1. The ventilatory sensitivity to CO2 obtained from a non-steady-state step-ramp CO2 challenge (analogous to the Read rebreathing method) was compared with the one of the steady-state method. 2. Experiments were performed during normoxia on twenty cats anaesthetized with chloralose-urethane. In eight of these cats additional measurements were carried out during metabolic acidosis and alkalosis. 3. The slope of the non-steady-state ventilatory response curve to CO2 was not significantly different from the steady-state one only if the ratio of the step-wise increase in end-tidal PCO2 (PET,CO2) (A) above its resting value and the subsequent rate of rise of the PET,CO2 (R) was equal to the time constant of the central chemoreflex pathway (tau c). This also held true during metabolic acidosis and alkalosis. 4. It is predicted that in human beings during hyperoxia the ventilatory response line obtained with Read's rebreathing method is to a fair approximation shifted to the right by a value of A with respect to the steady-state response line, provided A/R = tau c. 5. We argue that Read's prescription that a PET,CO2 equilibrium should be established between mixed venous blood, arterial blood and end-tidal gas has to be regarded as an experimental condition leading to stable-experiments rather than dictated by physiological mechanisms.
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