05.12.16

Dr Oli: Diving and excess CO2

Q: I've been advised to contact you by my dive instructor about some strange symptoms which occurred whilst I was diving a few days ago. This was off the South Coast in fairly rough conditions, but I was fine prior to the diving. During my first dive, at about 21m, I started to feel odd and then effectively "fainted", for a few minutes.  I had no vision or memory during this time. My instructor was with me and brought me to the surface. During the ascent I regained my senses, so we decided it could have been brought on by nitrogen narcosis.

The same thing happened on a subsequent dive, but I was unable to focus or concentrate and did not feel myself even during the safety stop at the end of the dive (i.e. 5 meters). We decided perhaps it was due to oxygen starvation, as I typically breathe very lightly whilst diving and use less air than the instructor expects.

Since then I have made a conscious effort to breathe more slowly and deeply and have felt fine, at similar depths. I take no medication, and am in good health as far as I am aware.

Do you think this is just down to lack of oxygen, or might there be another underlying medical issue?

A: Actually, rather than it being a lack of oxygen, I think the most likely explanation for this is an excess of another, often overlooked, gas: carbon dioxide (CO2). As well as being very useful in the fizzy drinks world, CO2 is the end product of cellular respiration, but is handily used by photosynthesising organisms to produce oxygen. In the diving world, the focus on nitrogen narcosis often diverts attention from the fact that CO2 is many times more narcotic, and even small increases in blood levels can cause potentially dangerous symptoms of this sort. Initially, modest rises in CO2 levels significantly reduce cognitive performance, eg. simple arithmetic and colour naming, as well as physical skills, eg. manual dexterity and hand-eye co-ordination. So narcosis is not always due to nitrogen. Further increases in CO2 cause dizziness, headaches, nausea and eventually loss of consciousness. Ordinarily, rising CO2 levels stimulate the respiratory centres in the brain, giving rise to the sensation of breathlessness and triggering hyperventilation. Paradoxically, however, when CO2 levels get to the point of affecting consciousness, they act as a respiratory depressant, reducing lung ventilation and therefore causing further CO2 retention. At this point a downward spiral is difficult to avoid. So the trick is obviously to avoid getting to this point in the first place. Ascent, avoidance or reduction of strenuous physical effort, and slow/measured breathing rates should reduce the risk of CO2 buildup.

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