A Comparison of the Effects of Steady-State and Progressive Hypoxia on the Ventilatory and Frequency Response

Abstract

During chemical stimulation of breathing the resulting increases in ventilation are achieved by changes in breath size and the duration of the phases of the respiratory cycle. Analysis of this inter-relationship in conscious man was started only recently. Hey, Lloyd, Cunningham, Jukes and Bolton¹ , who found that there was a unique linear relation between mean ventilation and tidal volume that is independent of the steady-state chemical stimulus. This relationship was found to be highly reproducible in an individual. Deviations from the steady-state Hey relation can occur, however, and several investigators^2–5 have found that following rapid changes in CO₂ and 0₂, tidal volume responded faster than frequency to yield a given ventilation. When the respiratory cycle is analyzed in terms of inspiratory and expiratory duration, it appears that there are differences between the responses to hypercapnia and hypoxia. Thus hypercapnia initially evokes increases in tidal volume with little change in inspiratory time, but with a decrease in expiratory time^6–1O. At high tidal volumes, hypercapnia appears to shorten inspiratory time⁶, ¹¹, although some controversy regarding this finding persists¹². In the cat this inspiratory shortening was found to be dependent on intact vagus nerves⁶.