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Respiratory disturbance index

The Respiratory Disturbance Index (RDI) is a clinical metric used in sleep medicine to assess the frequency and severity of sleep-disordered breathing, calculated as the total number of apneas, hypopneas, and respiratory effort-related arousals (RERAs) per hour of total sleep time during polysomnography.^[1] This index provides a more comprehensive evaluation of respiratory events than the Apnea-Hypopnea Index (AHI), which excludes RERAs, thereby capturing subtle disruptions that may contribute to fragmented sleep and associated health issues.^[2] RDI is primarily employed to diagnose and classify obstructive sleep apnea (OSA), a common disorder characterized by recurrent upper airway collapse during sleep.^[3] Severity is typically graded as normal (<5 events per hour), mild (5–14.9 events per hour), moderate (15–29.9 events per hour), or severe (≥30 events per hour), guiding treatment decisions such as positive airway pressure therapy for moderate to severe cases.^[4] Elevated RDI levels are linked to excessive daytime sleepiness^[5] and impaired cognitive function,^[6] as well as increased cardiovascular risks, including coronary artery disease mortality in untreated patients.^[7] In clinical practice, RDI is derived from overnight polysomnographic monitoring, which records physiological signals like airflow, respiratory effort, and oxygen saturation to identify events lasting at least 10 seconds.^[8] While the American Academy of Sleep Medicine (AASM) emphasizes AHI as the standard for OSA diagnosis, RDI is particularly valuable when AHI is borderline low but symptoms persist, as it incorporates RERAs—patterns of increasing respiratory effort without significant desaturation or airflow reduction—that can still cause arousals from sleep. As reaffirmed in 2025, the AASM position supports including arousal-based scoring in polysomnography, enhancing the utility of RDI for comprehensive assessment.^[2]^[9] Research indicates that higher RDI correlates with adverse outcomes, underscoring its role in risk stratification and long-term management of sleep apnea.^[7]

Overview

Definition

The Respiratory Disturbance Index (RDI) is a quantitative measure in sleep medicine used to evaluate the frequency and severity of respiratory disturbances during sleep, calculated as the average number of apneas, hypopneas, and respiratory effort-related arousals (RERAs) per hour of sleep.^[10] The term RDI has been used since the 1990s in sleep studies, initially often equivalent to the apnea-hypopnea index (AHI), but later extended to include RERAs for a broader assessment of subtle breathing irregularities that impact sleep quality.^[11] Around 2007, the American Academy of Sleep Medicine (AASM) formalized standardized scoring rules for respiratory events through its Manual for the Scoring of Sleep and Associated Events, which facilitated consistent calculation and reporting of the RDI in clinical sleep studies.^[10] Data for computing the RDI is primarily derived from polysomnography, the reference standard for monitoring sleep and respiratory function overnight.^[12]

Relation to sleep-disordered breathing

The respiratory disturbance index (RDI) serves as a key metric within the spectrum of sleep-disordered breathing (SDB), encompassing conditions such as obstructive sleep apnea (OSA), central sleep apnea (CSA), and upper airway resistance syndrome (UARS). In OSA, RDI quantifies the frequency of obstructive events and associated arousals, aiding in the identification of airflow limitations that disrupt breathing during sleep.^[13] For UARS, RDI is particularly relevant as it captures increased respiratory effort without full apneas or hypopneas, often through the inclusion of respiratory effort-related arousals (RERAs), distinguishing this syndrome from more severe OSA.^[8] While CSA primarily involves central respiratory pauses due to lack of neural drive, RDI contributes to broader SDB assessment by highlighting mixed or overlapping disturbances in comprehensive polysomnography evaluations.^[13] RDI plays a crucial role in detecting non-apneic disturbances, such as RERAs, that lead to sleep fragmentation without significant oxygen desaturation, thereby contributing to excessive daytime somnolence and heightened cardiovascular risks. These arousals interrupt sleep continuity, resulting in neurocognitive impairments and increased sympathetic activation, which exacerbate conditions like hypertension and arrhythmias.^[13] In patients with elevated RDI, even in the absence of overt apneas, fragmented sleep correlates with persistent daytime fatigue, underscoring RDI's utility in recognizing subtler SDB phenotypes that impact quality of life.^[14] Epidemiologically, elevations in RDI align with the rising prevalence of SDB in populations over 40 years old, where the condition affects approximately 10-20% of middle-aged adults (as of 2023 estimates), with rates increasing markedly after age 50.^[15] This pattern is more pronounced in males, who exhibit roughly double the prevalence compared to females, and in obese individuals, where body mass index emerges as the strongest predictor.^[16]

