Comprehensive list of all accidents

AM

Apologies, I have been away and just got back, therefore I will post a followup with responses to the questions/points raised in this thread (with reference data sources) just as soon as I get a chance. Many people have PM'd and emailed me - I will also respond to those when I can.

I am off diving in the States as of Saturday and probably will have no access, but when I return, I expect to be able to publish further data in respect of WOB discussion.

Regards

AnneMarie

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AM

Hi

As per our discussions offline, I don't believe any further response is warranted as the data referenced should imminently or now be transparent.

Regards

AnneMarie

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AM

Hi again

As per our offline discussion, no, the inference was not related to "all" eCCR accidents nor was there any consideration towards high-current. The inference related to only CCR heart attack deaths. Many eCCR accidents happen for other reasons and cannot accurately be assumed to be related to WOB-related factors.

What the statistics do show is that heart attacks occur in extremely greater multitudes on CCR than on OC.

As I believe we now have permission to publish plots from certain papers, a basic breakdown of the heart attack and stroke risk figures for divers in isolation should be ok. I understand these figures were already given public visibility at the Rebreather Fatal Accident meeting in Jan 2008 and further this work was described at the DAN Technical Conference.

In order to ensure that I am not rendering confidential data into the public domain, I have omitted the actual numbers until I receive email confirmation that I can publish these. I understand that the unpublished parts of the DAN paper will be published in autumn of this year therefore I cannot provide any detailed breakdown as yet, in regards to OC and Rebreather accident comparisons.

In respect to our extensive offline discussion, a brief reiteration of the conclusions so far in respect of the cardiac events/WOB connection: in a specified number of fatal O.C. accidents investigated by DAN between 1992 and 2000, cardiac incidents were implicated in a significant number of these. The Parento principle is within consideration so allocates the unknown cause accidents to the known causes pro-rata. It is my understanding that currently, this has not been done with rebreather accidents. Reversal of effect, would mean reducing the risk level of cardiac events on O.C. by a factor of between 2 and 3.

Of the total number of O.C. divers, again a proportion had prior Cardio Vascular Disease. Out of the rebreather accident group, few had known prior cardio-vascular disease (with some evident exceptions that are noted in the accident list).

Notably, in the O.C. list, cardiac incidents were considered potentially related by three factors:

1. Mammalian Dive Reflex, where heart rate and respiratory rate reduces
in water but blood pressure increases.

2. Heavy weight of equipment.

3. The UK HSE found that half of O.C. regulators fail to meet EN250:
this would cause CO2 retention and would promote high blood pressure,
triggering heart attacks to susceptible divers. This means the O.C.
incidence is probably higher than it should be if equipment was working
perfectly.

The incidence is offset by the reduction in hydrostatic pressure over the body.

In conclusion and consideration to general factors/principles, the incidence of heart attack does appear to be excessive in the rebreather accident list.

At this stage therefore, due consideration must be afforded to a prime potentiator, that of WOB implications on rebreathers. The WOB of rebreathers that are factored in the accident list is considered to be several times higher than safe limits, even within optimal loop parameters and five or so times higher during ascent and other non-optimal conditions. This could potentiate very significant CO2
retention, particularly in a current or where the diver has to work.

Rebreather WOB increases linearly with depth, so rebreathers taken to 100msw+ when they are in fact failing to meet the EN standards at 40msw, is a further risk that remains unmitigated by adding a small fraction of helium to the breathing loop.

On overall compound interpolation, the effect of risk varying with group size becomes non relevant when the risk is extrapolated in a linear fashion. So, straight average historical risks are compound extrapolated, and compound interpolated historical risks are linearly extrapolated.

This association between WOB and cardiac events in rebreather diving as compared with open circuit diving is currently being further documented in data compilation exercises. This particular aspect and related data, along with the other accident review data will formulate the basis of my paper (with the group as peer reviewers/editors).

From tomorrow I will have restricted access to the internet for some time, therefore my replies will again be delayed.

Regards

AnneMarie

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AD_ward9

There is no scientific link at the moment between WOB and heart attacks. There is a smoking gun, but that is all just now.

This is something several groups are looking at.

There are several steps, and different approaches being taken:
1. Some at DAN are looking at whether there is a link between WOB/Breathing Resistance/Acute Cardiac Illnesses. They may or may not find any link. Given the low incidence, the link will be hard to prove.

2. As an engineer involved in safety, I take measurements, look at the results, then apply the "HRDUBWD Principle" (we call it ALARP):

Measurements show many rebreathers have Work Of Breathing far in excess of known safe limits.

The safe limits are reducing over time, such as by the NEDU Jan 2007 report which is feeding into NATO STANAG 1410, being considered by SC7 (for CE regulations) etc.

