Ability to judge lung volumes at different CO2-drives for ventilation and the possible influence of such a judgment on the ventilatory CO2-responsiveness.

Possible influences of changes in the CO2-respiratory drive on the ability to judge lung volumes and the possible effects of such judgments on the ventilatory CO2-responsiveness were studied in healthy subjects. The psychophysical relationships between objective and subjective magnitudes of lung volumes were determined at normocapnia, at three different levels of hypercapnia (breathing 1-3% CO2 in air) and at hypocapnia (after volitional hyperventilation). The relationships, expressed by a power function with the mean exponent 1.5 (SD +/- 0.3), were analyzed with respect to slope, intercept and correlation coefficient of the regression lines. None of these parameters was affected by variations in end-tidal PCO2 or by the concomitant changes in ventilation. In the 'attentive' pre-judgment breathing (APJB) the ventilatory response to CO2 was lower than during non-attentive breathing in three or four subjects. It is speculated that in these subjects the CO2-responsiveness was depressed by the increased attention during APJB. The question whether volitional hyperventilation gave rise to any specific post-hyperventilatory breathing pattern was analyzed. The results showed that the majority of post-hyperventilatory breathing pattern was apnoeic or hypopnoeic in character.