RUNNING HEAD: Oxygen flow kinetics in hypoxia CORRESPONDING AUTHOR:

We tested the hypothesis that vagal withdrawal plays a role in the rapid (phase I) cardiopulmonary response to exercise. To this aim, on five males (24.6 + 3.4 years, 82.1 + 13.7 kg, maximal aerobic power 330 + 67 W), we determined beat-by-beat cardiac output (Q &), oxygen delivery (Q & aO2), and breath-by-breath lung oxygen uptake (V&O2), at light exercise (50 and 100 W) in normoxia and acute hypoxia (FIO2 = 0.11), because the latter reduces resting vagal activity. We computed Q & from stroke volume (Qst, by model flow) and heart rate (fH, electrocardiography), Q & aO2 from Q & and arterial O2 concentration. Double exponentials were fitted to the data. In hypoxia compared to normoxia, steady state fH and Q & were higher, Qst and V&O2 unchanged. Q & aO2 was unchanged at rest, lower at exercise. During transients, amplitude of phase I (A1) for V&O2 was unchanged. For fH, Q & and Q & aO2, A1 was lower. Phase I time constant (1) for Q & aO2 and V&O2 was unchanged. The same was the case for Q & at 100 W and fH at 50 W. Qst kinetics was unaffected. In conclusion, the results do not fully support the hypothesis that vagal withdrawal determines phase I