The perception of respiratory work and effort can be independent of the perception of air hunger

Dyspnea in patients could arise from both an urge to breathe and increased effort of breathing. Two qualitatively different sensations, "air hunger" a nd "respiratory work and effort," arising from different afferent sources a re hypothesized, in the laboratory, breathing below the spontaneous level m ay produce an uncomfortable sensation of air hunger, and breathing above it a sensation of work or effort. Measurement of a single sensory dimension c annot distinguish these as separate sensations; we therefore measured two s ensory dimensions and attempted to vary them independently. In five normal subjects we obtained simultaneous ratings of air hunger and of work and eff ort while independently varying Pco(2) or the level of targeted voluntary b reathing. We found a difference in response to the two stimulus dimensions: air hunger ratings changed more steeply when Pco(2) was altered and ventil ation was constant; work or effort ratings changed more steeply when ventil ation was altered and Pco(2) was constant. We conclude that "air hunger" is qualitatively different from "work and effort" and arises from different a fferent sources.