Surface oxygen and the risk of O2 toxicity

[richdrogpa]

Greetings all,
Over the past two days, we've treated two separate divers with decompression illness...

Diver number 1: ...The dives: 134'/34 minutes on 30.8% nitrox with a 5 minute stop at 15 feet using 40% nitrox. He had a 2 1/2 hour surface interval followed by a second dive, 124'/37 minutes, also on 30.8% nitrox...
Diver number 2: ...The dives: 106'/85 minutes using compressed air, with decompression on 100% O2 using his computer. He had a 90 minute surface interval, followed by a second dive to 106', this time with a bottom time of 100 minutes, again with decompression on 100% O2 per his computer....

Discussion: both divers were treated here at Duke within one day of one another. Both had been breathing hyperoxic decompression gases, both had decompression sickness, and both feared oxygen toxicity from surface O2.

Not sure about this but to my beginner - recreational diver's eyes it seems (based on the limited info) that they were pushing, if not exceeding, their NDLs. Especially the 2nd diver, on air, with limited SI, to basically the same depth (wonder if it was close to a square profile, probably not). I guess computers give you lots of leeway in setting dive parameters...

 

[vladimir]

Not sure about this but to my beginner - recreational diver's eyes it seems (based on the limited info) that they were pushing, if not exceeding, their NDLs. Especially the 2nd diver, on air, with limited SI, to basically the same depth (wonder if it was close to a square profile, probably not). I guess computers give you lots of leeway in setting dive parameters...

Since both divers were carrying higher oxygen mixes to accelerate decompression, we can safely assume these dives were planned (however poorly) as decompression dives, rather than NDL dives.

 

[DeepSeaExplorer]

I think a large part of the problem is that the industry as a whole has been overselling the dangers of oxygen so that a lot of divers see various guidelines, be it 1.2, 1.4, 1.6, 1.8 (resting), 2.0 (emergency), or whatever, as cliff edges that will instantly kill you whereas the industry now seems to be soft-pedaling DCS as a serious hazard, with the idea that, "what's the worst thing that can happen? A little O2, a run in the chamber and you'll be right as rain!"

You right, but I think the ultimate source is a little closer than the dive industry in general. The PO2 hysteria and misinformation is widespread on the Internet. It would be interesting to find out where these individuals got the idea that symptoms of DCS were instead symtoms of O2 Toxicity. I'd bet we wouldn't have to look very far...

 

[SC_Hoaty]

It would be interesting to find out where these individuals got the idea that symptoms of DCS were instead symtoms of O2 Toxicity.

It seems that if you take the O2 Toxicity hype on the net (and other places) and then look at their symptoms, that the divers simply jumped to the wrong diagnosis. (Colors added to illustrate this.)

Diver number 1: a middle-aged male with 30 years' diving experience who made two spearfishing dives on nitrox. The dives: 134'/34 minutes on 30.8% nitrox with a 5 minute stop at 15 feet using 40% nitrox. He had a 2 1/2 hour surface interval followed by a second dive, 124'/37 minutes, also on 30.8% nitrox. On his second dive, he reported that his ascent alarms were flashing while he was surfacing with a fish. He put the fish in the boat and returned to 15', where he completed a 5 minute stop, again on 40% nitrox. He reported swimming against the current and exerting himself more than normal but did not report any unusual symptoms during the dive. Following the dive, he experienced symptoms of spinal cord decompression sickness (tingling and weakness in both legs). He refused surface oxygen because he thought his symptoms were due to O2 toxicity. Of note, his calculated pO2 for his bottom mix at the deeper depth, 134 feet, was 1.56 ATA. This exceeds the normal recommended maximum pO2 of 1.2 ATA.
Diver number 2: a slightly younger man diving for shark's teeth. The dives: 106'/85 minutes using compressed air, with decompression on 100% O2 using his computer. He had a 90 minute surface interval, followed by a second dive to 106', this time with a bottom time of 100 minutes, again with decompression on 100% O2 per his computer. He reported no problems on his dive but surfaced with symptoms of inner ear decompression sickness (nausea, vertigo, tinnitus and hearing loss). He also refused surface O2 for fear of oxygen toxicity.

Discussion: both divers were treated here at Duke within one day of one another. Both had been breathing hyperoxic decompression gases, both had decompression sickness, and both feared oxygen toxicity from surface O2.

Symptoms of central nervous system O2 toxicity can be remembered using the acronym "VENTTID/C".
V: Vision. Blurred or tunnel vision, or other visual disturbances.
E: Ears. Ringing or roaring in the ears.
N: Nausea.
T: Twitching of the muscles, usually the facial muscles.
T: Tingling in the extremities, typically the fingers and toes.
I: Irritability. Any personality change.
D: Dizziness.
C: Convulsions.

Even with the incorrect diagnosis, it was their mistaken belief that "surface O2 would be harmful" that is the real error I see.

 

[knowone]

It would be interesting to find out where these individuals got the idea that symptoms of DCS were instead symtoms of O2 Toxicity. I'd bet we wouldn't have to look very far...

You would only need to use an otoscope.