A comparison between carbon dioxide inhalation and increased dead space ventilation in chickens

We compared the ventilatory response to elevated PICO2 with the response to elevation of dead space volume (delta VD) in anesthetized, hyperoxic chickens. In the first experimental series (I), PICO2 was varied between 0 and 26 Torr. In a second series (II), delta VD was raised between 0 and 17 ml by placing lengths of tubing between the bird and the source of inspired gas. In series I, ventilation increased and PaCO2 remained constant at levels of PICO2 below 20 Torr. In contrast, PaCO2 increased with low levels of delta VD in Series II. When changes in PaCO2 and ventilation were expressed as a function of the change in CO2 load reaching the lungs (delta L), the change in ventilation was greater, and that in PaCO2 less in Series I than in Series II at all levels of delta L below 25 ml (STPD) . min-1. The differences in ventilatory response to PICO2 and delta VD may qualitatively be explained by the distinct time patterns of CO2 concentration in the lungs which result in different discharge frequencies of CO2-sensitive intrapulmonary chemoreceptors and, possibly, by effects on ventilation resulting from differences in the timing of receptor discharge. Thus, these data provide additional evidence that avian intrapulmonary chemoreceptors may play a significant role in the chemical control of ventilation and regulation of PaCO2.