Reflex effects on human breathing of breath-by-breath changes of the time profile of alveolar$P_{CO_2 }$ during steady hypoxia

Abstract

The respiratory effects of forced changes of alveolar$P_{CO_2 }$ were studied in four healthy human subjects and in one anaesthetized cat. Solenoid valves, triggered by changes in mouth pressure, allowed changes from one inspiratory gas mixture to another, either during expiration (between-breath changes, BBC) or in the middle of inpiration (within-breath changes, WBC). In BBC the subject breathed CO₂-free gas in one inspiration, CO₂-rich gas in the next, and so on; end-tidal$P_{CO_2 }$ alternated regularly from breath to breath by 1.1 kPa. In WBC CO₂-free gas was given early in one inspiration and late in the next, with CO₂-rich gas late in the former and early in the latter, and so on end-tidal$P_{CO_2 }$ was nearly constant from breath to breath. Eight respiratory output variables were analysed. WBC induced small but significant alternation in most of the variables; these effects occurred almost exclusively in runs in hypoxia. The responses were not very different from those seen in BBC. The experiment on the cat showed that the alveolar$P_{CO_2 }$ changes predicted during WBC are reflected by changes in pH in the arterial blood. The results confirm predictions based upon observations in the steady state of tube- and reversed-tube breathing in man. It seems likely that the responses are mediated by the arterial chemoreceptors responding to small changes in the profile of the (CO₂, H⁺) oscillation.