Terms used in this guideline

This section defines terms that have been used in a particular way for this guideline.


A complete pause in breathing, defined as lasting 10 seconds or more on a sleep study. An obstructive apnoea is caused by blockage of the upper airway, whereas a central apnoea occurs when there is no respiratory effort.

Apnoea–hypopnoea index (AHI)

The number of apnoeas and hypopnoeas per hour, measured during a multi-channel sleep study.


A reduction in breathing, defined as lasting for 10 seconds or more on a sleep study. An obstructive hypopnoea is caused by partial obstruction of the upper airway.

Mandibular advancement splint

An oral device used to treat sleep-related breathing disorders. It is worn over the upper and lower teeth, and holds the lower jaw forward, thereby increasing space at the back of the mouth and decreasing snoring and sleep apnoea. A custom-made mandibular advancement splint is formed from a dental impression taken by a dentist, which is used to make the splint in a laboratory. It is then fitted by a suitably trained general dental practitioner. A semi-customised mandibular advancement splint is formed using a dental impression taken by the patient, which they send to the manufacturer to make the splint.

Nocturnal hypoventilation

Decreased breathing or under breathing during sleep, which can lead to varying severities of ventilatory failure (low oxygen levels and raised carbon dioxide). It can be caused by obesity, underlying lung disease, neuromuscular weakness and some medications such as opiates. Severe hypercapnia can be caused by nocturnal hypoventilation.

Oxygen desaturation index (ODI)

The ODI is defined as the number of episodes of oxygen desaturation per hour of sleep.

Positional modifier

An intervention to encourage patients not to sleep on their backs. There are several devices available such as the tennis ball technique, lumbar or abdominal binders, semi-rigid backpacks, full-length pillows and electronic sleep position trainers.

Positional OSAHS

A type of obstructive sleep apnoea/hypopnoea syndrome (OSAHS) that is affected by the person's sleep position. People with positional OSAHS have an apnoea–hypopnoea index (AHI) at least twice as high when lying face up (supine) as lying on their side (laterally).

Severity of OSAHS

This is determined using the AHI value, as follows:

  • Mild OSAHS: AHI of 5 or more to less than 15

  • Moderate OSAHS: AHI of 15 or more to less than 30

  • Severe OSAHS: AHI of 30 or more.

Sleep study

A test used to diagnose sleep disorders by recording multiple channels during sleep, such as brain activity, breathing rate, blood oxygen level, heart rate, and eye and leg movements. There are several different types of sleep study:

  • oximetry measures arterial oxygen saturation and heart rate while the person is asleep

  • respiratory polygraphy includes at least 4 channels such as oximetry, breathing rate, apnoeas and hypopnoeas, snoring and body position

  • polysomnography, which is more detailed and includes respiratory polygraphy measures combined with assessment of sleep quality and duration using additional brain activity, eye movement and muscle tone signals.


The use of information and communication technologies to monitor patients remotely and transmit data related to their health. It is used to provide information including respiratory events, pressure requirements, mask leak and adherence.