Question...

[Scuba446]

My wife and I were having a discussion, and we are curious...

If someone was on a 40-60 ft recreation dive, and dropped their weight belt and did an Emergency Ascent, BC inflated - for ANY reason, say, panic, choking - anyhting - and missed their 3 minute Safety Stop at 15 ft - but kept BREATHING on the way up - what would most likey happen? DCS? Embolism? But not a lung overexpansion injury....right?

How serious is it to miss that Safety Stop on recreational dives from 40-60 ft?

I know no one can say with certainty what WOULD happen, but we are just curious HOW bad it would be?

OK you experienced divers, jump in here at any time?

 

[Jonathan]

I'm not a doc but:

On my deep course coming up from 105 feet (seeing I'm even converting these days) I had a free flow at 45 feet (water temp 3 degrees - sorry centigrade don;t have that conversion to hand!) - whilst I was task loaded trying to sort this out I had a rapid ascent breathing off a pony reg - the first I knew was feeling like the michelin man as my dry suit got rather large..... I was fine - though my computer was a tad upset!

The important thing is to keep breathing - same day (not our group) someone decided to bolt from 6m holding their breath - very messy!

If diving within recreational limits you should be OK - though I welcome comments that may contradict this. What you should prbably do (which I did) was be checked out by a first aider and monitor for any signs of DCS. Probably not a wise idea to dive again that day - I didn't but then my computer was locked out as well.

Hope that helps and I look forward to the docs response.

Jonathan

 

[nicodaemos]

Scuba446 once bubbled...
How serious is it to miss that Safety Stop on recreational dives from 40-60 ft?

Since I'm studying the books for my open water class this weekend I can give you the theoretical stuff. According to the PADI book, the safety stop is optional (but recommended) on no decompression (aka no-stop) dives.

There are three circumstances in which this stop becomes mandatory:

1. Diving to 100 feet or deeper
2. Ending your dive at a pressure group within 3 pressure groups of the NDL
3. Hitting any limit on RDP or dive computer
   [/list=1]
   
   So it would seem to me that you should theoretically have little risk of DCS .... but then again all I'm doing is regurgitating.
   
   Okay I'll step back now and let the veterans weigh in with the real answers.

 

[Rick Murchison]

When I started diving, there was no such thing as a safety stop. Either the dive required a decompression stop, or it didn't. Indeed, we were taught that a 60 FPM rate of ascent was important for the tables to work, as a slower rate of ascent would cause us to continue to take on more nitrogen than the tables planned for. My buddies and I dove this way (60fpm rate of ascent, no safety stop) for two decades without incident. Never knew anyone who had a DCS hit either. I heard of a few, but never first hand.
Then came doppler studies - and it was discovered that many of us using the standard Navy profile were developing asymptomatic, or "silent" bubbles... we weren't getting DCS, but we were bubbling - on the veinous side, and there was no harm because the lungs were successfully filtering them out. But - bubbles, even asymptomatic ones, are not exactly good, so the researchers started looking for ways to reduce their formation. First, they cut back on the maximum recommended NDL times, calling the new times the "doppler" no decompression limits. For example, the 60 foot NDL was cut from 60 to the doppler limit of 50; the 50 foot NDL from 100 minutes to 70 minutes, etc.
Second, they looked at the effect of making precautionary stops on these silent bubbles, and found that low and behold, a three minute stop at 10 to 20 feet all but eliminated them. And so new "Doppler NDL" tables were cut, that included a recommendation for a three minute safety stop at fifteen feet.
Of further interest is that no modification to actual decompression stops were made, so, for example, a dive to fifty feet, which has a doppler NDL of 70 minutes, doesn't pick up any mandatory decompression obligation until bottom time exceeds 100 minutes, when a 3 minute stop at 10 feet is required. And while the PADI RDP language is "always make a safety stop for 3 min at 15 ft after any dive to 100 ft or greater," the language on the SSI tables, for example, is "It is recommended that you make a 3 to 5 minute safety stop at 15 feet on all dives over 30 feet."
My take on all this, based on the research I've read and on my experience diving, is that safety stops are a prudent, good thing to do, especially after deeper dives - but... missing one while still within the NDL is not likely to be a problem.
Rick

 

[The Iceni]

I feel a little cheeky piping up from the back of the class when Rick has given such an excellent reply but perhaps when I grow up I will learn to keep quiet. I also initially trained when safety stops were unheard of. Without BCs or dry suits we could not hold a stop easliy in any case. Thankfully much has changed.

Every second spent under pressure causes more inert gas to dissolve in the tissues. When the pressure is released slowly this gas simply diffuses out from the surface i.e from the lungs in the same way it got in. If the pressure is is released too rapidly it "boils" as bubbles and micronuclei form in the liquid phase.

IT IS ALL RELATIVE.

Simply put, insufficient excess gas is dissolved during a no-stop dive to cause problematic bubbles to form. The idea of a safety stop is to reduce the rate of pressure reduction at the point where the pressure changes are greatest - close to the surface. Similarly a deco profile does exactly the same, but more so!

From my perspective on such an ascent your veins will inevitably be full of small bubbles and micronuclei, which as Rick says, will be mopped up by the lungs and dissipate. Not so if the inert gas tissue loading is great. As Dr Deco has stated many, many times, in this situation the bubble seeds will ongas and grow.

Without pulmonry barotrauma I believe embolism is unlikely.

Of course, if the diver has a patent foramen ovale, even after a short shallow dive, some of these small bubbles could find their way into the arterial tree and cause the problems of embolisation.

Thus a constant rate of ascent of as little as 3 metres per minute after a dive that requires deco stops is a "rapid ascent " and a perfectly executive deco profile may become a "rapid ascent" in the presence of a right to left shunt.

