RANGE OF 1ST-BREATH VENTILATORY RESPONSES TO ADDED MECHANICAL LOADS IN NAIVE MEN

A mathematical model was derived to calculate wave forms representing the mechanical effects of added loads on inspiration in the absence of load-compensating mechanisms (i.e., the passive prediction). Load compensation on the first loaded breath was inferred in 80 naive awake men from the difference between actual and predicted responses to graded elastic and resistive loads. At every load, respiratory frequency and tidal volume (VT) responses from different individuals formed a continuum ranging from a rapid-shallow to a slow-deep breathing pattern. Individual VT responses ranged from values smaller than the passive prediction to values exceeding control indicating inspiratory motoneuron output was not constant but decreased in some subjects and increased in others. In support of this hypothesis: individual VT responses varied with both inspiratory duration (TI) (P < 0.001) and VT/TI (P < 0.01); TI response indicated some subjects prematurely terminated their inspiratory motoneuron output whereas others prolonged it; and VT/TI responses suggested some subjects modified the discharge frequency of their motoneuron output. These findings demonstrate that alterations in inspiratory motoneuron output are an important determinant of the immediate ventilatory response to loads in conscious man.