Mechanism of augmented exercise hyperpnea in chronic heart failure and dead space loading

摘要

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 CO2 production (V̇CO2) per se but is responsive to an apparent (real-feel) metabolic CO2 load (V˙CO2o) that also incorporates the adverse effect of physiological VD/VT on pulmonary CO2 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 CO2 breathing. The ventilatory effects of dead space loading are therefore similar to those of increased alveolar V_D/V_T and CO₂ breathing combined. These observations are consistent with the hypothesis that the increased series V_D/V_T in dead space loading adds to V˙CO2o as with increased alveolar V_D/V_T in CHF, but this is through rebreathing of CO₂ in dead space gas thus creating a virtual (illusory) airway CO₂ load within each inspiration, as opposed to a true airway CO₂ load during CO₂ breathing that clogs the mechanism for CO₂ elimination through pulmonary ventilation. Thus, the chemosensing mechanism at the respiratory controller may be responsive to putative drive signals mediated by within-breath Pa_CO2 oscillations independent of breath-to-breath fluctuations of the mean Pa_CO2 level. Skeletal muscle afferents feedback, while important for early-phase exercise cardioventilatory dynamics, appears inconsequential for late-phase exercise hyperpnea.