The Wesper Guide: Our Sleep & Respiratory Metrics
These definitions explore various sleep parameters measured during a sleep study. A sleep study is a test used to diagnose sleep disorders, and the results can help determine the presence and severity of sleep apnea.
One important measurement is the Apnea/Hypopnea Index (AHI), which indicates the average number of breathing interruptions per hour of sleep. The following also differentiates between central apneas (caused by the brain not sending breathing signals) and obstructive apneas (caused by a blockage in the airway). Other indices like the Oxygen Desaturation Index (ODI) and the Respiratory Disturbance Index (RDI) provide further details about sleep disruption due to breathing problems.
Blood oxygen levels, sleep stages, heart rate, and snoring time are also measured during a sleep study. All this data helps paint a comprehensive picture of a patient's sleep quality and identify potential sleep disorders.
Apnea/Hypopnea Index (AHI): The average number of apneas and hypopneas calculated per hour of sleep time. AHI may differ when the desaturation threshold is set at 3% or 4%. As per AASM guidelines, sleep apnea severity is calculated as follows: Normal/No Sleep Apnea: AHI = 0-4.99, Mild: 5-14.99, Moderate: 15-29.99, Severe: >30
Central Apnea: The total number of central apneas detected. A central apnea is defined as a respiratory event with a lack of respiratory effort, at least a 90% drop in airflow volume for 10 seconds or longer, and corresponds to an oxygen desaturation by at least 3% from baseline.
Central Apnea Index (CAI): The average number of central apneas and central hypopneas calculated per hour of sleep time. CAI may differ when the desaturation threshold is set at 3% or 4%.
Central Hypopnea: The total number of central hypopneas detected. A central hypopnea is defined as a respiratory event with a lack of respiratory effort, at least a 30% drop in airflow volume for 10 seconds or longer, and corresponds to an oxygen desaturation by at least 3% from baseline.
Deep Sleep Time: The total time the patient is detected in deep sleep.
Heart Rate: Heart beats per minute. Heart rate is reported as the maximum (highest) recorded, the mean (average) recorded, and minimum (lowest) recorded.
Hypoxic Burden: The total area under the respiratory event-related desaturation curve.
Light Sleep Time: The total time the patient is detected in light sleep.
Mixed Apnea: The total number of mixed apneas detected. A mixed apnea is defined as a respiratory event absent of respiratory effort and airflow in the first section of the event and respiratory effort without airflow in the last section.
Mixed Hypopnea: The total number of mixed hypopneas detected. A mixed hypopnea is defined as a respiratory event absent of respiratory effort and airflow in the first section of the event and respiratory effort without airflow in the last section.
NREM AHI: The apnea/hypopnea index (AHI) calculated only during Non-Rapid Eye Movement (NREM) sleep. NREM AHI may differ when the desaturation threshold is set at 3% or 4%.
Oxygen Desaturation Index (ODI): The average number of oxygen desaturations per hour of sleep time. ODI may differ when the desaturation threshold is set at 3% or 4%.
Obstructive Apnea: The total number of apneas detected that were determined to be obstructive. An obstructive apnea is defined as a respiratory event with increased respiratory effort, at least a 90% drop in airflow volume for 10 seconds or longer, and corresponds to an oxygen desaturation by at least 3% from baseline.
Obstructive Hypopnea: The total number of hypopneas detected that were determined to be obstructive. An obstructive hypopnea is defined as a respiratory event with increased respiratory effort, at least a 30% drop in airflow volume for 10 seconds or longer, and corresponds to an oxygen desaturation by at least 3% from baseline.
Other: The total number of undefined respiratory events that did not meet the criteria for an apnea, hypopnea, or RERA.
Percent Central Apnea (%CA): The percentage of apneas classified as central apneas, compared to obstructive apneas.
REM AHI: The apnea/hypopnea index (AHI) calculated only during Rapid Eye Movement (REM) sleep. REM AHI may differ when the desaturation threshold is set at 3% or 4%.
REM Sleep Time: The total time the patient is detected in REM sleep.
Respiratory Disturbance Index (RDI): The number of apneas, hypopneas, and respiratory effort-related arousals (RERAs) per hour of sleep time.
Respiratory Event Index (REI): The average number of apneas and hypopneas calculated per hour of recording time. REI is typically similar, but not identical, to AHI. REI may differ when the desaturation threshold is set at 3% or 4%.
Sleep Efficiency: The percentage of time the patient was asleep out of the time the patient attempted to sleep.
Sleep Latency: The time it took the patient to fall asleep for the first time.
Snore Time: The total time snoring was detected during sleep.
SPO2: Blood oxygen saturation percentage. SPO2 is reported as the maximum (highest) recorded, the mean (average) recorded, and minimum (lowest) recorded.
T88: The total amount of time the patient has an SPO2 of less than 88%
T90: The total amount of time the patient has an SPO2 of less than 90%.
Total Apnea: The total number of apneas detected. An apnea is defined as a respiratory event with at least a 90% drop in airflow volume that corresponds to an oxygen desaturation by at least 3% from baseline.
Total Hypopnea: The total number of hypopneas detected. A hypopnea is defined as a respiratory event with at least a 30% drop in airflow volume for 10 seconds or longer, that corresponds to an oxygen desaturation by at least 3% from baseline. Total Hypopnea may differ when the desaturation threshold is set at 3% or 4%.
Total RERA: The total number of respiratory event-related arousals detected. A RERA is defined as a series of breaths occurring for at least 10 seconds associated with an ever-increasing respiratory effort against a narrowed upper airway that terminates with arousal from sleep before criteria for a true apnea or hypopnea event are met.
Total Sleep Time: The total time the patients was detected as asleep.
Wake After Sleep Onset: The time spent awake after sleep onset and before the final awakening.