Analyses of the ventilatory response to carbon dioxide. Implications for clinical research

Baseline control estimations of the ventilatory response to carbon dioxide by the rebreathing method were attempted in 92 patients. When analysed retrospectively, only 53 were both technically and statistically satisfactory, although even these gave rise to some doubts. As the breath-by-breath correlation between ventilation (VI) and end-tidal carbon dioxide (PE'CO2) decreased there was a tendency for the gradients to become suspiciously low and for estimates in an individual gradient to become sensitive to the method of analysis. Even so, when responses were grouped according to correlation, the method of analysis did not alter the mean gradient within a group. Single estimates of the carbon dioxide response at each stage of a study may not be a very powerful technique in clinical research; multiple estimates, if they can be arranged, would be more satisfactory.