Normal Respiratory Disturbance Index (RDI) Levels on Sleep Studies
A normal RDI is considered to be less than 5 events per hour of sleep, with values of 5 or greater indicating varying degrees of obstructive sleep apnea (OSA). 1, 2
Understanding RDI and Its Components
RDI is a comprehensive metric that measures the average number of respiratory events per hour of sleep, including:
- Apneas (≥90% drop in airflow for at least 10 seconds)
- Hypopneas (≥30% decrease in airflow with either ≥3% oxygen desaturation or an EEG arousal)
- Respiratory effort-related arousals (RERAs)
This makes RDI more sensitive than the Apnea-Hypopnea Index (AHI), which only counts apneas and hypopneas 2.
Classification of OSA Severity Based on RDI
According to the American Academy of Sleep Medicine guidelines, OSA severity is classified as follows:
- Normal: RDI < 5 events/hour
- Mild OSA: 5 ≤ RDI < 15 events/hour
- Moderate OSA: 15 ≤ RDI ≤ 30 events/hour
- Severe OSA: RDI > 30 events/hour 1, 2
Age and Sex Considerations
It's important to note that RDI values tend to increase with age, particularly after age 50:
- In healthy individuals under 50 years of age, 95% have an RDI <15
- In healthy individuals over 65 years of age, only 50% have an RDI <15 3
Men typically have higher RDI values (median 10) compared to women (median 5), though the effect of aging on RDI is similar in both sexes 3.
Clinical Significance of Elevated RDI
Even mild elevations in RDI can be clinically significant:
- Progressive increases in daytime sleepiness (measured by Epworth Sleepiness Scale) correlate with increasing RDI levels 4
- The percentage of subjects with excessive sleepiness increases from 21% in those with RDI <5 to 35% in those with RDI ≥30 4
- Some patients with normal overall RDI (<10) but REM-specific RDI ≥15 may still experience excessive daytime sleepiness 5
Important Considerations in RDI Interpretation
- Different approaches to measuring RDI can lead to substantial variability in the assessment of sleep-disordered breathing
- The magnitude of median RDI can vary up to 10-fold depending on the specific criteria used to identify respiratory events 6
- When reviewing sleep study results, it's essential to understand the specific definition of RDI used in that particular study or laboratory
Clinical Implications
For patients with suspected sleep-disordered breathing, screening tools like the Berlin Questionnaire can help identify those at high risk for sleep apnea before proceeding to formal sleep studies 7. However, definitive diagnosis requires polysomnography with proper measurement and interpretation of indices like RDI.
The diagnosis of OSA is confirmed when the number of obstructive events (apneas, hypopneas + respiratory event related arousals) on polysomnography is greater than 15 events/hour, or greater than 5/hour in a patient who reports symptoms such as unintentional sleep episodes, daytime sleepiness, unrefreshing sleep, fatigue, insomnia, or waking up breath holding, gasping, or choking 1.