Breathing--homeostatic function and voluntary motor activity

In a magnitude production test, subjects of both sexes rated their breath depth under different conditions. In the first group, 50 subjects were examined at rest in the supine position and breathing spontaneously or assisted by a respirator. The second group, consisting of 33 subjects, was tested in the sitting position at rest and during mild bicycle exercise (females 35 W, males 40 W). Subjectively quantified breath depths and corresponding tidal volumes were compared and their relations were described by Stevens power function. The assessment of breath depth in spontaneous breathing at rest was remarkably precise (mean Stevens exponent 0.94 +/- 0.02). During assisted ventilation, rating was less accurate and tended to underestimate breath depth. Noticeable under both conditions was a tendency to overestimate voluntary tidal volume reductions. This was more pronounced in young (up to 35 years) women than in men or elderly women. During exercise, the subjects (men and women) overestimated both increases and reductions of tidal volume. Each voluntary tidal volume modification was accompanied immediately by involuntary changes of inspiration and expiration times, thus, partially compensating disturbances of ventilation. In addition, voluntarily reduced tidal volumes were quantitatively compensated within the first breath subsequent to the voluntary manoeuver. We conclude that breath depth sensation is more strongly related to proprioceptive signals than to visceroceptive signals and is influenced by the autonomous respiratory drive. Furthermore, proprioception is assumed to participate in the control of the autonomous respiratory drive.