Sleep apnea is a common disorder where breathing is repeatedly interrupted during sleep, leading to fragmented rest and reduced oxygen levels. Doctors use the Apnea-Hypopnea Index (AHI) to measure the severity of this condition. The AHI represents the average number of breathing pauses (apneas) and shallow breathing events (hypopneas) that occur each hour across the entire night of sleep. This fundamental measurement is derived from an overnight sleep study and provides a standardized way to diagnose and classify sleep-disordered breathing.
Calculating the REM Apnea-Hypopnea Index
The REM Apnea-Hypopnea Index (REM AHI) is a specialized calculation that separates the breathing events occurring only during rapid eye movement (REM) sleep from the rest of the night. This figure is calculated by taking the total count of apneas and hypopneas that happen during the REM stage and dividing that sum by the total number of hours spent in REM sleep. This provides a rate of respiratory disturbances specific to this particular sleep phase.
An apnea is defined as a complete or near-complete cessation of airflow lasting for at least ten seconds. A hypopnea is a partial collapse of the airway, typically lasting ten seconds or more, which results in either a measurable drop in blood oxygen saturation, often 3% or 4%, or an arousal from sleep. The total AHI averages these events over the entire period of sleep, but the REM AHI isolates the frequency of these events to the REM periods alone.
Polysomnography, the comprehensive overnight sleep study, is required to obtain this separate index because it records brain waves to distinguish the different sleep stages. Since REM sleep usually accounts for only about 20% to 25% of total sleep time, the REM AHI provides a stage-specific measure that can be very different from the overall AHI. Analyzing the index by sleep stage offers a more detailed understanding of the patient’s breathing mechanics.
Physiological Reasons Sleep Apnea Worsens During REM
The increase in breathing disturbances during REM sleep is primarily due to muscle atonia, a natural physiological process. During this stage, the brain actively inhibits motor neurons, resulting in temporary paralysis of most voluntary muscles. This inhibition significantly affects the upper airway muscles that normally work to keep the throat open.
The genioglossus muscle, the main tongue muscle responsible for keeping the airway patent, loses much of its tone during REM sleep. This reduced muscle activity allows the tongue and soft tissues to collapse backward more easily into the throat, causing the airway obstruction characteristic of sleep apnea. The loss of muscle tone during REM makes the airway most susceptible to collapse compared to the non-REM (NREM) stages.
Furthermore, the body’s respiratory control system becomes less responsive during REM sleep. The ventilatory drive, the reflex to breathe stimulated by rising carbon dioxide or falling oxygen levels, is attenuated. This decreased sensitivity means that apneic events must last longer or result in a greater drop in oxygen saturation before the brain triggers an arousal to restart proper breathing. Respiratory events during REM sleep are often longer in duration and associated with more severe drops in blood oxygen levels.
Impact on Diagnosis and Treatment
The separate calculation of the REM AHI holds significant clinical value for classifying and managing sleep apnea. When the REM AHI is substantially higher than the NREM AHI, the condition is classified as REM-predominant obstructive sleep apnea. This distinction is important because a patient might have an overall AHI score in the mild range, but a high REM AHI suggests a greater health risk.
A high REM AHI is independently associated with an increased risk of conditions like systemic hypertension, even when the total AHI is below the threshold for moderate or severe sleep apnea. This suggests that the extreme physiological stress and severe oxygen drops during REM sleep carry a disproportionate burden on the cardiovascular system. Identifying a REM-predominant pattern helps clinicians tailor the most effective treatment strategy.
Treatment protocols for REM-predominant apnea can involve specialized approaches beyond standard care. Since REM periods become longer and more concentrated in the second half of the night, patients using Continuous Positive Airway Pressure (CPAP) may need to ensure consistent use for longer than four hours to cover the most vulnerable period. Positional therapy, which encourages sleeping on one’s side, may also be effective because many people experience more severe apnea and longer REM periods when sleeping on their back.