So, do you know what you would change about the dive in order to Increase the NCS risk??
As others have said; Depth + Time = Nitrogen Absorption
Here's a very simplistic explanation:
Speed of ascent dictates how nitrogen is released. A recreational diver is limited to dives where nitrogen absorption is sufficiently low to enable a
direct ascent to the surface. However, that direct ascent must be completed at a specific speed (typically slower than 18m/60ft per minute). A technical diver is trained and equipped to complete dives that do not have a direct ascent to the surface - the amount and location of absorbed nitrogen dictates a much slower speed of ascent - this results in a 'staged' ascent, where the diver has to pause at pre-calculated depths to ensure that sufficient nitrogen is released. These 'pauses' are known as decompression stops.
Too Fast Ascent = Nitrogen cannot be removed quickly enough by the cardio-vascular system, thus it de-saturates
inside the body, forming bubbles.
The speed of ascent is determined by the
amount of nitrogen absorbed and
where it is absorbed into within the body.
More depth = more pressure. More pressure = more nitrogen. More time at pressure = even more nitrogen
As depth and time increase, the nitrogen is 'pushed' deeper in the body tissues. We categorise this absorption in terms of hypothetical 'tissue compartments'. There are 'fast' tissues and 'slow' tissues.
A recreational diver (shallow + short) is mainly going to absorb nitrogen into 'fast' tissues. These absorb nitrogen quickly, but also release it quickly. The recreational diving 'No-Deco Limits' ensure that the ascent/release of nitrogen is only limited by 'fast tissues'. The
speed of ascent reflects the faster release of nitrogen from the body.
A technical diver (deep and/or long) will absorb more nitrogen into 'slow' tissues. These release nitrogen slowly - which require the diver to make a much slower ascent to de-saturate. The very slow speed of that ascent dictates the need for 'stops'.
Killing the 'ficticious' diver:
Here is the dive schedule again:
1st. 40 minutes at 100ft (Mandatory 7minutes stop at 10msw is
ignored)
2nd. 45 minutes at 70ft
....the entire dive is conducted with Standard Normoxic Air.
Those dives would have a requirement for
deco stops, but missing those stops would not necessarily indicate DCI. They are relatively 'lightweight' deco dives. If there was DCI - it would be relatively mild, would probably onset shortly after surfacing. Most likely symptoms would be acute localised pain in one or more joints/limbs, possibly coupled with a slight skin rash. It would be treated by hyperbaric treatment with a high likelihood of complete recovery.
You've given us time, depth and breathing gas... and also defined that they 'missed' their deco stops. The only other variable would be their ascent speed. Even with a fast ascent - it'd be unlikely to be fatal.
So... to make the diver 'die'... you'd need to add something like this:
Now... given the dive you've described... to have the diver 'die', you could look at one of the following circumstances:
1. Diver completed the ascent on Dive 2, from 70ft to the surface, at an extremely fast pace. Possible cause of this would be panic; as divers are trained to share air with their buddy and/or complete a 'Controlled Emergency Swimming Ascent' to the surface where they maintain a safe ascent speed. The diver would have to panic to ignore either of those options. Panic would mean 'kicking for the surface as fast as humanly possible'. Panic
does kill divers. Another alternative for fast ascent would be a buoyancy malfunction. Maybe a stuck inflator button on a dry-suit or BCD that wasn't quickly dealt with (diver train to resolve this also). If the diver was raising an item from the sea-bed using a 'lift-bag', then they could get tangled in the bag and pulled up by it. Another option would be that the diver got caught in a fishing net/line and dragged up to the surface. The ascent to the surface would need to be in
seconds, to have the level of severity that you are looking for.
2. Diver made a rapid ascent from Dive 2, but somehow failed to breath/exhale on the way up. Air trapped in their lungs would expand (due to decreasing ambient water pressure). That expanding air would burst their lungs, resulting in a potentially fatal complication. The 'most lethal' complication from a Lung Over-Expansion Injury is an Arterial Gas Embolism (AGE) where air enters the circulatory system. There are other possible complications - just ask if you need more details on those.
3. Diver had a Patent Foramen Ovale (PFO). A medical condition otherwise known as a 'hole in the heart'. I believe that up to 1/4 of people have this (?). The PFO allows blood to re-circulate through the body
without passing the lungs. This means that nitrogen is released from the body, but a significant percentage of it won't pass the blood-lung barrier for release. Consequently, it accumulates...and is known to cause significantly higher risk of DCI.
4. The diver might have
other pre-disposing factors for DCI. Known pre-disposing factors would include: dehydration, exhaustion, lack-of-sleep, illness and injury or diving in extremely cold water. These conditions basically interfere with the cardio-vascular system - meaning the nitrogen 'off-gassing' would be slower than predicted. They'd increase the chance of DCI, but wouldn't necessarily make the DCI substantially more severe.
[h=3][/h]Otherwise... there's just
no way to make your hypothetical dive
more lethal. It's a bad dive plan - that could cause harm - but is likely to be survivable if medical care were forthcoming within a few hours of the incident. As it is, without an 'extra' factors - it wouldn't kill you in the water.
