Reading a Normal Airflow Waveform
Polysomnography records airflow as a continuous scrolling trace.
- <5: Normal AHI (events per hour)
- 30 sec: Recording epoch (per scored page)
- Nasal + oral: Airflow sensor (thermistor / pressure)
- PSG: Study type (gold-standard sleep study)
What the trace shows
A rhythmic rise and fall marks each breath cycle.
Why baseline matters
Every apnea and hypopnea is scored against this baseline.
Channels recorded
Airflow, chest effort, and oxygen saturation run together.
Scoring an Apnea Event
An apnea is a flat line lasting ten seconds or longer.
- 10 sec: Minimum duration (AASM scoring rule)
- ≥90%: Airflow drop (from baseline amplitude)
- Effort continues: Obstructive type (chest still moves)
- No effort: Central type (brain signal absent)
Visual signature
The waveform collapses to a near-flat segment.
Obstructive vs central
Effort belts distinguish a blocked airway from no drive.
Counting rule
Each qualifying flat segment counts as one apnea.
Scoring a Hypopnea Event
A hypopnea is a partial airflow drop paired with desaturation.
- ≥30%: Airflow drop (from baseline amplitude)
- 10 sec: Minimum duration (sustained reduction)
- ≥3%: Desaturation (SpO2 drop required)
- ≥4% drop: Alternative rule (no arousal needed)
Visual signature
The waveform shrinks but never fully flattens.
Desaturation pairing
An oxygen dip confirms the reduction was significant.
Arousal alternative
A brain-wave arousal can substitute for desaturation.
Calculating the Apnea-Hypopnea Index
AHI totals every scored event per hour of actual sleep.
- (A+H)/hrs: AHI formula (events over sleep time)
- Sleep time: Denominator (not total recording time)
- 6–8 hrs: Typical study length (overnight PSG)
- Sleep technologist: Scored by (epoch by epoch)
The core equation
Apneas plus hypopneas, divided by hours slept.
Why sleep time, not clock time
Wake periods would falsely dilute the rate.
Related index
RDI adds respiratory-effort arousals to the count.
Classifying OSA Severity from AHI
The AHI score sorts patients into standard severity bands.
- <5: Normal (events per hour)
- 5–15: Mild OSA (events per hour)
- 15–30: Moderate OSA (events per hour)
- >30: Severe OSA (events per hour)
Mild range
Often managed with lifestyle change or a dental device.
Moderate range
CPAP therapy is typically recommended here.
Severe range
CPAP is strongly indicated to cut cardiovascular risk.