VENTILATORY RESPONSES TO REPEATED SHORT HYPERCAPNIC CHALLENGES

In early phases of respiratory disease, patients are more likely to ex perience intermittent hypercapnia than a continuous increase in PCO2. The effect of intermittent arterial PCO2 elevation on subsequent breat hing patterns is unclear. To examine this issue, a series of six venti latory challenges (CH1-CH6), consisting of 2 min of breathing 5% CO2 i n O-2, followed by 5 min in room air (RA) were performed in 10 naive h ealthy subjects (age 12-39 yr). Minute ventilation (VE) increased from 11.9 +/- 1.0 (SE) l/min in RA to 27.6 +/- 3.0 l/min in 5% CO2 (P < 0. 0005) in each of the six hypercapnic challenges. Respiratory rate incr eased from 21.3 +/- 2.6 breaths/min on RA to 29.6 +/- 3.9 breaths/min during CH1 (P < 0.05). However, respiratory rate consistently decrease d with successive CO2 challenges (CH6: 21.5 +/- 2.6 breaths/min; P < 0 .02). Thus, maintenance of VE was achieved by gradual increases in tid al volume with each of the first four consecutive CO2 challenges (CH1: 1.05 +/- 0.09 liters; CH4: 1.44 +/- 0.13 liters; P < 0.002). Similarl y, the ratio of tidal volume to inspiratory time increased from CH1 (1 .16 +/- 0.16 l/s) to CH6 (1.57 +/- 0.21 l/s; P < 0.001). These changes in ventilatory strategy were not observed when RA recovery periods we re extended to 15 mill in five subjects. We conclude that during repea ted short hypercapnic challenges similar levels of VE are achieved. Ho wever, increased mean inspiratory flows are generated to maintain VE. We speculate that intermittent hypercapnia either modifies central con troller gain or induces a long-term modulatory effect to account for t he progressive changes in ventilatory components.