THE RESPIRATORY RESPONSES TO ANOXEMIA OF THE NORMAL UNANESTHETIZED DOG AND THEIR CAUSES

Abstract

Normal, unanesthetized dogs, trained to breathe through a mask and valves, were subjected to nine levels of anoxia, varying from 16% to 3% of oxygen in the inspired air. The concn. of inspired oxygen was constant during the anoxic period of any given expt. The data reveal that, in addition to the immediate and marked increase in the minute vol. of the respiration due to stimuli originating in the chemoreceptors of the carotid artery and the aortic arch, other stimuli may become manifest. At concns. of 6% of inspired oxygen and lower these stimuli, which follow the immediate chemoreceptor reflex, may be sufficiently powerful to overcome the factors which tend to depress the respiration following the initial chemoreceptor stimulation at concentrations of inspired oxygen above 6%. They may superimpose themselves upon the initial chemoreceptor stimulation and thus maintain or further increase the respiration, in spite of factors tending to decrease it. Further evidence of a stimulation in addition to the chemoreceptor reflex is evident when the dog is again permitted to breathe atmospheric air. A diminution in the respiratory vol. which bore a direct relationship to the pre-existing hyperpnea and hypocapnia would be expected. Instead, following the inspiration of 6% of oxygen or lower, the depression of the respiration below the control level is slight or insignificant. Apnea never occurred. The stimulation, additive to the chemoreceptor reflex, is most probably due to a chemical factor other than CO2.