All of them matter, to be sure. However, there are two which are of particular interest for those who have sleep breathing disorders.
AHI and RDI both measure the quality of your breathing during sleep, but they technically count on different aspects of breathing in order to arrive at their numbers.
So what is the difference between AHI and RDI?
What is AHI?
AHI stands for Apnea-Hypopnea Index. This is the more commonly understood term among obstructive sleep apnea (OSA) patients. This number tells them how many times (on average) they stop breathing during the night as they sleep as the result of apnea (complete obstruction of the upper airway) and hypopnea (partial obstruction of the upper airway). Together, the combination of both is referred to as "respiratory events."
Example: A person who has 50 apneas and 75 hypopneas over 6 hours' time (360 minutes) has an AHI of 21.
50 + 75 = 125
125 ÷ 360 (minutes) = .35
.35 × 60 (hour equivalency) = 21
An AHI score of 21 means the patient has been shown to have, on average, 21 respiratory events per hour, which means they are likely to be diagnosed with moderate sleep apnea by their sleep specialist.
What is the AHI severity gradient?
- None/Minimal: AHI < 5 per hour.
- Mild: AHI ≥ 5, but < 15 per hour.
- Moderate: AHI ≥ 15, but < 30 per hour.
- Severe: AHI ≥ 30 per hour.
What is RDI?
RDI stands for Respiratory Disturbance Index (some people call it the Respiratory Distress Index). This is less commonly discussed among sleep apnea patients because AHI is such a clear metric for understanding their disorder's severity.
However, for people with other sleep breathing concerns, such as upper airway resistance syndrome (UARS), RDI is one way to understand how problems with sleep breathing—even without apneas or hypopneas— can result in problematic sleep overall.
RDI measures not only apneas and hypopneas, but a third category, RERAs, or respiratory effort related arousals. RERAs measure disruptions in sleep that are not classifiable as apneas or hypopneas.
Someone can have a low AHI (under 5 per hour) but have a high RDI (15 or more). Why is this? While the airway is not obstructed, there's something happening during sleep that makes breathing harder for the sleeper. This could be snoring, allergies, asthma, congenital problems (such as deviated septum), or inflammation and swelling in the upper airway from illness, among other things.
Calculating both indices can give the sleep specialist a clearer picture of abnormal breathing patterns, including the more subtle ones which kick off RERAs even in the absence of apneas. Ultimately, the person who does not qualify for OSA but who has a high RDI is still in need of some kind of treatment because they are still waking up at least 15 times an hour... not exactly good, efficient sleep.