The so-called syncope of ascent is a cause of a transitory state of confusion, often described as either disorientation or lightheadedness and associated with a sensation of imminent loss of consciousness. It is caused by inadequate exhalation of the expanding lung gases during ascent, with resultant distension of the lungs and an increase in intrathoracic pressure causing an impairment of venous return. It is analogous to cough syncope.
Syncope of ascent most commonly occurs during rapid ascents, when the pressure gradients are magnified, and also when the diver attempts to retain the air in the lungs instead of exhaling it. In the past, free ascent training from 18 to 30 metres was carried out by divers and submariners, and this was a typical situation in which this disorder occurred – it caused considerable problems with differential diagnosis.
Because there is no actual lung disease, it is technically incorrect to describe this as PBT, but it could sometimes be a step in the progression to this disease.