Equipment
The test involves wearing sensors that monitor your oxygen level, pulse rate, chest and waist movements, snoring and nasal airflow, body position and, optionally, leg movements. The combination of sensors used can vary between sleep centres.
Home or in a sleep centre?
The kit is portable and can normally be taken home and worn overnight whilst you sleep in your own bed. However, you may be required to attend a sleep centre overnight where you can be monitored by staff. This varies between sleep centres.
Who attaches the equipment?
This depends upon which sleep centre you attend.
- You may be given a bag of equipment to take home and a sleep specialist will instruct you on how to attach the sensors to your body.
- A sleep specialist may attach the equipment to your body over a t-shirt or vest during a sleep centre visit. The equipment can be tested whilst you are wearing it to check it is working properly. You can then leave the sleep centre wearing the equipment so that when it is bedtime you are already set up for the sleep study.
- You may be asked to attend a sleep centre overnight as an inpatient and a sleep specialist will attach the equipment then.
How long does it take?
- 30 minutes to attach the equipment
- At least four hours of sleep (preferably all night)
- 10 minutes to remove the equipment in the morning
- Time to return the equipment the following day (if necessary).
What is monitored?
Airflow at the Nose / Snoring
A nasal cannula is placed on your face. This consists of two tiny prongs that sit just inside your nostrils and branch into left and right tubes. The tubes loop over each ear, re-join underneath the chin and feed back to the monitor. The nasal cannula can check for snoring and pauses in your breathing.
Effort to breathe
An elastic belt is worn around your chest and another around your waist. These can distinguish between obstructive and central apnoeas.
During obstructive apnoeas your airways close off, blocking the passage of air to your lungs. In response to the obstruction you show increasingly forceful efforts to breathe, so the belts move vigorously until you partially or fully wake up and your airways finally re-open.
During central apnoeas the muscles of your lungs fail to move and inflate them. You show no effort to breathe, so the belts barely move until you partially or fully wake up and your lung muscles are kick-started into working once again.
Body position
A sensor can show whether you are lying on your back, left or right sides or front. This can identify positional sleep apnoea, for example, where you have the majority of your apnoeas when lying on your back.
Amount of oxygen in the blood / pulse rate
A plastic or rubber finger monitor is worn to check whether you have sleep apnoea. After each pause in breathing, the level of oxygen in your body dips and your pulse rate rises before returning to normal. If this happens repeatedly, sleep apnoea can be diagnosed.
Leg movements (optional)
A sensor is placed near the shin bone on each leg. This can check whether your muscles are relaxed and you are asleep, or whether they are active and you are awake. The measurements can also help to diagnose periodic limb movement disorder, where your legs (and less commonly, arms) move repeatedly during sleep.
Results
The information is saved and memorised by a small, lightweight device, worn on a strap around the chest. It is downloaded the following day to give a reading in the form of several graphs.
The graphs are viewed and interpreted so that a sleep specialist can form a diagnosis. You will be given an appointment with the specialist to receive your results.
Limitations of semi-polysomnography
This test cannot diagnose some of the more unusual sleep disorders, such as narcolepsy. If you show more unusual behaviours during sleep you may be offered full polysomnography or be assessed by a neurologist.
