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

Abstract

Peripheral hypercapnic chemosensitivity (PHC) is assessed as the change in ventilation in response to a rapid change in carbon dioxide pressures (PCO2). The increase in chemoresponse from rest to sub-respiratory 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 (n=10 females) completed a maximal exercise test on one 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% CO2 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 post-exercise 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-1 mmHg-1 for 1st 40% Wmax, 85% Wmax, and 2nd 40% Wmax, respectively (p=0.45). There were no differences in PHC between pre-supra-RCP exercise rest and post-supra-RCP exercise rest (0.52±0.23 vs. 0.53±0.24 L min-1 mmHg-1, p=0.8003). Using a repeated measures correlation to account for within participants changes, there was a significant relationship between the end-tidal PCO2 and PHC for the 85% intensity (r=0.5, p<0.0001) when end-tidal PCO2 was dynamic between the trials. We conclude that the physiological changes (e.g. metabolic milieu, temperature, etc) produced with supra-RCP exercise do not further augment PHC, and that the pre-stimulus end-tidal PCO2 modulates the PHC.