OSA - Polysomnography:
What is it?
Polysomnography is a diagnostic test during which a number of physiologic variables are measured and recorded during sleep.
Physiologic sensor leads are placed on the patient in order to record:
- Brain electrical activity
- Eye and jaw muscle movement
- Leg muscle movement
- Respiratory effort (chest and abdominal excursion)
- Oxygen saturation
How does it work?
Information is gathered from all leads and fed into a computer and outputted as a series of waveform tracings which enable the technician to visualize the various waveforms, assign a score for the test, and assist in the diagnostic process.
Physiologic Variables measured and recorded (as done per the KUMC Sleep Disorders Center protocols)
Six electrodes (labeled C3, C4, A1, A2 O1, and O2) and one ground electrode are placed around the cranium to record electrical activity across the brain. These leads are used to determine the stage of sleep the patient is in during any given period of the night.
One electrode is placed above and to the outside of the right eye, and another electrode is placed below and to the outside of the left eye. These leads record the movements of the eyes during sleep and serve to help determine sleep stages.
EOG/chin EMG leads/ECG leads
Three leads are placed on the chin (one in the front and center and the other two underneath and on the jawbone) and two are placed on the inside of each calf muscle 2-4cm apart. These leads serve to demonstrate muscle movement during sleep. This is helpful in documenting a wake period, an arousal, or just a spastic movement.
EKG/ECG (Electrokardiogram - Electrocardiogram)
Two electrodes are placed on the upper chest near the right and left arms. These record the heart rate and rhythm and serve to alert the technician to a possible emergency situation. They also demonstrate whether apneic desaturation leads to arrhythmias or not.
Airflow (thermistor or thermocouple sensor)
A device that looks similar to a nasal cannula is secured just under the patients nose.
It senses the amount of air moving into and out of the airways and sends a signal to a physiological recorder. This tracing is used to determine the presence and extent of apneic episodes.
Respiratory Effort (piezo crystal effort sensor)
Two Velcro bands, one placed around the chest under the breasts and one around the abdomen, serve to determine chest wall and abdominal movements during breathing.
Each band is joined together by a piezo crystal transducer. The force of chest/abdominal expansion on the bands stretches the transducer and alters the signal to a physiological recorder. These leads, combined with the airflow sensor, are how apnea is demonstrated and categorized during the test.
Oxygen saturation (Pulse oximeter)
The O2 saturation is measured by a pulse oximeter probe placed on the patient i.e. finger, earlobe, etc.
If the sleep disorders center is equipped with video cameras in the patient rooms, the patient can be taped while sleeping. This allows the technician to review the tape at any time during the test and verify whether strange looking waveforms were caused by an actual arousal, a period of wake, or normal patient movement in bed.
Interpretation of test results:
Each sleep study is scored epoch by epoch both for stage of sleep and any abnormalities that can be seen. An epoch is a convenient time interval, usually equal to one page of record. Epoch durations should be 20 - 30 sec respectively depending on whether the recommended 15 or 10 mm/sec paper speeds are used. In the tracing on the right, nasopharyngeal pressures (Pn) of zero mark an apneic event.
- Obstructive sleep apnea is labeled and marked any time there is a greater than 50% decrease in airflow with continued efforts to breathe lasting over 10 seconds in duration. Cessation of airflow can be easily seen in the graphic below from medscape.com.
- Central sleep apnea is marked when there is a cessation in airflow as well as respiratory effort lasting at least 10 seconds in duration.
- Mixed sleep apnea is labeled if you see at least 10 seconds of central apnea followed by an obstructive component.
Sleep stages can be classed as follows:
Electro-oculographs in 6 sleep conditions
- Stages 1-4 (NREM Sleep)
- transition from wake to sleep
- light sleep
- transition into REM sleep
- deep sleep
- REM Sleep