A numerical and experimental study of compliance and collapsibility of preterm lamb tracheae

Knowledge of the mechanical behaviour of immature tracheae is crucial in order to understand the effects exerted on central airways by ventilatory treatments, particularly of Total Liquid Ventilation. In this study, a combined experimental and computational approach was adopted to investigate the compliance and particularly collapsibility of preterm lamb tracheae in the range of pressure likely applied during Total Liquid Ventilation (−30 to 30 cmH 2O). Tracheal samples of preterm lambs ( n=5; gestational age 120–130 days) were tested by altering transmural pressure from −30 to 30 cmH 2O. Inflation ( S i ) and collapsing ( S c ) compliance values were calculated in the ranges 0 to 10 cmH 2O and –10 to 0 cmH 2O, respectively. During the tests, an asymmetric behaviour of the Δ V/ V 0 vs. P curves at positive and negative pressure was observed, with mean S i =0.013 cmH 2O −1 and S c =0.053 cmH 2O −1. A different deformed configuration of the sample regions was observed, depending on the posterior shape of cartilaginous ring. A three-dimensional finite-element structural model of a single tracheal ring, based on histology measurements of the tested samples was developed. The model was parameterised in order to represent rings belonging to three different tracheal regions (craniad, median, caudal) and numerical analyses replicating the collapse test conditions were performed to evaluate the ring collapsibility at pressures between 0 and −30 cmH 2O. Simulation results were compared to experimental data to verify the model's reliability. The best model predictions occurred at pressures −30 to −10 cmH 2O. In this range, a model composed of median rings best interpreted the experimental data, with a maximum error of 2.7%; a model composed of an equal combination of all rings yielded an error of 12.6%.