Query on 'Off Effect'

Brad_Horn

G'Day,

From 'Oxygen and the diver: Kenneth Donald, pg 67' there is mentioned an 'Off Effect'.

Namely "The 'Off Effect' on returning to air breathing after exposure to toxic tensions of carbon dioxide is well known. there is a sudden exacerbation of symptoms often associated with headache, nausea and vomiting." "In one or two cases it appeared that convulsions were precipitated by reverting to air breathing"

Whilst, this appears to support either a good FFM or a BOV (& gag strap) that you can vomit into (& clear) and/or at least retain the ability to breathe from whilst convulsing post getting off the loop after/during a CO2 hit.

It doesn't appear to delve further into the subject with regard to air breathing, apart from that the effect is well known, does anyone know any further references/reading on the subject of the 'Off Effect'?

Have any studies been done, comparing going onto Trimix/Heliox as opposed to Air after a Co2 hit?

Regards
Brad

dave t

As I understand it the "off effect" is a convulsion induced by a rapid drop in po2 after breathing very elevated po2's for a time.
Having had a "practical" demostration of this effect and a subsequent conversation with the hyperbaric doctor I am led to belive that if a diver has been breathing very high po2 without convulsion switching to a gas with a much lower po2 can trigger a convulsion. I dont think which ever inert gas is also present makes much difference.

In my case it was suggested that had I taken the diver to the surface without him bailing out (ie stayed on the loop) he MAY not have convulsed as the po2 would have reduced more slowly!

HTH

Dave

Brad_Horn

Hi Dave,

Thanks for your realtime experience of the matter.

To clarify further quoting K.Donald "An 'Off Effect' is sometimes encountered after cessation of breathing oxygen at toxic tensions. it does not occur so frequently as that after breathing carbon dioxide"

Effectively having a BOV makes it easier to swap to OC if you get a Co2 hit = 'Off Effect', but conversely if you don't have a BOV and are forced to stay on the loop for <slightly {time taken to manually close loop, remove, find & purge 2nd stage & breather}> longer (flushing the loop et al) you may avoid the 'Off Effect'.
Thus take a little longer to clear up from the Co2 but avoid convulsing due to the 'Off Effect".

Therefore if one is diving and you have Co2 breakthrough is the 'Off Effect' instantaneous or is there a delay when you bail to OC for a sanity breath!!!

Regards
Brad

dave t

My experience was the diver took two or three breaths of air (at 44m) before convulsing, the reg fell out but I managed to put it back as the diver regained some composure quite quickly, he took three or four more breaths before convulsing again. Thats when we went for the surface...fast!!

I would questimate the diver had been breathing o2 at about 4bar p02 for about ten miniutes and the first signs I saw in him was confusion but thats probably cos he knew some thing was wrong but all three cells were at 1.3 bar or just under.

I understood that co2 convulsions were much rarer than o2 convulsions when it came to the off effect but I am no expert!!

best

Dave

apitkin

I have seen it a couple of times in chamber subjects. In both cases the subject was exhibiting symptoms suggestive of oxygen toxicity, which is why the oxygen was removed. Soon afterwards the convulsion started.

As Samuel Johnson once said: "It is incident to physicians, above all other men, to mistake subsequence for consequence."

My guess is that these people were going to convulse anyway. They merely had some concerning symptoms that resulted in the oxygen being removed shortly before the convulsion, which was going to happen anyway, started.

I am not aware of any research on the subject. I can't think of a mechanism by which a sudden reduction in PO2 might produce a seizure, but that doesn't mean much.

Andy

Jack

You may be focussing on the wrong gas. The `Off Effect` can occur when switching from mix to air as was the norm a few years back where ccr users would do an air dil flush @40m and use air dil for rest of deco. This can cause an `off effect` resulting in IEDCS. Its more commonly seen with victims of smoke inhalation when put on O2.

__________________
JACK
www.dublinbaydiving.com

dave t

Isnt that isobaric counter difusion?

the off effect is documented in Kenneth donalds book "oxygen and the diver"

apitkin

IEDCS = inner ear DCS due to isobaric inert gas counterdiffusion. Not the same as the off effect, as Dave said.

What? Explain please.

Andy

dteubner

You mean you've never seen inner ear DCI in peope with CO poisoning???

__________________
Dave T

Hanlon's Razor - Never attribute to malice that which can be adequately explained by stupidity.

apitkin

Oh. I don't know. Now I'm really confused. What are we talking about?