VENTILATORY STABILITY TO TRANSIENT CO2 DISTURBANCES IN HYPEROXIA AND NORMOXIA IN AWAKE HUMANS

Modarreszadeh and Bruce (J. Appl. Physiol. 76: 2765-2775, 1994) propos ed that continuous random disturbances in arterial Pco(2) are more lik ely to elicit ventilatory oscillation patterns that mimic periodic bre athing in normoxia than in hyperoxia. To test this hypothesis experime ntally, in nine awake humans we applied pseudorandom binary inspired C O2 fraction stimulation in normoxia and hyperoxia to derive the closed -loop and open-loop ventilatory responses to a brief CO2 disturbance i n terms of impulse responses and transfer functions. The closed-loop i mpulse 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 sig nificantly shorter 50% response duration (42.7 +/- 13.3 vs. 72.3 +/- 2 7.6 s, P = 0.020) in normoxia than in hyperoxia. Therefore, the ventil atory responses to transient CO2 disturbances are less damped (but gen erally 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). T he gain increased by 108.5, 186.0, and 240.6%, while phase delay decre ased by 26.0, 18.1, and 17.3%, at 0.01, 0.03, and 0.05 Hz, respectivel y. Changes in the same direction were found for the open-loop system. Generally, an oscillatory ventilatory response to a small transient CO 2 disturbance is unlikely during wakefulness. However, changes in para meters that lead to additional increases in chemoreflex loop gain are more likely to initiate oscillations in normoxia than in hyperoxia.