Hypoxaemic patient — blood-gas interpretation viva

This patient is hypoxaemic despite a high inspired fraction, which is the signature of a lung problem rather than hypoventilation. The PaO2 of 7.8 kPa on near-maximal inspired oxygen means the alveolar-to-arterial gradient is markedly widened: the expected PaO2 on this much oxygen is far higher, so the lung is failing to transfer oxygen. The low PaCO2 of 3.6 kPa with a mild respiratory alkalosis shows he is hyperventilating in compensation, which excludes hypoventilation as the mechanism. This is type 1 respiratory failure from severe ventilation-perfusion mismatch and shunt in the pneumonic lung, and it is partly refractory to oxygen.^[1] The raised lactate adds tissue hypoperfusion to the picture.

The hypoxaemia is refractory because shunted blood cannot be oxygenated; more oxygen alone will not fully correct it. The approach is to support oxygenation while treating the cause. Start high-flow nasal cannula for the hypoxaemic respiratory failure, which improves oxygenation and reduces the work of breathing and may avert intubation.^[2] Treat the pneumonia with antibiotics and fluids, position the patient, and monitor the respiratory rate, the effort and the saturation. Escalate to intubation and mechanical ventilation if he tires, becomes hypercapnic, or the hypoxaemia worsens despite high-flow support. Recheck the blood gas to track the response.