Overestimation of inspiratory upper airway resistance.

Inspiratory resistance was found to be 10-50% larger than the expiratory in a model of the upper airways. This was most likely caused by a high kinetic energy component in the airstream close to the constriction reducing the lateral pressure in excess of what could be explained by the frictional pressure drop. Consequently, the inspiratory resistance was overestimated. During expiratory flow, on the other hand, the kinetic energy component at the same site in the trachea was smaller and similar to that in the mouthpiece resulting in an apparently adequate assessment of expiratory resistance. These results may explain why previous authors have found equal inspiratory and expiratory upper airway resistance although the vocal cords abduct during inspiration and adduct during expiration. Pressures obtained by means of the tracheal puncture appeared to be more reliable compared to those obtained from puncture of the cricothyroid membrane.