The effects of carbon monoxide on respiratory chemoreflexes in humans

As protection against low-oxygen and high-carbon-dioxide environments, the respiratory chemoreceptors reflexly increase breathing. Since CO is also frequently present in such environments, it is important to know whether CO affects the respiratory chemoreflexes responsiveness. Although the peripheral chemoreceptors fail to detect hypoxia produced by CO poisoning, whether CO affects the respiratory chemoreflex responsiveness to carbon dioxide is unknown. The responsiveness of 10 healthy male volunteers were assessed before and after inhalation of approximately 1200 ppm CO in air using two iso-oxic rebreathing tests; hypoxic, to emphasize the peripheral chemoreflex, and hyperoxic, to emphasize the central chemoreflex. Although mean (SEM) COHb values of 10.2 (0.2)% were achieved, no statistically significant effects of CO were observed. The average differences between pre- and post-CO values for ventilation response threshold and sensitivity were -0.5 (0.9) mmHg and 0.8 (0.3) L/min/mmHg, respectively, for hyperoxia, and 0.7 (1.1) mmHg and 1.2 (0.8) L/min/mmHg, respectively, for hypoxia. The 95% confidence intervals for the effect of CO were small. We conclude that environments with low levels of CO do not have a clinically significant effect acutely on either the central or the peripheral chemoreflex responsiveness to carbon dioxide.