Figure 4

The illustration of end-tidal CO ₂ (EtCO ₂ ) concentration and corresponding FFT spectrum within 1 minute in one subject for example. The physiologic challenges of EtCO₂ include (a) baseline, (b) breathing with 6 cycles per minute, (c) breathing with 30 cycles per minute, (d) head-up tilt (HUT) and corresponding FFT spectrum during (e) baseline, (f) breathing with 6 cycles per minute, (g) breathing with 30 cycles per minute, (h) HUT.