Isocapnic augmented exercise ventilation in congestive heart failure: effect of apparent (real‐feel) metabolic CO2 load in respiratory chemosensing

CO 2 breathing introduces an airway CO 2 load that clogs the mechanism of pulmonary CO 2 elimination, inevitably causing hypercapnia when the inspired PCO 2 is high enough to prohibit isocapnia regardless of the ventilation level. The control law (Comroe's law) evidencing such self‐imposed permissive hypercapnia has been dubbed “CO 2 tolerance curve” (rather than CO 2 response curve) by Fenn and Craig (1963). By contrast, muscular exercise introduces a metabolic CO 2 load that is readily countered with proportionate increase in ventilation without CO 2 retention. Isocapnia from rest to exercise is maintained by the controller with the exercise ventilatory response being well adjusted for corresponding decrease in anatomical VD/VT, resulting in a small positive Y‐intercept in the ventilation vs. metabolic CO 2 output relationship (Whipp's law). Remarkably, patients with congestive heart failure compensate for their abnormally large alveolar VD/VT with augmented ventilation, maintaining isocapnia at rest and during exercise without CO 2 retention despite their decreased exercise tolerance. Thus, 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. (Supported by HL067966 and RR028241)