Polysomnography: Definition, Medical Uses, Mechanism, Procedure, Interpretation and Examples

It is a comprehensive record of the biophysiological changes that occur during sleep.

Polysomnography (PSG), a type of sleep study , is a multiparametric test used in the study of sleep and as a diagnostic tool in sleep medicine.

The test result is called a polysomnogram, also abbreviated polysomnography.

The name is derived from Greek and Latin roots: the Greek πολύς (polus for “many, much”, indicating many channels), the Latin somnus (“dream”), and the Greek γράφειν (graphein, “to write”).

It is typically done at night when most people sleep, although some labs can accommodate shift workers and people with circadian rhythm sleep disorders and test at other times of the day.

Polysomnography monitors many body functions, including brain activity (electroencephalogram), eye movements (electrooculography), muscle activity or skeletal muscle activation (electromyography), and heart rhythm (electrocardiography), during sleep.

After the identification of the sleep apnea sleep disorder in the 1970s, breathing functions, respiratory airflow, and respiratory effort indicators were added along with peripheral pulse oximetry.

Medical uses

Polysomnography is used to diagnose or rule out many types of sleep disorders, including narcolepsy, idiopathic hypersomnia, periodic limb movement disorder (PLMD), REM behavior disorder, parasomnias, and sleep apnea.

Although it is not directly helpful in diagnosing circadian rhythm sleep disorders, it can be used to rule out other sleep disorders.

Polysomnography should not be used routinely to detect sleep disorders in workers who complain of insomnia or fatigue, but who do not have other symptoms of sleep apnea.

Polysomnography should be avoided unless a person has indications for polysomnography. (These indications include sleep apnea, obesity, a risky neck diameter, or fullness of risk from the meat in the oropharynx.)

Alternatives to try first are to change your work schedule to allow enough time for sleep and improve sleep hygiene.


A polysomnogram will generally record a minimum of 12 channels requiring a minimum of 22 lead connections for the patient. These channels vary by laboratory and can be tailored to meet physician requests.

There are a minimum of three channels for the EEG:

  • One or two to measure air flow.
  • One or two for chin muscle tone.
  • One or more for leg movements.
  • Two for eye movements (electrooculography).
  • One or two for heart rate and pace.
  • One for oxygen saturation.
  • One for belts, which measures chest wall movement and upper abdominal wall movement.

Belt movement is typically measured with piezoelectric sensors or respiratory inductance plethysmography.

This movement equates to effort and produces a low-frequency sinusoidal waveform as the patient inhales and exhales. Because movement equates to effort, this measurement system can produce false positives.

It is possible, especially during obstructive apneas, that the effort is made without measurable movement.

Cables for each channel of recorded data lead from the patient and converge into a central box, which in turn is connected to a computer system to record, store, and display the data.

During sleep, the computer monitor can display multiple channels continuously. Also, most labs have a small video camera in the room so the technician can visually observe the patient from an adjoining room.

The electroencephalogram (EEG) will generally use six “scan” electrodes and two “reference” electrodes, unless a seizure disorder is suspected, in which case more electrodes will be applied to document the onset of seizure activity. .

The scanning electrodes are usually attached to the scalp near the front, central (upper), and occipital (back) portions of the brain through a paste that will conduct electrical signals originating from neurons in the cortex.

These electrodes will provide a reading of brain activity that can be ‘scored’ at different stages of sleep (N1, N2, and N3, which combined are called Non-REM sleep) and Stage R, which is rapid eye movement sleep, or REM and wakefulness).

EEG electrodes are placed according to the international 10-20 system.

The electrooculogram (EOG) uses two electrodes; one that is placed 1 cm above the outer edge of the right eye and another that is placed 1 cm below the outer edge of the left eye.

These electrodes capture the activity of the eyes by virtue of the electropotential difference between the cornea and the retina (the cornea is positively charged with respect to the retina).

