Assessment of peripheral chemoreflex contributions to exercise hyperpnea in humans

The inaccessibility of the peripheral chemoreceptors (carotid bodies) precludes their direct investigation in exercising humans. As a result, a range of indirect techniques has been developed to assess the carotid body contribution to ventilatory control in exercise. The considerable controversy surrounding their involvement in this control process is a reflection, in part, of the limitations associated with current estimators of their reflex responsiveness. Most estimators of carotid chemosensitivity rely on the premise that, as the carotid bodies appear to be the sole mediators of the ventilatory response to hypoxia in humans, the entire ventilatory response to an hypoxic challenge necessarily should be ascribed to carotid chemoreceptor activation. Other estimators rely on the assumption, in humans, inhalation of 100% oxygen effectively "silences" the carotid chemoreflex. This article will review these estimators, and present a critical evaluation of their relative merits and limitations.