Within-breath PCO2 levels in the airways and at the pulmonary stretch receptor sites.

The discharge frequency of pulmonary stretch receptors (PSRs) shows an inverse responsiveness to the CO2 partial pressure (PCO2), which is limited to an extremely hypocapnic range. During inspiration extremely hypocapnic PCO2 levels are obtained in a large part of the respiratory tract due to the diffusion limited gas mixing. The question remains whether PSRs in combination with these low levels of PCO2 are involved in the regulation of breathing. As a necessary first step to be able to answer this question, this paper is devoted to the calculation of the within-breath PCO2 transients in the respiratory tract and the corresponding PCO2 oscillations in the superficial airway tissue. For PSRs located in the smooth muscles of large bronchi, the calculations predict a time delay of a few seconds to adapt their discharge frequency to a change in PCO2 in the airway lumen. The result is in good agreement with the observed time delay reported in the literature. For the PSRs located in the acini the calculated time constant of their discharge response to PCO2 variations in the lumen is much smaller than 250 ms. This implies a within-breath response to the oscillating luminal PCO2. Further, the calculations show that a CO2 diffusion front is established within the acini during early inspiration. This diffusion front penetrates further and further into the acini with increasing work load due to the concomitant increase in inspiratory flow. As a consequence, the discharge frequency vs. volume response curve of PSRs, especially those located in distal airways, may be modified by a flow-induced PCO2-related contribution.(ABSTRACT TRUNCATED AT 250 WORDS)