Inhalation of 6% CO₂ produces reliable hypercapnia and elevation of ventilation.

<p>WT mice were exposed to a range of inspired CO₂ levels to determine optimal conditions for testing hypercapnic ventilatory responses. <b>(A)</b> Representative traces of whole-body plethysmography illustrate frequency and depth of breathing in conscious, unrestrained mice exposed to normal air at baseline (21% O₂, 0% CO₂, balance N₂), and to increasing levels of hypercapnia (3.7, 6.0, and 9.8% CO₂). Summary data shows <b>(B)</b> respiratory frequency (breaths/min), <b>(C)</b> tidal volume (μL/breath/g), and <b>(D)</b> minute ventilation (mL/min/g) at each inspired CO₂ level. Arterial blood samples taken from WT mice at baseline (normal air) or during exposure to 6% CO₂ were analyzed for <b>(E)</b> P_aO_2, <b>(F)</b> P_aCO₂, and <b>(G)</b> pH_a. Values are means ± SE; n = 4–8 animals/group. *<i>P</i> < 0.05 vs. baseline (1-way ANOVA; Dunnett’s post-hoc test (B) or two-tailed, paired Student’s t-test (C-E)).