Hypercapnic augmented exercise ventilation in dead space loading: effect of virtual (illusory) airway CO2 load in respiratory chemosensing

Patients with congestive heart failure (CHF) suffer increased alveolar dead space, yet they demonstrate compensatory augmentation of ventilation such that arterial PCO 2 remains remarkably normal (isocapnic) from rest to maximal exercise despite their decreased exercise tolerance. This paradoxical effect suggests that the control law governing isocapnic exercise ventilatory response is not merely determined by metabolic CO 2 output per se but is responsive to an apparent (real‐feel) metabolic CO 2 load that also accounts for the adverse effect of physiological VD/VT on pulmonary CO 2 elimination. By contrast, healthy individuals subjected to dead space loading with increased series dead space also experience augmented ventilation at rest and during exercise as with increased alveolar dead space in CHF, but the resultant response is hypercapnic instead of isocapnic, as with CO 2 breathing. The ventilatory effects of dead space loading are therefore similar to those of increased alveolar dead space and CO 2 breathing combined. These observations are consistent with the hypothesis that the increase in total VD/VT in dead space loading adds to the apparent metabolic CO 2 load as with increased alveolar dead space in CHF, but this is through rebreathing of CO 2 in dead space gas thus creating a virtual (illusory) airway CO 2 load in respiratory chemosensing that mimics CO 2 breathing. (Supported by HL067966 and RR028241)