Differentials
Interstitial lung disease
SIGNS / SYMPTOMS
Patients with interstitial lung disease often complain of shortness of breath, especially exertional dyspnoea. In addition, they may complain of a non-productive cough. Patients can present with a rapid, shallow breathing pattern; inspiratory crackles on examination of the lungs; and clubbing of the digits.
INVESTIGATIONS
While pulse oximetry may reveal an SaO₂ <90%, arterial blood gas analysis demonstrates hypocapnia rather than hypercapnia associated with hypoxaemia.
CXR demonstrates interstitial changes and fibrosis, which can be confirmed with a CT scan.
Obstructive sleep apnoea (OSA) without associated alveolar hypoventilation
SIGNS / SYMPTOMS
Patients with OSA without associated alveolar hypoventilation present with symptoms of excessive daytime sleepiness and loud snoring. On examination, obesity without signs of cor pulmonale such as lower-extremity oedema is common, although one third of patients have a BMI <30 kg/m².
INVESTIGATIONS
Arterial blood gas analysis demonstrates a normal PaCO₂, and pulse oximetry while awake reveals a SaO₂ >90%.
Overnight polysomnogram shows obstructive apnoeas and hypopnoeas with nocturnal oxygen desaturation purely associated with the sleep-disordered breathing events.
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