Lung Hysteresis: Its Origins and the Role of Pulmonary Surfactant

Lung stability during respiratory maneuvers is due to the presence of the pulmonary surfactant at the air-liquid interface of the lungs. The presence of this surfactant layer was discovered more than eighty years ago and has been shown to reduce the surface tension of alveolar fluid to less than 10 mN/m upon expiration, thereby preventing complete collapse of the lungs. An important, but often overlooked, property of the pulmonary surfactant is its ability to change surface tension more rapidly upon expansion than compression. In in vitro studies, this phenomenon leads to considerable hysteresis in surface tension-surface area isotherm. This process is considered critical for the expansion of small alveoli during inspiration. Although this phenomenon is regarded as vital for lung mechanics, there is a scarcity of studies that have used hysteresis area as a measure of surfactant functionality.

In this presentation, we will clarify the relationship between the observed hysteresis phenomenon during dynamic surface tension experiments and those measured during pressure-volume experiments in the lungs. Topics discussed will include: the origin of hysteresis and its implications for lung mechanics, the relationship between surfactant components and hysteresis area, and the interplay between hysteresis area, transpulmonary pressure and surface tension. Finally, the functionality of lung derived surfactants will be evaluated based on their hysteresis behavior at 37ºC obtained from in vitro surface tension experiments.