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% O 2 or O 2 -enriched mixture (O 2 divers) to those measured in eight recreational divers who dived occasionally using compressed air/21% O 2 (air divers). The O 2 divers completed a 6-h HBO exposure in which the inspired oxygen pressure (PiO 2 ) varied from 1.15 to 2.7 absolute atmospheres (ATA), PiO 2 being maintained at 1.15 ATA throughout the first 2-h period, whereas the air divers only completed a 2-h HBO exposure with PiO 2 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 O 2 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 O 2 divers, similar HBO-induced M-wave changes were only observed when PiO 2 was greater than 1.50 ATA. We conclude that HBO elicites neuromuscular hyperexcitability, attenuated in elite O 2 divers.