Ribcage contribution to CO2 response during rebreathing and steady state methods

Disagreements exist between previous studies of the contribution of the rib cage (RC) and abdomen-diaphragm (AD) components to CO2-stimulated ventilation. These studies used dissimilar techniques of CO2 stimulation and varying methods of data processing and presentation, thus precluding direct comparisons. We have therefore studied two methods of CO2 stimulation in 12 subjects, using a Read's rebreathing method and a modified steady-state technique. Respiratory inductive plethysmography was used to assess the RC and AD contributions to ventilation. Mean slopes for the ventilatory response to CO2 were the same for both methods (mean 2.56 L.min-1.mmHg-1), and the intercepts were significantly different (43.7 mmHg for rebreathing and 38.0 for modified steady state: P less than 0.001). There was a small, but significant, increase in the percentage RC contribution to ventilation during hypercapnia of 0.97%/mmHg PCO2 for rebreathing and 0.62 for steady state (P less than 0.01 and P less than 0.05, respectively), and these values were not significantly different from each other. Using our data in comparison with other studies, we have been able to show that differences in processing and presentation of data have given rise to wide variations in conclusions.