Transpulmonary pressure is the difference between the
alveolar pressure and the
intrapleural pressure in the
pleural cavity
The pleural cavity, or pleural space (or sometimes intrapleural space), is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enabl ...
. During human ventilation, air flows because of
pressure gradient
In hydrodynamics and hydrostatics, the pressure gradient (typically of air but more generally of any fluid) is a physical quantity that describes in which direction and at what rate the pressure increases the most rapidly around a particular locat ...
s.
P
tp = P
alv – P
ip
Where P
tp is transpulmonary pressure, P
alv is alveolar pressure, and P
ip is intrapleural pressure.
Physiology
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 the
alveoli.
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 (a
pneumothorax
A pneumothorax is collection of air in the pleural space between the lung and the chest wall. Symptoms typically include sudden onset of sharp, one-sided chest pain and dyspnea, shortness of breath. In a minority of cases, a one-way valve is ...
), the lungs collapse as a result of their inherent
elastic 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 as
hysteresis
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
. Lung volume at any given pressure during inhalation is less than the lung volume at any given pressure during exhalation.
Measurement
Transpulmonary pressure can be measured by placing
pressure transducer
Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressur ...
s. The alveolar pressure is estimated by measuring the pressure in the airways while
holding one's breath.
[ The intrapleural pressure is estimated by measuring the pressure inside a balloon placed in the ]esophagus
The esophagus (American English), oesophagus (British English), or œsophagus (Œ, archaic spelling) (American and British English spelling differences#ae and oe, see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), c ...
.[Ronald B. George (2005) ''Chest medicine: essentials of pulmonary and critical care medicine''. . p. 96 .]
Measurement of transpulmonary pressure assists in spirometry
Spirometry (meaning ''the measuring of breath'') is the most common of the pulmonary function tests (PFTs). It measures lung function, specifically the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled. Spirometry is h ...
in availing for calculation of static lung compliance.
References
{{reflist
Respiratory physiology