Acute respiratory distress syndrome (ARDS) is a life-threatening condition characterized by severe breathing difficulties affecting 200,000 people in the US annually with no known cure and 40% mortality. ARDS begins with trauma to the lung either by disease (including COVID-19 induced pneumonia), injury, or trauma. These underlying factors trigger an inflammatory response that leads to increased permeability of alveolar-capillary barriers. Due to the enhanced permeability, phospholipases, serum proteins, and other components of the innate immune system flood the alveolar spaces. Of particular interest is the action of phospholipase A2, which degrades bacterial, viral, and native membrane double-chain lipids into soluble single-chain lysolipids and fatty acids. The orders of magnitude increased solubility of the lysolipids cause them to dissolve out of the membranes, leading to bacterial and viral cell lysis and death. However, the resulting increased concentration of surface-active lysolipids in the alveolar fluids leads to alterations in the interfacial properties of native lung surfactants, which in turn can lead to mechanical instabilities in lung inflation. We hypothesize that this evolution of composition leads to a subsequent pulmonary collapse through lung surfactant displacement by these immune system actors. The changing composition of the interface causes an order of magnitude decrease in the dilatational modulus of the lung surfactant monolayer. This decrease in the dilatational modulus can lead to the Laplace Instability, in which smaller alveoli deflate and larger alveoli are distended, which are typical symptoms of ARDS.
Measurements of the dilatational modulus with a conventional Langmuir trough are complicated by the combined dilatation and shear deformations of the interface that result from the motion of the trough barriers. However, for lung surfactant monolayers, the dilatational modulus is 2- 3 orders of magnitude greater than the shear modulus, so contributions from shear are negligible. This allows us to perform dilatational measurements while simultaneously imaging the interface using fluorescence microscopy. In addition, complex insoluble monolayers are easier to apply to the trough interface than to bubble microtensiometers. We find that adding lysopalmitoyl phosphatidylcholine, a model lysolipid, to our in vitro model lung surfactant monolayers, the dilatational modulus spontaneously decreases by orders of magnitude, The interfacial morphology and phase behavior also change dramatically, transitioning from solid-like to liquid-like, consistent with our initial hypothesis that the elevated concentration of these inflammatory products is one of the dominant mechanisms in ARDS progression. For sufficiently small dilatational moduli, the interfacial tension cannot adjust with changes in the interfacial area and avoid the Laplace instability. We find that the dilatational modulus is an extremely sensitive measure of the interfacial composition and this information can be invaluable in determining the effects of competing surface-active compounds in the subphase. The confirmation of this hypothesis is crucial in providing a mechanistic view of ARDS progression which can lead to new therapeutic interventions to treating ARDS.
