep Medicine clarified this issue on their website for AASM accredited sleep centers. Here is an excerpt.

Although the AASM requires a ≥3 percent desaturation when scoring hypopneas, a number of payers, including Medicare, still require ≥4 percent desaturation for reimbursement.  In such cases, Version 2.0 of the AASM Scoring Manual recommends reporting of hypopneas according to both definitions in order to comply with both accreditation and payer requirements. Complying with payer requirements is critical for initial coverage of diagnostic testing and continued coverage of PAP therapy and supplies. 

American Academy of Sleep Medicine, Hypopnea scoring: Meeting requirements of payers and the AASM Scoring Manual, 3/28/2013

In Closing…

I hope this article helps clarify the definitions of apnea, hypopnea and RERA while shedding light on the sources of confusion regarding those terms. These breathing abnormalities are the foundation of the sleep disorder known as obstructive sleep apnea which affects roughly 1 in 5 adults worldwide. With the vast majority of sleep apnea sufferers undiagnosed, we can only hope that, as education and awareness expand, health care systems everywhere will get better at diagnosing and treating patients afflicted with this debilitating condition.