Figure 4: (a) Relative insensitivity of breathing at rest to artificially lowering PO2 in Man. Minute ventilation (± se solid bars, ± sd open bars) in normal subjects [87] as inspired oxygen is artificially lowered (strictly hypocapnic hypoxia exists once hyperventilation occurs). Equivalent PaO2 points are aligned on the FiO2 scale, with PaO2 estimated before hyperventilation occurs using the alveolar gas equation (assuming 760 mmHg barometric pressure, RQ = 0.8, PaO2 = PAO2, and PaCO2 = PACO2), and the point afterwards is estimated based on dynamic forcing experiments in isocapnia (courtesy of Dr. G. A. Balanos). Reproduced with permission from Dripps et al., [87]. (b) Sensitivity of breathing at rest to artificially raising PaCO2 in Man. Minute ventilation and PaCO2 (femoral) in 8 healthy men [88] while inhaling 0–6% CO2 in air at atmospheric pressure (mean slope is 2.5 L·min−1 mmHg−1 artificial PaCO2 rise). Reproduced with permission from Lambertsen et al. [88].