This helps determine when REM sleep occurs, of which rapid eye movements are characteristic, and also essentially helps determine when sleep occurs.

The electromyogram (EMG) generally uses 4 electrodes to measure muscle tension in the body and monitor an excessive amount of leg movements during sleep (which may indicate periodic limb movement disorder, periodic limb movement disorder, extremities).

Two leads are placed on the chin with one above the jawline and one below. This, like electrooculography, helps determine when REM sleep and sleep occurs.

Sleep generally includes relaxation, so there is a marked decrease in muscle tension. There is a further decrease in skeletal muscle tension in REM sleep.

A person becomes partially paralyzed and makes acting out of dreams impossible, although people who do not have this paralysis may suffer from REM conduct disorder.

Finally, two more leads are placed on the tibialis anterior of each leg to measure leg movements.

Although a typical EKG would use ten electrodes, only two or three are used for a polysomnogram. They can be placed below the collarbone on each side of the chest or one below the collarbone and the other six inches above the waist on either side of the body.

These electrodes measure the electrical activity of the heart as it contracts and expands, recording characteristics such as the “P” wave, the “QRS” complex, and the “T” wave.

These can be analyzed for any abnormality that may be indicative of an underlying cardiac pathology.

Nasal and oral airflow can be measured using pressure transducers and / or a thermocouple, installed in or near the nostrils; the pressure transducer is considered the most sensitive.

This allows the clinician / researcher to measure the rate of respiration and to identify interruptions in breathing. Respiratory effort is also measured in conjunction with nasal / oral airflow through the use of belts.

These belts expand and contract with respiratory effort. However, this breathing method can also produce false positives. Some patients will open and close their mouth while obstructive apneas occur.

This forces air in and out of the mouth while air does not enter the airways and lungs.

Therefore, the pressure transducer and thermocouple will detect this decreased airflow and the respiratory event may be falsely identified as hypopnea, or a period of reduced airflow, rather than obstructive apnea.

Pulse oximetry determines the changes in blood oxygen levels that often occur with sleep apnea and other breathing problems. The pulse oximeter fits the fingertip or earlobe.

Snoring can be recorded with a sound probe on the neck, although more often the sleep technician will simply note the snoring as “mild,” “moderate,” or “heavy,” or give a numerical estimate on a scale of 1 to 10. .

Also, snoring indicates airflow and can be used during hypopneas to determine if the hypopnea may be obstructive apnea.


For the standard test, the patient goes to a sleep lab early in the evening and for the next 1-2 hours is entered into the setup and “wired” so that multiple channels of data can be recorded when they fall asleep.

The sleep laboratory can be in a hospital, a separate medical office, or a hotel. A sleep technician must always be present and is responsible for attaching the electrodes to the patient and supervising the patient during the study.

During the study, the technician observes the sleep activity by looking at the video monitor and the computer screen that shows all the data, second by second.

In most labs, the test is completed and the patient is discharged at 7 a.m. unless a multiple sleep latency test (MSLT) is performed during the day to assess sleepiness. excessive daytime.

More recently, healthcare providers may prescribe home studies to improve patient comfort and reduce expenses.

The patient is instructed after using a screening tool, uses the kit at home, and returns it the next day.

Most screening tools consist of an airflow measuring device (thermistor) and a blood oxygen monitoring device (pulse oximeter).

The patient would sleep with the sensing device for one to several days, then return the device to the healthcare provider. The provider would retrieve data from the device and could make assumptions based on the information provided.

For example, a series of drastic desaturations of oxygen in the blood during the night periods can indicate some type of respiratory event (apnea). The equipment monitors, at a minimum, oxygen saturation.

More sophisticated home study devices have most of the monitoring capabilities of your sleep lab technicians and their counterparts, and can be complex and time-consuming to set up for self-monitoring.


Once the test is complete, a “annotator” analyzes the data by reviewing the study in “epochs” of 30 seconds. The score consists of the following information:

Onset of sleep from the moment the lights go out : This is called ‘sleep onset latency’ and is typically less than 20 minutes. (Note that determining “sleep” and “wake up” is based solely on EEG.

