13th January 2024

Sleep Studies

Sleep studies are required to investigate sleep-related breathing disorders. There are various sleep studies of increasing degrees of complexity dependent on the requirements (and resources available to the particular centre). Tests can be done at home or overnight at the testing centre.

The gold standard is the Polysomnogram (PSG) which has multiple tools to measure different physiologic signals that help stage sleep, monitor respiratory airflow, monitor respiratory effort and monitor oximetry and heart rate and rhythm. Optionally, there are mechanisms to measure ventilation as well.

Components of a full Polysomnogram

A full sleep study will include all the components below and be done overnight in a centre.

Staging Sleep

• Electroencephalography (EEG): to monitor brain waves and help stage sleep.
• Electrooculogram (EOG): to pick up the movement and activity of the eyes
• Electromyogram (EMG): to measure muscle tension and movement

They measure for:

• Arousals: changes in sleep stage to full wakefulness or a lighter stage of sleeping. These are counted and then divided by total sleep time to give an Arousal Index (AI)

Monitor Respiratory Airflow

• Oronasal thermistor and nasal pressure transduction: to monitor for changes to airflow

They measure for:

• Apnoeas: absence or near-complete absence of airflow. These are classified as obstructive (apnoea with evidence of respiratory effort), central (apnoea with absence of any respiratory effort) or mixed (apnoea with a period of absent respiratory effort followed by evidence of some effort).
• Hypopnea: Reduction of airflow, not sufficient to be classified as an apnoea

Respiratory Effort

• Thoracic and abdominal Respiratory Inductance Plethysmography (RIP): bands across the abdomen and thorax to measure chest and abdominal wall movement and thus respiratory effort

Oximetry

• Pulse oximeters: the most basic element to measure the oxygen saturations through sleep

They measure for:

• Hypoxaemia during sleep 

Cardiac Rate and rhythm

• Electrocardiogram (ECG): 1-3 lead ECG to measure heart rate and monitor for any cardiac dysrhythmias

Ventilation

Monitoring of ventilation is often only performed if there is a high clinical suspicion of hypoventilation.

• Transcutaneous CO2 monitoring
• End-tidal CO2 monitoring

From the above measures, one can make interpretations on:

• Sleep quality: disturbance/ fragmented sleep will exhibit a higher proportion of N1 or N2 sleep. The timing of apnoeas can also help differentiate central from obstructive apnoeas.
• Apnoea- Hypopnea Index (AHI): Apnoea + hypopneas divided by the total sleep time. This can be subdivided into an obstructive AHI and central AHI.
• Degree of Hypoxaemia

Home studies

Home studies tend to be a cheaper and more available alternative. The portable devices can be taken home and used to assess airflow, respiratory effort, oximetry and cardiac rhythm (or even used just to measure overnight oximetry!). The sleep staging component is the absent measure. Home studies are particularly useful in patients with a high pre-test probability of diagnosis.

Portable or home studies will not measure sleep and stages. From the other data points though we can derive:

• Respiratory Event Index (REI): the number of apnoeas or hypopneas during recorded time. Because this time doesn’t record sleep itself (the recorded time is longer than the total time spent), this can underestimate the ratio of apnoeas/hypopneas to actual time asleep.

Image Source: Wikimedia Commons (2017). Polysomnography tester, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/0/0d/Polysomnography_tester.jpg  (Accessed: 9 August 2022).

References:

• Schulman D. Polysomnography in the evaluation of sleep-disordered breathing in adults [Internet]. Uptodate.com. 2022 [cited 8 August 2022]. Available from: https://www.uptodate.com/contents/polysomnography-in-the-evaluation-of-sleep-disordered-breathing-in-adults
• Understanding the Results (Diagnosing OSA) [Internet]. Sleep.hms.harvard.edu. 2021 [cited 8 August 2022]. Available from: https://sleep.hms.harvard.edu/education-training/public-education/sleep-and-health-education-program/sleep-health-education-34
• Obstructive sleep apnoea in adults [Internet]. BMJ Best Practice. 2020 [cited 8 August 2022]. Available from: https://bestpractice.bmj.com/topics/en-gb/215