Rapid descent

[Dr Deco]

Dear AquaTech:

HPNS

The High Pressure Nervous Syndrome or HPNS was something observed by commercial divers and laboratory test subjects in dives deeper than about 400 fsw (on helium/oxygen) when the compression rate was fast, that is, 60 to 100 feet per minute. HPNS manifested itself as dizziness and very noticeable tremors of the hand, to the point where some types of underwater work would be difficult.

It could be alleviated, to a large degree, by compressions that were very slow, about one foot per minute. While commercial divers sought to compress at 100 feet/min, 1000 fsw dive would take taking almost one day (17 hours). Stopping every several hundred feet for an hour was also successful.

Some attempted to alleviate the problem, with considerable success, by adding nitrogen to the heliox bottom mix. Much of this work was performed at the F G Hall Laboratory at Duke University. Since nitrogen has some narcotic properties, it was reasoned that it would serve as a mild “sedative.” Whether the gas is truly acting as a sedative was not resolved, as I understand, but it seems to make some sense.

Recreational Diving

Since recreational divers seldom go as deep as 400 feet, the problem will not arise. As divers do go deeper, however, slow descents will be required. Since commercial diving is often performed in the “saturation” mode, rapid descents (100 feet/min) are not really necessary, but bottom time is more critical for “bounce diving” of technical divers. We see that this is really becoming saturation-like for many tech diving scenarios.

Dr Deco :doctor:

 

[The Iceni]

I believe that rapid descents are reported to increase the risks-of, and the severity of, (nitrogen?) narcosis.

I have never seen this fully explained but once again carbon dioxide may play a part.

Forgetting the problems of gas changes, if you descent at 100 feet per minute you will pass 100 feet (30 metres)at the end of the first minute. The ambient pressure at 100 feet is near enough 4 bar and the original gas in the lungs will be compressed by ambient pressure to reduce lung volume to a quarter of their surface size unless gas is added from the diver's regulator.

It would be possible to breath hold for this minute, or even to take a long, slow inspiration throughout the duration of the descent to 100 feet. During this time there will be no excretion of the carbon dioxide originally held in the lungs, the partial pressure of which would either rise as the lungs are compressed, or remain the same as it diffuses into the gas added to make up lung volume to what it was on the surface during the prolonged inpiration. To this CO2 is added by metabolism.

In practice, of course, a scuba diver will be breathing but, using the same reasoning, the net movement of gas into the lungs will be greater than the net movement of gas out of the lungs during a rapid descent. It is possible therefore, that there could be a considerable build-up of carbon dioxide in the body . . . .

at least during the initial stages of such a rapid descent, where the pressure changes are greatest.

Any thoughts?

 

[AquaTec]

Dr Deco once bubbled...
Dear AquaTech:

HPNS

The High Pressure Nervous Syndrome or HPNS was something observed by commercial divers and laboratory test subjects in dives deeper than about 400 fsw (on helium/oxygen) when the compression rate was fast, that is, 60 to 100 feet per minute. HPNS manifested itself as dizziness and very noticeable tremors of the hand, to the point where some types of underwater work would be difficult.

It could be alleviated, to a large degree, by compressions that were very slow, about one foot per minute. While commercial divers sought to compress at 100 feet/min, 1000 fsw dive would take taking almost one day (17 hours). Stopping every several hundred feet for an hour was also successful.

Some attempted to alleviate the problem, with considerable success, by adding nitrogen to the heliox bottom mix. Much of this work was performed at the F G Hall Laboratory at Duke University. Since nitrogen has some narcotic properties, it was reasoned that it would serve as a mild “sedative.” Whether the gas is truly acting as a sedative was not resolved, as I understand, but it seems to make some sense.

Recreational Diving

Since recreational divers seldom go as deep as 400 feet, the problem will not arise. As divers do go deeper, however, slow
descents will be required. Since commercial diving is often performed in the “saturation” mode, rapid descents (100 feet/min) are not really necessary, but bottom time is more critical for “bounce diving” of technical divers. We see that this is really becoming saturation-like for many tech diving scenarios.