Measurement

Components

The Respiratory Disturbance Index (RDI) comprises three primary types of respiratory events observed during sleep: apneas, hypopneas, and respiratory effort-related arousals (RERAs). These events are defined based on standardized criteria established by the American Academy of Sleep Medicine (AASM) to ensure consistent identification in polysomnographic studies. Apneas represent a complete or near-complete cessation of airflow, scored when there is a drop in the peak signal excursion by at least 90% of the pre-event baseline, using an oronasal thermal sensor for diagnostic studies or positive airway pressure (PAP) device flow for titration studies, lasting for 10 seconds or longer.^[10] This reduction must occur during respiratory channels and is further classified as obstructive (with continued or increasing inspiratory effort throughout the event), central (absent inspiratory effort throughout the event), or mixed (a combination of central and obstructive features).^[10] Hypopneas involve a partial reduction in airflow and are scored in adults when the peak signal excursions drop by at least 30% of the pre-event baseline, measured using a nasal pressure sensor for diagnostic studies or PAP device flow for titration, for a duration of 10 seconds or longer.^[10] To qualify as a hypopnea, the event must be associated with either at least a 3% oxygen desaturation from the pre-event baseline or an arousal from sleep, as detected by electroencephalographic (EEG) criteria.^[10] This definition, recommended by the AASM, aims to capture clinically significant partial airflow limitations that contribute to sleep fragmentation and hypoxemia without meeting the stricter criteria for apneas.^[17] Respiratory effort-related arousals (RERAs) are scored when a sequence of breaths lasts at least 10 seconds, characterized by increasing respiratory effort—evidenced by progressive hyperpnea or flattening of the inspiratory portion of the nasal pressure or PAP device flow waveform—culminating in an EEG arousal, without fulfilling the criteria for an apnea or hypopnea.^[10] These events highlight subtle increases in upper airway resistance that disrupt sleep continuity through arousal, often in the absence of significant desaturation, and are particularly relevant in identifying upper airway resistance syndrome.^[10] According to the AASM Scoring Manual, RERA scoring is optional, but when performed, it allows for calculation of RDI, which includes such events unlike the AHI.^[18] The RDI excludes isolated phenomena such as snoring without associated airflow reduction, desaturation, or arousal, as well as periodic breathing patterns that do not meet the duration or severity thresholds for the defined events.^[10] These exclusions ensure that only events with demonstrable physiological impact on breathing or sleep architecture are incorporated, focusing the index on disturbances that correlate with clinical symptoms of sleep-disordered breathing.^[19]

Calculation

The respiratory disturbance index (RDI) is calculated by summing the total number of apneas, hypopneas, and respiratory effort-related arousals (RERAs) observed during a polysomnography study and dividing by the total sleep time (TST) in hours, yielding the number of events per hour of sleep.^[10]^[20] Formally, the formula is:

\text{RDI} = \frac{\text{Number of apneas} + \text{Number of hypopneas} + \text{Number of RERAs}}{\text{TST (hours)}}

^[21]^[10] TST represents the actual duration of sleep determined through EEG-based sleep staging during polysomnography, excluding wake periods and artifacts, rather than the total recording time from lights out to lights on; this normalization accounts for variability in sleep efficiency across individuals.^[22]^[20] For example, in a study recording 30 apneas, 20 hypopneas, and 10 RERAs over 6 hours of TST, the RDI would be (30 + 20 + 10) / 6 = 10 events per hour.^[20] The AASM Scoring Manual provides separate scoring rules for pediatric (under 13 years) and adult patients, which can affect event identification—such as shorter duration thresholds for apneas and different criteria for hypopneas in children—but the overall RDI computation follows the same aggregation method. Version 2.0 (2012) refined these rules to improve consistency in diagnosing sleep-disordered breathing across age groups, with the current version 3.0 (2023) maintaining them with minor clarifications.^[21]^[10]^[23]^[24]

Clinical Applications

Diagnosis

The Respiratory Disturbance Index (RDI) is integral to the comprehensive evaluation of sleep-disordered breathing, including obstructive sleep apnea (OSA) and upper airway resistance syndrome, via its calculation during polysomnography (PSG). While the American Academy of Sleep Medicine (AASM) uses the apnea-hypopnea index (AHI) as the standard for diagnosing OSA, RDI provides additional insight by incorporating respiratory effort-related arousals (RERAs), particularly useful when AHI is borderline but symptoms persist. Conducted overnight in a sleep laboratory, PSG simultaneously records multiple physiological signals to identify respiratory events. Key monitored parameters include electroencephalography (EEG) for sleep staging, airflow using nasal pressure transducers or thermistors, respiratory effort via thoracic and abdominal inductance plethysmography belts, and oxygen saturation with pulse oximetry. These data enable the quantification of apneas, hypopneas, and RERAs, from which the RDI is derived as the average number of such events per hour of sleep.^[25]^[10] Home sleep apnea testing (HSAT) offers an alternative for initial screening but is limited in its ability to compute a reliable RDI. HSAT devices typically capture airflow, respiratory effort, and oxygen desaturation but omit EEG and other channels needed to detect RERAs, which require evidence of cortical arousals. Consequently, HSAT often underestimates RDI by excluding these events, potentially missing milder or non-apneic disturbances that PSG would identify.^[26]^[19] In the diagnostic workflow, RDI results from PSG or HSAT are reviewed post-study and correlated with patient-reported symptoms to confirm sleep-disordered breathing. Tools such as the Epworth Sleepiness Scale (ESS), a validated eight-item questionnaire assessing the likelihood of dozing in various situations, help quantify excessive daytime sleepiness—a hallmark symptom of OSA. An abnormal RDI combined with elevated ESS scores and clinical history strengthens the diagnostic case.^[19] The American Academy of Sleep Medicine (AASM) guidelines endorse PSG as the preferred method for accurate assessment in suspected OSA cases due to its comprehensive monitoring capabilities. Although earlier guidelines (e.g., 2005) emphasized RDI, current standards as of the 2023 AASM Scoring Manual primarily use AHI, with RDI serving as an optional measure for broader respiratory disturbances. This recommendation underscores PSG's role in ensuring precise event detection over less detailed alternatives.^[19]

