Transpulmonary pressure and lung elastance can be estimated by a PEEP-step manoeuvre

BackgroundTranspulmonary pressure is a key factor for protective ventilation. This requires measurements of oesophageal pressure that is rarely used clinically. A simple method may be found, if it could be shown that tidal and positive end-expiratory pressure (PEEP) inflation of the lungs with the same volume increases transpulmonary pressure equally. The aim of the present study was to compare tidal and PEEP inflation of the respiratory system. MethodsA total of 12 patients with acute respiratory failure were subjected to PEEP trials of 0-4-8-12-16cmH(2)O. Changes in end-expiratory lung volume (EELV) following a PEEP step were determined from cumulative differences in inspiratory-expiratory tidal volumes. Oesophageal pressure was measured with a balloon catheter. ResultsFollowing a PEEP increase from 0 to 16cmH(2)O end-expiratory oesophageal pressure did not increase (0.54.0cmH(2)O). Average increase in EELV following a PEEP step of 4cmH(2)O was 230 +/- 132ml. The increase in EELV was related to the change in PEEP divided by lung elastance (El) derived from oesophageal pressure as PEEP/El. There was a good correlation between transpulmonary pressure by oesophageal pressure and transpulmonary pressure based on El determined as PEEP/EELV, r(2)=0.80, y=0.96x, mean bias -0.4 +/- 3.0cmH(2)O with limits of agreement from 5.4 to -6.2cmH(2)O (2 standard deviations). ConclusionPEEP inflation of the respiratory system is extremely slow, and allows the chest wall complex, especially the abdomen, to yield and adapt to intrusion of the diaphragm. As a consequence a change in transpulmonary pressure is equal to the change in PEEP and transpulmonary pressure can be determined without oesophageal pressure measurements.