Diagnosing sleep-disordered breathing requires a polysomnography, a formal sleep study that objectively measures physiological functions during sleep. Metrics, or indices, quantify the frequency and severity of breathing disturbances occurring throughout the night. One such specific metric is the Periodic Respiratory Disturbance Index (PRDI), which provides a comprehensive measure of how often a person’s breathing is disrupted during sleep time. Understanding this index helps patients interpret their sleep study results and grasp the nature of their diagnosis.
Defining the Periodic Respiratory Disturbance Index
The Periodic Respiratory Disturbance Index (PRDI), often referred to simply as the Respiratory Disturbance Index (RDI), is a specialized metric calculated from a sleep study report. It represents the average number of respiratory-related events that cause a disruption in sleep per hour of total sleep time recorded. Clinicians use this value as a key measure to diagnose and assess the overall severity of sleep-disordered breathing (SDB).
This index functions as a rate, providing a standardized way to compare the frequency of breathing problems across different individuals, regardless of their total sleep duration. The PRDI offers a broader and more inclusive view of breathing problems compared to other indices, as it specifically captures events that fragment sleep architecture.
The Components of PRDI: What Events Are Counted?
The calculation of the PRDI is based on adding together the total count of three distinct types of events that occur during sleep and dividing that sum by the total hours of sleep. These events are Apneas, Hypopneas, and Respiratory Effort-Related Arousals (RERAs). An Apnea is defined as a complete cessation of airflow that lasts for at least ten seconds.
A Hypopnea involves a partial reduction in airflow, typically a decrease of 30% or more for at least ten seconds, which is also associated with either a brief awakening or a drop in blood oxygen saturation levels. The inclusion of RERAs is what primarily differentiates the PRDI from the AHI, making it a more sensitive measure of sleep disruption. RERAs are subtle breathing disruptions where increased respiratory effort is recorded, leading to a brief arousal from sleep.
This arousal, a shift from deeper sleep to a lighter sleep stage, is noted on the electroencephalogram (EEG) portion of the sleep study. RERA events occur because the body is exerting greater effort to breathe against a narrowed upper airway, a condition often associated with Upper Airway Resistance Syndrome (UARS). By counting these milder, effort-based events, the PRDI captures sleep fragmentation that might otherwise be missed.
Interpreting Your PRDI Score
The PRDI score is translated into clinical meaning using established numerical thresholds to classify the severity of sleep-disordered breathing. A higher score indicates more frequent breathing disturbances and a greater degree of sleep fragmentation. Clinicians use four categories to interpret the index, providing a clear framework for diagnosis and treatment planning.
A PRDI score of fewer than five events per hour is considered within the normal range for adults. Scores between five and 15 events per hour are classified as mild sleep-disordered breathing. Moderate severity is indicated by a PRDI between 15 and 30 events per hour.
A PRDI score exceeding 30 events per hour is categorized as severe sleep-disordered breathing. This high score correlates with increased daytime symptoms like excessive fatigue or sleepiness. The numerical result guides the physician in determining the necessity and type of intervention needed, such as continuous positive airway pressure (CPAP) therapy.
PRDI vs. AHI: Understanding Key Differences
The Apnea-Hypopnea Index (AHI) is the most commonly cited metric for diagnosing Obstructive Sleep Apnea (OSA), but it does not tell the full story of sleep disruption for every patient. The AHI calculation includes only Apneas and Hypopneas that meet strict criteria, often requiring a significant drop in blood oxygen levels. The PRDI, however, is a more comprehensive measure because it includes all the events counted in the AHI, plus the Respiratory Effort-Related Arousals (RERAs).
This difference is particularly significant for patients whose sleep is severely fragmented by breathing difficulty, but whose oxygen levels do not drop enough to qualify for an AHI event. For instance, a patient might have a normal AHI score but an elevated PRDI due to frequent RERAs, which could explain persistent symptoms of daytime fatigue. The PRDI is therefore often reported in addition to the AHI to give a fuller picture of sleep disruption.