Fig. 2.

Apparent metabolic CO₂ load in chronic heart failure (CHF) and dead space loading. a1: Linear V̇_E − V̇_CO2 relationship in a healthy subject and a CHF patient from rest to moderate exercise. The V̇_E − V̇_CO2 slope in CHF is significantly larger as a result of increased physiological (mainly alveolar) V_D/V_T, without any appreciable change in Y-intercept. Vertical broken lines indicate the corresponding respiratory compensation points when the linear V̇_E − V̇_CO2 relationship begins to curve upwards. Adapted from Mezzani et al. (2009) with permission. a2: In both healthy subject and CHF patient, exercise V̇_E is tightly coupled to apparent metabolic CO₂ load ${\dot{V}}_{\mathrm{C}{\mathrm{O}}_2}^o={\dot{V}}_{\mathrm{C}{\mathrm{O}}_2}/(1-V_D-V_T)$. a3: Control system block diagram corresponding to the control law shown in panel a2. b1: Dead space loading increases the slope as well as Y-intercept of the V̇_E − V̇_CO2 relationship in healthy subjects. b2: “Alveolar ventilation” (=V̇_E − V̇_D) is tightly coupled to V̇_CO2 for varying sizes of external dead space. Panels b1 and b2 are adapted from Ward and Whipp (1980) with permission. b3: Control system block diagram corresponding to the control law shown in panel b2.