OP
Yes I agree it is a main parameter, in case of major deco, unatended ascent with no real surface support is a guaranteed death (if not from bent, from drowning)
Oxygen poisoning seizure > how to react ?
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OP
Additional question : suppose you brought your buddy up and go back to your deco immediately; what would a standard buhlman computer display?
I assume it should pick up your deco just fine based on your gf. I saw some agencies recommand return to 1/2 max depth for 5 min in case of rapide ascent such as the one you would perform here, and a total surface time of 3 min max.
I assume it should pick up your deco just fine based on your gf. I saw some agencies recommand return to 1/2 max depth for 5 min in case of rapide ascent such as the one you would perform here, and a total surface time of 3 min max.
Hello,
Lots of interesting ideas and themes in this thread. I would like to address a couple of them, but first, I have attached 3 papers that address various topics relevant to this discussion.
First, the expert consensus review of just about all the issues being raised in relation to rescue published in UHM in 2012. It has been mentioned several times in the thread.
Second, a paper describing a loss of consciousness event in a rebreather diver who had been at 105m and was sent to the surface in an unconscious, unescorted buoyant ascent with omission of around 50 min of deco, and who survived.
Third, a paper describing the Thailand cave rescue and the subsequent evaluation of the full face mask employed, with some commentary around the implications for management of an unconscious diver underwater.
For those of you interested in this topic all three publications contain relevant information.
One issue I would like to address is a pervading assumption that it is possible to safely manage the airway of an unconscious breathing diver underwater to complete a period of decompression. Apart from in the presence of a full face mask (see the Thailand paper) this is an untested and somewhat implausible assumption. Even if the regulator is in place (which it usually isn't after a seizure) and even if the diver appears to be breathing (which you can't assess if the diver is on a rebreather), it is extremely unlikely that the mouthpiece can be maintained in place with an effective seal and ensuring the airway remains patent, all while maintaining buoyancy for both rescuer and victim, and controlling an ascent to comply with a decompression prescription, maybe in mid water with no reference or line. You get the point. For perspective, maintaining an airway in the ideal circumstances of controlled induction of unconsciousness in an operating room during at the start of anesthesia can be difficult even for a trained airway expert (anesthesiologist). Its easy to talk about doing it underwater, but it would be extremely difficult (I won't say impossible) underwater.
It was these sorts of considerations that led the expert committee (see the rescue consensus paper) to recommend that it not be attempted, more or less under any circumstances. Even an unescorted unconscious buoyant emergency ascent with a decompression obligation probably carries a greater chance of survival, especially if there is some sort of surface support (see the loss of consciousness event case report). Maybe you might try it in a cave, or if the circumstances dictate absolutely no hope of help at the surface. No guideline can cover every eventuality.
Another assumption that has emerged in this discussion is that it is important to wait until the seizure has passed before surfacing with the diver or sending them to the surface. This is based on a perception that a seizure will cause the vocal cords to spasm closed, thus obstructing the airway and increasing the risk of pulmonary barotrauma during ascent. It was an issue that the committee paid very close attention to (see the rescue consensus paper). The reality is that a seizure does not mean that the airway is immutably obstructed, particularly to expiration (which is what we are interested in during an ascent). I have manually ventilated patients having prolonged seizures (meaning the airway can't be completely obstructed), and video evidence of non-obstruction in experimental seizures in pigs is discussed in the consensus paper. It follows that waiting until the end of a seizure to initiate ascent is probably the greater hazard, particularly if the regulator is out because the first thing the victim will do at the end of the seizure is attempt to take a large breath and drown. Thus, in the consensus paper we recommended coming straight to the surface if the mouthpiece is out, and only waiting until the seizure is over if the mouthpiece is in. More recently, another group has gone a step further and recommended that ascent be initiated immediately during a seizure no matter whether the mouthpiece is in or out (which I happen to agree with). You can find the abstract of their opinion here.
The unfortunate truth is that if someone has a seizure at any sort of significant depth and particularly if they and their buddies have a decompression obligation that prevents an escorted ascent to the surface, it is an incredibly dangerous situation no matter what you do.
Simon M
Lots of interesting ideas and themes in this thread. I would like to address a couple of them, but first, I have attached 3 papers that address various topics relevant to this discussion.
