VENTILATORY VARIABILITY INDUCED BY SPONTANEOUS VARIATIONS OF PA-CO2 IN HUMANS

We tested the hypothesis that breath-to-breath variations in arterial CO2 partial pressure (Pa-CO2) during spontaneous breathing of awake hu mans cause a significant portion of spontaneous ventilatory variabilit y (including periodic oscillations). This hypothesis was tested in two ways. First, using a recently developed adaptive Pa-CO2 buffering tec hnique we reduced the spontaneous variability in Pa-CO2 of six awake n ormal young human subjects during hyperoxia and observed a correspondi ng decrease in their breath-to-breath ventilatory variations. Second, we predicted the ventilatory responses to CO2 disturbances by using a model of chemical control of ventilation, both examining the hyperoxic condition (similar to experimental studies) and predicting the respon ses to CO2 variations of a normal subject breathing room air. In all e xperimental and theoretical studies, we found that small random distur bances to Pa-CO2 have significant effects on ventilation, including th e potential for such Pa-CO2 disturbances to elicit oscillatory fluctua tions in ventilation even though the ventilatory chemical control syst em was stable (i.e., a brief disturbance to Pa-CO2 did not elicit sust ained ventilatory oscillations). On the basis of these results we prop ose that the stability of chemoreflex ventilatory control loops depend s on both ''loop gain'' factors and the characteristics of random dist urbances to Pa-CO2.