I hope that helps.

:doctor:

 

[Scuba446]

Thanks all for the fine efforts. We were just curious in a situation like an Emergency Ascent (that BOW teaches) how likely it would be someone would take a DCS 'hit' or not....

We will ALWAYS do a 3 min Safety Stop' as recommended - but just in case of a TRUE emergency, its nice to know that having to miss that stop may not result in a automatic DCS hit.

Thanks...

 

[Dr Deco]

Dear scuba 446:

Bends/no bends limit

This question is not dissimilar to one posted earlier concerning DCS and “limits.” What happens if you are 60 fsw and ascend to the surface, without a breath hold, will depend on you nitrogen gas load, i.e., how long you were at depth. The ATTACHMENT shows in FIGURE 1 that the incidence of DCS is dependent on bottom time (gas load). If you were down for only 12 minutes, the estimated DCS rate is 0.5%. If you were down for 50 minutes, the calculated rate is 2.2%; for 95 minutes, it is 5%. This is because more dissolved nitrogen is available in the tissues to diffuse into the tissue micronuclei. For the poor fellows in The Last Dive , they were down so long (at 200 fsw) that they were at virtually 100% probability of DCS. Unfortunately, as the gas load increases, the risk of all problems increases. That is, so many gas bubbles will form in some cases that the heart will fill with bubbles in the vena cava and the diver will expire from an “air lock” in the right ventricle. When this occurs, the blood is no longer pumped; the bubbles are simply compressed and reexpand with each heartbeat. Death ensues.

The information for this graph was prepared by researchers from the US Navy while preparing their calculations for new dive tables. It is adapted from a similar graph that appeared in The Alert Diver several years ago. It is evident that as the dose of nitrogen is increased, the risk of DCS rises is a continuous fashion.

Limits to Tables

In FIGURE 2 of the ATTACHMENT is illustrated what most divers believe occurs when they reach the table limits. They think that the risk of DCS rises extremely rapidly. This is, in fact, incorrect. There is not any sharp rise, no right angle bend in the risk curve. It is up to the table designer to determine what is the appropriate risk for the users of the tables and design the NDLs accordingly.

These risk estimates are generally greater than those encountered by recreational scuba divers – fortunately.

Risk and Diving

These estimates are determined for divers in a chamber (generally) and some activity in the water is assumed. Recreational divers can reduce the risk by refraining from heavy physical exercise on the surface, reducing the venous bubble loads (safety stops), and not sleeping during the surface interval on the boat. :boom:

If we wished to INCREASE the incidence of DCS, the best way would be to have the dive subjects perform exercise following the dive. In decompression at NASA, the workloads are always carefully monitored to assure that they are within the limits reached by astronauts during extravehicular activity (“space walks”).:mean:

Dr Deco :doctor:

 

[Scuba446]

Dr. Deco,
Thanks....I am a Cardica Monitor Tech, former EMT and current Rn student. I sure know what happens when the ventricle fails to pump...basically, CHF....inability to pump and perfuse the body is about as bad as it can get - minus tissue death.

Of course, an MD sure knows more than I - and thanks for the medical insight!

:boom:

 

[The Iceni]

Dr Deco once bubbled...
Dear scuba 446:
For the poor fellows in The Last Dive , they were down so long (at 200 fsw) that they were at virtually 100% probability of DCS. Unfortunately, as the gas load increases, the risk of all problems increases. That is, so many gas bubbles will form in some cases that the heart will fill with bubbles in the vena cava and the diver will expire from an “air lock” in the right ventricle. When this occurs, the blood is no longer pumped; the bubbles are simply compressed and reexpand with each heartbeat. Death ensues.

Don't forget the coronary arteries.

I agree 100%, Dr Deco. I hope I am not splitting hairs here. However, I do suspect that death from cardiac arrest secondary to acute myocardial infarction is much more likely to occur before such a vapour lock developes.

In a situation where there are so many small bubbles in the peripheral veins I am sure a significant number will also develop in the pulmonary veins (after all the pulmonary circulation contains almost exactly the same nitrogen load as the peripheral circulation and they are both at a similarly low pressure). Even when a right to left shunt is not present these bubbles will embolise to vital organs, the most important of which is the heart.

For example a right coronary artertery gas embolus will lodge in and obstruct the arterial blood supply to an area of the right side of the heart and inevitable ischaemia and myocardial infarction will follow. This is the site of the atrioventricular node - the natural pacemaker. Any infarction in the right coronary artery frequently leads to arrythmias, including ventricular tachycardia and fibrillation - the commonest form of cardaic arrest.

For a vapour lock to cause cardiac arrest in the manner descibed above the bubbles need to be very large indeed so I suspect such a vapour lock would occur later.

Post mortem evidence, the presence of the ineviatble nitrogen bubbles in the right ventricle will be unable to differentiate between these two possible causes of a fatal cardiac arrest in a diver.

:doctor:

 

[Rick Murchison]

Dr Paul Thomas once bubbled...


Don't forget the coronary arteries.In a situation where there are so many small bubbles in the peripheral veins I am sure a significant number will also develop in the pulmonary veins (after all the pulmonary circulation contains almost exactly the same nitrogen load as the peripheral circulation and they are both at a similarly low pressure).

Don't think so, Doc. The gas exchange in the alveoli is efficient and fast. Blood leaving them will have offgassed to, essentially, ambient partial pressures; the gradient for bubble formation and growth just isn't there. It is after the blood makes its capillary excursion through nitrogen soaked tissue that it picks up a nitrogen load sufficient for bubble growth.
Rick