Stress-relaxation due to respiratory system tissue viscoelasticity: An update

This article deals with an unclear physiological phenomenon interesting respiratory system tissues, i.e. stress-relaxation. Due to their viscoelastic properties, the tissues do not maintain constant stress under constant deformation. Rather, the stress slowly relaxes and falls to a lower value. The exact molecular basis of this complex viscoelastic behavior are not well defined, but it has been suggested that it may be generated because of the anisotropic mechanical properties of elastin and collagen fibers in the alveolar septa and their interaction phenomena, such as reciprocal sliding, also in relation to interstitial liquid movements. The experimental approaches and methods of analysis of experimental results in the study of respiratory system stress-relaxation are briefly described. The effects on stress-relaxation of various biochemical and physical factors are reviewed, including the consequences of body temperature variations, respiratory system inflammations and hyperbaric oxygen exposure, endocrinal factors, circulating blood volume variations, changes in inflation volume and/or flow, changes in intra-abdominal pressure because of pneumoperitoneum or Trendelenburg position. The effects of these factors on stress-relaxation may have practical consequences because, depending on the viscoelastic pressure amount which is requested to inflate the respiratory system in different conditions, respiratory muscles have to produce different values of inspiratory pressure during spontaneous breathing. High inspiratory pressure values might increase the risk of respiratory failure development on a mechanical basis.