We conclude that spirometric measurements of inspiratory-expiratory tidal volumes agree well with impedance changes monitored by EIT and can be used bedside to estimate PEEP-induced changes in EELV.

PEEP 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.

A PEEP increase resulted in a less than expected increase in end-expiratory oesophageal pressure, indicating that the chest wall and abdomen gradually can accommodate changes in lung volume. The rib cage end-expiratory spring-out force stretches the diaphragm and prevents the lung from being compressed by abdominal pressure. The increase in transpulmonary pressure following a PEEP increase was closely related to the increase in PEEP, indicating that lung compliance can be calculated from the ratio of the change in end-expiratory lung volume and the change in PEEP, ΔEELV/ΔPEEP.