Effects of Inhalation of CO2, Muscular Exercise and Epinephrine on Maximal Breathing Capacity

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Abstract
The maximum breathing capacity (MBC) is increased during the inhalation of 7.5% CO2 and during the hyperpnea following exercise. It is increased by the injection of epinephrine but not by breath holding. It is decreased by previous hyperventilation. Neither reflex drive to respiration nor "warming up" of the respiratory muscles can explain the increase in MBC. At similar minute volumes of ventilation, the subjects often noted dyspnea when breathing 7.5% CO2 but not during muscular exercise. The dyspnea caused by CO2 inhalation is still unexplained but does not appear to be related to an increase in the work of breathing that is disproportionate to the minute volume of ventilation. Neither respiratory drive alone nor prior warm-up of the respiratory muscles alone seems to be a satisfactory explanation of the increased MBC during exercise and CO2 inhalation. Injection of epinephrine, however, produces a similar increase and liberation of endogenous epinephrine may be an important factor in the increased MBC noted during inhalation of 7.5% CO2 and after muscular exercise.