During exercise, healthy individuals are able to maintain arterial oxygenat
ion, whereas highly-trained endurance athletes may exhibit an exercise-indu
ced arterial hypoxaemia (EIAH) that seems to reflect a gas exchange abnorma
lity, The effects of EIAH are currently debated, and different hypotheses h
ave been proposed to explain its pathophysiology. For moderate exercise, it
appears that a relative hypoventilation induced by endurance training is i
nvolved. For high-intensity exercise, ventilation/perfusion ((V)over dot (A
)/(Q)over dot) mismatching and/or diffusion limitation are thought to occur
, The causes of this diffusion limitation are still under debate, with hypo
theses being capillary blood volume changes and interstitial pulmonary oede
ma, Moreover, histamine is released during exercise in individuals exhibiti
ng EIAH, and questions persist as to its relationship with EIAH and its con
tribution to interstitial pulmonary oedema, Further investigations are need
ed to better understand the mechanisms involved and to determine the long t
erm consequences of repetitive hypoxaemia in highly trained endurance athle
tes.