Acute neuromuscular respiratory failure: Viva

Q1. Why is his saturation 98 percent when he is clearly failing?

Because neuromuscular respiratory failure is a failure of the respiratory pump, not the lungs. The lung tissue is structurally normal, so the alveoli that are still ventilated continue to oxygenate the blood reaching them and the saturation stays high. What fails is the bellows: the weak diaphragm and intercostals move less air, so carbon dioxide climbs while oxygenation is preserved. The saturation is therefore reassuring in the wrong direction, and the child can arrest with a normal saturation. ^[4][1]

Probe: So what does the saturation tell you? It tells me almost nothing useful at this stage. I drive the decision from the bedside forced vital capacity and the bulbar assessment, never from the saturation or a single blood gas, which are both late and confirmatory. ^[3]

Q2. How do you monitor him, and at what point do you intubate?

I measure serial bedside spirometry every two to six hours, recording forced vital capacity, maximum inspiratory pressure, and maximum expiratory pressure, plotting them on a trend, and acting on the Lawn thresholds. A forced vital capacity under 20 mL per kilogram, a fall of over 30 percent in twenty-four hours, a maximum inspiratory pressure weaker than minus 30 cmH2O, and a maximum expiratory pressure under 40 cmH2O each predict the need for ventilation. He meets them all, so I intubate now. ^[1][2]

Probe: What if the bulbar function were intact? Then I could trial non-invasive bilevel positive airway pressure for pure pump failure, with cough augmentation and close spirometry, intubating only if it failed or bulbar weakness developed. But this child has bulbar weakness, so the mask will not protect his airway, and the answer is intubation. ^[9]