Are multiple dives more risky for DCS than just 1 dive?

[boulderjohn]

John, that is a very interesting finding. If it is true, it would suggest that the primary cause of DCS in rec dives is not the dive profiles per se (or related things like gas choice or equipment failure), but basically rustiness. Spacing out, forgetting how to use your equipment correctly, struggling to control your ascents with new or unfamiliar gear, struggling with poor weighting, etc. Do you interpret it along these lines as well?

In any case, if you have a link to that DAN presentation or it underlaying research, I would be eager to see it.

I am not an expert, and I wrote to DAN. I am awaiting a response.

There is another theory that could account for it. Years ago, it was commonly believed that the act of diving acclimated your body to the process of decompression. An analogy would be the occasional diver who has trouble equalizing on the first day of a dive trip but has no trouble at all by day 3-4. I remember a SB thread many years ago on that topic.

 

[inquis]

The DCS risk may be higher for multiple shallow dives compared to a single deep dive, but tons of people do both safely. On an absolute scale, the probability is extremely low for either. Stay within NDL, control your ascent rates, and enjoy the dive(s).

 

[Kendall Raine]

I am not an expert, and I wrote to DAN. I am awaiting a response.

There is another theory that could account for it. Years ago, it was commonly believed that the act of diving acclimated your body to the process of decompression. An analogy would be the occasional diver who has trouble equalizing on the first day of a dive trip but has no trouble at all by day 3-4. I remember a SB thread many years ago on system activation across multiple

As long as you’re in touch with DAN it would be great to learn their latest thinking on complement system activation across multiple dives/days/provocative exposures.

In any event please let us know what they say.

 

[Johnoly]

, supporting the idea human physiology can build some resistance against getting bent.

Or being a Monday,,,,,,,, they are hung over and dehydrated from the weekend.

 

[Chavodel8en]

Thanks everyone for this information. Gives me something to think about and frankly alot of it is counter to what I expected. But I can see how just one isolated deep dive may be more of a "shock to the system."

My thinking was: we know the algorithms are really essentially educated guesses - there's always a possibility they will be wrong. And if you do 16 dives in 4 days, there are 16 possibilities that the algorithms aren't actually accurate to your particular circumstances -- and I thought that it may be exponential as well.

But probably most of you doing 16 dives in 4 days aren't "riding the NDL" on every dive, so maybe I overstate the risk in my head. I mean if you do one deep dive riding the NDL, and then a bunch of shallow dives (always avoiding NDL), how much greater risk do you incur.

I guess I was hoping for assurance that because I dont cram alot of other dives around my deep dive (like some of you crazed folks!), that I'm at less risk. There doesn't seem to be evidence to support that.

But of course I have less risk just bc 1 only have one real opportunity to get bent, as opposed to some y'all who cram 8 opportunities to get bent in 4 days.

And @gamon, I took it to mean he was showing an NDL of 1 minute remaining.

Yes, thats what I meant

 

[beldridg]

This is where knowing your surfacing gradient factor can be a potential benefit.

I'm guessing that most recreational divers never really look at their surfacing GF. If you know you are going to be doing a lot of dives over a number of days, you could just make sure your safety stops are long enough at the end of each dive to reduce your SurfGF to whatever you are comfortable with given tolerances, conditions, how you are feeling, etc.

It obviously won't get rid of all risk and it is subjective, but I find it a useful data point.

- brett

 

[inquis]

I can see how just one isolated deep dive may be more of a "shock to the system."

I view two dives as more of a shock. If you're not great on air consumption, a 100 ft dive will be 16 minutes and end in PADI's Group K. Even two seemingly trivial 40 ft dives (gas-limited to, say, 39 mins), separated by 1 hr surface interval, surfaces at 4 groups lower (Group O). Not to mention the unknown impact of any bubbles that may have formed on the first dive that were compressed.

If you're like many divers and just pop up to the surface after the safety stop, doing that in Group O will be far more of a shock to your system. I also believe most people are more likely to be careful during the final ascent after going to 100 ft compared to "only" 40 ft. (Just speaking to my impression of the masses here -- the mere fact you're asking the question suggests you probably aren't the "typical" diver.)

 

[Compressor]

Looking at the original poster's question here is my answer:
NDL is a risk on every dive. You should look at it as a function of your nitrogen load taking into account any nitrogen loading from previous dives. Stay away from the NDLs unless you want to do mandatory decompression stops. But remember that every dive is a decompression dive but sometime you need to make a mandatory stop.

 

[dmaziuk]

Thanks everyone for this information. Gives me something to think about and frankly alot of it is counter to what I expected. But I can see how just one isolated deep dive may be more of a "shock to the system."

My thinking was: we know the algorithms are really essentially educated guesses - there's always a possibility they will be wrong. And if you do 16 dives in 4 days, there are 16 possibilities that the algorithms aren't actually accurate to your particular circumstances -- and I thought that it may be exponential as well.

Last I checked the one probabilistic planner: SAUL, didn't do repeat dives so about the only source of information on them remains the 1993 DSAT report. It does show gas accumulation in the slow tissues by day 6, however, it's not clear whether that is real or an artifact of the exponential model (the exponent never hits the asymptote i.e. by numbers you never off-gas completely). Their test schedules could be described as "riding the NDL" I suppose, although they were validating the tables that come with a bit more "padding" than a computer. Anecdotally, a lot of people do liveaboards and not many of them get bent so I'd say the models are doing quite well.

 

[boulderjohn]

It took a while, but I got this reply from Emmanuel Dugrenot, senior researcher for DAN.

Thank you very much for your questions! You are probably mentioning a presentation about a study from our team (A study of decompression sickness using recorded depth-time profiles - PubMed (nih.gov)). Indeed, from that study, it seems that DCS is more likely to happen at the beginning of the trip… but we must consider these data with caution, as they are only considering declared DCS which can be more severe and far from the real number (A survey of scuba diving-related injuries and outcomes among French recreational divers - PubMed). If you also look at DCS associated with high multi day repetitive dive counts, the dive guides are more at risk as they are diving the most.

One of the hypothesis to explain that, is the micro bubble seeds from which bubbles are growing, are eliminated after the first dives, so we indeed observe a decrease in high bubble grades after the first days (Influence of repeated daily diving on decompression stress - PubMed (nih.gov)). Nevertheless, bubbles are not the only risk factor of decompression sickness, which is also highly correlated to inter and intra individual parameters like dehydration, stress and fatigue or inflammatory responses. From that, we can also easily imagine that diving really often can promote dehydration and a basal inflammatory state which on the contrary is not good.

Hoping my answer was clear, otherwise don’t hesitate to come back to me.

Best regards,

Emmanuel