Transpulmonary pressure is the difference between thealveolar pressure and theintrapleural pressure in thepleural cavity. During human ventilation, air flows because ofpressure gradients.
Ptp = Palv – Pip
Where Ptp is transpulmonary pressure, Palv is alveolar pressure, and Pip is intrapleural pressure.
Since atmospheric pressure is relatively constant, pressure in the lungs must be higher or lower than atmospheric pressure for air to flow between the atmosphere and thealveoli. If 'transpulmonary pressure' = 0 (alveolar pressure = intrapleural pressure), such as when the lungs are removed from the chest cavity or air enters the intrapleural space (apneumothorax), the lungs collapse as a result of their inherentelastic recoil. Under physiological conditions the transpulmonary pressure is always positive; intrapleural pressure is always negative and relatively large, while alveolar pressure moves from slightly negative to slightly positive as a person breathes. For a given lung volume, the transpulmonary pressure is equal and opposite to the elastic recoil pressure of the lung.
The transpulmonary pressure vs volume curve of inhalation (usually plotted as volume as a function of pressure) is different from that of exhalation, the difference being described ashysteresis. Lung volume at any given pressure during inhalation is less than the lung volume at any given pressure during exhalation.[1]
Transpulmonary pressure can be measured by placingpressure transducers. The alveolar pressure is estimated by measuring the pressure in the airways whileholding one's breath.[2] The intrapleural pressure is estimated by measuring the pressure inside a balloon placed in theesophagus.[2]
Measurement of transpulmonary pressure assists inspirometry in availing for calculation ofstatic lung compliance.