CO2 homeostasis is maintained in conscious humans by regulation of tidal volume, but not of respiratory rhythm

Automatic regulation of tidal volume (V-T) maintains CO2 homeostasis when spontaneous respiratory rhythm is replaced with a cortically triggered-rhythm. We examined whether automatic regulation of respiratory frequency (f(R)) could maintain CO2 homeostasis at rest if the V-T is cortically designated in experiments performed in 21 conscious humans. First, volitionally controlled f(R) at levels lower than baseline resulted in a larger V-T, maintaining end-tidal CO2 fraction constant at eupneic levels. However, when f(R) was volitionally controlled at levels higher than baseline, end-tidal CO2 fraction decreased unexpectedly. Next, when the V-T was volitionally constrained but f(R) was freely chosen, end-tidal CO2 fraction decreased. The present study revealed some limitations in the control of CO2 homeostasis by automatic regulation of f(R), probably because respiratory rhythm is susceptible to non-metabolic factors. This study also showed the importance of automatic regulation of V-T in maintaining CO2 homeostasis at rest. Nevertheless, automatic regulation of V-T was incomplete when f(R) was volitionally imposed at high levels. (C) 2013 Elsevier B.V. All rights reserved.