Effective resistance of the respiratory system studied by a quick release technique

A human subject with mouthpiece in place makes inspiratory or expiratory effort against a closed shutter which abruptly opens when a predetermined pressure (P 0) is developed. Pressure at the mouth (P ao and flow (V̇) are continuously recorded. When the shutter opens, P ao changes toward atmospheric — first abruptly and then gradually, while V̇ rapidly rises to a peak value (V̇ peak) and then gradually declines. In any series of trials P 0 is held constant, but resistance external to the subject (R ext) is changed with each trial. In different series P 0 is varied. In any series of trials at constant P 0 the relationship between P ao and V̇ was linear. The slope (K) of the straight line fitted to the experimental points was interpreted as being equal in magnitude to the ‘effective resistance’ (R eff) of the respiratory system. R eff has two components — a true resistance (R int) and a virtual one (R fv) related to the force-velocity characteristics of the respiratory muscles. On the assumption that R int is independent of P 0 but that R fv varies linearly with it, R int and R fv can be evaluated.