Inaccuracy of oesophageal pressure for pleural pressure estimation in supine anaesthetized subjects.

Oesophageal pressure was measured, using a balloon and catheter system, at three or four positions in the oesophagus of eight supine subjects anaesthetized with 1-1.5% halothane in 67% nitrous oxide. Airway pressure and the difference between airway and oesophageal pressures were recorded during occlusion of inspiration, occlusion of expiration and occlusion of expiration followed by inspiratory occlusion. Lung volume could not change appreciably during these manoeuvres, so the difference between airway and oesophageal pressure should not have changed. The change in oesophageal pressure was less than the change in airway pressure in 86 of 90 measurements. The change in oesophageal pressure was expressed as a fraction of the change in airway pressure: the maximum fraction was obtained in each patient, and the mean of these maximum values was 82%. This suggests that changes in the difference between airway and oesophageal pressures will overestimate the change in transpulmonary pressure during artificial ventilation in supine subjects, and that lung compliance would be underestimated.