About Sleep Studies
If you snore, have daytime fatigue, and have hypertension; it is worth querying your family physician about the possibility of sleep apnea. Obesity is a risk factor as is Diabetes, even though sleep apnea can occur in people of ideal weight. In addition, a large neck size is associated with increased risk of apnea. Men with a neck size greater than 17 inches and women with neck sizes greater than 16 inches are at increased risk.
Following a complete history and physical examination, your doctor may suspect that you are suffering from sleep apnea. The primary objective test for obstructive sleep apnea is a sleep study, known as Polysomnography (PSG)
You will be offered the option of an at - home sleep study or sent to a sleep laboratory for a night of sleep monitoring.
Polysomnography (sleep study) is a comprehensive recording of the bio-physiological changes that occur during sleep. The traditional PSG monitors many body functions including brain (EEG), eye movements (EOG), muscle activity or skeletal muscle activation (EMG) and heart rhythm (ECG) during sleep as well as respiratory airflow and respiratory effort indicators along with peripheral arterial oxygen saturation.
Polysomnography is used to diagnose, or rule out, many types of sleep disorders including Narcolepsy, Periodic Limb Movement Disorder (PLMD), Parasomnias, and Sleep Apnea. It is most commonly ordered for patients with complaints of daytime fatigue or sleepiness that may be caused by interrupted sleep.
After the test is completed the data is reviewed and scored based upon the following:
- Onset of sleep from time the lights were turned off; this is called "sleep onset latency" and normally is less than 20 minutes.
- Sleep efficiency - the number of minutes of sleep divided by the number of minutes in bed. Normal is approximately 85 to 90% or higher.
- Sleep stages - Sleep is divided into two distinct types: Non-REM (NREM) sleep and rapid eye movement, or REM sleep.
- The Apnea-Hypopnea index and Respiratory-Disturbance Index.
What is the Apnea-Hypopnea Index (AHI)?
In severe Sleep Apnea cases, people are briefly awakened hundreds of times every night due to oxygen deprivation leading to poor sleep and extreme fatigue during the day. The number of these episodes per hour of sleep is called the Apnea-Hypopnea index, or the AHI. This index specifically calculates sleep apnea severity based on the total number of complete cessations (apnea) and partial obstructions (hypopneas) of breathing per hour of sleep.
Some patients have 50 or more episodes per hour; most sleep experts use a cutoff of 10 events or more per hour before considering treatment with a continuous positive airway pressure (CPAP) device or some other sleep apnea therapy.
What is the Respiratory Disturbance Index (RDI)?
The RDI — or Respiratory Distress Index — is a formula used in reporting sleep studies finding. Similar to the Apnea-Hypopnea index AHI, it reports on respiratory events during sleep, but unlike the AHI, it also includes respiratory-effort related arousals (RERAs). RERAs are arousals from sleep that do not technically meet the definitions of apneas or hypopneas, but do disrupt sleep. They are abrupt transitions from a deeper stage of sleep to a shallower stage. A RERA is characterized by increasing respiratory effort for 10 seconds or more leading to an arousal from sleep.
The score consists of the following information:
- Any breathing irregularities mainly apneas and hypopneas. Apnea is a complete or near complete cessation of airflow for at least 10 seconds followed by an arousal and/or 3% oxygen desaturation; hypopnea is a 50% decrease in airflow for at least 10 seconds followed by an arousal and/or 3% oxygen desaturation. (Medicare requires a 4% desaturation in order to include the event in the report.)
- "Arousals" are sudden shifts in brain wave activity. They may be caused by numerous factors, including breathing abnormalities, leg movements, environmental noises, etc. An abnormal number of arousals indicates "interrupted sleep" and may explain a person's daytime symptoms of fatigue and/or sleepiness.
- Body position during sleep.
- Oxygen saturation during sleep.
Once scored, the test recording and the scoring data are sent to the sleep medicine physician for interpretation. Ideally, interpretation is done in conjunction with the medical history, a complete list of drugs the patient is taking, and any other relevant information that might impact the study. Once interpreted, the sleep physician writes a report which is sent to the referring physician, usually with specific recommendations for the patient.
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