The Iceni
Medical Moderator
Hi again Doug,
I am only exploring possibilities and playing physiological gymnastics. Certainly this is not a major problem. However, perhaps you really do need to read a text on basic respiratory physiology. The following thread may also help
http://www.scubaboard.com/showthread.php?s=&threadid=13630
as will the attachement, which shows the anatomical lung volumes.
Forget about oxygen, nitrogen and DCI for a minute and think only about carbon dioxide. It is constantly produced by the body and is excreted by the lungs. The rate of excretion depends on both the breathing rate and the volume of each breath, which determines the "volume" of gas into which the CO2 diffuses. As you rightly say the lung volumes remain relatively constant during normal balanced breathing on the surface and when diving.
So, for simplicity once more consider a tidal volume of 2 litres and a respiratory rate of 10 breaths per minute is adequate for the level of excercise at the surface and during the descent and that the alvelolar end-expiratory pp CO2 is stable at 0.1 bar (again for the sake of simplicity). Total lung volume varies from, say, 6 litres to 4 litres - the lungs NEVER empty completely. (By the way, an SAC of 20 litres per minute is a bit on the high side.)
On the surface and at any constant depth 2 litres of fresh breathing medium is inhaled and 2 litres of stale air is exhaled, together with two litres worth of CO2 at the pp CO2 in the veins of 0.1 bar. The system is in equilibrium the amount of CO2 excreted exactly equalling the amount of CO2 produced by the body and the diver is comfortably controlling his breathing rate and lung volmes at 6 litre inhalation and 4 litres exhalation.
Now he is on the surface, takes a normal breath to inflate his lungs to 6 litres and instantly descends to 3 metres before he has had a chance to exhale.
How much stale air will the diver exhale in order to return his actual lung volume to the comfortable 4 litres?
Work that out before reading on, Doug.
Ambient pressure at 3 metres is 1.3 bar, so the 6 litres of (now stale) air in his lungs has been compressed to 6/1.3 = 4.6 litres. This does two things. Firstly CO2 in the lungs has been compressed by ambient pressure raising its partial pressure to 0.13 bar and secondly, in answer to the question, the diver will only exhale 600 mls of stale air before his chest wall receptors tell him that he has exhaled to the comfortable end-expiratory volume of 4 litres.
This repesents an expiratrory deficite of 2,000 - 600 = 1,400 mls or a reduction to 600/2000 = 30% efficiency.
600 mls of CO2 at 0.13 bar = 78
2 litres of CO2 at 0.1 bar = 200.
With this first breath the diver has excreted about a third of the CO2 he normally would.
The diver now takes a normal breath at 3 metres to reinflate the lungs to 6 litres. He inhales 2 litres of breathing medium which dilutes the remaining CO2 in the lungs but to this has been added the CO2 produced by metabolism and so the end -expiratory pp CO2 will be higher than it was on the surface, simply because less has been excreted with the previous breath.
The diver now descends from 3 meters (1.3 bar) to 6 metres (1.6 bar) with a pressure increase of 1.6/1.3 = 1.23. The 6 litres of stale air in his lungs is compressed to 6 x 1.3/1.6 = 4.875 litres. The diver will only exhale 875 mls of stale air; a deficite of 2,000 - 875 = 1,125 mls. 875/2000 = 44% efficiency.
875 mls of CO2 at 0.123 bar = 107
2 litres of CO2 at 0.1 bar = 200.
CO2 excretion is half normal.
So back to my original post. Over the whole excursion from the surface to 100 feet (4 bar) using these figures the respiratory deficite is as follows
TLC = 6 litres, at 4 bar = 1.5 litres
6 - 1.5 = 4.5 litres over the period of the descent.
Divide by ten if ten 2-litre breaths are taken during the descent, however long it takes, and the average deficite is 450 mls per breath.
Calculate the average exhale;2,000 - 450 = 1,550.
1,550/2,000 = 78%
Divide by twenty if twenty 2-litre breaths are taken during the descent, however long it takes, and the average deficite is 225 mls per breath. 2,000 - 225 = 1,775. 1,775/2,000 = 88%
So, in answer to your original question Doug, in addition to the other problems, there will be a transient increase in pp CO2 unles the diver increases his respiratory rate to comensate for reduced ventilatory efficiency during a fast descent.
