“When the transpulmonary pressure (alveolar pressure minus pleural pressure) decreases, airways tend to close. This was first observed by Laennec1 and demonstrated by Dollfuss2 and Hedenstierna3 in ventilated patients. The proportion of the lung that is closed is named closing capacity, and this is known to increase with age and due to obesity. Airway closure due to rising pleural pressure is a natural behaviour and protects the alveolus against full collapse. Airway closure therefore is to be expected in all occasions that raise pleural pressure such as increased positive airway pressure during positive pressure ventilation.”
— Jan van Egmond, Clinical physicist & Member of the Exovent Development team
Materials and Methods
Description of the respiratory system by standard physics modelling.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 is commonly described as a balloon (the lungs) enclosed by a balloon (thorax cavity). For both balloons the relation between volume V and transmural pressure P is described by V(P) = V(0) + C.P, in which C denotes the compliance. If pleural pressure (the pressure between the balloons) is designed as P , airway pressure by P and extra-thoracic pressure by P , the transmural pressure for thorax cavity is P – P and for the lung P -P . Pleural pressures will change by increasing airway pressure as well as decreasing extra-thoracic pressure. Solving these two situations for thorax cavity and lungs and denoting lung compliance by C and thorax wall compliance as C , results in ΔP = ΔP C /(C +C ) for negative extra-thoracic pressure and ΔP = ΔP C /(C +C ) for positive airway pressure: any increase in airway pressure will increase pleural pressure proportionally, but decreased extra-thoracic pressure lowers pleural pressure.
Conclusion
Negative extra-thoracic pressure results in a reduction in pleural pressure which will help small peripheral airways to open up. Positive
airway pressure increases pleural pressure and therefore induces airway closure. Many problems during positive airway pressure mechanical ventilation, atelectasis, pneumonitis, pulmonary emphysema, pneumothorax and other air leaks are related to raised pleural pressure4. Negative extra-thoracic pressure ventilation is expected to avoid many of these problems.
Jan van Egmond
References