Dr Deco :doctor:

:mean: =-)

well lets talk about HPNS on trimix dives with descent rates of 100 ft/min at depths between 400 and 500 ft.

I have not heard of there being any problems as long as there is N2 envolved. all complaints have been on heliox which i think is generaly used only on much deeper dives.

 

[AquaTec]

Dr Paul Thomas once bubbled...
I believe that rapid descents are reported to increase the risks-of, and the severity of, (nitrogen?) narcosis.

I have never seen this fully explained but once again carbon dioxide may play a part.

Forgetting the problems of gas changes, if you descent at 100 feet per minute you will pass 100 feet (30 metres)at the end of the first minute. The ambient pressure at 100 feet is near enough 4 bar and the original gas in the lungs will be compressed by ambient pressure to reduce lung volume to a quarter of their surface size unless gas is added from the diver's regulator.

It would be possible to breath hold for this minute, or even to take a long, slow inspiration throughout the duration of the descent to 100 feet. During this time there will be no excretion of the carbon dioxide originally held in the lungs, the partial pressure of which would either rise as the lungs are compressed, or remain the same as it diffuses into the gas added to make up lung volume to what it was on the surface during the prolonged inpiration. To this CO2 is added by metabolism.

In practice, of course, a scuba diver will be breathing but, using the same reasoning, the net movement of gas into the lungs will be greater than the net movement of gas out of the lungs during a rapid descent. It is possible therefore, that there could be a considerable build-up of carbon dioxide in the body . . . .

at least during the initial stages of such a rapid descent, where the pressure changes are greatest.

Any thoughts?

You bring up some interesting points. about CO2 buildup but it does not seem to be an issue in reality. i think i know why but i will ask you some questions first. isn't the normal breathing rate something like 15 breaths a minute. if i am right about this then you would take 15 breaths while traveling the 100 feet. I think the other main thing is to make sure to have a complete exhale with every breath, I personaly have a habit of counting to myself to manage my breathing.

Narcosis is not an issue, the first 100 feet is on 36% and the balance is on bottom gas usualy 10/50 but as much as 7/65 leaving very little N2 for narcosis

 

[The Iceni]

AquaTec once bubbled...
You bring up some interesting points about CO2 buildup but it does not seem to be an issue in reality. i think i know why but i will ask you some questions first. isn't the normal breathing rate something like 15 breaths a minute.

Lets say that you do do 15 normal full breaths in the minute you descent to 100 feet (30M) and 4 bar.

75% of lung volume more is inhaled by the breathing mix than is exhaled over the entirity of the descent. If this takes a minute and 15 breathing cycles each inhalation/exhalation cycle consists of 75%/15 more gas inhaled than exhaled = 5%.

Edit On reflection it is likely to be much, much more than this because tidal volume is much less than total lung capacity, but I will leave the rest of the post as it is while I work on the true figures.

Respiratory efficiency of each breathing cycle is reduced by 5%. This does not affect oxygen transport because the pp O2 is increased but, as you know, the pp CO2 does not change. It is entirely dependent on metabolism. The net effect is that CO2 excretion is reduced by 5% during such a descent and this will surely be sufficient to cause a transient increase in pp CO2.

On the other hand, if the descent is reduced to a slow 20 feet per minute and the respiratory rate is increased to 20 breaths per minute it would take five minutes to descend to the same depth during which you would take 5 x 20 = 100 breaths.

Each inhalation/exhalation cycle consists of 75%/100 more gas inhaled than exhaled. = 0.75%, which I suggest would have little perceiveable effect on CO2 excretion.

Conversely as I suggested above, if the diver simply holds his lung volume constant, by a slow inhalation, CO2 excretion will be zero. However if he does a "free dive" to depth the compression of the CO2 in the lungs will cause a reversal of the normal CO2 gradient marginally increasing basal pp CO2 in the blood.

