Sleep Indices

Sleep Indices are a quantitative way to evaluate various aspects of sleep. There are many factors of sleep, therefore there are many measurements that can be made to numerically describe the quality of sleep a person experiences.

Apnea-Hypopnea Index

The Apnea-Hypopnea Index (AHI) is used as a measure of the severity of the condition known as sleep apnea. The AHI is the sum of the number of apneas (pauses in breathing) plus the number of hypopneas (periods of shallow breathing) that occur, on average, each hour. To count in the index apneas and hypopneas, collectively called events, must have a duration of at least 10 seconds. Apnea events may be further classified as Obstructive sleep apnea (OSA) or Central sleep apnea (CSA), abbreviated OA and CA respectively.

The AHI, as with the separate Apnea Index and Hypopnea Index, is calculated by dividing the number of events by the number of hours of sleep.

Therefore,

         AHI = (OA + CA + H) ÷ # of hours of sleep

ie. A person sleeps 8 hours. In 8 hours the individual experiences 2 obstructive apnea, 5 central apnea and 10 hypopneas.

         AHI = (2 OA + 5 CA + 10 H) ÷ 8 hours of sleep
               (17) ÷ 8 = 2.125
         AHI = 2.13

As we can see, AHI is proportional to the number of apnea events and inversely proportional to the number of hours logged as sleep. Naturally, if a person experiences any combination of apnea events then they will see a corresponding increase in their AHI, while an increase in hours slept will cause the AHI to decrease if the number of events stays the same.

This poses another problem. While an increase in sleep hours should cause a lower AHI, the increase in sleep hours will also increase the time that apnea events have the potential to occur.

For instance, a patient who sleeps for 6 hours on night 1, and experiences 5 obstructive apnea, 2 central apnea and 7 hypopneas.Their AHI will look like:

         AHI = (5 OA + 2 CA + 7 H) ÷ 6 hours of sleep
               (14) ÷ 6 = 2.33
         AHI = 2.33

If the same patient sleeps 8 hours on night 2 there are 2 more hours of sleep that the potential for apnea may occur. So the patient may experience 6 obstructive apnea, 4 central apnea, and 9 hypopneas. The patient will have an AHI on night 2 that looks like this:

         AHI = (6 OA + 4 CA + 9 H) ÷ 8 hours of sleep
               (19) ÷ 8 = 2.375
         AHI = 2.38

While the patient achieved 2 more hours of logged sleep on night 2, the patient also experienced more apnea events because of the 2 extra hours that apnea events have the potential to occur. So the AHI only varied slightly even though the patient slept 25% more on night 2 versus night 1.

Although the reduction of AHI is a good indicator of CPAP efficacy, AHI alone is not necessarily an indicator of sleep quality. Many new CPAP patients will report feeling tired even though they report AHI numbers consistently in the normal range (AHI 0-5). There are many factors that affect sleep quality. AHI is certainly one aspect of sleep quality but should not be considered as the empirical value of sleep. While less than 5 AHI is considered treated, many feel "the lower the better" as long as comfort is considered and you are not chasing unrealistic numbers such as zero.

Apnea Index

The Apnea Index (AI) is the component of the Apnea Hypopnea Index (AHI) that represents the average number of apnea events that occur during each hour of sleep. Apnea events may be further classified as Obstructive sleep apnea (OSA) or Central sleep apnea (CSA), abbreviated OA and CA respectively.

Unlike the Apnea-Hypopnea Index, the apnea index only counts obstructive apnea (OA) and central apnea (CA) events and does not include hypopneas (H) as part of the equation. AI is derived from the sum of obstructive apnea and central apnea events divided by the number of hours of sleep.

Therefore,

           AI = (OA + CA) ÷ # of hours of sleep

If a person sleeps for 8 hours and experiences 5 OA and 5 CA, then the math will look like this:

           AI = (5 OA + 5 CA) ÷ 8 hours of sleep
                (10) ÷ 8
           AI = 1.25

The apnea index is a useful tool in the characterization of apnea type. By neglecting to consider hypopnea a more preferred approach to treatment can be ascertained as no individual will have exactly the same statistics for apnea events and apnea event types. By examining apnea index one may draw conclusions for the optimization of individual machine settings when other event types are compared.

Central Apnea Index

The Central Apnea Index (CAI) is a component of the Apnea Hypopnea Index (AHI) that represents the average number of central apnea events that occur during each hour of sleep.

Central apnea is when the effort to breathe is diminished or absent, typically for 10 to 30 seconds, either intermittently or in cycles and is usually associated with a reduction in blood oxygen saturation.

The central apnea index is the sum of all central apnea events divided by the number of hours of sleep.

Therefore,

             CAI = # of central events ÷ # of hours of sleep

If a patient sleeps for 6 hours and experiences 10 central events, the equation will look like this:

             CAI = 10 central events ÷ 6 hours of sleep
             CAI = 1.67

The central apnea index is useful in determining the appropriate type and level of therapy for a patient who suffers from sleep apnea when other types of apnea may or may not be present.

Obstructive Apnea Index

The Obstructive Apnea Index (OAI) is a component of the Apnea Hypopnea Index (AHI) that represents the average number of obstructive apnea events that occur during each hour of sleep.

