The effect of shape, age and extension on the compliance of equine tracheal segments. Veterinary Research Communications, 15 (2), [135][136][137][138][139][140][141][142][143][144][145][146] The influence of the age and weight of the animal as well as that of the extension and the shape of the trachea on the mechanical properties of the cervical trachea was studied in 33 isolated tracheal segments obtained from freshly slaughtered horses. The relationship between intraluminal pressure and volume was determined in the cervical tracheal segments positioned firstly under normal longitudinal tension and secondly in hyperextension. At the same time, changes in the area of the cross-section of the lumen (X-SA) at the midpoint of the segment were measured using photographs obtained by slit-lamp transillumination and endoscopy. The sagittal (DS) and transverse (DT) diameters were determined for each segment and the DT/DS ratio was calculated to give an estimate of the shape of the X-SA. The results showed that (i) neither the age nor the weight of the horses had any influence on the mechanical properties of the trachea; (ii) extension decreases the compressibility of the tracheal segment in vi&o; (iii) there is a wide variation in the extrathoracic tracheal X-SA shape in horses; and (iv) the shape of the X-SA has a major influence on the mechanical properties of the trachea. It was concluded that (i) hyperextension of the neck will partly facilitate respiration at high levels of ventilation by elongating the trachea and by decreasing its collapsibility; and (ii) the tracheal collapse which may occur during high levels of ventilation will be more or less important depending on the individual X-SA shape.