OK in free divers, but you then take a breath at 100 metres and a mainline slug of high partial pressure nitrogen.:boom:

Perhaps this is indeed why rapid descents provoke narcosis - N2 plus CO2? :idea:

In addition, Aquatec, forgive me but in your question you said nothing about using Nitrox, doing any gas switches or indeed stopping your descent at 100 feet. Indeed in the proposed 3 minutes of descent you would get to 300 feet!! ut:

 

[AquaTec]

Dr Paul Thomas once bubbled...

In addition, Aquatec, forgive me but in your question you said nothing about using Nitrox, doing any gas switches or indeed stopping your descent at 100 feet. Indeed in the proposed 3
minutes of descent you would get to 300 feet!! ut:

Sorry I add that for clarity, I just assumed that it would be understood that you would not dive to 300 feet on air.

the rest of your post I am still digesting

 

[AquaTec]

Note I apologise in advance for my lack of formating, I still havent figured it out. I will put "Doug" in front of my statements within the quote

Dr Paul Thomas once bubbled...
Lets say that you do do 15 normal full breaths in the minute you descent to 100 feet (30M) and 4 bar.

75% of lung volume more is inhaled by the breathing mix than is exhaled over the entirity of the descent. If this takes a minute and 15 breathing cycles each inhalation/exhalation cycle consists of 75%/15 more gas inhaled than exhaled = 5%.

Doug - would that 5% be the O2 that is matabalized


Edit On reflection it is likely to be much, much more than this because tidal volume is much less than total lung capacity, but I will leave the rest of the post as it is while I work on the true figures.


Respiratory efficiency of each breathing cycle is reduced by 5%.

Doug - Why would the efficience be reduced with each breath, what happens after 20 breaths. please explain as this does not make since to me.

This does not affect oxygen transport because the pp O2 is increased but, as you know, the pp CO2 does not change.

Doug - it seems that with the lawss of gas all gasses should change in partial pressure. this is interesting concept as the gas is created by your own matabalism and then enters the lungs to be exhaled, I would think it would be in direct proportian to the depth you where at.

It is entirely dependent on metabolism. The net effect is that CO2 excretion is reduced by 5% during such a descent and this will surely be sufficient to cause a transient increase in pp CO2.

Doug - why is the co2 excreation reduced

On the other hand, if the descent is reduced to a slow 20 feet per minute and the respiratory rate is increased to 20 breaths per minute it would take five minutes to descend to the same depth during which you would take 5 x 20 = 100 breaths.

Doug - Hyperventalation....at 20 you are on the upper level of acceptable breathing rate, which i know will increase co2 so by increasing the breathing rate it seems that you would be increasing the co2 production. and your descent rate seems very slow which i would think would increase your overall time at depth therefore increasing your nitrogen intake, granted you would be shallower longer so the PN2 would be less. confusing issue

Each inhalation/exhalation cycle consists of 75%/100 more gas inhaled than exhaled. = 0.75%, which I suggest would have little perceiveable effect on CO2 excretion.

Doug - on the surface we inhale 21% O2 and exhale 16% O2 so that difference should be the same underwater, the nitrogen is the same in and out, and you add to the mix CO2 I am not sure how much but it can't be much

Conversely as I suggested above, if the diver simply holds his lung volume constant, by a slow inhalation, CO2 excretion will be zero. However if he does a "free dive" to depth the compression of the CO2 in the lungs will cause a reversal of the normal CO2 gradient marginally increasing basal pp CO2 in the blood.