First, the expert consensus review of just about all the issues being raised in relation to rescue published in UHM in 2012. It has been mentioned several times in the thread.
Second, a paper describing a loss of consciousness event in a rebreather diver who had been at 105m and was sent to the surface in an unconscious, unescorted buoyant ascent with omission of around 50 min of deco, and who survived.
Third, a paper describing the Thailand cave rescue and the subsequent evaluation of the full face mask employed, with some commentary around the implications for management of an unconscious diver underwater.
For those of you interested in this topic all three publications contain relevant information.
One issue I would like to address is a pervading assumption that it is possible to safely manage the airway of an unconscious breathing diver underwater to complete a period of decompression. Apart from in the presence of a full face mask (see the Thailand paper) this is an untested and somewhat implausible assumption. Even if the regulator is in place (which it usually isn't after a seizure) and even if the diver appears to be breathing (which you can't assess if the diver is on a rebreather), it is extremely unlikely that the mouthpiece can be maintained in place with an effective seal and ensuring the airway remains patent, all while maintaining buoyancy for both rescuer and victim, and controlling an ascent to comply with a decompression prescription, maybe in mid water with no reference or line. You get the point. For perspective, maintaining an airway in the ideal circumstances of controlled induction of unconsciousness in an operating room during at the start of anesthesia can be difficult even for a trained airway expert (anesthesiologist). Its easy to talk about doing it underwater, but it would be extremely difficult (I won't say impossible) underwater.
It was these sorts of considerations that led the expert committee (see the rescue consensus paper) to recommend that it not be attempted, more or less under any circumstances. Even an unescorted unconscious buoyant emergency ascent with a decompression obligation probably carries a greater chance of survival, especially if there is some sort of surface support (see the loss of consciousness event case report). Maybe you might try it in a cave, or if the circumstances dictate absolutely no hope of help at the surface. No guideline can cover every eventuality.
Another assumption that has emerged in this discussion is that it is important to wait until the seizure has passed before surfacing with the diver or sending them to the surface. This is based on a perception that a seizure will cause the vocal cords to spasm closed, thus obstructing the airway and increasing the risk of pulmonary barotrauma during ascent. It was an issue that the committee paid very close attention to (see the rescue consensus paper). The reality is that a seizure does not mean that the airway is immutably obstructed, particularly to expiration (which is what we are interested in during an ascent). I have manually ventilated patients having prolonged seizures (meaning the airway can't be completely obstructed), and video evidence of non-obstruction in experimental seizures in pigs is discussed in the consensus paper. It follows that waiting until the end of a seizure to initiate ascent is probably the greater hazard, particularly if the regulator is out because the first thing the victim will do at the end of the seizure is attempt to take a large breath and drown. Thus, in the consensus paper we recommended coming straight to the surface if the mouthpiece is out, and only waiting until the seizure is over if the mouthpiece is in. More recently, another group has gone a step further and recommended that ascent be initiated immediately during a seizure no matter whether the mouthpiece is in or out (which I happen to agree with). You can find the abstract of their opinion here.
The unfortunate truth is that if someone has a seizure at any sort of significant depth and particularly if they and their buddies have a decompression obligation that prevents an escorted ascent to the surface, it is an incredibly dangerous situation no matter what you do.
Simon M
GF99/99
Contributor
Some very interesting points being made.
Just to through another discussion point into the mix what are others thoughts and actions for how to prevent O2 poisoning ultimately leading to seizures underwater related to high CNS levels. I think breathing the wrong gas at depth resulting in high Po2 level leading to O2 poisoning is avoidable and should never happen unless an accident is made. But high CNS levels is something that is more of a realistic problem (especially for CCR divers) that you consciously choose to do if you are doing long dives either deep dives with long decompression obligations or far penetration overhead dives. Its amazing how easy it is to get over the 100% CNS mark on a rebreather. So what are some mitigations, safety measures, etc. in place for situations like this other then air breaks, diving lower setpoints, etc. and as Dr. Mitchell has leaded to above maintaining a sealed mouth piece and patent airway is next to impossible.
IMO the only way to safely conduct dives like this would be with FFM or the use of habitats for the decompression phase. However how feasible is this for everyone.
