A model of the chemoreflex control of breathing in humans: model parameters measurement

We reviewed the ventilatory responses obtained from rebreathing experiments on a population of 22 subjects. Our aim was to derive parameter estimates for an 'average subject' so as to model the respiratory chemoreflex control system. The rebreathing technique used was modified to include a prior hyp erventilation, so that rebreathing started at a hypocapnic P-CO2 and ended at a hypercapnic P-CO2. In addition, oxygen was added to the rebreathing ba g in a controlled manner to maintain iso-oxia during rebreathing, which all owed determination of the response at several iso-oxic P-O2 levels. The bre ath-by-breath responses were analysed in terms of tidal volume, breathing f requency and ventilation. As P-CO2 rose, ventilation was first steady at a basal value, then increased as P-CO2 exceeded a breakpoint. We interpreted this first breakpoint as the threshold of the combined central and peripher al chemoreflex responses. Above, ventilation increased linearly with P-CO2. With tidal volume usually contributing more than frequency to the increase . When breathing was driven strongly, such as in hypoxia, a second breakpoi nt P-CO2 was often observed. Beyond the second breakpoint, ventilation cont inued to increase linearly with P-CO2 at a different slope. with frequency usually contributing more than tidal volume to the increase. We defined the parameters of the variation of tidal volume, frequency and ventilation wit h P-O2 and P-CO2 for an average subject based on a three-segment linear fit of the individual responses. These were incorporated into a model of the r espiratory chemoreflex control system based on the general scheme of the 'O xford' model. However, instead of considering ventilatory responses alone, the model also incorporates tidal volume and frequency responses. (C) 2090 Elsevier Science B.V. All rights reserved.