Respiratory neuromuscular output during breath holding.

The involuntary respiratory muscle contractions that occur during breath holding were found in almost all of 52 subjects and were regular in a majority. In detailed studies, subjects rebreathed a mixture of 8% CO2 in O2 and then held their breath on an occluded mouthpiece, with glottis open, at functional residual capacity. Contractions monitored as waves of negative pressure were reproducible and increased in amplitude and frequency through the breath hold, but the breakpoint did not always correspond to the same pressure or frequency. Frequency and the time derivative of pressure (dP/dt) of contractions were much higher during breath holding than frequency of breathing and dP/dt of occluded breaths at the same gas tensions during rebreathing. Contractions were reduced in amplitude after the subject took three breaths without altering gas tensions. The results are consistent with the hypothesis that contractions contribute to dyspnea in breath breath holding, but there is not a simple correlation between their magnitude and the degree of dyspnea.