Obstructive apnea events are the most common type of sleep apnea and is caused by obstruction of the upper airway. There are many ways that the airway can become blocked and the apnea must have a duration of 10 seconds or more to be considered as an event.

The obstructive apnea index is the sum of all obstructive events divided by the number of hours of sleep.

Therefore,

              OAI = # of obstructive events ÷ # of hours of sleep

If a patient sleeps for 9 hours and experiences 27 obstructive apneas, then the math will look like this:

              OAI = 27 obstructive events ÷ 9 hours of sleep
              OAI = 3

The obstructive apnea index is useful in determining the appropriate type and level of therapy for a patient who suffers from sleep apnea when other types of apnea may or may not be present.

Hypopnea Index

The Hypopnea Index (HI) is the component of the Apnea-Hypopnea Index (AHI) that represents the average number of hypopnea events that occur during each hour of sleep.

A Hypopnea is a partial apnea event, defined as a 30% or greater reduction in airflow (compared to baseline), and a 3% or greater reduction in Oxygen saturation (a drop in the oxygen level or desaturation) during a 10-second event. The hypopnea index is the sum of all hypopnea divided by the number of hours of sleep.

Therefore,

              HI = # of hypopnea ÷ # of hours slept

If an individual sleeps for 8 hours and they experience 10 hypopnea, the equation will look like this:

              HI = 10 H ÷ 8 hours of sleep
              HI = 1.25

The hypopnea index is useful in determining the appropriate therapy while considering any other types of apnea that may or may not be present.

Respiratory-effort Related Arousal

A Respiratory-effort Related Arousal (RERA) is an abnormal breathing event which does not meet the criteria of an Apnea or a Hypopnea but is an “arousal” event associated with a respiratory effort as noted by electroencephalogram (EEG) during the sleep study. An EEG determines the stages of sleep, and the “arousal” is noted by EEG. Unlike apnea and hypopnea events RERAs are not typically conscious events and a patient will never know they have them.

RERA events are important as any RERA event will likely disturb any of the cycles of sleep, thus, affecting sleep quality.

RERA events are not usually averaged and generally are not conveyed in an index. A polysomnogram (PSG) usually tags the individual events and a sum of RERA events is noted in the PSG report.

Respiratory Disturbance Index

The Respiratory Disturbance Index (RDI) is similar to AHI, but the RDI also considers respiratory-effort related arousals (RERA).

RDI is calculated as the sum of apnea events (central and obstructive), hypopnea events and RERA divided by the number of hours of sleep.

Desaturation Index

The Desaturation Index (desat index, ODI) is the average number of times blood oxygen levels drop per hour. To be considered a desaturation event, the blood oxygen level must drop 3% from a baseline. For example, a desaturation occurs when blood oxygen drops from 99% to 96% spO2.

ODI is the sum of desaturation events divided by the number of hours of sleep.

Therefore,

             ODI = # of desaturation events ÷ # of hours of sleep

If a patient sleeps for 7 hours and experiences 27 desaturation events, the equation looks like this:

             ODI = 27 desaturation events ÷ 7 hours of sleep
             ODI = 3.86

The desaturation index is important because oxygen desaturation is detrimental to a patient's health in many ways other than sleep quality. Oxygen is critical to the proper function of all the bodily systems and any significant drop in blood oxygen has consequences that have short and long term effects. Decreased brain function, heart stress, organ damage, circulatory damage..are all examples of hypoxia (low oxygen levels). This is important to sleep apnea sufferers as any apnea-hypopnea event will likely see a coinciding desaturation event. The severity of the desaturation event is increased as the duration of the apnea-hypopnea event increases. Chronic desaturation can lead to disease, organ failure and death.

Periodic Limb Movement of Sleep Index

Periodic Limb Movement of Sleep (PLMs) is characterized as the movement of the legs during sleep. Generally, only the legs are monitored however the arms may be affected as well. This limb movement results in in decreased sleep quality and insomnia.

To be characterized as a PLMs event, the limb movement is recorded as an EEG signal from the leg muscle contraction with a duration greater than 0.5 seconds and less than 5 seconds, and there must be 4 marked contractions within a 90 second period. So, if a person moves their leg for at least half a second and no more than 5 seconds, 4 times in 90 seconds, they will have had a PLMs event.

Peridic Limb Movement Index (PLMI) is the sum of PLMs events divided by the number of hours of sleep.

Therefore:

            PLMI = # of PLMs events ÷ # of hours of sleep

If a patient sleeps for 9 hours and experiences 37 PLMs events, the equation will look like this:

            PLMI = 37 PLMs events ÷ 9 hours of sleep
            PLMI = 4.11

PLMs is important to sleep apnea sufferers because PLMs is sometimes a side effect of sleep apnea and upper airway resistance syndrome (UARS). Many diagnosis of PLMs has been made with the underlying cause being mild to moderate obstructive sleep apnea or UARS. In severe cases of obstructive sleep apnea, the PLMs is discovered only after treatment. Generally, PLMs is greatly improved with CPAP treatment for obstructive sleep apnea.

Periodic Limb Movement of Sleep Arousal Index

Limb Movement Index

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