Expansion
Hypoxia, central stimulation, lung disease, drugs and anxiety
Expansion
- Hypoxia: altitude, pneumonia, pulmonary oedema, pulmonary embolism, severe anaemia
- Central stimulation: pain, anxiety, fever, sepsis, head injury, stroke, meningitis, hepatic encephalopathy
- Drugs and toxins: salicylates (direct respiratory centre stimulation), progesterone, doxapram, catecholamines
- Pulmonary: interstitial lung disease, asthma (early), pneumothorax
- Pregnancy: progesterone-driven, a normal finding
- Iatrogenic: excessive mechanical ventilation
- Compensatory: for a metabolic acidosis
Two clinical points deserve emphasis.
Salicylate poisoning classically produces a mixed picture: an early respiratory alkalosis from direct stimulation of the respiratory centre, combined with a high anion gap metabolic acidosis. Finding both should prompt the diagnosis.
Pulmonary embolism typically gives hypoxaemia with a low carbon dioxide and a widened A-a gradient, and a normal carbon dioxide in a breathless patient with suspected embolism is concerning rather than reassuring.