Why a sigh feels like relief

Feeling a sense of relief after a sigh could be related to how lung movements during a deep breath reduce surface tension of the organ’s fluid, making it easier to breathe, a study has found.

Researchers, including those from Switzerland’s ETH Zurich, observed how the lung fluid behaves when it is stretched and compressed through experiments in the laboratory.

Lung movements during inhaling and exhaling subject the lung fluid to similar stresses.

“The fluid coats the entire surface, making the lungs more deformable or — with a more technical word — compliant,” Jan Vermant, professor of soft materials at ETH Zurich and author of the study published in the journal Science Advances, said.

Simulating lung movements of a normal and deep breath, the team measured surface tension — or surface stress — of the fluid, which was found to reduce significantly after a deep breath.

“This surface stress influences how compliant the lungs are. The more compliant the lungs are, the less resistance there is to expansion and contraction — and the easier it is to breathe,” Vermant said.

The result offers a physical explanation for experiencing a feeling of relief in the chest that often accompanies a deep sigh, the researchers said.

They added that a thin film of the lung fluid formed on the surface of one’s lungs has multiple 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,” first author Maria Novaes-Silva, a doctoral student in Vermant’s research group at ETH Zurich, said.

The layering returns to its equilibrium, or balanced, configuration over time when the fluid does not move at all or moves only slightly during shallow breathing, Novaes-Silva said.

A deep breath is required every now and then to restore this ideal layering, with a pronounced stretching and relaxing of the lung fluid changing the composition of the outer layer, the researchers said.

“There is an enrichment of saturated lipids, this results in a more densely packed interface,” Novaes-Silva said.

Vermant added, “(The densely packed interface) is a state outside of the boundaries of the thermodynamic equilibrium that can only be maintained through mechanical work.”

“Sighs enrich the air-liquid interface with saturated lipids, triggering structural rearrangements. This periodic ‘reset’ transforms the layer into a mechanically robust, DPPC-rich film, where compressional hardening counteracts tension,” the authors wrote.

A DPPC-rich film is a lipid layer with a phospholipid ‘dipalmitoylphosphatidylcholine’ being the main component of pulmonary surfactant and is important for lung function.

The findings highlight how an occasional shake-up of layers of the lung fluid — by means of a deep breath or sigh — is important to lung mechanics, the authors said.

Insights from the study could also help guide protective ventilation strategies following a lung trauma and possibilities to optimise treatment, they added.