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J Appl Physiol 83: 466-476, 1997;
8750-7587/97 $5.00
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Journal of Applied Physiology
Vol. 83, No. 2, pp. 466-476, August 1997
CONTROL OF BREATHING, CIRCULATION, AND TEMPERATURE

Ventilatory stability to transient CO2 disturbances in hyperoxia and normoxia in awake humans

Jie Lai and Eugene N. Bruce

Center for Biomedical Engineering, University of Kentucky, Lexington, Kentucky 40506-0070

Received 30 October 1995; accepted in final form 11 April 1997.

Lai, Jie, and Eugene N. Bruce. Ventilatory stability to transient CO2 disturbances in hyperoxia and normoxia in awake humans. J. Appl. Physiol. 83(2): 466-476, 1997.---Modarreszadeh and Bruce (J. Appl. Physiol. 76: 2765-2775, 1994) proposed that continuous random disturbances in arterial PCO2 are more likely to elicit ventilatory oscillation patterns that mimic periodic breathing in normoxia than in hyperoxia. To test this hypothesis experimentally, in nine awake humans we applied pseudorandom binary inspired CO2 fraction stimulation in normoxia and hyperoxia to derive the closed-loop and open-loop ventilatory responses to a brief CO2 disturbance in terms of impulse responses and transfer functions. The closed-loop impulse response has a significantly higher peak value [0.143 ± 0.071 vs. 0.079 ± 0.034 (SD) l · min-1 · 0.01 l CO2-1, P = 0.014] and a significantly shorter 50% response duration (42.7 ± 13.3 vs. 72.3 ± 27.6 s, P = 0.020) in normoxia than in hyperoxia. Therefore, the ventilatory responses to transient CO2 disturbances are less damped (but generally not oscillatory) in normoxia than in hyperoxia. For the closed-loop transfer function, the gain in normoxia increased significantly (P < 0.0005), while phase delay decreased significantly (P < 0.0005). The gain increased by 108.5, 186.0, and 240.6%, while phase delay decreased by 26.0, 18.1, and 17.3%, at 0.01, 0.03, and 0.05 Hz, respectively. Changes in the same direction were found for the open-loop system. Generally, an oscillatory ventilatory response to a small transient CO2 disturbance is unlikely during wakefulness. However, changes in parameters that lead to additional increases in chemoreflex loop gain are more likely to initiate oscillations in normoxia than in hyperoxia.

periodic breathing; central chemoreflex; peripheral chemoreflex; closed-loop response; open-loop response; impulse response; transfer function; pseudorandom binary sequence

0161-7567/97 $5.00 Copyright © 1997 the American Physiological Society




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