Severity assessment

The severity of respiratory disturbances is assessed using standardized classification scales based on RDI values, which guide clinical decision-making and risk stratification. According to the American Academy of Sleep Medicine (AASM), obstructive sleep apnea (OSA) associated with RDI is categorized as mild for values ≥5 and <15 events per hour, moderate for ≥15 and <30 events per hour, and severe for ≥30 events per hour.^[27] These thresholds align closely with Medicare guidelines, which use similar RDI cutoffs to determine eligibility for diagnostic and therapeutic interventions.^[28] Higher RDI values carry significant prognostic implications, correlating with elevated risks of comorbidities and adverse outcomes. Elevated RDI is independently associated with hypertension, as demonstrated in population-based studies linking increasing RDI levels to higher prevalence of high blood pressure alongside factors like obesity and diabetes.^[29] In patients with heart failure, higher RDI predicts increased rates of death and cardiovascular hospitalizations.^[30] Furthermore, untreated OSA with RDI ≥10 events per hour more than doubles the risk of cardiovascular mortality in individuals with coronary artery disease, with risks escalating further at higher thresholds such as >20 events per hour.^[31] RDI severity informs treatment strategies, particularly the initiation and monitoring of continuous positive airway pressure (CPAP) therapy. An RDI ≥15 events per hour typically prompts the recommendation for CPAP in symptomatic patients, as it indicates moderate to severe disease warranting intervention to mitigate health risks.^[28] Post-treatment RDI reductions are monitored via follow-up studies to evaluate efficacy, with successful therapy often lowering values below 5 events per hour to alleviate symptoms and reduce long-term complications.^[27] Recent updates to AASM scoring rules have refined hypopnea criteria, potentially influencing RDI-based severity assessments. In the 2023 release of the AASM Manual for the Scoring of Sleep and Associated Events (Version 3), scoring hypopneas using ≥4% oxygen desaturation from pre-event baseline was reclassified from acceptable to optional, while the recommended criteria remain ≥3% desaturation or arousal; this shift may lead to variations in reported RDI values depending on the chosen rules, particularly in home sleep apnea testing.^[18]

Comparisons

With apnea-hypopnea index

The apnea-hypopnea index (AHI) counts only apneas and hypopneas occurring per hour of total sleep time, excluding respiratory effort-related arousals (RERAs).^[11] A key methodological difference is that RDI includes RERAs alongside apneas and hypopneas, often resulting in higher values—typically by 5–10 events per hour—in patients exhibiting upper airway resistance, thereby enhancing sensitivity for detecting obstructive sleep apnea (OSA).^[32]^[20] This discrepancy has clinical implications, as AHI may overlook mild OSA cases, particularly in non-obese individuals where upper airway resistance syndrome predominates; research demonstrates that RDI more effectively predicts associated symptoms like fatigue and depression in such patients.^[11]^[33] While AHI suffices for evaluating severe OSA, RDI is favored in comprehensive polysomnography reporting to account for subtler respiratory disturbances.^[11]

With respiratory event index

The respiratory event index (REI) is a metric derived from unattended home sleep apnea testing (HSAT), analogous to the apnea-hypopnea index but calculated as the number of apneas and hypopneas per hour of total recording time rather than sleep time, excluding respiratory effort-related arousals (RERAs) and lacking electroencephalography (EEG) for arousal detection. In contrast, the respiratory disturbance index (RDI), obtained from laboratory polysomnography (PSG), incorporates RERAs alongside apneas and hypopneas, providing a more comprehensive assessment of respiratory disturbances during verified sleep periods. REI often underestimates OSA severity compared to RDI, primarily due to the absence of thoracic and abdominal effort belts, EEG monitoring, and direct sleep staging in HSAT, which can miss subtle arousals and overestimate recording time relative to actual sleep. Validation studies indicate mean absolute errors between REI and PSG-derived indices (such as AHI, akin to RDI components) of 6.7 to 13.9 events per hour, with greater discrepancies in mild and moderate cases where arousal-related events are overlooked.^[34] Clinically, REI serves as a practical screening tool for high-risk patients in home settings, facilitating accessible initial evaluation of moderate to severe OSA in uncomplicated cases, while RDI from PSG is preferred for definitive diagnosis, particularly in ambiguous or comorbid presentations requiring detailed analysis. A 2023 study reported strong correlations between REI and PSG-derived AHI (r = 0.92–0.99), with higher misclassification rates in mild (up to 48.1%) and moderate OSA cases, underscoring REI's limitations compared to comprehensive PSG indices like RDI.^[34]

References

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