Trans-Airway pressure (P TA ) : It is the pressure gradient between the air opening and the alveolus.
P TA = P awo - P A
P TA produces air way movement in the conductive airways. It represents the pressure caused by resistance to gas flow in the airways (i.e. airflow resistance).
Trans-Thoracic pressure(P w or P tt ) It is the pressure difference between the alveolar space(lung) and the Body surface.
P w = P A - P bs
Pw represents the pressure needed to expand or contract
the lungs and the chest wall at the same time.
Trans-Pulmonary pressure (P L or P TP ), or Trans-Alveolar pressure : It is the pressure difference between the alveolus and the pleural space.
P L = P A - P pl
P L maintains alveolar inflation and is sometimes called the alveolar distending pressure. All modes of ventilation increase P L either by decreasing P pl (negative pressure ventilators) or increasing P A by increasing pressure at the upper airway (Positive pressure ventilators).
Trans-respiratiory pressure (P TR ): It is the pressure gradient between airway opening and the body surface
P TR = P awo - P bs
It is the pressure required to inflate the lungs and airways during positive pressure ventilation. In this situation the body surface pressure is atmospheric and is usually given the value zero; thus Pawo becomes the pressure reading on a ventilator gauge.