Normoxic hypercapnic ventilation. Typical increase in minute ventilation as an effect of step addition of 5% CO₂ to the inspiratory gas flow during the 8-min non–steady-state hypercapnic ventilation in one subject. Inspired carbon dioxide (Fico₂) level was 5%, and inspired oxygen fraction was 21%, i.e., room air. The y-axis has for some parameters been adjusted to display maximal resolution of the data, i.e., does not start from 0. ETco₂ = end-tidal carbon dioxide.