1. Distribution of lung volume, pulmonary ventilation and perfusion were studied in supine patients before and during anaesthesia with paralysis and artificial ventilation. Inspired gas and pulmonary blood flow were measured with 133xenon and the chest was scanned with vertically moving counters at a lung volume of 1 litre above functional residual capacity.

2. Ventilation/unit lung volume was slightly greater and perfusion/unit lung volume substantially greater during anaesthesia in the dependent parts of the lungs. The spread of ventilation/perfusion ratios in supine conscious patients was small in comparison with that reported in upright conscious patients. During anaesthesia and artificial ventilation, the inequality of ventilation to perfusion was marginally increased in three of the four patients.

3. Ventilation/perfusion inequality alone was insufficient to explain the alveolar—arterial Po2 difference usually observed during anaesthesia.

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