Ventilatory response to fixed acid evaluated by ‘iso-Pco₂’ technique

¹ Department of Physiology, University of Chicago, Chicago, Illinois

In order to avoid changes in a second stimulus to ventilation, i.e. carbon dioxide, while measuring the response to fixed acid, a technique was utilized whereby alveolar carbon dioxide tension (Pa_COCO2) could be held constant. This technique includes continuous recording of Pa_COCO2 with an infrared type analyzer and addition of sufficient CO₂ to the inspired air to keep Pa_COCO2 at its control value (near 40 mm Hg). The response of anesthetized dogs to infusion of 0.5 m HCl was measured when the Pa_COCO2 was held at the control value and also at various other levels. Other experiments measured the effect of CO₂ when it was not allowed to change arterial hydrogen ion concentration [H⁺]. It was found that both these substances are potent respiratory stimuli and that their effects may be considered essentially separate and additive, as suggested by Gray. The experiments also demonstrated a slightly increased sensitivity to CO₂ at increased arterial [H⁺], but this effect was not found necessary to explain the response to acid. Responses to successive small increments in Pa_COCO2 failed to reveal any ‘threshold,’ even with CO₂ tensions as low as 15 mm during acidosis.