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]

SAQ 2 — The infant with bronchiolitis (20 marks, ~15 minutes)

A 5-month-old, previously well, is admitted with bronchiolitis. He has increasing work of breathing, is feeding poorly, and his oxygen saturation is 89 percent in air. The ward is considering high-flow nasal cannula. [10]

Questions

1. What does high-flow nasal cannula do, and what does the trial evidence show about its role in bronchiolitis? (7 marks) [10]
2. What is the next step if he continues to deteriorate on high-flow, and what features would make you intubate? (7 marks) [2]
3. Why is non-invasive support preferred to early intubation in this setting, and what must be monitored? (6 marks) [2] [10]

Model answer (must-hit)

1. High-flow nasal cannula delivers heated, humidified gas above the infant's inspiratory demand, washing out dead space and giving a small distending pressure that reduces the work of breathing. In the PARIS trial, early high-flow oxygen reduced escalation of care compared with standard oxygen in infants with bronchiolitis, although it did not shorten the illness — so it lowers treatment failure rather than curing the disease faster. [10]
2. If he continues to deteriorate, CPAP is the next step to splint the airway and recruit the lung. Intubation is indicated when he cannot maintain oxygenation or ventilation despite non-invasive support, has a depressed conscious state, cannot protect the airway, has apnoeas, or has copious secretions he cannot clear — failure to improve is the signal to intubate rather than to persist. [2]
3. Non-invasive support avoids the risks of intubation and mechanical ventilation and preserves the infant's own airway defences while the self-limiting illness resolves. It requires close monitoring of work of breathing, oxygen saturation, and — where hypoventilation is a concern — carbon dioxide, with frequent reassessment so that failure is recognised early. [2] [10]