Maximal frequency of breathing at various tidal volumes in airways obstruction

To determine the relative importance of control system and of mechanical and metabolic properties of the motor system on the maximal frequency of breathing (1/t) at various tidal volumes (VT), we have studied the relationship between cycle period (t) and tidal volume in 10 normal subjects, in 9 patients with chronic obstructive lung disease, and in a normal subject breathing against different resistive loads. The results, expressed according to the equation t = t0 + b VT (where t0 represents the time required for a breathing cycle at zero VT and b means the additional time required for a breathing cycle with a VT equal to vital capacity), indicate that t0 is independent from the load either in unloaded or loaded normal subjects or in obstructive patients. In fact, although b was found significantly different, t0 remained essentially constant. These findings suggest that the level of maximal voluntary ventilation may be mainly related to metabolic and mechanical properties of the motor system (muscles of breathing and load imposed on them).