Question A sudden feeling of dread and fear during a simple CCR dive

[Cragene]

Hi everyone,

I don't know if this is the right section, but since no incident occurred, I don't believe this categorizes as a "near miss" (well, at least I hope so )
I'm posting this more as a question to understand if something like this occurred to anyone else, and if it was indeed just a psychological/narcosis effect, or something more serious (pre-hypercapnia or bad gas).

A little context about me and the previous dives​

A bit of context about me: I'm a lake diver, I dive quite often (2/3 times a week), and I recently started CCR diving. I've just shy of ~50 hours with my rebreather, so I probably qualify as a super-beginner and still have a lot to perfect compared to when I dive OC (and proper weighting is one of the things where I still struggle with, and this usually leads to being over-weighted).

The dive itself was a really simple and pretty "standard" one for me, in a spot that I know like the back of my hand and where I've dived several times, both on OC and CC; it was in the afternoon (so no morning stress or whatever), and with two buddies I trust and I often dive with.
It was a recreational dive; the other buddies were on OC, and we all used air.
I used the rebreather so that I could practice trim and buoyancy, as I had been using a new configuration on my last two dives (a steel 12-liter bailout tank and a lighter undersuit).
It was the third dive on the same scrubber: the first one lasted ~54 minutes with air, the second one ~43 minutes with 20/30; both with an avg depth of 20 meters (66 fsw).
All dives (including this one) were over the same week, and the scrubber was always carefully sealed between each dive; by the way, the three-dives / three-hour rule on a single scrubber is what I normally follow and never had any issues so far.
Setpoint was 1.2 on the bottom, and CNS before the dive was (obviously) 0% with a surface interval of about 42 hours.

The dive​

It all started without problems and was pretty straightforward: simple shore entry, then we went down to our target depth (40m - 133 fsw), stayed there just for a couple of minutes, then ascended to ~30m (100 fsw) and continued our dive.
They were slowly riding their NDL being on OC, I was staying more or less at the same depth; I was the last one in the group, the visibility was ok below 25m (82 fsw), and the thermocline was around 16m (52 fsw), above water temp was around 20-22 °C (68-72 °F), below was 9 °C (48 °F).
After 25 minutes or so, the first buddy who was leading the dive signaled to turn back and slowly ascend, and that's where we got separated, due to poor visibility and me being a bit deeper than them, and I didn't think they would turn back so fast.
But, anyway, no big deal, I searched them for a minute, then continued alone to reach the entry point and the anchor we left at 5m (16 fsw) at the beginning of the dive.
I knew that they wouldn't panic or anything, and in our pre-dive briefing, we decided that we would be autonomous in case we got separated and we would reunite either at 5m (16 fsw) or the surface.

Since I was on CC, I decided to stay at the same depth (it was around 25m - 82fsw) to have better visibility.
In hindsight, I probably swam a bit too fast and felt very heavy. In fact, after just 15 minutes, I had already reached the entry point (there is a fixed rope that goes from ~3m (10 fsw) down to 50m (165 fsw), to ease navigation).

"Something bad is going to happen"​

And this was when something strange happened: after a quick dip to 35 meters (to see another little part of the rocky wall before ascending and ending the dive), I suddenly felt a sense of dread and fear, as if something bad was going to happen and that I needed to get out of the water quickly.
I don't know if this accounts for the term "a sense of impending doom", but it was unpleasant for sure.
In addition, I felt my heart rate increasing rapidly, and I could sense my heart pounding through the drysuit.

But again, no panic and no "grasping for air" or whatever. I then decided to ascend, and once I reached 16m (52 fsw) all symptoms subsided; so I decided to continue diving following another wall around 18m (60 fsw) for 4-5 minutes, so maybe I would find my buddies and end the dive together (since that wall is the "return" route we usually take).
I didn't find them, but I felt, again, the same sense of dread, this time when I was just 20m (65 fsw) deep; so I decided to go straight up following the floor instead of going back to the rope, then continued to the anchor at 5m (16 fsw), and holding my safety stop.
This time, the sense of dread didn't subside (even though it was "lighter" than the first time at 35m/114 fsw), and I also bailed out to see if something changes (but after a minute or so nothing changed so I decided to go back on the loop).
(and, thinking back about the fact that I was able to hold the 5m/16 fsw stop with a normal volume in the counterlungs, while breathing from the bailout regulator, makes me think that maybe I was surely over-weighted...)

Finally, I exited the water by slowly ascending from 5m (16 fsw) to the surface, and found my buddies waiting for me there (they finished the dive just 5 minutes before me), and almost immediately, all symptoms disappeared.
And no after-dive effects as well: no headaches (as I think I would have had if I had increased CO2 retention), no restlessness or flu-like symptoms like a subclinical DCS, and no "long-lasting" fear.

I have no idea what happened and why; underwater, I thought of everything, including:

• Bad scrubber or channelling
• Increased ppO2 and possible CNS toxicity (but the ppO2 stayed at 1.2 with few spikes to 1.3 when descending)
• Bad loop due to something in the tank (maybe carbon monoxide or any other contaminant)
• Bad loop due to me burping twice during the dive, and so I was rebreathing some hydrogen or methane in the mix
• And honestly, I didn't think about narcosis, when on OC I dove down to 55m (180 fsw) quite often and with no issues; yet I know that increased WOB on the CCR at depth is dangerous, that's why I limit the use of air to a maximum of 35-40m (132 fsw) and use light trimix for everything deeper (or with longer bottom times).

