1. To investigate whether changes in Paco2 can be detected independently of the CO2-induced changes in pulmonary ventilation, we tested five normal subjects for the ability to distinguish different levels of end-tidal Pco2 (PETco2) 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 PETco2 = 50 mmHg (6.7 kPa). End-tidal Pco2 was held at 40 mmHg (5.3 kPa) during the first 2 min of each test trial ('control period'); then, for 4 min ('test period'), PETco2 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 PETco2 was raised during the test period to 50 mmHg, the subjects detected the changes. There was one false positive result (3%), when PETco2 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 PETco2 from 40 to 50 mmHg was statistically significant.

4. We conclude that increases in PETco2 can be detected independently of changes in the absolute level of ventilation.

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