An apnea and a hypopnea are both scored breathing events, but they have different criteria. An apnea is a near-complete interruption of airflow. A hypopnea is a qualifying reduction in airflow accompanied by other evidence under the scoring rule used.
The following explanation concerns adult sleep studies. Children's scoring criteria differ and should be interpreted by a team with paediatric sleep expertise.
Apnea: near-complete loss of airflow
Adult scoring generally requires at least a 90% reduction in the relevant airflow signal for at least 10 seconds. Breathing-effort signals help classify the event as obstructive, central or mixed. The airflow pattern alone does not settle that classification.
An oxygen drop is not required for every adult apnea to be counted. Event duration, the surrounding breathing pattern and the quality of the sensors still affect interpretation.
Hypopnea: partial reduction plus a qualifying change
The AASM's recommended adult rule uses at least a 30% airflow reduction lasting at least 10 seconds, together with either at least a 3% oxygen desaturation or an EEG-defined arousal. An alternative acceptable rule requires at least a 4% desaturation and does not count an arousal alone.
These rules can count different events in the same recording. Ask which was used, especially when comparing reports. There is no reliable fixed percentage that converts one result into the other.
A hypopnea is not necessarily unimportant because some airflow remains. Repeated oxygen drops and sleep disruption can matter clinically.
RERAs and arousals
A respiratory effort-related arousal, or RERA, is a qualifying sequence of disturbed breathing ending in a brain arousal without meeting apnea or hypopnea criteria. RERAs may contribute to a respiratory disturbance index, or RDI, rather than AHI. Check how the report defines its indices.
A PAP estimate uses different information
A PAP machine usually detects events from its airflow and pressure signals. Without the full sleep-study sensors, it cannot apply all laboratory criteria directly. Its hypopnea flag is therefore not equivalent to either laboratory rule simply because the label is similar.
The treatment state also matters: a diagnostic study usually describes untreated breathing, while a PAP report describes breathing during therapy. Use the device report to follow treatment in context, not to reconstruct an untreated laboratory AHI or independently choose pressure.
Related reading: How AHI is calculated: events, hours and different reports