Maximum expiratory flow changes induced by longitudinal tension on trachea in normal subjects.

Maximal expiratory flow (Vmax) was noticed to increase in some subjects during neck hypertension. Maximal expiratory flow volume (MEFV) curves were obtained in 15 normal young subjects at regular and hyperextended neck posture. Eleven of the subjects had consistently higher Vmax during neck hyperextension at high lung volumes, accompanied by MEFV configuration changes in the form of the obliteration of a concavity towards the volume axis that existed in the curve at regular neck posture. Effort independency was documented at lung volumes where the changes occurred. Radiographic studies indicate tracheal elongation with a relatively fixed carina during neck hyperextension. We propose that at high lung volumes in normal young subjects, the flow-limiting mechanism resides in the trachea and that the increased Vmax with neck hyperextension. We propose that at high lung volumes in normal young subjects, the flow-limiting mechanism resides in the trachea and that the increased Vmax with neck hyperextension reflects the effect of tracheal elongation which stiffens the trachea under dynamic conditions and increases its tube-wave speed. This concept was confirmed by MEFV curves obtained from anesthetized tracheostomized dogs when increased tracheal longitudinal tension resulted in an increase of Vmax.