2 Modeling the ventilatory response to carbon dioxide in humans after bilateral and unilateral carotid body resection (CBR)

IT IS AXIOMATIC that the respiratory chemoreceptors sense and respond to changes in the composition of their immediate microenvironment.78 In humans, the ventilatory response to a step change in end-tidal CO2 yields a fast (τ ∼10 s) and a slow component τ ∼120 s).12,40 Two sets of chemoreceptors are thought to elicit these two components: the peripheral chemoreceptors, causing the fast component and located in the carotid bodies at the bifurcation of the common carotid artery, and the central chemoreceptors, causing the slow component and located in the ventral medulla.12,40,55 Validation of the (carotid body)-origin of the fast component in humans is a difficult task and has not been accomplished satisfactorily as yet. Studies in animals,55 and patients who have had bilateral carotid body resection (CBR) for the relief of asthmatic symptoms,12,91 or bilateral carotid endarterectomy for transient cerebral ischemia,208 suggest that the fast component of the ventilatory response to CO2 arises from carotid body activity. However, it is questionable whether animal studies apply directly to humans, and in case of patients with underlying disease of the vessels and lungs, it is also possible that the effect on the V̇i-CO2 response was related to any underlying process. In this study, we sought to examine the ventilatory response to CO2 of adult human subjects who had undergone bilateral and unilateral carotid body resection for carotid body tumors. Testing in patients with carotid body tumors prior to resection had revealed that the carotid body function had not been altered by the tumor formation. Furthermore, all of the tested subjects were otherwise healthy with normal lung and cardiovascular function.