Expansion
The lung follows a different path on inflation than on deflation
Expansion
Plotting lung volume against pressure gives a loop rather than a single line: at any given pressure, volume is greater during deflation than during inflation.
Two causes:
- Surfactant behaves differently as the surface film is compressed and expanded
- Recruitment: collapsed alveoli require a high opening pressure to snap open, but once open they remain so at much lower pressures
The clinical application is in ventilating the injured lung:
- The inflation limb may show a lower inflection point, historically taken as the pressure at which recruitment occurs, and used to guide PEEP
- Once recruited, alveoli can be held open with a lower pressure, which is the rationale for a recruitment manoeuvre followed by adequate PEEP
- An upper inflection point marks the onset of overdistension
The general principle, that it is easier to keep alveoli open than to reopen them, is the argument for avoiding repeated collapse and reopening, which causes atelectrauma.