Effect on ventilation of carbon dioxide delivered in early inspiration in man

Four subjects breathed an air-CO2 mixture, obtained by injecting pure CO2 0.4 l.min-1 into the inspiratory line of an open breathing circuit. The effects of injecting CO2 directly into the tubing (Fenn-Craig technique, FC), with a resulting large CO2 bolus in early inspiration, were compared with those of injecting into a mixing chamber (MC) and thus spreading the CO2 load more evenly over inspiration. Ensemble averaging of three experiments for MC and FC in each subject was used to clarify the relatively small changes in ventilation (V), about 5 l.min-1, and end-tidal PCO2 (PETCO2), about 5 mmHg, which were produced by the stimulus. Change in V and in PETCO2 were both systematically less for MC than FC, by about 1 l.min-1 and 0.5 to 1 mmHg respectively, suggesting that the CO2 load to the alveoli was larger for FC, with V then appropriately higher by the normal CO2 response. The increased CO2 load in FC could be accounted for by the lower proportion of CO2 left in the dead space, compared to MC. Thus, in man breathing air, no systematic effect on CO2 sensitivity was produced by an early inspiratory bolus of CO2.