Conclusions​

First of all, sorry for my wall-of-text (you could have guessed it would be a long write-up by looking at my other posts on SB ).
Secondly, have you ever experienced something similar? And do you think this was a real near miss, and I risked by staying underwater so long after the first symptoms?

Maybe it was "just" a bad episode of dark narc, probably triggered by swimming faster than usual, being over-weighted, a bit tired, and a little cold after ~40 minutes in 9 °C (48 °F) water for about ~45 minutes with a lighter-than-usual undersuit.
Or maybe it was something else, but I still have no idea what

 

[Steel_Stalk]

I’m in a pretty similar boat as you, just under 50 hours on a Meg and have had two instances of “something feels weird”. Both times I was less than 100’ in depth and opted to flip the lever on my BOV and breathe OC for a minute or so just to “catch my breath”. Once I got my head refocused, I switched back onto the loop. For me, I don’t believe it was nitrogen narcosis or scrubber failure or even task overload.

The only other times in my 17 years of diving that I have felt a similar way would probably be my intro to cave class about 8 years ago and a trimix class on OC about 5 years ago.

 

[davehicks]

This sounds an awful lot like you overtaxed your scrubber and started to build up CO2. A CO2 headache is not guaranteed afterward if it was not a prolonged exposure. You were on the 3rd dive, said you were working a bit hard with the swim, and started to feel anxious with elevated heart rate or breathing. Classic CO2 symptoms.

It's good you bailed out, but not so good you went back on loop. At minimum you should do a full flush of the loop and slow down.

I've had a CO2 hit from a likely breakthrough and my experience was similar. Anxious, irritable, high heartrate and respiration.

 

[Mobulai]

Bad loop due to me burping twice during the dive, and so I was rebreathing some hydrogen or methane in the mix

Could be minor CO2 buildup? Like anxiety triggered shallow slow breathing + scrubber had already 90min on it + being below thermocline
I’m a noob myself so can’t guess much right here without knowing more; but I did experience something like that on OC when I was trying to unpanick (mentally) and conserve gas — heck even on land (edit 2: which I later learned contributes to co2 retention)

Edit: I see Dave beat me to it

 

[ginti]

About narcosis -> The most narcotic gas is CO2. So if you start building up CO2 with the rebreather, you will automatically be way more under narcosis effect than if you were breathing air OC.

To me it also looks like CO2 was building up, with all the consequences... out of curiosity, did you have any physical sensation before building this huge anxiety? Like light legs, or something else?

 

[grantctobin]

What unit?
What water temperature?

 

[rjack321]

That's a whole lot of swimming at 30+m on air dil. I am leaning heavily on WOB and CO2 retention as well - moreso than any kind of breakthrough

 

[Cragene]

It's good you bailed out, but not so good you went back on loop. At minimum you should do a full flush of the loop and slow down.

Actually, I did not.
I stayed on the loop the whole time (even after the first symptoms), I only bailed out for under a minute during my safety stop, to check if the symptoms would go away.

I don’t know why, but I felt confident to stay on the loop, because apart from anxiety, fear and elevated heart rate, I had zero issues in breathing (and maybe my breathing pattern was so “natural” that I breathed too shallowly during the last part of the dive, because I was very relaxed being a soft and simple dive for me)

To me it also looks like CO2 was building up, with all the consequences... out of curiosity, did you have any physical sensation before building this huge anxiety? Like light legs, or something else?

No physical sensation, just a “build-up” of anxiety (e.g., I started to think “hey it’s really dark here”, “I’m feeling cold”, “I’m alone and the bottom here is over 80m deep, who would find me if something happens?”) and then few seconds or minutes later it exploded in that feeling of dread and tachycardia.

What unit?
What water temperature?

Unit is a Divesoft Liberty (classic back mount with standard scrubber)
Water temperature was 9° C / 48 °F below the thermocline at 16m.

 

[davehicks]

Actually, I did not.
I stayed on the loop the whole time (even after the first symptoms), I only bailed out for under a minute during my safety stop, to check if the symptoms would go away.

I don’t know why, but I felt confident to stay on the loop, because apart from anxiety, fear and elevated heart rate, I had zero issues in breathing (and maybe my breathing pattern was so “natural” that I breathed too shallowly during the last part of the dive, because I was very relaxed being a soft and simple dive for me)



No physical sensation, just a “build-up” of anxiety (e.g., I started to think “hey it’s really dark here”, “I’m feeling cold”, “I’m alone and the bottom here is over 80m deep, who would find me if something happens?”) and then few seconds or minutes later it exploded in that feeling of dread and tachycardia.


Unit is a Divesoft Liberty (classic back mount with standard scrubber)
Water temperature was 9° C / 48 °F below the thermocline at 16m.

The next time this happens or if you even feel like you're working too hard just stop flush your loop and then monitor. One of the common issues on CCR is a high workload if you have to kick into something for over a minute definitely do a preemptive flush.