A Clinical Method for Assessing the Ventilatory Response to Carbon Dioxide

Summary A simple method of studying the regulation of ventilation has been developed so that rapid serial observations can be made in sick patients. The method is based on the progressive increase of Pco 2 and the resulting stimulation of ventilation which occurs during rebreathing from a bag. A small bag is used in which the initial Pco 2 is close to the Pco 2 of mixed venous blood. With this technique, a Pco 2 equilibrium is rapidly established between mixed venous blood, arterial blood and end‐expiratory gas. Subsequently, parallel changes develop in end‐expiratory Pco 2 and ventilation. Under these conditions, changes in end‐expiratory Pco 2 reflect the changes of medullary chemoreceptor Pco 2 and the developing carbon dioxide stimulus. Experiments are reported which establish the linearity, reproducibility and normal range of response‐curves relating ventilation (V̇ E ) to end‐expiratory Pco 2 . These V̇ E /Pco 2 response‐lines were only slightly disturbed by simulated abnormality of the body carbon dioxide storage capacity or acute depletion of the body carbon dioxide stores. V̇ E /Pco 2 response‐lines obtained by rebreathing and by the prolonged steady‐state method differed significantly in position but not in slope. The similarity in slope implies that the same proportion, Δ end‐expiratory Pco 2 /Δ chemoreceptor Pco 2 , is established for the two methods. The difference in position indicates a smaller Pco 2 gradient between arterial blood and chemoreceptor tissue in the rebreathing method. The method is particularly suited for clinical use, being rapid, tolerable and easily repeated. It avoids the necessity for sampling arterial or internal jugular venous blood, and can be performed by the bedside with the use of simple apparatus.