Fig. 2

Normalized phase 2 and 3 slopes obtained from time (Sn2T, Sn3T) and volumetric capnograms (Sn2V, Sn3V), Fowler’s anatomical (VnDF) and Bohr’s physiological (VnDB) dead space fractions measured with both mainstream (empty boxes) and sidestream (hatched boxes) techniques in spontaneously breathing subjects (n = 35) at low (blue), medium (white) and high (red) breathing frequencies. #: p < 0.05, $: p < 0.01, *: p < 0.001.