Patients with chronic heart failure (CHF) suffer increased alveolar VD/VT (dead-space-to-tidal-volume ratio), yet they demonstrate augmented pulmonary ventilation such that arterial PCO2 (PaCO2) remains remarkably normal from rest to moderate exercise. This paradoxical effect suggests that the control law governing exercise hyperpnea is not merely determined by metabolic CO 2 production (V˙CO2) per se but is responsive to an apparent (real-feel) metabolic CO 2 load (V˙CO2o) that also incorporates the adverse effect of physiological VD/VT on pulmonary CO 2 elimination. By contrast, healthy individuals subjected to dead space loading also experience augmented ventilation at rest and during exercise as with increased alveolar VD/VT in CHF, but the resultant response is hypercapnic instead of eucapnic, as with CO 2 breathing. The ventilatory effects of dead space loading are therefore similar to those of increased alveolar VD/VT and CO 2 breathing combined. These observations are consistent with the hypothesis that the increased series VD/VT in dead space loading adds to V˙CO2o as with increased alveolar VD/VT in CHF, but this is through rebreathing of CO 2 in dead space gas thus creating a virtual (illusory) airway CO 2 load within each inspiration, as opposed to a true airway CO 2 load during CO 2 breathing that clogs the mechanism for CO 2 elimination through pulmonary ventilation. Thus, the chemosensing mechanism at the respiratory controller may be responsive to putative drive signals mediated by within-breath PaCO2 oscillations independent of breath-to-breath fluctuations of the mean PaCO2 level. Skeletal muscle afferents feedback, while important for early-phase exercise cardioventilatory dynamics, appears inconsequential for late-phase exercise hyperpnea.