What RDI Stands for in Sleep Studies
In sleep studies, 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 Calculation
The Respiratory Disturbance Index (RDI) is calculated using the following formula:
RDI = AHI + RERA index
Where:
- AHI (Apnea-Hypopnea Index) = number of apneas and hypopneas per hour of sleep
- RERA index = number of respiratory effort-related arousals per hour of sleep 1
Comparison with Other Sleep Metrics
It's important to distinguish RDI from other common sleep study metrics:
AHI (Apnea-Hypopnea Index): Counts only apneas and hypopneas per hour of sleep 2
- Apneas: ≥90% drop in airflow for at least 10 seconds
- Hypopneas: ≥30% decrease in airflow with either ≥3% oxygen desaturation or an EEG arousal
ODI (Oxygen Desaturation Index): Measures the number of ≥3% arterial oxygen desaturations per hour of sleep 1
Clinical Significance
RDI is used for:
OSA Diagnosis and Classification: RDI helps diagnose and classify the severity of obstructive sleep apnea 1:
- No OSA: RDI <5
- Mild OSA: 5 ≤ RDI < 15
- Moderate OSA: 15 ≤ RDI < 30
- Severe OSA: RDI ≥ 30
Treatment Decisions: The RDI value guides treatment decisions for sleep-disordered breathing 1
Prognostic Value: Higher RDI values have been associated with increased mortality and cardiovascular hospitalizations in patients with heart failure 3
Important Considerations
More Comprehensive Than AHI: RDI includes RERAs, which are breathing events that don't meet the criteria for apneas or hypopneas but still cause arousals. This makes RDI potentially more sensitive for detecting subtle breathing disturbances 1, 4
Reporting Variability: According to the American Academy of Sleep Medicine, reporting of the RDI metric is considered optional, as scoring RERAs is also optional 1
Historical Confusion: The literature has been inconsistent in defining RDI, with many articles defining it simply as the number of apneas and hypopneas per hour of sleep (equivalent to AHI) 1, 5
Medicare Definition: The Centers for Medicare and Medicaid define RDI as the number of apneas and hypopneas per hour of monitoring (not specifically sleep) 1
By including RERAs in addition to apneas and hypopneas, RDI provides a more comprehensive assessment of sleep-related breathing disturbances than AHI alone.