Oxygen kinetics and debt during recovery from expiratory flow-limited exercise in healthy humans

In healthy subjects expiratory flow limitation ( EFL) during exercise can lower O-2 delivery to the working muscles. We hypothesized that if this affects exercise performance it should influence O2 kinetics at the end of exercise when the O2 debt is repaid. We performed an incremental exercise test on six healthy males with a Starling resistor in the expiratory line limiting expiratory flow to similar to 1 l s(-1) to determine maximal EFL exercise workload (W-max). In two more square-wave exercise runs subjects exercised with and without EFL at W-max for 6 min, while measuring arterial O-2 saturation (% SaO(2)), end-tidal pressure of CO2 (PETCO2) and breath-by-breath O-2 consumption (<(V)over dot > O-2) taking into account changes in O-2 stored in the lungs. Over the last minute of EFL exercise, mean PETCO2 (54.7 +/- 9.9 mmHg) was significantly higher (P < 0.05) compared to control (41.4 +/- 3.9 mmHg). At the end of EFL exercise % SaO(2) fell significantly by 4 +/- 3%. When exercise stopped, EFL was removed, and we continued to measure <(V)over dot > O-2. During recovery, there was an immediate step increase in <(V)over dot > O-2, so that repayment of EFL O-2 debt started at a higher <(V)over dot > O-2 than control. Recovery <(V)over dot > O-2 kinetics after EFL exercise was best characterized by a double-exponential function with fundamental and slow time constants of 27 +/- 11 and 1,020 +/- 305 s, compared to control values of 41 +/- 10 and 1,358 +/- 320 s, respectively. EFL O-2 debt was 52 +/- 22% greater than control (2.19 +/- 0.58 vs. 1.49 +/- 0.38 l). We conclude that EFL exercise increases the O-2 debt and leads to hypoxemia in part due to hypercapnia.