DYNAMIC VENTILATORY RESPONSES TO CO2 IN THE AWAKE LAMB - ROLE OF THE CAROTID CHEMORECEPTORS

In awake lambs we investigated the role of the peripheral chemoreceptors in producing dynamic ventilatory (VE) responses to CO2. The immediate VE response, within 15 s, to transient CO2 inhalation was studied in two groups: 1) five lambs before carotid denervation and 2) the same lambs after carotid denervation. The time course of VE responses during the first 60 s after a step change to 8% inspired CO2 was also studied in lambs after carotid denervation and in a group of six carotid body-intact lambs 10-11 days of age. Acute CO2 responses were asessed using step changes to various concentrations of CO2 + air and CO2 + O2, while VE was recorded breath by breath. Intact lambs exhibited a brisk VE response to step changes in CO2, beginning after 3-5 s. Hyperoxia altered but did not suppress the dynamic VE CO2 response when the carotid chemoreceptors were intact. Carotid denervation markedly reduced the VE response during the first 25 s after a CO2 step change, revealing the time delay required for the central chemoreceptors to produce an effective VE response. The residual VE response remaining after CD was thought to be mediated by the remaining aortic body chemoreceptors and was eliminated by adding O2 to the CO2 challenges. However, after carotid denervation, even with CO2 + hyperoxia, the onset of a small tidal volume response was apparent by 10-12 s. We conclude for the awake newborn lamb that 1) dynamic VE responses to sudden changes in CO2 are mediated predominantly by the peripheral chemoreceptors, 2) the carotid chemoreceptors retain their CO2 responsiveness even under hyperoxia, 3) the central chemoreceptor VE response to CO2 appears to begin approximately 25 s after a CO2 step change, and 4) 10 s after sudden onset of CO2 breathing a tidal volume response begins that is not mediated by peripheral chemoreceptors. These results are consistent with the hypothesis that rapid VE responses to transient disturbances such as apnea, sigh, and movement are mediated by the peripheral chemoreceptors.