Polysomnography

All about the most used sleep lab test for diagnosing sleep apnea...

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Polysomnography is a multi-parametric test that is used to study/record in detail all the biophysiological changes that occur in the human body when the person is asleep. The PSG or polysomnogram, as the test is referred to, measures or monitors many body functions including the eye movements (EOG), brain (EEG), heart rhythm (ECG), skeletal muscle activation (EMG), and breathing or respiratory effort during sleep, and based on the observations, the doctor will decide whether the person who had undergone PSG suffers from obstructive sleep apnea (OSA) or any of its namesakes or not. Even though PSG can be done at any time, usually it is performed at night when everybody usually sleeps except in certain special cases when the tests are done over two days time.

When a patient feels that he/she has some sleep problem and consults a specialist for diagnosis, he may refer him/her to undergo a polysomnogram in a sleep lab. A standard polysomnogram procedure starts by the late evening, and the first 1-2 hrs will be spent on "wiring up" the patient, putting all the required electrodes and channels in place. Even though there is no fixed rule that specifies the number of channels that should be fixed to the patient, for a typical polysomnogram, it is eleven. That is the minimum number of channels required for efficiently carrying out the test. If to observe more attributes, like the snoring volume or airflow using a transducer, a higher number channels may be required, but then it is entirely at the doctor's discretion based on the patient's condition.

Of the eleven channels, two are for EEG - to determine if the person is indeed sleeping and in what stage of sleep he/she is at a given time - one channel to measure air flow, one channel each to sense chin and leg movements, two channels to measure REM, one channel each for EKG and oxygen saturation, and one for recording the chest and abdominal wall movements. All these channels are then wired to a computer where it is deciphered into readable attributes and stored for future reference or record. A live video of the sleeping patient is also recorded so that the supervising technician could observe the patient from an adjacent room. The same clips may also be used by the doctor for further diagnosis of the patient. Despite all the wires attached to the patient, interestingly, it has been found that few patients ever have any problems sleeping whilst under observation. Most of them stated they even slept better in the lab than at home.

Usually the PSG will be over by 7 AM and the patient can go home or straight to work. After the test is done, a scorer - usually a different person than the technician - analyzes the collected data by reviewing the study in "epochs" of 30 seconds each, looking for:

  • Sleep latency, the moment the patient slept since the lights were switched off. Remember, it is the EEG that says whether the patient was awake or was sleeping.
  • Sleep efficiency, which is the minutes of total sleep divided by the minutes spent on bed. In normal cases, it falls between 85%-90%.
  • Sleep stages, which are based on the data coming from the five channels, EOG (2), EEG (usually 2 channels), and chin EMG. Depending on the collected information, each 30 second "epoch" is classified as either "awake" or any one of the five sleep stages - 1,2,3,4, and REM. The sleep stages 1 and 2 are further classified as "light sleep" while 3 and 4 as "deep sleep".


Here, it should be kept in mind that the sleep pattern differs with age. For example, for older people the duration of the REM stage will be lesser in comparison with younger people. REM generally occupies 25%-35% of the total sleep time while the most observed sleep stage across all ages is stage 2 (except in infancy). Consumption of drugs (anti-depressants) and alcohol could also affect the duration of sleep stages.

  • Breathing irregularities like apnea or hypopnea, if there is any. For those who are not familiar with the terms, the former is the complete cessation of breathing for at least 10 seconds in sleep while the latter refers to partial cessation, again lasting for at least 10 seconds. Apnea/ hypopnea index (API) is calculated from these observations. For normal persons, it must be below 5.
  • "Arousals" indicated by a sudden shift in the brain wave activity. It could be due to a variety of reasons such as leg movements, atmospheric conditions, breathing issues etc. A higher than normal number of "arousals" could be indicative of disturbed sleep or other symptoms like fatigue and/or sleeplessness.
  • Cardiac rhythm abnormalities, body position during sleep, leg movement patterns, and oxygen saturation.


Once the observation has been analyzed and "scored", the results are then sent to the sleep physician for interpretation. A sleep physician reaches a conclusion after also considering the patient's medical and drug history, and the results are in turn sent across to the referring physician with specific recommendations based on the analysis for further treatments or tests.