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  • Misc. Heart
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  • Polysomnography

  • Polysomnography LeadsSleep apnoea (apnea - US spelling) is a breathing disorder that occurs in sleep. There are two main types of sleep apnoea; obstructive sleep apnoea and central sleep apnoea, though it is possible to have a combination of both causes. Narcolepsy is another sleep disorder but sleep apnoea must be ruled out before it can be diagnosed along with other more specialised tests (see Neurophysiology).

    The most commonest sleep apnoea is obstructive sleep apnoea (OSA). This is where there is a partial obstruction (hypopnoea) or complete obstruction (apnoea) causing a significant reduction in the airflow or complete occlusion in the presence of normal breathing effort. The most likely cause for this is upper airway collapse. During these apnoeas Oxygen desaturation is seen and can lead to arousal from sleep. 

    Central sleep apnoea is much less common than OSA and this is where the autonomic system stops sending signals to the muscles that control breathing, as the breathing is stopped oxygen desaturation can also occur. This type of apnoea occurs when there is damage to the brainstem from infection, haemorrhage or trauma to the cervical spine (neck). Other causes can include Parkinson's disease, cardiac failure and side effects of some medications such as narcotic painkillers.

    As previously mentioned there can be a combination of OSA and central sleep apnoea and this is most likely caused by obesity amongst other causes

    Testing for sleep apnoea falls into two general categories: polysomnography and limited channel monitoring (also known generically as 'sleep apnoea testing'). Sleep Questionnaires and Diaries also provide useful information to understand a person's sleep habits and patterns. The commonest in use in UK is the Epworth Sleepiness Scale. However, by themselves they are insufficient to confirm or exclude the diagnosis of OSA.

    Epworth Sleep Scale


    The main investigation into sleep apnoea is polysomnography (literally 'poly' - many, 'somn' - sleep and 'graphy' - recording). This investigation will enable sleep specialists to decide what is the best treatment for the condition diagnosed. Experienced technicians usually perform and monitor this test and the patient is expected to sleep in a specialised room or sleep laboratory.

    Polysomnography simultaneously records multiple physiological signals during sleep. The parameters that are recorded during polysomnography may include all or most of the following:

Electroencephalography (EEG) Electrodes on your face and scalp This monitors your brain waves/Sleep Stage
Chin Electromyogrpahy (EMG) Sensor on chin To record mouth movement
Limb Electromyography (EMG) Sensor on lower limb This monitors muscle tone
Electrooculogram (EOG) Electrodes adjacent to eyes To record eye movement
Thoracoabdominal Movements Band around chest/abdomen To record chest/abdomen breathing
Oronasal Airflow Electrode/sensor near mouth/nose Records airflow
Pulse Oximetry Oxygen sensor on finger Blood oxygen levels
Electrocardiography (ECG) 3 electrodes on chest Heart rate
Sound Microphone To record snoring/breathing sounds
Video Digital night camera To record general whole body movement

Limited Channel Monitoring or Sleep Apnoea Test

    This type of investigation records a smaller number of physiological signals compared to polysomnography. Generally the signals recorded for this test are focused on breathing and blood oxygen levels. With these monitors alone, sleep cannot be precisely determined but with the EMG sensors it can be estimated. This test is usually performed in the home, it may be set up and prepared in the laboratory but for the recording the technician will be absent.

    Increasingly, people who have a high likelihood of OSA and have few other medical conditions after a clinical consultation, are tested via limited channel monitoring. This method is also used as a means to determine whether their treatments are effective.

Limited Channel Test

Technician present to record optimally
Natural sleep environment

Other sleep conditions recorded
Lower costs

Multiple physiological signals recorded
Patient convenience

Recorded in unfamiliar environment
Not Technician supervised

Expensive (equipment & night time staffing)
Greater test failure rate

Patient Inconvenience
Less recorded signals


    The results of any investigation must be taken in the context of the clinical consultation and history but the main results taken from Polysomnogrpahy are the Apnoea Hypopnoea Index (AHI) or Respiratory Disturbance Index (RDI), see below for explanation. These indexes are calculated from the average number of apnoea/hyponea events, oxygen desaturation levels. Additionally the heart rate variability, frequency of snoring and how these related to body position are also considered.

    Apnea Hypopnea Index (AHI). The AHI is the number of apnoeas or hypopnoeas that are recorded during the study per hour of sleep. It is generally expressed as the number of events per hour.

    Respiratory Disturbance Index (RDI. This is slightly different to the AHI as the RDI includes not only apnoeas and hypopnoeas, but it also includes respiratory effort related arousals (RERA's).  RERAs are arousals from sleep that do not technically meet the definitions ofapnoeas/hypopnoeas but do disrupt sleep. They are abrupt transitions from a deeper stage of sleep to a shallower one.

    This means a person's RDI can be higher than their AHI.

    Both the AHI and RDI sleep apnoea severity is classified as follows:

    None/Minimal:     AHI < 5 per hour
    Mild:                    AHI ≥ 5, but < 15 per hour
    Moderate:           AHI ≥ 15, but < 30 per hour
    Severe:               AHI ≥ 30 per hour

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