What does RDI (Respiratory Disturbance Index) stand for in a sleep study?

What Does RDI Stand for in a Sleep Study?

RDI stands for Respiratory Disturbance Index, which is the average number of apneas, hypopneas, and respiratory effort-related arousals (RERAs) that occur per hour of sleep. 1

Definition and Components of RDI

The Respiratory Disturbance Index (RDI) is a key metric used in sleep studies to diagnose and classify the severity of obstructive sleep apnea (OSA). It consists of three main components:

1. Apneas: Complete upper airway obstructions with ≥90% drop in airflow lasting at least 10 seconds
2. Hypopneas: Partial upper airway obstructions (typically ≥30% reduction in airflow)
3. RERAs (Respiratory Effort-Related Arousals): Breathing events that don't meet criteria for apneas or hypopneas but cause arousals from sleep

RDI vs. AHI (Apnea-Hypopnea Index)

It's important to distinguish RDI from AHI:

• AHI: Counts only apneas and hypopneas per hour of sleep
• RDI: Counts apneas, hypopneas, AND RERAs per hour of sleep

This means RDI = AHI + RERA index 1

Clinical Significance of RDI

RDI is used for:

• OSA diagnosis: RDI <5 indicates no OSA
• OSA severity classification:
  • 5 ≤ RDI < 15: Mild OSA
  • 15 ≤ RDI < 30: Moderate OSA
  • RDI ≥ 30: Severe OSA 1

Measurement Considerations

The American Academy of Sleep Medicine (AASM) provides guidelines on measuring respiratory events:

• For apnea detection, an oronasal thermal sensor is recommended
• For hypopnea detection, nasal pressure is the recommended sensor
• During PAP titration studies, the PAP device flow sensor should be used 1

Clinical Implications

RDI is often higher than AHI in the same patient since it includes RERAs. This can be clinically significant as some patients with normal AHI may have elevated RDI, potentially explaining symptoms like excessive daytime sleepiness despite a "normal" AHI 2.

Common Pitfalls

1. Terminology confusion: Many articles and clinicians incorrectly define RDI as equivalent to AHI, leading to potential misdiagnosis 1
2. Inconsistent measurement: Different sleep labs may use varying criteria for scoring respiratory events, affecting the final RDI value 3
3. Age considerations: RDI naturally increases with age, even in healthy individuals without OSA symptoms, which should be considered when interpreting results 4

When reviewing sleep study results, always verify which definition of RDI the laboratory is using to ensure accurate interpretation and appropriate clinical decision-making.