Why Deep Sighs Are Actually Good For Us

More than half of all premature babies born before the 28th week of pregnancy develop respiratory distress syndrome shortly after birth. As their lungs are not yet fully developed, they produce too little of the seemingly magical fluid that reduces surface tension in the lungs. As a result, some alveoli collapse - and the lungs are unable to get enough oxygen.  

Lungs become more deformable

Until 40 years ago, this usually spelled death. But then, in the late 1980s, pediatricians developed a life-saving procedure: they extracted the fluid from animal lungs and injected it into the lungs of premature babies. "This works very well in newborns," says Jan Vermant, Professor of Soft Materials at ETH Zurich. "The fluid coats the entire surface, making the lungs more deformable or - with a more technical word - compliant."  

But even in adults, lungs can fail. During the coronavirus pandemic, around 3,000 people in Switzerland developed acute respiratory distress syndrome. Injecting surface-active fluid from animal lungs into the lungs of adults, however, does not help. "This shows that it's not just about reducing surface tension," as Vermant states. "We believe that mechanical stresses within the fluid also play an important role."  

In collaboration with scientists from Spain, Belgium and the USA, his research group harnessed sophisticated measurement techniques to investigate precisely how lung fluid behaves when it is stretched and recompressed in the laboratory. The fluid in our bodies is also subjected to similar movements when the lungs expand during inhalation and contract again during exhalation. The researchers have just published their findings in the journal Science Advances.  

Explanation for the feeling of relief in the chest  

In their experiments, the researchers simulated the movements of normal and particularly deep breaths - measuring the surface stress of the fluid in each case. "This surface stress influences how compliant the lungs are," explains Vermant. The more compliant the lungs are, the less resistance there is to expansion and contraction - and the easier it is to breathe.  

With the help of their measuring instruments, the researchers found that surface stress decreases significantly after deep breaths. Apparently, there is a physical explanation for the feeling of relief experienced in the chest that often occurs after a deep sigh. The explanation starts from realizing that the thin film formed by the lung fluid on the surface of the lungs actually consists of several layers.  

"Directly at the boundary with the air, there is a slightly stiffer surface layer. Underneath, there are several layers that should be softer than the surface layer," explains Maria Novaes-Silva, a doctoral student in Vermant's research group and first author of the study. As she has proven in experiments, this layering returns to the equilibrium configuration over time when the fluid does not move at all or moves only slightly during shallow breathing.  

Reconstructing multilayered structures  

A deep breath is needed from time to time to restore this ideal layering. Based on their analyses, the researchers have discovered that the pronounced stretching and compression of the pulmonary fluid causes the composition of the outer layer to change. " There is an enrichment of saturated lipids, this results in a more densely packed interface," says Novaes-Silva. Vermant adds: "This is a state outside of the boundaries of the thermodynamic equilibrium that can only be maintained through mechanical work."  

It is also known from clinical practice that lung compliance gradually changes over time - and that breathing becomes increasingly difficult in connection with constant shallow breathing. The measurements in the laboratory therefore seems to reflect observations from the clinic. Novaes-Silva concludes: "These similarities are indications that we have captured real properties with our experimental setup."  

Can the new insights gained by materials scientists also be used to derive expedient conclusions and insights for lung failure in adults? "A promising approach is to identify components that can artificially reconstruct multilayered structures," the researchers note in their technical article. In conversation, Vermant points to therapies involving foam that are currently being developed and researched in greater depth by other groups.