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A to Zzz's of Sleep Definitions and Abbreviations


APNOEA:  Without Breath (also spelt Apnea in some countries).  Therefore, Sleep Apnoea means pauses in breathing whilst sleeping.

APNOEA/HYPOPNOEA INDEX (AHI):  The number of apnoea/hypopnoea incidents per hour.

APNOEA INDEX (AI):  The number of apnoea events per hour.

AROUSAL:  Going from sleep to wakefulness, or from a deeper to lighter stage of sleep.

APAP:  Automatic Positive Airway Pressure Machine (Automatic CPAP), which automatically adjusts the level of pressure needed on a breath-by-breath basis.   Useful for people on high pressures or who find it difficult to tolerate the pressure needed.

BI-PAP:  Bi-level Pressure Machine with 2 pressures – a lower pressure for exhalation and a higher pressure for inhalation.  (Useful for people who find it difficult to exhale against the pressure). 

BRUXISM:  Teeth grinding during sleep.

CENTRAL SLEEP APNOEA (CSA):  Absence of breath caused by irregularity in the brain’s control of breathing. 

CPAP:  Continuous Positive Airway Pressure Machine with one fixed pressure giving constant same-pressure therapy to keep the airways open.

EPAP:  Expiratory Positive Airway Pressure prescribed for the expiratory (breathing out) phase of a person on Bi-level CPAP therapy.

FLOW LIMITATION:  Partial closure of the upper airway impeding the flow of air into the lungs.

GERD (GASTROESPHAGEAL REFLUX DISEASE):  Flow of stomach acid upwards into the esophagus that can cause arousals and disrupt sleep. 

HST:  Home sleep study


HYPERSOMNIA:  Excessive daytime sleepiness.

HYPNOTICS:  Sleep-inducing medication.

HYPOPNOEA:  Shallow breathing, often accompanied by oxygen desaturation.

HYPOVENTILATION:  Decreased rate of breathing.

INSOMNIA:  Difficulty in falling and/or staying asleep.

INSPIRATORY POSITIVE AIRWAY PRESSURE (IPAP):  Pressure prescribed for the inspiratory phase of a person on Bi-level CPAP therapy.

LASER ASSISTED UVULOPALATOPLASTY (LAUP):   Operation using a laser to make some small cuts in your soft palate to make it firmer and hopefully reduce snoring. 

MICROSLEEP:  Sleep episode lasting from a fraction of a second up to 30 seconds – normally as a result of sleep deprivation. 

MIXED SLEEP APNOEA: A combination of Central and Obstructive Sleep Apnoea.

NARCOLEPSY:  A sleep disorder that causes excessive sleepiness and frequent daytime sleep attacks. 

NOCTURIA: Excessive, often frequent, urination during the night.

OBSTRUCTIVE SLEEP APNOEA (OSA):  Sleep disorder where the sufferer stops breathing for at least 10 seconds, caused by an obstruction, such as the uvula or tongue blocking the windpipe, soft palate dropping etc.


OBSTRUCTIVE HYPOPNOEA:  An episode of overly shallow breathing or an abnormally low respiratory rate, which is different from an apnoea as there does remain some flow of air.   It is also caused by the same obstructions as OSA.


OXYGEN SATURATION:  Oxygen saturation refers to the percentage of oxygen found in the  blood.  Normal values range between 90-100%.




PERIODIC LIMB MOVEMENT DISORDER (PMLD):  A sleep disorder where the sufferer involuntarily moves their limbs during sleep.


PSG:   Polysomnography or the Sleep Study


PULSE OXIMETER:  Medical device which monitors the oxygen saturation and heart rate of the wearer.  


RAINOUT:  Term used to describe a build-up of condensation/water in the CPAP hose tube from the humidification.  (A hose cover normally fixes this!).

RESPIRATORY DISTURBANCE INDEX (RDI):  Includes all respiratory events per hour. 

REM SLEEP (RAPID EYE MOVEMENT SLEEP):  The sleep stage where there is rapid eye movement (hence it’s name), loss of body movement, more brain activity and where dreaming occurs.

RESTLESS LEGS SYNDROME (RLS): A sleep disorder characterized by a tingling, aching, crawling and itching of the limbs which can last for hours and usually gets worse at night, preventing the sufferer from getting the sleep they need. There is an almost irresistible urge to move the legs to ease the discomfort. 


RPSGT:  Registered Polysomnography Technologist or Sleep Tech for short.

SEPTOPLASTY:  Surgery to straighten the nasal septum (dividing the nasal passage).  A deviated septum can cause nasal blockage and sinus problems.

SLEEP APNOEA:  Cessation of breathing for 10 or more seconds during sleep, and takes the form of Obstructive Sleep Apnoea, Central Sleep Apnoea or Mixed Sleep Apnoea.

SLEEP DEBT:   A build-up (often accumulated over many years) of sleep deprivation, and the body requires this to be paid back.  In the case of Sleep Apnoea, this debt does not usually start to get paid back until the sufferer is on successful treatment.

SLEEP PARALYSIS:  An experience of waking and not being able to move at all for (usually) a short period of time.  Whilst this is not harmful, it can be very distressing.

TONSILLECTOMY:  Surgical removal of the tonsils which can alleviate the symptoms of obstructive Sleep Apnoea, or in the case of children, removal can cure Sleep Apnoea until they are adults, in most cases.

TRACHEOTOMY: Surgical procedure to create an opening in the trachea (windpipe) so that one can breathe.  At present this is the only known complete cure for Sleep Apnoea.

UPPER AIRWAY RESISTANCE SYNDROME (UARS):  A sleep disorder caused by blockages to breathing from the upper airways – most often caused by nasal congestion.  It is closely related to Sleep Apnoea, and certainly mimics most of the symptoms.  Unlike Sleep Apnoea, 50% of the sufferers are female. 

UVULOPALATOPHARYNGOPLASTY (UPPP):  Surgical procedure to remove the uvula, pharyngeal arches and often partial removal of the soft to help improve the symptoms of Sleep Apnoea.

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