There is a known link between CO2 retention and large increases in blood pressure. It is well documented in anaesthesia.

There does appear to be an excessive frequency of heart attacks on rebreathers. It appears to be of the order 500 times higher than expected, by comparison with Open Circuit. This is not explained by the fact that rebreather divers are typically 5 years older than the average O.C. diver.

Divers are subject to other factors that raise blood pressure, including the mammalian dive reflex, carrying heavy equipment, activity in an alien environment, and other factors. It is expected therefore that there are more cardiac problems for divers than for non-divers per hour, but this does not explain the difference between rebreather divers and O.C. divers.

The numbers involved are still small. Random events have clusters when observed by a human that looks for associations by nature, but the probability is already too low for this observation to be plausible as a random event.

The source data is not very good, for either O.C. or rebreather mortality. However, they are are similar to each other. The bottom line seems to be that in about 16,000 rebreather diver years of exposure, there is about the same number of heart attacks as for 8 million O.C. diver years of exposure.

Applying the "HRDUBWD Principle" means equipment designers should tackle the plausible causes. To assist that process, we will publish figures for the main rebreathers in detail as benchmarks in the Open Revolution WOB report, and reveal the details of how to reduce the WOB: explaining why each unit has the WOB it has. In most cases the WOB can be reduced a lot by straight forward changes to the mouthpiece, which usually accounts for about half the total, with counterlung designing making up most of the rest for a given type of scrubber (radial should always have lower gas resistance than axial, Micropore EACs have lower gas resistance than anything we measure in a scrubber designed specifically for them).

As a final note, one thing that stands out when looking at these cases is that accidents tend to occur to larger people more often than normal sized people: overweight from either very large build or more commonly, obesity. Diving is not for grossly unfit people. Very large people also need to take special care on rebreathers as their metabolism is faster, so PPO2 changes faster: monitor that gas more frequently, change your scrubber more often.

Alex

PhilSiswick

Fascinating - thanks both (AM and Alex) for these posts.

Cheers,

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Phil

No comment on open circuit... it's an evolutionary dead end not really worth discussing here. Dave Sutton, 2007

I have always felt that the dive I am on is not nearly important as the dives I plan to be on the rest of my life. Tom Rose, 2007

www.hugsac.org.uk

paulraymaekers

I agree that WOB must be lowered, clear.
Problem is that we try to match it with what a reasonable fit large male can do for some time: 75l/min, and even more, at depth. So here we start having some 'discrimination'

you start designing to meet the limits, and mostly as a consequence the unit becomes 'bigger': bigger hoses, bigger mouthpiece, larger volumes. Al this means you get a heavier unit, with often more drag under water.

now here comes a 50kg, 1.55m female diver who want to start diving CCR, and wants do buy a unit that meets the safety limits....

?? how many small female boris divers are around?

regards

paul

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www.rEvo-rebreathers.com

.... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...

Drmike

s
yes but big bore doesnt necessarily have to mean heavy unit

The Sentinel (which apears to have the lowest wob (in upright position) is lighter than some mainstream units.

Putting a bigger bore dsv, good check valves and big hoses on a unit isnt going to make any real difference from a diveability or weight viewpoint at all but can in some cases significantly improve wob

the average ccr diver is a fat ugly man - i see very few babes -which is as shame :-(

bigger does mean more drag - so you need a good wob to swim it ! viscious circle

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Get a girlfriend you sad twat - a Rebreather is an unfaithful mistress - dont blind yourself to her faults just because she goes down on you

AD_ward9

The difference in drag from low WOB features, is absolutely minute. A few mm on a hose diameter, a few mm on a mouthpiece, use of a single layer counterlung construction, use of either EACs or a radial instead of axial granular scrubber, minor changes to connectors, use of lower durometer flapper valves etc. It might add up to 150 grams of extra weight, but that is about all.

There is some extra volume needed, but under a litre.

Alex

paulraymaekers

hello Alex, that is possible in theory, but at the moment the units with the least WOB ar by far the biggest on the market
it does not mean that we must not try to change that!
paul

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www.rEvo-rebreathers.com

.... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...

silent running

Hi Paul, no big changes are necessary. The Prism has a lower WOB than any other CCR on the CCRB chart except possibly Boris and weighs only 47 lbs/21.5 kilos, fully charged, is only 53.4 cm tall, 43.2 cm wide and at 23 cm front to back, thinner than all other CCRs in the CCRB chart, except the Kisses and rEvo.

A CCR can have very low WOB and still be compact with low hydro drag. I couldn't do the kind of diving in strong currents so easily without the low WOB and low profile of the Prism... -Andy