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):
Quote: (Originally Posted by
hrdubwd)
Of course ...
My point is that the calculations made are wrong (make that 99.999...%). It is not a case of waiting for enough to die, it is simply that the formality of the prediction attempted cannot be done with the data we have (or could realistically have), assumptions notwithstanding. What we have is an association between certain kinds of event and the equipment involved. This is entirely enough to trigger questions about equipment design, its usage, diving practices, and the people who dive that kit, including their purposes, quality of training, health and so on.
Consider CO-poisoning from gas-fired water heaters. Whilst a probabilistic approach could be taken, it is entirely inappropriate to do so. The association of death (with a clearly identifiable proximate cause, in this case) with those devices led to queries about installation, safety mechanisms, education of the user, and so on. Changes occurred. If I understand it correctly, the burst of problems with the Meg is a parallel situation. Calculating risk there is pointless.
One preventable death is enough to trigger those questions, and the onus is on the producer, the trainer, and the user to honestly appraise - and fix - problems. There is no other threshold for action. Denial is counterproductive.
BWD
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