BREAKING POINT OF BREATH HOLDING AND TOLERANCE TIME IN REBREATHING

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The physical and mental factors which concern the determination of the breaking point of breath holding and of rebreathing were investigated in normal human subjects.<BR>The hyperbolic intensity-duration relationship of ventilatory stimulation of CO2, which has been revealed about the breaking point of breath holding, applies also in the onset of diaphragm activity during breath holding.<BR>The basic tolerable CO2 level, which corresponds to the rheobase in electrical stimulation, was almost the same as, or a little lower than, the normal resting PA, CO<SUB>2</SUB> level for the breaking point as well as for the onset of diaphragm activity, and showed few individual variations and training effects.<BR>It is considered that in the intensity-duration relationship the basic tolerable CO2 level dominantly related to the chemosensibility of the respiratory central structures, and the time factor may be in larger part related to mental or psychological events. The onset of diaphragm activity during breath holding is mainly attributed to physical factors.<BR>The tolerance time of rebreathing with a restricted tidal volume and at a definite rate was longer than breath holding time, even when the tidal volume was as tightly restricted as to 0.11. Single deep rebreathing at the breaking point caused a great relief from distress and made several succssive breath holding possible.<BR>In the successive breath holding, however, the period of absence of the rhythmic respiratory activity could hardly be observed.<BR>These increasing effects of respiratory movement upon the tolerance time are not ascribed to changes in physical conditions, but probably relate to some unexplained neural mechanisms.

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