Just to through another discussion point into the mix what are others thoughts and actions for how to prevent O2 poisoning ultimately leading to seizures underwater related to high CNS levels. I think breathing the wrong gas at depth resulting in high Po2 level leading to O2 poisoning is avoidable and should never happen unless an accident is made. But high CNS levels is something that is more of a realistic problem (especially for CCR divers) that you consciously choose to do if you are doing long dives either deep dives with long decompression obligations or far penetration overhead dives. Its amazing how easy it is to get over the 100% CNS mark on a rebreather. So what are some mitigations, safety measures, etc. in place for situations like this other then air breaks, diving lower setpoints, etc. and as Dr. Mitchell has leaded to above maintaining a sealed mouth piece and patent airway is next to impossible.
IMO the only way to safely conduct dives like this would be with FFM or the use of habitats for the decompression phase. However how feasible is this for everyone.
@Dr Simon Mitchell, thanks so much for taking the time to help us with this problem..! I really appreciate it.
jadairiii
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Thank you so much for chiming in on this topic with facts.
I have addressed this on multiple topics, always have good, competent surface support and always dive with a marking buoy so surface support can follow the team. This also lends itself to the topic, always dive with a team.
Hello,
Yes, slightly off the topic of the thread, but an important issue. There are two relevant (and related) points to remember.
First, the 'CNS' limits are a very imprecise risk measure, with a very poor positive predictive value.
Second, part of the reason for the poor positive predictive value is that risk is so context-sensitive; that is, it's dependent on other factors beyond a simple consideration of time and inspired PO2. For example, one of the reasons we seem much more vulnerable to oxygen toxicity in diving compared to a dry hyperbaric environment is that in diving we are often exercising, and breathing dense gas through underwater breathing apparatus; factors which lead to CO2 retention which in turn is a major risk factor for CNS oxygen toxicity. However, the periods you are concerned about, when 'CNS levels' are peaking beyond 100%, most typically occur when we are decompressing, at rest, shallow, not breathing very dense gas, and therefore (in theory) unlikely to be retaining CO2.
We investigated this in a field study at Bikini Atoll in 2015 (paper attached) and found no general tendency for rebreather divers to retain CO2 during decompression. This is obviously not to say that suffering a CNS oxygen toxicity event during decompression is impossible, but it is a reassuring finding and helps explain why such events seem very rare despite the fact that rebreather divers often exceed the 100% CNS limit.
A point which obviously follows from this is that you would not want to be exercising hard, breathing dense gas, or using a rebreather with an depleted CO2 scrubber when your CNS percentage is high!
Simon M
I don’t think much evolved since you left Jeddah; the technical community in KSA is still exceptionally small and lining up skilled folks to be on the same dive is a challenge.
Decompression diving in KSA has significant risks due to limited talent and experience in the water, in the boat and at the chamber. Profiles should be planned accordingly. Fortunately, every single one of my technical dives were relatively tame wall dives on DPV with a simple bucket profile. I was just enjoying the scenery, looking for sharks and bedding in my skills with planning, deep DSMB shots, DPV skills, gas switching procedures and a comprehensive debrief (event if by myself).
I know the chamber in Jeddah, the only one on KSA’s Red Sea coast, works because we had to send someone there in 2020. However, between the point of evacuation to the point of actually getting the diver into the chamber was pure chaos, unhelpful conjecture, unnecessary drama and idle speculation rather than a trained clinical medical response.
Regarding boat operators, one of my last dives in KSA was up in Yanbu where I was supporting a Jeddah dive shop to develop a B2B relationship to expand their services and locations. The charter we picked allowed other boats to back up right over the top of our DSMBs. I kid you not…crystal clear water, calm conditions, we were at 6m near our boat and here comes another boat with its prop spinning at limb-chopping speeds backing up right towards us. Fortunately nobody was hurt but the outrageous part was our boat captain didn‘t recognize any of this as a problem.
When I dived on LOBs with the small handful of GUE folks in KSA, we had buckets of O2 on board and the tech dives were typically the only one of the day.
I would have assumed that a computer like a Shearwater would not lock you but continue to off gas faster (according to the model).
So when you go back to your stop, it would just continue to apply the tissue on/off gassing as it would have never stopped but I do not know for sure.
I am gonna ask them.
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