Sleep Apnoea
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Direct answer
Five or more apnoea-hypopnoea events per hour on polysomnography define obstructive sleep apnoea in a symptomatic patient — mild 5-15, moderate 15-30, severe above 30. An apnoea is cessation of airflow for at least 10 seconds with respiratory effort persisting (obstructive) or abolished (central); a hypopnoea is a flow reduction of at least 30% for 10 seconds with a 3-4% oxygen desaturation or an arousal. Collapse happens at the retropalatal pharynx when the genioglossus and other dilators lose their waking reflex tone against negative intraluminal pressure. Each obstruction ends in arousal, and the desaturation-arousal cycling drives sympathetic surges — the physiological route to systemic hypertension, atrial fibrillation, pulmonary hypertension and insulin resistance. Continuous positive airway pressure splints the airway open, typically titrated between 5 and 20 cm of water.
What you must remember
- Scoring rules: apnoea 10 seconds or more of absent flow; hypopnoea 30% flow drop for 10 seconds plus 3-4% desaturation or arousal; AHI grades 5-15, 15-30, above 30.
- Airway physiology: the genioglossus (hypoglossal nerve) is the key dilator; its tone falls in sleep, lowest in rapid-eye-movement sleep, which is why events cluster in REM.
- Risk architecture: obesity (pharyngeal fat pads), male sex, retrognathia, menopause, alcohol and sedatives; the obese, sleepy, snoring middle-aged man is the standard Indian short case.
- Arousal cost: each event ends with a three-second-or-longer EEG arousal; fragmented sleep produces daytime sleepiness — Epworth score above 10 — rather than the patient noticing nocturnal waking.
- Cardiovascular cascade: repeated desaturation fires catecholamine surges — resistant hypertension, arrhythmias, right ventricular strain, impaired glucose tolerance; obesity hypoventilation syndrome adds daytime PaCO2 above 45 mmHg (the old Pickwickian label).
- Obstructive versus central: thoracoabdominal belts and diaphragmatic EMG distinguish preserved effort (obstructive) from absent effort (central); mixed events begin central and end obstructive.
- Treatment logic: CPAP is a pneumatic splint, not a ventilator; mandibular-advancement appliances pull the tongue base forward; 10% weight loss meaningfully reduces AHI.
Walking through a polysomnography strip
Read one minute of a tracing the way a sleep technician does. The nasal flow trace flattens for 25 seconds; the thorax and abdominal belts keep heaving, so the event is obstructive. Oxygen saturation slides from 94 to 82% over the same window. The EEG shows abrupt alpha intrusion — an arousal — the belt excursions resume with a loud snort on the microphone, and saturation climbs back over the next half-minute. Repeat that 40 times an hour and the architecture is shattered: slow-wave sleep shrinks, REM fragments, and the morning report prints an AHI of 42 — severe. The same strip with absent belt movement during flow loss would read central, prompting a search for heart failure or opioid effect. Titration night follows: CPAP raised stepwise from 5-6 cm until flow flattening and arousals abolish, commonly settling near 8-12 cm.
How the examiner frames it
The classic probe is the belt question: how do you know an apnoea is obstructive and not central? Because effort channels keep moving — the diaphragm fights a closed airway — whereas central events show silent belts. The second is the paradox everyone should be able to explain: why do the worst desaturations occur in REM, when the apnoea is anatomical? Because REM paralyses the accessory dilators most and depresses arousal responses longest. And a clinical caution worth voicing in Indian settings, where polysomnography access is limited: home sleep apnoea testing is acceptable for high-probability obstructive cases without major comorbidity, but not for suspected central apnoea, heart failure or neuromuscular disease — an equity-aware answer that examiners reward.
Frequently asked questions
What apnoea-hypopnoea index defines obstructive sleep apnoea?
An AHI of 5 or more events per hour in a symptomatic patient, graded mild 5-15, moderate 15-30 and severe above 30 events per hour.
How is an obstructive apnoea distinguished from a central one on polysomnography?
Obstructive events show continued thoracoabdominal effort against a closed airway, while central events show absent respiratory effort with the airflow loss.
Which muscle is the principal upper airway dilator in obstructive sleep apnoea?
The genioglossus, supplied by the hypoglossal nerve; its sleep-related loss of tone allows retropalatal collapse under negative luminal pressure.
What is the Epworth Sleepiness Scale cut-off suggesting pathological sleepiness?
A score above 10 out of 24 indicates excessive daytime sleepiness and supports evaluation for sleep-disordered breathing.
What pressure range is used for CPAP therapy?
Continuous positive airway pressure is titrated typically between 5 and 20 cm of water to splint the pharynx and abolish apnoeas, hypopnoeas and flow limitation.