To determine whether the intensity of dyspnea at a given level of respiratory motor output depends on the nature of the stimulus to ventilation, we compared the sensation of difficulty in breathing during progressive hypercapnia (HC) induced by rebreathing, during incremental exercise (E) on a cycle ergometer, and during isocapnic voluntary hyperventilation (IVH) in 16 normal subjects. The sensation of difficulty in breathing was rated at 30-s intervals by use of a visual analog scale. There were no differences in the level of ventilation or the base-line intensity of dyspnea before any of the interventions. The intensity of dyspnea grew linearly with increases in ventilation during HC [r = 0.98 +/- 0.02 (SD)], E (0.95 +/- 0.03), and IVH (0.95 +/- 0.06). The change in intensity of dyspnea produced by a given change in ventilation was significantly greater during HC [0.27 +/- 0.04 (SE)] than during E (0.12 +/- 0.02, P less than 0.01) and during HC (0.30 +/- 0.04) than during IVH (0.16 +/- 0.03, P less than 0.01). The difference in intensity of dyspnea between HC and E or HC and IVH increased as the difference in end-tidal PCO2 widened, even though the time course of the increase in ventilation was similar. No significant differences were measured in the intensity of dyspnea that occurred with changes in ventilation between E and IVH. These results indicate that under nearisocapnic conditions the sensation of dyspnea produced by a given level of ventilation seems not to depend on the method used to produce that level of ventilation.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of ventilatory-control abnormalities in predisposing to familial sleep-disordered breathing (SDB) was assessed in 31 subjects 28 +/- 10 yr of age (mean +/- SD). Subjects with (n = 10) and without SDB (n = 12) were recruited from 13 families having two or more members with SDB. Nine age- and gender-matched controls were recruited from families having no member with SDB. Respiratory responses to eucapnic hypoxia, and ventilatory and occlusion pressure responses to hyperoxic hypercapnia with and without added resistive loads (6.5 cm H2O/L/s), were assessed through rebreathing. Age, FEV1, and FVC did not differ among the groups. Hypoxic responses (delta VE/delta SaO2) were significantly lower among the first-degree relatives of SDB families than among controls (-0.76 +/- 0.47 L/min/% SaO2, and -1.32 +/- 0.92 L/min/% SaO2, respectively, p < 0.05). Respiratory responses to hypercapnia during unloaded conditions were similar among the groups. With resistive loading, inspiratory impedance, as measured through the relationship of mouth occlusion pressure (P100) to inspiratory flow (VT/TI), increased with increasing hypercapnia to a greater extent in members of SDB families than in controls (0.169 +/- 0.054 cm H2O/L/min versus 0.122 +/- 0.051, respectively, p < 0.05). These data suggest that familial SDB may be based partly on a familial abnormality in ventilatory control associated with blunting of the hypoxic ventilatory response. The greater increase in impedance during inspiratory loading in members of affected families also suggests a propensity for dynamic airway narrowing.

The present study examined the effects of age on the ability to quantitate changes in inspiratory resistive loads using signals related to the size of the load, per se. Magnitude scaling of inspiratory resistive loads was performed in different trials during breathing at small, large, and varied size tidal volumes. Subjects were specifically instructed to scale the magnitude of the airflow resistance. In both young and older adults, the perceived magnitude of a given resistance was the same in the small-, large-, and varied-sized breath trials despite substantial differences in inspiratory duration and peak inspiratory airway pressure. The change in sensation for a given change in resistance, however, was less in the older than in the younger adults. These results indicate that airflow resistance can be scaled independently of the effort used in breathing. The perception of airflow resistance is blunted in elderly adults probably as a result of an impairment in the central nervous system processing of separate signals of pressure and flow.