Hyperbaric hyperoxia induces a neuromuscular hyperexcitability: assessment of a reduced response in elite oxygen divers.

We compared the changes in compound muscle mass action potential (M-wave) recorded in vastus lateralis in response to hyperbaric hyperoxia (HBO) in nine combat divers who dived daily while breathing 100% O2 or O2-enriched mixture (O2 divers) to those measured in eight recreational divers who dived occasionally using compressed air/21% O2 (air divers). The O2 divers completed a 6-h HBO exposure in which the inspired oxygen pressure (PiO2) varied from 1.15 to 2.7 absolute atmospheres (ATA), PiO2 being maintained at 1.15 ATA throughout the first 2-h period, whereas the air divers only completed a 2-h HBO exposure with PiO2 constant at 1.15 ATA. Before HBO exposure, there were no intergroup differences between baseline M-wave characteristics (amplitude and duration), but the conduction time was significantly shorter in O2 divers compared with air divers. After 90 min of HBO (1.15 ATA) the air divers demonstrated neuromuscular hyperexcitability, as evidenced by an increased M-wave amplitude (13%, P<0.01 versus baseline), shortened M-wave duration (5%, P<0.05 versus baseline), and reduced conduction time (5%, P<0.01 versus baseline). In O2 divers, similar HBO-induced M-wave changes were only observed when PiO2 was greater than 1.50 ATA. We conclude that HBO elicites neuromuscular hyperexcitability, attenuated in elite O2 divers.