Breathing responses to small inspiratory threshold loads in humans

To investiage the effect of inspiratory threshold load (ITL) on breathing, all previous work studied loads that were much greater than would be encoun tered under pathophysiological conditions. We hypothesized that mild ITL fr om 2.5 to 20 cmH(2)O is sufficient to modify control and sensation of breat hing. The study was performed in healthy subjects. The results demonstrated that with mild ITL 1) inspiratory difficulty sensation could be perceived at an ITL of 2.5 cmH(2)O; 2) tidal volume increased without change in breat hing frequency, resulting in hyperpnea; and 3) although additional time was required for inspiratory pressure to attain the threshold before inspirato ry flow was initiated, the total inspiratory muscle contraction time remain ed constant. This resulted in shortening of the available time for inspirat ory flow, so that the tidal volume was maintained or increased by significa nt increase in mean inspiratory flow. On the basis of computer simulation, we conclude that the mild ITL is sufficient to increase breathing sensation and alter breathing control, presumably aiming at maintaining a certain le vel of ventilation but minimizing the energy consumption of the inspiratory muscles.