Doug - Holding a constant lung volume does not make since as you want to completly emty your lungs with each breath at any depth to remove as much CO2 as possible so there need to be a difference in lung volume to empty and then refill


OK in free divers, but you then take a breath at 100 metres and a mainline slug of high partial pressure nitrogen.:boom:

Perhaps this is indeed why rapid descents provoke narcosis - N2 plus CO2? :idea:


Doug - rapid descents increase narcosis because of the rapid increase of PN2 each breath could be a whole ata difference, it is a nitrogen hit that causes it, whereas a slow increase in PN2 still results in the same effects it does not hit as dramaticy my oppinion

In addition, Aquatec, forgive me but in your question you said nothing about using Nitrox, doing any gas switches or indeed stopping your descent at 100 feet. Indeed in the proposed 3 minutes of descent you would get to 300 feet!! ut:

Doug - Yes you are right, i clarified it later for those who thought i would do this dive on just air

 

[The Iceni]

Here are the true figures, Doug!

For the sake of this argument let us assume total lung capacity (TLC) is 7.5 litres, a diver's tidal volume (TV) is 2 litres and you do 15 such "normal" breaths in the minute you descent to 100 feet (30M) to 4 bar.

Surface TLC is reduced by ambient pressure to 7.5/4 = 1.875 litres. Thus 7.5 - 1.875 = 5.625 litres must be added by the breathing medium during the entirety of the descent. This is added in the fifteen inhalations which on average means that each inhalation contains 5.625/15 = 375 mls more gas than each exhalation.

For each inhalation of 2 litres, there will be and expiration of 1.625 litres (An alternative way of looking at this is the gas already in the lung needed for expiration has been compressed to this lower volume by the 375 mls added). A ratio of 1.625/2= 81.25%. Respiratory expiratory efficiency is reduced BY 18.75%.

This does not affect oxygen transport because the pp O2 is increased but, as you know, the pp CO2 does not change. It is entirely dependent on metabolism. The net effect is that CO2 excretion is reduced by about 18% during such a descent and this WILL cause a transient increase in pp CO2 at the bottom of the fast descent to 100 feet even if you breath 15 times in that minute.

On a slow 20 feet per minute descent and an increased respiratory rate to 20 breaths per minute it would take five minutes to descend to the same depth during which time you would take 5 x 20 = 100 breaths. The same 5.625 litres must be added but in this case over 100 breaths, which on average means that each inhalation contains 5.625/100 = 56.25 mls more gas than each exhalation.

Expiratory respiratory efficiency is reduced TO 1.944/2 = 97.2%. Thus carbon dioxide excretion is still reduced but only BY 2.8% during such a slow descent.

This adds to my conviction that a rapid descent provokes narcosis by the synergistic effects of CO2 retention on an elevated pp N2 by this mechanism of "asymmetrical respiration".

It would seem this is one good reason to avoid such a rapid descent!

Needless to say, in the same way, a rapid ascent would increase expiratory resiratory efficiency blowing off CO2, but I suggest this not to be recommended for other reasons we all know about! :boom:

I think this makes sense.

 

[The Iceni]

Doug - would that 5% be the O2 that is metabolised?

Nothing to do with oxygen. There is more than enough. I think I explained it above. On the way down each actual breath in contains more gas than each breath out in order to compensate for the compression caused by ambient pressure, but lung excursion is the same two litres each time. I am not sure there is a good anaology to use but think what happens to a baloon or football as it is pressurised on descent. It collapses.

Why would the efficiency be reduced with each breath, what happens after 20 breaths. Please explain as this does not make since to me.

On the surface, each breath in is the same volume as each breath out. What you are effectively doing is adding air to the balloon on the way down to keep it's volume the same. You are not adding it all at the bottom, you are adding it in 15 portions simply because you are breathing 15 times in that minute it took you to reach 100 feet.

It seems that with the laws of gas all gasses should change in partial pressure. this is interesting concept as the gas is created by your own matabolism and then enters the lungs to be exhaled, I would think it would be in direct proportian to the depth you where at.

