Incident Report

1970-01-01I was ascending from -54 m to the level of the decompression stops. Whilst at shallow depth I suddenly felt I would faint for no reason. Then this feeling went away I completed the decompression properly and I surfaced without incident.At the time I was far less experienced than nowadays. After this somewhat alarming incident I consulted various diving medical texts and a couple of doctors with diving experience. The most likely reason for the problem was that I was breathing too deeply whilst ascending. As the ambient pressure was gradually decreasing on the ascent this would slightly overfill my lungs. The problem is at its most dangerous at shallow depths as here there is the biggest volume change per unit depth. The hyperextension of the lungs due to unnecessary deep ventilations and exacerbated by expanding air whilst ascending probably pressed on my aorta (the main artery leaving the heart to supply all organs including the brain with oxygenated blood). A temporary interruption of oxygenated blood to the brain probably caused this experience. I believe this phenomenon is known assyncope of ascent. The remedy is to breathe normally - and deliberately avoid deep inhalations whilst ascending.