Ventilatory response to hypercapnia by rebreathing in successive trials

The ventilatory response to carbon dioxide at rest was determined repeatedly by the rebreathing method; three successive CO2 response lines, with 30-min intervals, were examined in 12 healthy male subjects. Overall mean slopes obtained at 30, 60 and 90 min were 2.95, 2.47 and 2.28 liters/min . mmHg, respectively, the difference being statistically not significant. However, the slopes of four high responders, who were subjects with higher slopes than that of the mean values obtained here, decreased significantly (p less than 0.05) at 60 min as compared with the 30 min test, but this did not occur in the other subjects. Better reproducibility in obtaining the CO2 response slopes in the high responders was observed in the first rather than the 60 or 90 min trails, suggesting incomplete recovery in the humoral agents released during the CO2 test. These results indicated that the CO2 response curve by rebreathing should be determined carefully, especially in the high responder, taking into consideration its individual variability and and interval in consecutive measurements.