A Polysomnography (PSG), or sleep study, is the tool physicians use to diagnose sleep-disordered breathing. Results often contain technical acronyms like AHI, RDI, and RERA. Understanding these metrics is crucial for grasping the nature and severity of a sleep issue. This article focuses on demystifying the Respiratory Effort Related Arousal, or RERA.
Defining Respiratory Effort Related Arousal
A Respiratory Effort Related Arousal (RERA) describes a sequence of breaths indicating a struggle against a narrowed upper airway during sleep. This event lasts for a minimum of ten seconds and is defined by a measurable increase in the work of breathing. Crucially, this increased effort does not cause a significant drop in blood oxygen levels or a major reduction in airflow, which would classify it as a different event.
The defining characteristic of a RERA is that the respiratory effort is significant enough to cause an arousal, a brief awakening recorded by the electroencephalogram (EEG). This arousal is typically a change in brain wave frequency, such as a shift to an alpha wave pattern. The individual often does not become consciously aware of waking up, but the repeated interruptions fragment sleep continuity.
The event signals increasing resistance in the upper airway, forcing the chest and diaphragm muscles to work harder to pull air past the obstruction. This crescendo of effort triggers the brain to briefly lighten its sleep stage to normalize the breathing pattern. This highlights that sleep fragmentation, rather than oxygen deprivation, is the primary source of the problem caused by RERAs.
RERA vs. Apnea and Hypopnea
RERAs are distinguished from apneas and hypopneas by the specific criteria they fail to meet. An apnea represents the most severe event, defined as a complete cessation of airflow (a drop of 90% or more) lasting at least ten seconds. Obstructive apneas involve continued respiratory effort against this total blockage.
A hypopnea is a partial reduction in breathing, scored when airflow decreases by 30% or more for ten seconds or longer. To be counted, this reduction must be accompanied by either a clear arousal on the EEG or a drop in blood oxygen saturation of 3% or 4%, depending on the scoring rules used.
In contrast, a RERA involves increased resistance and effort that is not severe enough to meet the airflow reduction or oxygen desaturation thresholds of an apnea or a hypopnea. The event is identified purely by the increased effort leading directly to the arousal. This distinction means RERAs represent the subtle, yet disruptive, end of the spectrum of sleep-disordered breathing.
Identifying RERA During a Sleep Study
Identifying a RERA relies on specialized Polysomnography equipment, as the event does not depend on oxygen desaturation. Sleep technicians monitor subtle changes in breathing resistance using a nasal pressure transducer. This sensor is far more sensitive to changes in airflow limitation than the thermistors typically used to detect apneas.
During a RERA, the inspiratory waveform recorded by the nasal pressure transducer begins to flatten, adopting a characteristic plateau shape that signifies flow limitation. This flattening indicates the patient must apply greater negative pressure to maintain airflow against the resistance. If this pattern of increasing effort lasts for ten seconds or more and is immediately followed by a change in the EEG, it is scored as a RERA.
While esophageal manometry, which measures pressure directly in the esophagus, was historically considered the most accurate method, it is invasive and rarely used today. The nasal pressure transducer has become the practical and American Academy of Sleep Medicine (AASM) recommended surrogate for reliably detecting flow limitation and subsequent arousal.
Clinical Implications of RERA Scores
The presence and frequency of RERAs are significant because they contribute to the patient’s overall Respiratory Disturbance Index (RDI). The RDI is a comprehensive metric that includes the total number of apneas, hypopneas, and RERAs per hour of sleep. This differs from the Apnea-Hypopnea Index (AHI), which only includes apneas and hypopneas.
A high RERA score, particularly when the AHI is low, is often the defining feature of Upper Airway Resistance Syndrome (UARS). This syndrome describes individuals who experience excessive daytime sleepiness and fatigue due to frequent sleep fragmentation from RERAs, even without the severe oxygen drops typical of advanced sleep apnea.
The RERA score directly informs the physician’s treatment strategy, especially when prescribing positive airway pressure (CPAP) therapy. If a patient has a high RERA count, CPAP pressure settings must be high enough to overcome the subtle upper airway resistance causing the effort and subsequent arousals. Failing to address RERAs can result in persistent sleep fragmentation and the continuation of symptoms, even after treating apneas and hypopneas.