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John Gibbons said: > I think that sometimes people forget that CO2 has a significant effect at > pressure on the CNS and can cause sudden blackout in poorly ventilated > divers working at depth. Some of the symptoms felt at low ppO2 (less than > 1.6) might be attributed to CO2 and not O2 to which Jason replied: > I agree entirely. At the time I assumed that they were CO2 related. > Trouble is I don't have any evidence for it being CO2 acting alone, > CO2 acting with high O2, or just high O2. Tunnel vision fits right > in with the VENTID thing but I may well be something of a CO2 retainer. > I am able to hold my breath till I pass out (only ever tried this on > the surface ;) ) and I have found that I can double my breath holding > time by breathing pure O2 first. (7 minutes, no hyperventilation) > I have had considerable personal experience of tunnel vision underwater (no, I don't need a larger mask :-)). After swimming flat out for a couple of minutes during underwater orienteering competitions, I sometimes get tunnel vision. The effect is completely controllable - reduce swimming effort if the tunnel starts to close out. There is no associated loss of mental acuity (sure, with me its hard to tell...) and no sense of impending blackout. At least one of my orienteering buddies has exactly the same effect - we both have the sense that we can "close the tunnel" until we can just see the compass, and then regulate the tunnel diameter by the amount of swimming effort. Significantly, this is at very shallow depths - less than five metres, which suggests that PO2 is not playing a big role. Regards Jon Dr. J. Tapson email: jtapson@no*.ct*.ac*.za* Lecturer ph: (27) 21 460 3042 School of Electrical Engineering fax: (27) 21 451 698 Cape Technikon Box 652 Cape Town 8000
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