Ventilatory response of peripheral chemoreceptors to hypercapnia during exercise above the respiratory compensation point.

Peripheral hypercapnic chemosensitivity (PHC) is assessed as the change in ventilation in response to a rapid change in carbon dioxide pressures (Pco). The increase in chemoresponse from rest to subrespiratory compensation point (RCP) exercise intensities is well-defined but less clear at intensities above the RCP when changes in known ventilatory stimulants occur. Twenty healthy subjects ( = 10 females) completed a maximal exercise test on 1 day, and on a subsequent day, transient hypercapnia was used to test PHC at multiple exercise stages. The transient hypercapnia involved two breaths of 10% CO repeated five times during each of the following: sitting at rest on the cycle ergometer, cycling at 40% wmax, cycling at 85% Wmax, at rest on the cycle ergometer immediately following the 85% stage, and cycling at 40% Wmax again following the postexercise rest. The PHC was not different across exercise intensities (0.98 ± 0.37 vs. 0.91 ± 0.39 vs. 0.92 ± 0.42 L·min·mmHg for first 40% wmax, 85% wmax and second 40% Wmax, respectively ( = 0.45). There were no differences in PHC between presupra-RCP exercise rest and postsupra-RCP exercise rest (0.52 ± 0.23 vs. 0.53 ± 0.24 L·min·mmHg, = 0.8003). Using a repeated-measures correlation to account for within-participant changes, there was a significant relationship between the end-tidal Pco and PHC for the 85% intensity ( = 0.5, < 0.0001) when end-tidal Pco was dynamic between the trials. We conclude that the physiological changes (e.g., metabolic milieu and temperature) produced with supra-RCP exercise do not further augment PHC, and that the prestimulus end-tidal Pco modulates the PHC. Exercise at intensities above the respiratory compensation point did not further augment peripheral hypercapnic chemosensitivity (PHC). Moreover, the PHC was not different during a preexercise resting state compared with rest immediately after intense exercise. The lack of differences across both comparisons suggests that exercise itself appears to sensitize the PHC.