Materials and Methods
The compliance C in the linear part of the inspiratory Pressure Volume relation of the respiratory system allows calculation of the pleural pressure P .
Results and discussion
The respiratory system can be considered to consist of one balloon (the lungs) inside a second balloon (thoracic cavity), as
depicted in figure 1. For both balloons the relation between volume V and transmural pressure P is described by V(P) = V(0) + C.P . If pleural pressure (the pressure between the balloons) is P , airway pressure is P and extra-thoracic pressure is P , the transmural pressure for thorax cavity is P – P and for the lung P -P . Solving these two relations for the thorax cavity and the lungs, and denoting lung compliance by C and thorax wall compliance by C , results in ΔP = ΔP C /(C +C ) for NPV and ΔP = ΔP C /(C +C ) for PPV. Hence any increase in airway pressure, which occurs in PPV, will increase pleural pressure proportionally. This is in much the same way as increased pressure from the outside, such as excessive abdominal weight against the diaphragm does. Conversely, it stands to reason that decreased extra-thoracic pressure, as occurs during NPV, will always lower pleural pressure. Restriction must be made that the alveoli are openly connected with the airway (then P is alveolar pressure).
Conclusion
Application of negative extra-thoracic pressure will always result in the pleural pressure going down, whereas increased positive airway pressure will always result in an increase in pleural pressure. This is a fundamental difference between PPV and NPV, and can be expected to have significant consequences for both ventilation and perfusion, as well the risk of pneumothorax.
Jan van Egmond
References