Finite element determination of the forces exerted by endotracheal tubes on the upper airways

Accidents occurring during intubations often originate in the mechanical interaction between the tube and the upper airways. A review of these accidents, including their possible complications, shows that they are mainly due to the forces exerted by the tube on the anatomical structures during the long intubation periods. The structural design of currently available tubes is presented, and the viscoelastic properties of the constituent PVC materials are analysed. It is shown how the long-term viscoelastic behaviour of the polymer can be assessed from accelerated relaxation tests in uniaxial tension at temperatures higher than the body temperature and how the variation in the relaxation modulus can be modelled through a simple constitutive equation with a minimum number of parameters. From these data, the mechanical interaction of the tubes with the trachea are computed by means of the finite element method. The relative efficiency of different types of endotracheal tubes is thus discussed and operational criteria are eventually defined for the choice of tubes in clinical situations.