Not at all. The partial pressure of CO2 is entirely dependent on metabolism and respiratory excretion. The only source of CO2 is from active tissues burning glucose to produce CO2 and water molecules. The number of molecules produced is directly proportional to activity and the (partial) pressure of a gas is directly proportrional to the number of molecules of that gas in unit volume. As its is excreted by the lungs as soon as it is produced its partial pressure IN THE LUNGS (and consequntly the arteries and veins) does not rise by very much UNLESS excretion is reduced, in which case it can rise to toxic levels as more and more is produced by the body. More molecules in the same (lung) space means increased partial pressure

Why is the co2 excreation reduced?

I can well understand why this is a difficuly physiological concept to grasp. Think of it as a train with a CO2 molecule as the passengers with 100 seats per carriage and a maximum of ten carriages, which runs 15 times a day. An eight carriage train carries less seated passengers than a ten-carriage train; It is 80% as efficient.

On the surface ten carriages travel each way; full out and empty on the return journey. On the way down in the dive only eight carriages can be used on the outward journey (expiration) because two empty carriages must be added for the return journey as they have priority and the train can only take ten carriages.

On the way up in the dive it is the other way round. Eight empty carrriages can only be returned because room has to be made for the two extra full carriages that then have priority.

I am not sure if that is a good analogy but cannot think of a better way of explaining it.

Hyperventalation....at 20 you are on the upper level of acceptable breathing rate, which i know will increase co2

Quite the opposite, Doug! Hyperventilation causes more trains to run and more passengers to be carried so it lowers the number of passengers (CO2 molecules) left in the body. In fact it can cause severe metabolic disturbances.

so by increasing the breathing rate it seems that you would be increasing the co2 production.

no!

and your descent rate seems very slow which i would think would increase your overall time at depth therefore increasing your nitrogen intake, granted you would be shallower longer so the PN2 would be less. confusing issue

Now that's a different subject, altogether!

On the surface we inhale 21% O2 and exhale 16% O2 so that difference should be the same underwater, the nitrogen is the same in and out, and you add to the mix CO2 I am not sure how much but it can't be much.

On the surface. (very simply)

Vol in = Vol out

In 21% oxygen, + 79% nitrogen = 100%.

Out 16% oxygen + 79% nitrogen + 4% carbon dioxide + 1% water vapour = 100%

Holding a constant lung volume does not make since as you want to completly emty your lungs with each breath at any depth to remove as much CO2 as possible so there need to be a difference in lung volume to empty and then refill

of course! but you cannot COMPLETELY empty you lungs.

Rapid descents increase narcosis because of the rapid increase of PN2 each breath could be a whole ata difference, it is a nitrogen hit that causes it, whereas a slow increase in PN2 still results in the same effects it does not hit as dramaticy my oppinion.

but as far I am aware CO2 is known to increase the narcotic effects of nitrogen and rapid descents are notorious for causing more narcosis.

 

[AquaTec]

well this is all very interesting, I appreciat you bearing with me as you are trying to explain physiology to a layman. so I appologise if i do not understand some basics.


ok, my first question - how do you format your reply that way so that it is easy to distinguish the quote form the responce

2nd question. are you saying that your lungs reduce in size, that is what i am getting out of it. that doesn;'t make since to me.

my understanding is that the air molcules compress together so that at 10 ata it would take 10 times the volume of air to fill the same space as it would fill on the surface.

I understand that you still fill your lungs up with the same volume of air at depth as you do at the surface, it just requires more breathing gas to do so. in this case 10 times.

am I totaly off track with the point you where trying to make.

For the sake of this argument let us assume total lung capacity (TLC) is 7.5 litres, a diver's tidal volume (TV) is 2 litres and you do 15 such "normal" breaths in the minute you descent to 100 feet (30M) to 4 bar.

quote
Surface TLC is reduced by ambient pressure to 7.5/4 = 1.875 litres. Thus 7.5 - 1.875 = 5.625 litres must be added by the breathing medium during the entirety of the descent. This is added in the fifteen inhalations which on average means that each inhalation contains 5.625/15 = 375 mls more gas than each exhalation.

OK so here are you saying that the lung reduces in size or that it takes more gas to fill the lung, the TLC being reduced is whats stumping me here