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

Per Persson,Stefan Lundin,Ola Stenqvist

doi:10.1186/s40635-016-0103-4

Per Persson, Stefan Lundin + Show 1 more

Open Access

https://doi.org/10.1186/s40635-016-0103-4

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Abstract

BackgroundWe 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.MethodsTest lungs with a compliance of 19, 38, or 57 ml/cmH2O 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 cmH2O. Chest wall elastance was increased by strapping the plastic container. Pressure was measured in the airway and pleura. Changes in end-expiratory lung volume (ΔEELV), during PEEP steps of 4, 8, and 12 cmH2O, 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.ResultsLung pressure/volume curves of an isolated lung coincided with lung P/V curves in the complete model irrespective of chest wall stiffness. ΔEELV was equal to the size of the PEEP step divided by lung elastance (EL), ΔEELV = ΔPEEP/EL. The end-expiratory “pleural” pressure did not increase after PEEP inflation, and consequently, transpulmonary pressure increased as much as PEEP was increased.ConclusionsThe 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.

Highlights

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 consequence is that the endexpiratory pleural pressure does not change when end-expiratory lung volume is increased by raising PEEP
Note that the isolated lung P/V curve without chest wall, where airway pressure is equal to transpulmonary pressure, coincides with the endexpiratory airway P/V points irrespective of the stiffness of chest wall

Summary

Introduction

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. We have recently shown that it is possible to determine lung elastance without using esophageal pressure measurements using a simple PEEP step procedure [8, 9]. End-expiratory transpulmonary pressure increases as much as PEEP is increased This has previously been described by Rahn and coworkers 1946, where the chest wall pressure/volume curve was determined during end-expiratory relaxation at different lung volumes [10] without using esophageal or pleural pressure measurements. The fundamental effects of the expansive chest wall on lung mechanics have been discussed [11], but skepticism concerning the clinical effects seems to prevail

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