Sleep respiratory definitions

What is the Apnea-Hypopnea Index (AHI)?

In severe sleep apnea, people are briefly awakened up to hundreds of times every night due to sleep disordered breathing due to airway collapse (imagine someone sitting at your bedside, tapping you on the shoulder every minute or so to awaken you), leading to poor sleep and extreme fatigue during the day. The number of these episodes per hour of sleep—as measured with WatchPAT or a sleep-laboratory study (PSG)—is called the Apnea-Hypopnea index (AHI). This index specifically calculates sleep apnea severity based on the total number of complete cessations (apnea) and partial obstructions (hypopneas) of breathing per hour of sleep.
The AHI is used to classify the severity of your sleep apnea, according to the following criteria:

  • Normal: < 5 events per hour of sleep
  • Mild: 5-15 events per hour of sleep
  • Moderate: 15-30 events per hour of sleep
  • Severe: > 30 events per hour of sleep

Some patients have 50 or more episodes per hour; most sleep experts use a cut-off of 10 events or more per hour before considering treatment with a continuous positive airway pressure (CPAP) device or some other sleep apnea therapy.

What is the Apnea-Hypopnea Index (AHI)?

Respiratory Disturbance Index (RDI) is the number of apnea, hypopnea, and RERA events per hour of sleep. The RDI is the recommended measurement for the severity of Obstructive Sleep Apnea (OSA), by the AASM (American Academy of Sleep Medicine).
The index includes the following respiratory indices:

  • Apnea: Cessation of breathing during sleep for 10 seconds or more
  • Hypopnea: Reduction of breathing followed by desaturation of 3% or more and/or an arousal
  • RERA: Obstructive event that does not meet the criteria for apnea or hypopnea but causes arousal from sleep

What is Oxygen Desaturation Index (ODI)?

The Oxygen Desaturation Index (ODI) measures the number of times that the oxygen level in blood drops 4% or more during sleep. These drops in oxygen levels are called desaturations. ODI is measured by an oximeter, placed in the probe.
When breathing becomes disrupted during sleep, as may occur in obstructive sleep apnea, the oxygen levels of the blood may repeatedly fall. These drops are typically associated with collapses of the upper airway, events called apnea or hypopnea. Drops occur less frequently in snoring or upper airway resistance syndrome (UARS), two conditions in which breathing is disturbed, yet to a lesser degree.

The Stages of Sleep

Sleep is divided into two distinct types: Non-REM (NREM) sleep and rapid eye movement, or REM sleep. When you first go to sleep, you begin NREM sleep, also known as “quiet sleep,” and then go through four distinct phases, each one progressively deeper than the other.
In Stage I, your muscles begin to relax, your breathing becomes regular, and body temperature starts to drop. You can be awakened easily from this early and shallow stage of sleep.
During Stage II and III, your sleep becomes deeper, with your body processes slowing down. It’s difficult to awaken you, and if you are suddenly awakened, you may be groggy or startled and not know where you are.
Delta sleep, or Stage IV sleep, is thought to be the restorative stage when the body goes about its various repair functions.
During REM sleep, the next major sleep phase, your mind is hyperactive but focused. At this time, your blood pressure soars and then sinks low. Your heart rate races and slows, and your breathing pattern is uneven. Blood flow to your brain increases. Dreams are common and often vivid. Erections are also common, as are engorgements of the clitoris. Except for a quick fluttering back and forth of your eyelids, there is no noticeable physical motion while you are in REM sleep because your body’s motor system has been shut down.
A typical sleep cycle begins with NREM sleep and ends with REM sleep, with the two combined cycles lasting a total of 90 to 100 minutes. This sleep cycle is repeated four to five times during the night. Early at night, the REM period may only be five minutes long but it may stretch to an hour as the night progresses. However, if you are a poor sleeper—perhaps because of sleep apnea—you may spend less time in Stage IV and REM states, you’re your cycles fragmented and disrupted by periods of wakefulness.


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