Effects of voluntary constraining of thoracic displacement during hypercapnia.

The study evaluated the interrelationships between the extent of thoracic movements and respiratory chemical drive in shaping the intensity of the sensation of dyspnea. Normal subjects rated their sensations of dyspnea as PCO2 increased during free rebreathing and during rebreathing while ventilation was voluntarily maintained at a constant base-line level. Another trial evaluated the effects on the intensity of dyspnea, of voluntary reduction in the level of ventilation while PCO2 was held constant. During rebreathing, there was a power function relationship between changes in PCO2 and the intensity of dyspnea. At a given PCO2, constraining tidal volume and breathing frequency to the prerebreathing base-line level resulted in an increase in dyspnea. The fractional differences in the intensity of dyspnea between free and constrained rebreathing were independent of PCO2. However, the absolute difference in the intensity of dyspnea between free and constrained rebreathing enlarged with increasing hypercapnia. At PCO2 of 50 Torr, this difference correlated significantly with the increase in both minute ventilation (r = 0.675) and tidal volume (r = 0.757) above the base line during free rebreathing. Similarly, during steady-state hypercapnia at 50 Torr PCO2, the intensity of dyspnea increased progressively as ventilation was voluntarily reduced from the spontaneously adopted free-breathing level. These results indicate that dyspnea increases with the level of respiratory chemical drive but that the intensity of the sensation is further accentuated when ventilation is constrained below that demanded by the level of chemical drive. This may be explained by a loss of inhibitory feedback from lung or chest wall mechanoreceptors acting on brain stem and/or cortical centers.