Concept and measurement of ventilatory sensitivity to carbon dioxide

¹ Laboratoires de Physiologie, Faculté de Médecine et Centre Marie-Lannelongue, Paris, France

Four resting subjects breathed 0, 1, 2.5, and 4% CO₂ diluted in air for 25 min. Ventilation, Po₂ and Pco₂ of alveolar and expired gases were measured. One can represent on a Po₂-Pco₂ diagram, as Rahn and Fenn have done, some features of the reaction to CO₂, and plot lines of "ventilatory isosensitivity to CO₂." The relation between experimental points and these lines shows why "ventilatory CO₂ sensitivity," Va/Pa_COCO2, for normal conditions cannot be quantified by the classical procedure of giving one or several percent CO₂ to breathe. CO₂ sensitivity nonetheless exists normally, since in subjects breathing mixtures containing a few mm Hg CO₂, Pa_COCO2 is practically unchanged, while Pa_OO2 rises by several mm Hg. The difficulties of accurately measuring Va/Pa_COCO2 suggest caution in accepting some mathematical developments often applied to raw data, and in taking this ratio as a true index of ventilatory CO₂ sensitivity, particularly when measurements made with high inspired CO₂ concentration are used to interpret the regulation of normal respiration.

Submitted on December 11, 1964

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