Repeated Ischemic Preconditioning Effects on Physiological Responses to Hypoxic Exercise

INTRODUCTION: Repeated ischemic preconditioning (IPC) can improve muscle and pulmonary oxygen on-kinetics, blood flow, and exercise efficiency, but these effects have not been investigated in severe hypoxia. The aim of the current study was to evaluate the effects of 7 d of IPC on resting and exercising muscle and cardio-pulmonary responses to severe hypoxia. METHODS: A total of 14 subjects received either: 1) 7 d of repeated lower-limb occlusion (4 x 5 min, 217 +/- 30 mmHg) at limb occlusive pressure (IPC) or SHAM (4 x 5 min, 20 mmHg). Subjects were tested for resting limb blood flow, relative . microvascular deoxyhemoglobin concentration ([HHB]), and pulmonary oxygen ((V)over doto(2p)) responses to steady state and incremental exercise to exhaustion in hypoxia (fractional inspired O-2 = 0.103), which was followed by 7 d of IPC or SHAM and retesting 72 h post-intervention. RESULTS: There were no effects of IPC on maximal oxygen consumption, time to exhaustion during the incremental test, or minute ventilation and arterial oxygen saturation. However, the IPC group had higher delta efficiency based on pooled results and lower steady state Delta[HHB] (IPC similar to 24% vs. SHAM similar to 6% pre to post), as well as slowing the [HHB] time constant . (IPC similar to 26% vs. SHAM similar to 3% pre to post) and reducing the overshoot in [HHB]: (V)over dotO(2) ratio during exercise onset. CONCLUSIONS: Collectively, these results demonstrate that muscle O-2 efficiency and microvascular O-2 distribution can be improved by repeated IPC, but there are no effects on maximal exercise capacity in severe hypoxia.