4.2 Respiratory Event Scoring
Key Takeaways
- An adult apnea is scored when peak signal excursion drops 90% or more from baseline on the oronasal thermal sensor for at least 10 seconds, with the drop occupying at least 90% of the event.
- Obstructive apnea shows continued or increased effort, central apnea has absent effort throughout, and mixed apnea begins with absent effort and ends with resumed effort within a single event.
- The AASM recommended adult hypopnea rule scores a 30% or greater drop in nasal pressure for at least 10 seconds with a 3% or greater oxygen desaturation or an arousal; the 4% rule is now optional in Version 3.
- A respiratory effort-related arousal (RERA) is a 10-second-plus sequence of increasing effort or nasal-pressure flattening leading to an arousal that does not meet apnea or hypopnea criteria, and counts toward the RDI only.
- Pediatric scoring uses two respiratory cycles as the minimum event duration for apneas and hypopneas rather than the adult fixed 10 seconds.
Why Respiratory Scoring Matters
Respiratory event scoring drives the apnea-hypopnea index (AHI) that determines sleep-disordered-breathing severity and treatment decisions. The RPSGT exam tests exact AASM thresholds, the correct sensor for each event type, and the differences between adult and pediatric rules. A single swapped sensor or threshold is the most common trap, so anchor each rule to its signal first.
Required Sensors
For a diagnostic study, apneas are identified using the oronasal thermal sensor (thermistor) and hypopneas are identified using the nasal pressure transducer. Effort is measured with respiratory inductance plethysmography (RIP) belts on the thorax and abdomen. Oxygen saturation (SpO2) comes from pulse oximetry with a short averaging time (3 seconds or less at a heart rate of 80). On positive airway pressure (PAP) studies, the device flow signal replaces the thermal sensor and nasal pressure transducer for event detection.
Apnea
An apnea is scored when peak signal excursion on the thermal sensor drops 90% or more of the pre-event baseline for at least 10 seconds in adults, with the 90% reduction occupying at least 90% of the event. Apneas are classified by respiratory effort:
- Obstructive apnea - continued or increased inspiratory effort throughout the period of absent airflow.
- Central apnea - absent inspiratory effort throughout the entire event.
- Mixed apnea - absent effort initially, followed by resumption of effort in the second portion, within one event.
Hypopnea
The AASM recommended adult hypopnea rule scores a hypopnea when the nasal pressure (or alternative) signal drops 30% or more from baseline for at least 10 seconds, associated with either a 3% or greater oxygen desaturation from the pre-event baseline or an arousal. In Version 3 (2023), the older rule requiring a 4% or greater desaturation with no arousal credit was reclassified from acceptable to optional. Read the stem carefully: the recommended (3% or arousal) rule is the default unless a question explicitly specifies the 4% rule.
Respiratory Effort-Related Arousal (RERA)
A RERA is a sequence of breaths lasting at least 10 seconds characterized by increasing respiratory effort or flattening of the nasal pressure waveform that leads to an arousal, when the sequence does not meet apnea or hypopnea criteria. RERAs contribute to the respiratory disturbance index (RDI) but not to the AHI.
Cheyne-Stokes Breathing
Cheyne-Stokes breathing is scored with a crescendo-decrescendo change in breathing amplitude accompanied by central apneas or central hypopneas, requiring 3 or more consecutive cycles plus either 5 or more central apneas/hypopneas per hour of sleep or the cyclic pattern lasting at least 10 consecutive minutes. The ventilatory cycle is characteristically long, often 40 seconds or more, distinguishing it from idiopathic central apnea.
Hypoventilation
In adults, sleep-related hypoventilation is scored when arterial or surrogate carbon dioxide (PaCO2 or transcutaneous/end-tidal CO2) increases to greater than 55 mmHg for 10 or more minutes, or there is a rise of 10 mmHg or more during sleep above the awake supine value to a level exceeding 50 mmHg for 10 or more minutes.
Pediatric Respiratory Rules
Pediatric scoring (for children below the locally defined age cutoff) differs primarily in duration: obstructive, mixed, and central apneas and hypopneas require a minimum of two missed or abnormal respiratory cycles rather than a fixed 10 seconds. Pediatric central apneas are scored when effort is absent and the event lasts 20 seconds or longer, or lasts at least two breaths and is associated with an arousal, a 3% desaturation, or (in infants) bradycardia.
Comparison of Respiratory Events
| Event | Signal & threshold | Min duration (adult) | Effort | Desat/arousal | Counts toward |
|---|---|---|---|---|---|
| Obstructive apnea | Thermal, >= 90% drop | 10 s | Present/increased | Not required | AHI |
| Central apnea | Thermal, >= 90% drop | 10 s | Absent throughout | Not required | AHI |
| Mixed apnea | Thermal, >= 90% drop | 10 s | Absent then present | Not required | AHI |
| Hypopnea (recommended) | Nasal pressure, >= 30% drop | 10 s | Variable | >= 3% desat or arousal | AHI |
| RERA | Nasal pressure flattening / rising effort | 10 s | Increasing | Arousal required | RDI only |
Expect stems that swap the thermal sensor and nasal pressure roles, switch the 90% versus 30% thresholds, or test the central-versus-obstructive effort distinction. Pediatric questions almost always hinge on the two-respiratory-cycle duration rule, and a frequent distractor offers the obsolete "acceptable 4%" wording rather than the current "optional" status.
Effort Channels and Differentiating Event Types
The respiratory inductance plethysmography (RIP) thorax and abdomen belts are the key to classifying apneas. In an obstructive event, the belts continue to move (often paradoxically, with thorax and abdomen out of phase) while airflow is absent. In a central event, both belts are flat. In a mixed event, the belts are flat early then resume motion before airflow returns. Esophageal pressure (Pes) is the gold standard for effort but is rarely used clinically; the exam expects you to read RIP belt motion as the practical effort signal.
Oxygen Desaturation and the ODI
Desaturation is referenced to the pre-event baseline, the SpO2 immediately before the event. The oxygen desaturation index (ODI) counts desaturations (commonly 3% or 4%, per protocol) per hour of sleep and is reported alongside the AHI. Use a short pulse-oximetry averaging time (3 seconds or less) so transient desaturations are not blunted; a long averaging time is a classic technical error that artificially lowers the ODI.
Severity Cut Points and Why Sensors Matter
The AHI maps to severity using the familiar 5 / 15 / 30 cut points: an AHI of 5 to under 15 is mild, 15 to under 30 is moderate, and 30 or more is severe obstructive sleep apnea. Because the hypopnea definition (3% versus 4% desaturation) changes the event count, two labs using different rules can assign different severities to the same study. This is exactly why the exam insists you know which rule a stem specifies, and why the recommended 3%-or-arousal rule is treated as the default. The sensor-to-event pairing (thermal for apnea, nasal pressure for hypopnea) is the single most repeated fact in this domain.
During a diagnostic adult study, the oronasal thermal signal drops 95% from baseline for 14 seconds. Respiratory effort belts show continued chest and abdominal movement throughout the event. How is this event scored?
Under the AASM recommended adult hypopnea rule, which combination correctly scores a hypopnea?
How does pediatric respiratory event scoring differ from adult scoring with respect to event duration?
A breathing sequence lasts 18 seconds with progressively increasing inspiratory effort and flattening of the nasal pressure waveform, terminating in a cortical arousal. Airflow never drops enough to meet apnea or hypopnea criteria. What is scored, and which index does it affect?