Modeling Long-Term Facilitation of Respiration During Interval Exercise in Humans

Abstract

AbstractLong-term facilitation (LTF) of respiration has been mainly initiated by intermittent hypoxia and resultant chemoreceptor stimulation in humans. Comparable levels of chemoreceptor stimulation can occur in combined exercise and carbon dioxide (CO2) inhalation and lead to LTF. This possibility was supported by data collected during combined interval exercise and 3% inhaled CO2 in seven normal subjects. These data were further analyzed based on the dynamics involved using mathematical models in this study. Previously estimated peripheral chemoreceptor sensitivity during light exercise (40 W) with air or 3% inhaled CO2 approximately doubled resting sensitivity. Ventilation after a delay increased by 17.0 ± 2.48 L/min (p < 0.001) during recovery following 45% maximal oxygen uptake ($$V_{{{\text{O}}_{2} \max }}$$ V O 2 max ) exercise consistent with LTF which exceeded what can be achieved with intermittent hypoxia. Model fitting of the dynamic responses was used to separate neural from chemoreceptor-mediated CO2 responses. Exercise of 45% $$V_{{{\text{O}}_{2} \max }}$$ V O 2 max was followed by ventilation augmentation following initial recovery. Augmentation of LTF developed slowly according to second-order dynamics in accordance with plasticity involving a balance between self-excitatory and self-inhibitory neuronal pools.