Patterns of breathing in response to alternating patterns of alveolar carbon dioxide pressures in man

The time profile of alveolar PCO2 within the respiratory cycle has been forced to follow contrasting patterns in alternate breaths, in two different ways. Within-breath changes (w.b.c., with a CO2-rich inspirate supplied early or late in alternate inspirations) involved minimal alternation of end-tidal PCO2. Between-breath changes (b.b.c., with whole inspirates of CO2-free or CO2-rich gas) involved large swings of end-tidal PCO2. As previously reported (Metias, Cunningham, Howson, Petersen & Wolff, 1981), both patterns of forcing were associated with alternation of ventilation, but only when hypoxia was present. The patterns of the alternating reflex responses in 118 runs on four human subjects in steady hypoxia are described in terms of alternation of inspiratory and expiratory tidal volume, time and mean flow. These patterns often disappeared, or changed unpredictably in mid-run. The inspiratory pattern of reflex alternation depended in part on the type of forcing, but alternation of inspiratory tidal volume was usually observed with both types. No single pattern of expiratory alternation emerged as predominant. The pattern of reflex expiratory alternation was surprisingly independent of the pattern of inspiratory alternation: indeed, in w.b.c., but not in b.b.c., alternation of mean expiratory flow and of mean inspiratory flow were mutually exclusive. It is concluded that in man, as in cats and dogs, the arterial chemoreceptor pathway has access to various parts of the respiratory pattern generator, the exact response depending to some extent on the timing within the respiratory cycle. In particular, expiratory variables may be influenced directly through the arterial chemoreceptor pathway, i.e. without any supposedly mediating inspiratory alternation being demonstrable. The results are discussed briefly in relation to some current views on the organization of respiratory control.