Respiratory Gas Exchange, Acid-Base Balance, and Electrolytes during and after Maximal Work Breathing 15 mm Hg $\text{PI}_{\text{CO}_\text{2} }$

Abstract

It is generally accepted on the basis of practical experience in submarines and other confined spaces that accumulation of CO₂ in the inspired air amounting to a partial pressure of 15 mm Hg is subjectively acceptable—if even noticeable—and compatible with ordinary physical and mental activities. Nevertheless, measurable changes in ventilation and alveolar Pco₂ have been reported by Lambertsen [1960] in resting subjects at 15 mm Hg Pco₂ and in light exercise by Froeb [I960]. Schaefer and his associates [1963] have described alterations in respiration, acid-base, and electrolyte balance in the course of acclimatization and de-acclimatization to an environment with a $\text{PI}_{\text{CO}_\text{2} }$ of 11 mm Hg. Relatively little factual information exists, however, on the effects of CO₂ under conditions of strenuous exertion verging on the limits of work capacity as might be encountered in emergency situations in space operations, in submarines, or in diving activities. An excellent investigation by Menn, Sinclair, and Welch [1970] with inspired CO₂ tensions ranging from 8 to 30 mm Hg revealed a consistent and proportionate increase in ventilation, but reduced CO₂ output and respiratory exchange ratio (R.E.R.) in submaximal exercise. In exhausting exercise, with $\text{PI}_{\text{CO}_\text{2} }$ 21 mm Hg ventilation was not significantly different from the controls, while maximal O₂ intake was only slightly less. But there was a highly significant reduction in CO₂ output. These authors concluded that strenuous exercise with exogenous hypercapnia leads to CO₂ retention, adding respiratory acidosis to the metabolic one, thus forcing the respiratory system to its limits.