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

Abstract

Background: 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/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.

Results: Lung 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.

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.

Keywords: Expanding chest wall; Lung elastance; PEEP step maneuver; Transpulmonary pressure.

Figures

Fig. 1

Respiratory system model. Left panel: model with no expanding force of the chest wall, where the lung volume is at residual volume and pleural pressure is zero. Right panel: model with evacuated pleural space, with a pressure of −5 cmH2O, which causes a higher fluid level in the plastic container than in the abdominal sack. As a consequence, the fluid in the plastic container wants to flow towards the sack, imitating the rib cage spring-out force, while the lung is inflated by a positive transpulmonary pressure of 5 cmH2O (zero airway pressure minus −5 cmH2O pleural pressure), to functional residual capacity (FRC). Nominal lung compliance of 19, 38, and 57 ml/cmH2O was achieved by using one, two, or three test lungs in parallel. A stiff chest wall was achieved by strapping hard board on the outside of the pleural space

Fig. 2

Left panels: shows airway pressure/volume (P/V) curve in an isolated lung (no CW), with nominal compliance of 38 ml/cmH2O (two test lungs), where the airway pressure equals the transpulmonary pressure. Mid panels: tidal airway (total respiratory system) P/V curves at 0, 4, 8, and 12 cmH2O of end-expiratory airway pressure (PEEP; filled circles). Upper mid panel: normal chest wall. Lower mid panel: stiff chest wall. Right panels: tidal airway (respiratory system) P/V curves with superimposed isolated lung P/V curve (dashed line) from end-expiration at ZEEP to end-inspiration at PEEP 12 cmH2O. Upper right panel: normal chest wall. Lower right panel: stiff chest wall. Note that irrespective of whether the chest wall is normal or stiff, the isolated lung P/V curve is aligned along the tidal end-expiratory airway P/V points

Fig. 3

Shows airway pressure and lung volume changes before, during, and after a PEEP step up and down in the model with one test lung and normal chest wall. PAW airway pressure (red), PTP transpulmonary pressure (dark blue), PPL pleural pressure (green). Volume change (light blue). Black arrows shows the change in PEEP and PTP, which indicate that end-expiratory PTP changes as much as end-expiratory airway pressure (PEEP) is changed

Fig. 4

Best fit lung P/V curves from PEEP steps of 4, 8, and 12 cmH2O in isolated lung (red), normal chest wall stiffness (long dash, black), and extra stiff chest wall (short dash, black). Filled circles indicate end-expiratory P/V points and open circles end-inspiratory P/V points of isolated lung. Note that end-expiratory and end-inspiratory transpulmonary P/V points are aligned on a common transpulmonary P/V curve, and consequently, the transpulmonary pressure at a certain lung volume is independent of whether this specific lung volume is a result of a tidal inflation or PEEP inflation

Fig. 5

Tidal airway P/V curves (red) starting from PEEP levels of 0, 4, 8, and 12 cmH2O and lung P/V curves (blue) starting from from ZEEP to end-inspiration at the highest PEEP level. Filled circles = end-expiratory airway P/V points. Note that the lung P/V curves have been transpositioned in parallel from their actual starting point at ≈5 cmH2O to zero to visualize that the slope of the lung P/V curve is identical to the slope of the end-expiratory airway P/V points (for actual position of lung P/V curves, see Fig. 7). Note that the lung P/V curve during PEEP inflation is unaffected by stiffness of the chest wall

Fig. 6

Left panel: isolated lung, right panel: lung and chest wall. Tidal airway P/V curves (red arrows) and tidal lung (transpulmonary) P/V curves (blue arrows). In the isolated lung without a chest wall, the airway P/V curve is equal to the transpulmonary P/V curve. The tidal volume from the low PEEP was almost equal to the end-expiratory lung volume change between the ZEEP and a PEEP of 8 cmH2O when the chest wall stiffness was normal and lung compliance was 38 ml/cmH2O. The end-inspiratory transpulmonary pressure of the tidal volume from ZEEP is close to the end-expiratory transpulmonary pressure at PEEP of 8 cmH2O. The lung P/V curves of the isolated lungs (red arrows, left panel) are identical to the lung P/V curves (blue arrows, right panel) of the complete model (lung, chest wall and abdomen) in the right panel. The end-inspiratory airway pressure of a tidal volume from ZEEP is right shifted from the end-expiratory airway P/V point of PEEP 8 cmH2O due to the influence of the chest wall with a pressure equal to the change in pleural pressure (ΔPPL). Note that the lung P/V curves have been transpositioned in parallel from its actual starting point at ≈5 cmH2O to zero in order to visualize that the slope of the lung P/V curve is identical to the slope of the end-expiratory airway P/V points (for the actual position of lung P/V curves, see Fig. 7)

Fig. 7

Tidal airway P/V curves (red), chest wall P/V curves (green), and lung P/V curves (blue) in model with normal (left panel) and stiff chest wall (right panel). End-expiratory P/V points are marked with filled circles. Blue dashed line: best fit lung P/V curve transpositioned to start at zero pressure. Note that the best fit lung P/V curve is passing through the end-expiratory airway P/V points and that the end-expiratory pleural pressure remain unchanged and negative when increasing PEEP, indicating an negligible end-expiratory chest wall elastance

Fig. 8

Tidal airway P/V curves of tidal volumes of 297 ml (red arrows) at ZEEP and 8.4 cmH2O of PEEP in the model with two test lungs (nominal lung compliance 38 ml/cmH2O). Red circles: end-expiratory airway P/V points, arrows: end-inspiratory airway P/V points. Transpulmonary P/V curve (blue arrow) of tidal volume of 297 ml. To determine the elastance of the lung, a volume change must be induced and the increase in transpulmonary pressure caused by the change in volume must be measured. A change in volume can be achieved by tidal inflation or by changing the end-expiratory airway pressure, PEEP inflation. In this case, changing PEEP by 8.4 cmH2O resulted in an increase in end-expiratory lung volume (281 ml) of the same amount as the tidal volume used (297 ml). In the experiment in the figure, the end-inspiratory transpulmonary pressure calculated conventionally as ΔPAW-ΔPPL is 14.1–5.3 = 8.8 cmH2O and calculated based on a PEEP step maneuver as (ΔPEEP/ΔEELV) × VT is (8/281) × 297 = 8.9 cmH2O