Transpulmonary and pleural pressure in a respiratory system model with an elastic recoiling lung and an expanding chest wall

We have shown in acute lung injury patients that lung elastance can be determined by a positive end-expiratory pressure (PEEP) step procedure and proposed that this is explained by the spring-out force of the rib cage off-loading the chest wall from the lung at end-expiration. The aim of this study was to investigate the effect of the expanding chest wall on pleural pressure during PEEP inflation by building a model with an elastic recoiling lung and an expanding chest wall complex.

Methods: Test lungs with a compliance of 19, 38, or 57 ml/cmH 2 O were placed in a box connected to a plastic container, 3/4 filled with water, connected to a water sack of 10 l, representing the abdomen.

The space above the water surface and in the lung box constituted the pleural space. The contra-directional forces of the recoiling lung and the expanding chest wall were obtained by evacuating the pleural space to a negative pressure of 5 cmH 2 O.

Chest wall elastance was increased by strapping the plastic container. Pressure was measured in the airway and pleura.

Changes in end-expiratory lung volume (?EELV), during PEEP steps of 4, 8, and 12 cmH 2 O, were determined in the isolated lung, where airway equals transpulmonary pressure and in the complete model as the cumulative inspiratory-expiratory tidal volume difference. Transpulmonary pressure was calculated as airway minus pleural pressure.

Results: Lung pressure/volume curves of an isolated lung coincided with lung P/V curves in the complete model irrespective of chest wall stiffness.

?EELV was equal to the size of the PEEP step divided by lung elastance (EL), ?EELV?=??PEEP/EL. The end-expiratory “pleural”pressure did not increase after PEEP inflation, and consequently, transpulmonary pressure increased as much as PEEP was increased.

Conclusions: The rib cage spring-out force causes off-loading of the chest wall from the lung and maintains a negative end-expiratory “pleural”pressure after PEEP inflation.

The behavior of the respiratory system model confirms that lung elastance can be determined by a simple PEEP step without using esophageal pressure measurements.

Published on: 2017-03-29

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