Fig. 1.

A: experimental setup. Each subject breathed through a face mask attached to a low-resistance 1-way valve with a built-in hot-wire flowmeter. Valve mechanism allowed subjects to inspire room air or a selected gas mixture from a 200-liter Douglas bag. TCD, transcranial Doppler; , end-tidal Pco₂; , end-tidal Po₂; V̇e, minute ventilation; Vt, tidal volume. B: experimental protocol. CO₂ was administered to each subject at rest (i) and during exercise (ii). Order of the rest and exercise trials was randomized for each subject. V̇o₂, O₂ uptake.