Non-invasive ventilation and home respiratory support — formative SAQs

SAQ 1 — The boy with Duchenne muscular dystrophy and morning headaches (20 marks, ~15 minutes)

A 13-year-old boy with Duchenne muscular dystrophy, now using a wheelchair, is reviewed in clinic. He describes waking with headaches, is sleepy during the day, and his mother notes his cough has become weaker and he took a fortnight to recover from his last cold. His daytime blood gas is normal. ^[6]

Questions

1. What respiratory problem do his symptoms suggest, and why can the daytime blood gas be normal? (5 marks) ^[6]
2. What investigation would you use to confirm it, and what is the key limitation of overnight oximetry alone? (5 marks) ^[2]
3. State the appropriate ventilatory treatment and the evidence for its timing. (5 marks) ^[5]
4. His cough is weak. What would you assess and what intervention follows? (5 marks) ^{[3] [7]}

Model answer (must-hit)

1. The picture is chronic nocturnal hypoventilation: morning headache reflects overnight carbon dioxide retention, and daytime sleepiness reflects disturbed sleep. The daytime blood gas can be normal because hypoventilation appears first in sleep, particularly rapid-eye-movement sleep when accessory muscles are inhibited and a weak diaphragm works alone, long before daytime failure develops. ^[6]
2. Confirmation is by an overnight sleep study with carbon dioxide monitoring — attended polysomnography with capnography where available, or overnight oximetry combined with transcutaneous or end-tidal carbon dioxide. The key limitation of oximetry alone is that it can miss hypoventilation: the oxygen saturation may be maintained while the carbon dioxide rises, so carbon dioxide must be measured directly. ^[2]
3. The appropriate treatment is nocturnal bilevel non-invasive ventilation, which augments the breath and corrects nocturnal gas exchange. Ward and colleagues showed that starting nocturnal ventilation once nocturnal hypoventilation is present prevents progression, whereas starting it prophylactically before hypoventilation has appeared confers no benefit — so timing is guided by the sleep study. ^[5]
4. Cough is assessed clinically and, in a cooperative older child, by cough peak flow; a value falling toward the low hundreds of litres per minute warns that cough will fail during infection. The intervention is airway clearance including mechanical insufflation-exsufflation (cough assist), which Chatwin and colleagues showed increases cough peak flow, introduced before the next chest infection rather than during it. ^{[3] [7]}