Apnea –Hypopnea Index- Way to Diagnose Apnea
June 13, 2008
Hypopnea is a medical term for abnormally shallow breathing or slow respiratory rate. This differs from apnea in that there remains some flow of air.
Hypopnea comes from the Greek roots hypo- (meaning low, under, beneath, down, below normal) and pnoe (meaning breathing). Literally it means under breathing.
In the diagnosis and treatment of sleep disorders, a hypopnea event is not considered to be clinically significant unless there is a 50% (or greater) reduction in flow and an associated 3% (or greater) de-saturation in the person’s O2 levels for 10 seconds or longer, or if it results in arousal or fragmentation of sleep.
The direct consequence of hypopnea (as well as apnea) is that the CO2 in the blood increases and the oxygen level in the patient’s blood decreases proportionate to the severity of the airway obstruction. This disruptive pattern of breathing generates disruptive sleep patterns, the consequences of which being that those individuals exhibit increased fatigability, lethargy, decreased ability to concentrate, increased irritability, and morning headaches. Basically, those individuals are extremely tired due to their inability to get a good night’s sleep.
Hypopnoeas can be either central (i.e., as part of a waxing and waning in breathing effort) or obstructive in origin. During an obstructive hypopnoea, in comparison to an obstructive apnea, the airway is only partially closed. However, this closure is still enough to cause a physiological effect (i.e., an oxygen de-saturation and/or an increase in breathing effort terminating in arousal).
Apnea- Hypopnea Index:-
Apnea is usually measured during sleep (preferably in all stages of sleep) over a two-hour period. An estimate of the severity of Apnea is calculated by dividing the number of Apnea by the number of hours of sleep, giving an Apnea index (AI). The greater the AI, the severe will be the Apnea.
A hypopnea is a decrease in breathing that is not as severe as an Apnea. So, if normal breath airflow is 100% to 70%, a hypopnea is 69% to 26% of a normal breath. Like Apnea, hypopneas are associated with a 4% or greater drop in the saturation of oxygen in the blood and usually occur during sleep. Also like Apnea, hypopneas usually disrupt the level of sleep. A hypopnea index (HI) can be calculated by dividing the number of hypopneas by the number of hours of sleep. Greater the HI, severe will be the hypopnoea.
The apnea-hypopnea index (AHI) is an index of severity that combines apneas and hypopneas. Combining them both gives an overall severity of sleep apnea including sleep disruptions and de-saturations (a low level of oxygen in the blood). The apnea-hypopnea index, like the apnea index and hypopnea index, is calculated by dividing the number of apneas and hypopneas by the number of hours of sleep.
Another index that is used to measure sleep apnea is the respiratory disturbance index (RDI). The respiratory disturbance index is similar to the apnea-hypopnea index, however, it also includes respiratory events that do not technically meet the definitions of apneas or hypopneas, but do disrupt sleep.
Sleep Apnea is formally defined as an Apnea-hypopnea index of at least 15 episodes/hour in a patient without medical problems that may be related to the sleep Apnea. (That is the equivalent of one episode every 4 minutes.)
In a patient with high blood pressure, stroke, daytime sleepiness, ischemic heart disease (low flow of blood to the heart), insomnia, or mood disorders—all of which can be caused or worsened by sleep Apnea–sleep Apnea is defined as an Apnea-hypopnea index of at least 5 episodes/hour. This definition is stricter because the patient may be already experiencing the negative medical effects of sleep Apnea, and it may be important to begin treatment at a lower Apnea-hypopnea index.
Summary:
The apnea-hypopnea index (AHI) is an index of severity that combines apneas and hypopneas. Combining them both gives an overall severity of sleep apnea including sleep disruptions and de-saturations (a low level of oxygen in the blood). The apnea-hypopnea index, like the apnea index and hypopnea index, is calculated by dividing the number of apneas and hypopneas by the number of hours of sleep.
Whatever may be the merits or demerits of apnea hypopnea index, it is still an index of how severe is both apnea and hypopnea and by combining these two disorders, gives an overall severity of the sleep apnea condition that also includes disruptions in sleep as well as when levels of oxygen in the blood have fallen to a low level.
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