Effect of perception of mechanical loading on human respiratory pattern regulation.

We applied external flow resistive (R) and elastic (E) mechanical loads over the entire respiratory cycle to five normal subjects by using a pseudorandom loading protocol. Loads ranged in magnitude from imperceptible (R0/E0) through just perceptible (R1/E1) to large (R2/E2) and resulted in respiratory pattern responses that were due to reflex responses alone (R0/E0) or to a combination of reflex responses and behavioral reactions to the perception of impeded breathing (R1/E1 and R2/E2). Pattern regulation dynamics were estimated from the computed impulse responses of tidal volume and inspiratory and expiratory durations. We anticipated that emergence of behavioral contributions would be marked by increased variability in response strategies and by increased nonlinearity in the observed responses. Regarding the immediate pattern response to loading, there was a tendency for increased qualitative variation across subjects as the load size increased, but the within-subject variability (coefficient of variation) was unaffected. We found no evidence for increased nonlinearity as loads became perceptible. The emergence of behavioral control in some instances seemed to be marked by reduction of complexity of the impulse response to one dominated by the zeroth-order lag, leading to dynamically simpler responses compared to control.