Patients sometimes feel like they were awake when the EEG shows they were sleeping. This may be due to misperception of the sleep state, the effects of medications on brain waves, or individual differences in brain waves).

Sleep efficiency : the number of minutes of sleep divided by the number of minutes in bed. Normal is about 85 to 90% or more.

Stages of sleep : they are based on 3 data sources from 7 channels: Electroencephalography (generally 4 channels), Electrooculography (generally 2 channels) and chin EMG (generally 1 channel).

Based on this information, each 30-second epoch is scored as “awake” or one of the 4 stages of sleep: 1, 2, 3 and REM, or rapid eye movement, sleeping. Stages 1-3 are together known as non-REM sleep. Non-REM sleep is distinguished from REM sleep, which is completely different.

In non-REM sleep, stage 3 is called “slow wave” sleep because of the relatively wide brain waves compared to other stages; Another name for stage 3 is “deep sleep.”

In contrast, stages 1 and 2 are “light sleep.” The figures show stage 3 sleep and REM sleep; each figure is a 30-second epoch of a nocturnal polysomnography.

The percentage of each stage of sleep varies by age, with decreasing amounts of REM and deep sleep in older people. Most people of all ages (except childhood) are stage 2. REM normally occupies about 20-25% of sleep time.

Many factors besides age can affect both the amount and percentage of each stage of sleep, including medications (especially antidepressants and pain relievers), alcohol taken before bed, and lack of sleep.

Any irregularity in breathing, mainly apneas and hypopneas. Apnea is a complete or nearly complete cessation of airflow for at least 10 seconds followed by excitation and / or desaturation with 4% oxygen.

Hypopnea is a 30% or more decrease in airflow for at least 10 seconds followed by excitation and / or oxygen desaturation at 4%.

The national insurance program Medicare in the US requires a desaturation of 4% to include the event in the report.

“Awakenings” are sudden changes in brain wave activity. They can be caused by numerous factors, including respiratory abnormalities, leg movements, ambient noises, etc.

An abnormal number of awakenings indicates “interrupted sleep” and may explain a person’s daytime symptoms of fatigue and / or drowsiness.

  • Heart rhythm abnormalities.
  • Leg movements.
  • Body position during sleep.
  • Oxygen saturation during sleep.

Once scored, the test record and scoring data are sent to the sleep medicine practitioner for interpretation.

Ideally, the interpretation is done in conjunction with the medical history, a complete list of the medications the patient is taking, and any other relevant information that may affect the study, such as the pre-test nap.

Once interpreted, the sleep physician writes a report that is sent to the referring provider, usually with specific recommendations based on the test results.

Summary report examples

The following sample report describes the patient’s condition, the results of some tests, and mentions continuous positive airway pressure as a treatment for obstructive sleep apnea.

Continuous positive airway pressure is continuous positive airway pressure and is delivered through a mask to the patient’s nose or the patient’s nose and mouth. (Some masks cover one, some both.)

Continuous positive airway pressure is usually prescribed after the diagnosis of obstructive sleep apnea is made from a sleep study (that is, after a polysomnography test).

To determine the correct amount of pressure and the appropriate mask type and size, and also to ensure that the patient can tolerate this therapy, a “continuous positive airway pressure titration study” is recommended.

This is the same as a ‘Polysomnography’ but with the addition of the applied mask so that the technician can increase the airway pressure within the mask as needed, until all or most obstructions in the airway are removed. patient’s airway.

Mr. J —-, 41 years old, 1.72 cm tall, 120 kilos, came to the sleep laboratory to rule out obstructive sleep apnea. Complains of some snoring and daytime sleepiness.

His score on the Epworth Sleepiness Scale rises to 15 (out of a possible 24 points), confirming excessive daytime sleepiness (<10/24 is normal).

