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Obstructive sleep apnea basics

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SLEEP STUDIES

Figure 2. Polysomnogram excerpts with normal sleep, obstructive apnea, obstructive hypopnea, and respiratory event-related arousal waveform findings.

Figure 2. Polysomnogram excerpts with normal sleep, obstructive apnea, obstructive hypopnea, and respiratory event-related arousal waveform findings.

Polysomnography (PSG) is the gold standard of evaluation for OSA. The more recently availabile home sleep apnea test (HSAT) is convenient for select patients as a confirmatory test but results may underestimate the severity of sleep-related breathing disorders.

Polysomnography

Table 3. Polysomnogram parameters and associated leads
PSG is a monitored, 8-hour sleep study conducted in a laboratory with an established scoring criteria for OSA-related respiratory events.24 The test can be tailored to a patient’s clinical history to determine the need for supplemental oxygen and positive airway pressure titration, detect elevated carbon dioxide (hypercapnia or hypoventilation) due to shallow breathing, and monitor for seizures or parasomnias. The PSG also records REM and nonREM sleep for REM-related sleep disorders, body position (supine and off supine), and variability in muscle tone that corresponds to the different stages of sleep (Figure 2, Table 3).
Figure 3. A hypnogram depicts the recorded levels of sleep over time.

Figure 3. A hypnogram depicts the recorded levels of sleep over time.

Hypnogram. A hypnogram is a type of polysomnography that illustrates the different stages of sleep over time: wake, stage 1, stage 2, and stage 3, and REM sleep (Figure 3). In a typical night’s sleep of 7 to 9 hours, patients cycle through the sleep stages 4 to 5 times. A hypnogram can also include waveforms for other parameters such as body position, respiratory events (apnea and hypopneas), microarousals, continuous positive airway pressure therapy, and oxygen saturation.

Home sleep apnea test

HSATs record 4 to 7 parameters including airflow (thermal and nasal pressure), effort (inductive ple­thysmography), and oximetry. No electroencephalogram is used, so sleep is not recorded; it is assumed the patient is sleeping for the duration of the test. As such, respiratory events are based on oxygen desaturations and reduced airflow and pressure as well as chest and abdomen effort. The raw data are edited and manually scored and reviewed by a sleep specialist.25

Although the HSAT is convenient for many patients, it underestimates the severity of sleep-related breathing disorders. HSAT is intended to confirm OSA in patients with a high likelihood of OSA based on their sleep history.26 It is ideally employed for adult patients with no major medical problems or other sleep problems who are at high risk for moderate to severe OSA based on the STOP-BANG questionnaire or those with daytime sleepiness and 2 of the 3 symptoms of snoring, witnessed apnea, or hypertension.27

A negative or inconclusive HSAT warrants a PSG to ensure the patient does not have OSA. Use of HSAT is contraindicated in patients with

  • Significant cardiopulmonary disease
  • Potential weakness due to a neuro­muscular condition
  • Awake hypoventilation or high risk for sleep-related hypoventilation (severe obesity)
  • History of stroke
  • Chronic opioid use
  • Severe insomnia
  • Symptoms of other significant sleep disorders
  • Environmental/personal factors that would preclude adequate acquisition and interpretation of data (disruptions from children, pets, other factors).27

DIAGNOSTIC CRITERIA

Table 4. Obstructive sleep apnea diagnostic criteria
Results from a PSG or HSAT inform the diagnosis of OSA and the need for treatment. The current diagnostic criteria for OSA were established in 2014 by the American Academy of Sleep Medicine (Table 4).28

Respiratory events captured on a PSG or HSAT

The OSA diagnostic criteria are based on the occurrence of obstructive respiratory events recorded during sleep such as apneas, hypopneas, and respiratory event-related arousals.

Figure 4. Apneas can be obstructive, mixed, or central.

Figure 4. Apneas can be obstructive, mixed, or central.

Apneas. An apnea is a respiratory event resulting in a complete lack of airflow as measured by a greater than 90% reduction in thermal sensor for 10 or more seconds. Apneas can be obstructive, central, or mixed (Figure 4). Obstructive apneas occur when the airway is closed and respiratory effort is present in the chest and abdomen (Figure 2). In central apnea, there is no airflow and no respiratory effort, meaning the brain is not directing the body to breathe. Mixed apneas cause a lack of airflow with and without respiratory effort.

Hypopneas. A hypopnea is a respiratory event resulting in reduced airflow. The America Association of Sleep Medicine’s preferred definition is a reduction in nasal pressure of at least 30% for 10 seconds or longer with 3% or greater oxygen desaturation or an electroencephalogram arousal. Another acceptable definition is at least 30% reduction in thoracoabdominal movement or airflow with 4% or greater oxygen desaturation, which is used by the Centers for Medicare and Medicaid Services and other insurers.29,30 Hypopnea requires greater oxygen desaturation and is not dependent on arousals, which can sometimes make it more challenging to identify OSA (Figure 2).

Respiratory event-related arousals. Respiratory event-related arousals are respiratory events not meeting apnea or hypopnea criteria. They are measured as a sequence of breaths of 10 or more seconds with increasing respiratory effort or flattening of the nasal pressure waveform leading to arousal (Figure 2).29 Respiratory event-related arousals are disruptive to sleep and have many of the same consequences as apneas and hypopneas.

Severity

Table 5. Obstructive sleep apnea severity
A diagnosis of OSA should include a measure of severity (mild, moderate, or severe) as the severity may determine if a patient with OSA is treated or not. Severity is determined by AHI, respiratory disturbance index, or respiratory event index (Table 5).29 For any of the 3 indexes, a value 5 to 14.9 is considered mild, 15 to 29.9 is considered moderate, and 30 or greater is considered severe.

SUMMARY

OSA results from airway collapse and obstruction during sleep, often causing arousal from sleep with or without oxygen desaturation. The prevalence of OSA is underestimated and it is underdiagnosed despite known risk factors and comorbid conditions. Screening for OSA with a sleep history, simple upper airway examination, and quick validated screening tool like the STOP-BANG or Epworth Sleepiness Scale aid in identifying the need for testing for OSA. A laboratory sleep study with a PSG can confirm the diagnosis and severity of OSA. HSATs are available to confirm the diagnosis of OSA in patients at high risk for moderate to severe OSA.

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Sleep apnea and the heart

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