Regulation of the tidal volume and ventilatory responses to CO2 in normal man and in scoliosis

Abstract

The variation of the ventilatory response to CO2 and its pattern among normals and in the presence of pulmonary disease is wide. In normal subjects the relationship between the slopes of the ventilatory (delta V/delta PCO2), tidal volume (delta VT/delta PCO2) and the frequency (delta f/delta PCO2) responses and body size, metabolic rate, resting ventilation and pattern, lung volumes or mechanical properties of the respiratory system have only been demonstrated in a few studies. In idiopathic scoliosis there is a positive correlation between the delta V/delta PCO2 and delta VT/delta PCO2 and body size, resting ventilation and tidal volume, lung volumes and compliance of the respiratory system. Although there were significant correlations between the delta V/delta PCO2 and the delta VT/delta PCO2 and th delta f/delta PCO2 the correlation between the delta VT/delta PCO2 and delta f/delta PCO2 was not significant. Correlations between the delta f/delta PCO2 and lung volumes, compliance or body size were also not significant. Therefore variation in the frequency response to CO2 contribute to the variation between individuals of the delta V/delta PCO2 and this variation is unrelated to respiratory mechanics or body size. We conclude that in human studies any examination of possible relationships between ventilatory response to CO2 and body size, lung volumes and mechanics should examine separately the tidal volume and frequency response to CO2. We postulate that the tidal volume response is the most appropriate variable to normalize for lung volumes, e.g., vital capacity (delta VT/VC/delta/ PCO2).