Detection of hypercapnia by normal subjects.

1. To investigate whether changes in P aco 2 can be detected independently of the CO 2 -induced changes in pulmonary ventilation, we tested five normal subjects for the ability to distinguish different levels of end-tidal P co 2 ( P ETco 2 ) while holding minute ventilation constant. 2. Helped by a visual feedback system, the subjects maintained a constant ventilation targeted at a level that was higher than that dictated by the chemical drive at P ETco 2 = 50 mmHg (6.7 kPa). End-tidal P co 2 was held at 40 mmHg (5.3 kPa) during the first 2 min of each test trial (9control period9); then, for 4 min (9test period9), P ETco 2 was either elevated to 50 mmHg or kept at 40 mmHg. Twelve runs were performed by each subject. 3. In 24 out of the total 30 trials (80%) in which P ETco 2 was raised during the test period to 50 mmHg, the subjects detected the changes. There was one false positive result (3%), when P ETco 2 kept at 40 mmHg during the test period was reported as different from control. In four out of the five subjects the ability to detect the change in P ETco 2 from 40 to 50 mmHg was statistically significant. 4. We conclude that increases in P ETco 2 can be detected independently of changes in the absolute level of ventilation.