Effects of exercise and CO2 inhalation on intersubject variability in ventilatory and heart rate responses to progressive hypoxia.

Although the ventilatory and heart rate responses to hypoxia are known to vary widely among subjects, it is not known how exercise or hypercapnia influence the intersubject variability of these responses. If the intersubject variability increases under such conditions, the inherent response of individuals will have more impact on ventilation and heart rate under a variety of hypoxic conditions during exercise or with hypercapnia than at rest or with normocapnia. Seventeen healthy male volunteers underwent tests to measure ventilatory response to isocapnic progressive hypoxia three times respectively: at rest; during CO2 inhalation (end-tidal carbon dioxide tension (PET,CO2) raised by 5 torr from the baseline level); and during mild exercise with a cycle ergometer (12.5 W) in a supine position. The mean (SEM) value of hypoxic ventilatory response (HVR) (delta minute ventilation (V'E)/delta arterial oxygen saturation (Sa,O2) was significantly increased both in the exercise and hypercapnic runs compared with that in the control run (0.45 +/- 0.12, 0.34 +/- 0.08, respectively, vs 0.12 +/- 0.02 L.min-1/% fall), although the respiratory pattern was different under the two loaded conditions. The intersubject variation in HVR was also significantly increased during the two loaded conditions compared with the control, although a significant correlation remained between the control value and that obtained during either loaded condition (r = 0.66 and r = 0.60, respectively). The heart rate (HR) response evaluated by the slope factor (delta HR/delta Sa,O2) was not significantly different either in the mean value or in the intersubject variability among the three experimental conditions. In conclusion, exercise or CO2 inhalation not only increase the slope value of HVR but also amplify the intersubject variability of the response. In contrast, the HR response to hypoxia evaluated as a slope factor does not change with exercise or CO2 inhalation.