Investigations of OSA

Laboratory Studies

• A thyrotropin test (if patient has other signs or symptoms of hypothyroidism)
• An arterial blood gas determination for patients presenting with cor pulmonale, in order to rule out daytime hypoxemia or hypercapnia.

Other Tests

• Pulmonary function tests (Spirometry)
• An overnight sleep study, or polysomnography (PSG)
• Sleep stages are recorded via an electroencephalogram (EEG), electrooculogram (EOG), and chin electromyogram (EMG).
• Heart rhythm is monitored with a single-lead ECG.
• Leg movements are recorded via an anterior tibialis EMG.
• Breathing is monitored in terms of:
• Airflow at the nose and mouth (using both a thermal sensors and a nasal pressure transducer)
• Effort (using inductance plethysmography)
• Oxygen saturation.

The breathing pattern is analyzed for the presence of apneas and hypopneas. Definitions have been standardized by the American Academy of Sleep Medicine (AASM).

• Obstructive apnea is the cessation of airflow for at least 10 seconds with persistent respiratory effort.
• Central apnea is the cessation of airflow for at least 10 seconds with no respiratory effort.
• Mixed apnea is an apnea that begins as a central apnea and ends as an obstructive apnea.
• Hypopnea is a 30% or greater decrease in flow lasting at least 10 seconds and associated with a 4% or greater oxyhemoglobin desaturation.

• Respiratory event–related arousal is an event in which patients have a series of breaths with increasing respiratory effort or flattening of the nasal pressure waveform leading to an arousal from sleep that does not otherwise meet the criteria for an apnea or hypopnea.
• The AHI is derived from the total number of apneas and hypopneas divided by the total sleep time.
• Recommendations for cutoff levels on AHIs are as follows:
• Mild - Five to 15 episodes per hour
• Moderate - Fifteen to 30 episodes per hour
• Severe - More than 30 episodes per hour

PSG is often followed by a Multiple Sleep Latency Test (MSLT). The MSLT is considered an objective measurement of Excessive Daytime Sleepiness (EDS). It is used to see how quickly you fall asleep in quiet situations during the day.

• The MSLT consists of 4-5 naps of 20-minute duration every 2 hours during the day. The latency to sleep onset for each nap is averaged to determine the daytime sleep latency.
• Normal daytime sleep latency is greater than 10-15 minutes. OSAHS is generally associated with latencies of less than 10 minutes.