This one-night sleep study shows evidence of obstructive sleep apnea (OSA). For the entire night, his apnea + hypopnea index rose to 18.1 events / hour. (normal <5 events / hour, this is “moderate” obstructive sleep apnea).

While sleeping in the supine position, his apnea + hypopnea index was twice as high, at 37.1 events / hour. He also had some oxygen desaturation; During 11% of the sleep time, his oxygen saturation was between 80% and 90%.

The results of this study indicate that Mr. J —- would benefit from continuous positive airway pressure. To this end, it is recommended that you return to the laboratory for an ongoing positive airway pressure titration study.

The report recommends that Mr. J —- return for a continuous positive airway pressure titration study, which means a return to the laboratory for a second overnight polysomnogram (this one with the mask applied).

Often, however, when a patient develops obstructive sleep apnea within the first 2 to 3 hours of the initial polysomnogram, the technologist will interrupt the study and apply the mask at that time; the patient is awake and equipped for a mask.

The remainder of the sleep study is then a “continuous positive airway pressure titration.” When both diagnostic polysomnography and continuous positive airway pressure titration are performed on the same night, the entire study is termed “split night.”

The split night study has these advantages:

  1. The patient only has to go to the lab once, so it is less disruptive than two different nights.
  2. It is “half as expensive” for those who pay for the study.

The split night study has these disadvantages:

There is less time to make a diagnosis of obstructive sleep apnea (Medicare in the US requires a minimum of 2 hours of diagnosis before applying the mask).

There is less time to ensure a continuous positive airway pressure titration. If the titration begins with only a few hours of sleep, the remaining time may not guarantee a continuous positive airway pressure titration, and the patient may have to return to the laboratory.

Because of the costs, more and more studies for ‘sleep apnea’ are attempted as split-night studies when there is early evidence of obstructive sleep apnea.

Note that both types of studies, with and without continuous positive airway pressure mask, are polysomnograms. However, when using the continuous positive airway pressure mask, the flow measurement lead in the patient’s nose is eliminated.

Instead, the continuous positive airway pressure machine transmits all flow measurement data to the computer.

The following report is an example report that could be produced from a split night study:

Mr. B____, 38 years old, 1.82 cm tall, 157 kilos, came to the Sleep Lab Hospital to diagnose or rule out obstructive sleep apnea.

This polysomnogram consisted of:

  • Overnight recording of left and right electrooculography.
  • Submental electromyography.
  • Left and right anterior electromyography.
  • Central and occipital electroencephalogram.
  • Electrocardiograph.
  • Air flow measurement.
  • Respiratory effort.
  • Pulse oximetry.

The test was performed without supplemental oxygen. His latency to onset of sleep was slightly prolonged at 28.5 minutes. Sleep efficiency was normal at 89.3% (413.5 minutes of sleep time of 463 minutes in bed).

During the first 71 minutes of sleep, Mr. B____ manifested 83 obstructive apneas, 3 central apneas, 1 mixed apnea and 28 hypopneas, for a high apnea + hypopnea index (AHI) of 97 events / hour (obstructive sleep apnea « serious”).

Their lowest oxygen saturation during the period of pre-continuous positive airway pressure was 72%.

Continuous positive airway pressure was applied at 5 cm H2O and titrated sequentially at a final pressure of 17 cm H2O.

At this pressure, his apnea + hypopnea index was 4 events / hour. And the low oxygen saturation had risen to 89%. This final titration level occurred while I was in REM sleep. The mask used was a Respironics Classic nasal (medium size).

In summary, this split-night study shows severe obstructive sleep apnea in the period of pre-continuous positive airway pressure, with definite improvement at high levels of continuous positive airway pressure.

At 17 cm H2O, his apnea + hypopnea index was normal at 4 events / hour. And low oxygen saturation was 89%. Based on this split night study, I recommend that you start with a continuous positive nasal airway pressure of 17 cm H2O in conjunction with heated moisture.