Measurements in sleep lab
A sleep study (polysomnogram) is performed in a sleep disorder center and is typically used to diagnose sleep apnea. It is also possible for the condition to be diagnosed based on home sleep apnea testing.
A lot of information is collected, and part of the purpose of these studies consists of tracking your breathing patterns through the night. This is accomplished with several sensors:
- Nasal cannula (or thermistor) with plastic prongs that sit in the nostrils
- Respiratory effort belts that stretch across the chest and/or stomach
- An oximeter clip that measures continuous oxygen and pulse rate by shining a laser light through a fingertip
All of this information is analyzed to determine how many times you breathe shallowly (flow limitation and Hypopnea) or stop breathing altogether(complete Obstructive) during the night. Any partial obstruction of the airway is called a hypopnea. Hypopnea refers to a transient reduction of airflow (often while asleep) that lasts for at least 10 seconds. Shallow breathing or an abnormally low respiratory rate may be called hypoventilation.
A complete cessation in breathing is called apnea (from the Greek meaning “no breath”). Hypopnea is less severe than apnea (which is a more complete loss of airflow). It may likewise result in a decreased amount of air movement into the lungs and can cause oxygen levels in the blood to drop. Sleep apnea is more commonly due to partial obstruction of the upper airway.
To count in the AHI, these pauses in breathing must last for 10 seconds and more which is associated with a decrease in the oxygen levels of the blood or cause an awakening called arousal. These awakenings may fragment sleep, make it unrefreshing, and lead to daytime sleepiness.
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Some sleep facilities use other measures to assess this degree of severity. The respiratory disturbance index (RDI) may be used if a measurement of airway resistance with a pressure oesophagal manometer is also included in the study. The oxygen-desaturation index (ODI) attempts to calculate the number of apnea or hypopnea events per hour that lead to an oxygen drop of at least 3%. This is thought to be important in assessing the risk of long-term cardiovascular (high blood pressure, heart attack, and heart failure) or neurocognitive (stroke and dementia) consequences.
If your sleep study does not contain these more specific measures, this is nothing to worry about.