This may partly explain the cause the severe narcosis reported with fast decents.
I hope that explains how I see it
I am only exploring possibilities and playing physiological gymnastics. Certainly this is not a major problem. However, perhaps you really do need to read a text on basic respiratory physiology. The following thread may also help
http://www.scubaboard.com/showthread.php?s=&threadid=13630
as will the attachement, which shows the anatomical lung volumes.
I have sent you a private message with a few tips.
This boils down to Boyles law! and you are right in saying 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. . . it just requires more breathing gas to do so. in this case 10 times.
Perhaps I should not have started this train of thought because it is somewhat esoteric?
Forget about oxygen, nitrogen and DCI for a minute and think only about carbon dioxide. It is constantly produced by the body and is excreted by the lungs. The rate of excretion depends on both the breathing rate and the volume of each breath, which determines the "volume" of gas into which the CO2 diffuses. As you rightly say the lung volumes remain relatively constant during normal balanced breathing on the surface and when diving.
So, for simplicity once more consider a tidal volume of 2 litres and a respiratory rate of 10 breaths per minute is adequate for the level of excercise at the surface and during the descent and that the alvelolar end-expiratory pp CO2 is stable at 0.1 bar (again for the sake of simplicity). Total lung volume varies from, say, 6 litres to 4 litres - the lungs NEVER empty completely. (By the way, an SAC of 20 litres per minute is a bit on the high side.)
On the surface and at any constant depth 2 litres of fresh breathing medium is inhaled and 2 litres of stale air is exhaled, together with two litres worth of CO2 at the pp CO2 in the veins of 0.1 bar. The system is in equilibrium the amount of CO2 excreted exactly equalling the amount of CO2 produced by the body and the diver is comfortably controlling his breathing rate and lung volmes at 6 litre inhalation and 4 litres exhalation.
Now he is on the surface, takes a normal breath to inflate his lungs to 6 litres and instantly descends to 3 metres before he has had a chance to exhale.
How much stale air will the diver exhale in order to return his actual lung volume to the comfortable 4 litres?
Work that out before reading on, Doug.
Ambient pressure at 3 metres is 1.3 bar, so the 6 litres of (now stale) air in his lungs has been compressed to 6/1.3 = 4.6 litres. This does two things. Firstly CO2 in the lungs has been compressed by ambient pressure raising its partial pressure to 0.13 bar and secondly, in answer to the question, the diver will only exhale 600 mls of stale air before his chest wall receptors tell him that he has exhaled to the comfortable end-expiratory volume of 4 litres.
This repesents an expiratrory deficite of 2,000 - 600 = 1,400 mls or a reduction to 600/2000 = 30% efficiency.
600 mls of CO2 at 0.13 bar = 78
2 litres of CO2 at 0.1 bar = 200.
With this first breath the diver has excreted about a third of the CO2 he normally would.
The diver now takes a normal breath at 3 metres to reinflate the lungs to 6 litres. He inhales 2 litres of breathing medium which dilutes the remaining CO2 in the lungs but to this has been added the CO2 produced by metabolism and so the end -expiratory pp CO2 will be higher than it was on the surface, simply because less has been excreted with the previous breath.
The diver now descends from 3 meters (1.3 bar) to 6 metres (1.6 bar) with a pressure increase of 1.6/1.3 = 1.23. The 6 litres of stale air in his lungs is compressed to 6 x 1.3/1.6 = 4.875 litres. The diver will only exhale 875 mls of stale air; a deficite of 2,000 - 875 = 1,125 mls. 875/2000 = 44% efficiency.
875 mls of CO2 at 0.123 bar = 107
2 litres of CO2 at 0.1 bar = 200.
CO2 excretion is half normal.
So back to my original post. Over the whole excursion from the surface to 100 feet (4 bar) using these figures the respiratory deficite is as follows
TLC = 6 litres, at 4 bar = 1.5 litres
6 - 1.5 = 4.5 litres over the period of the descent.
Divide by ten if ten 2-litre breaths are taken during the descent, however long it takes, and the average deficite is 450 mls per breath.
Calculate the average exhale;2,000 - 450 = 1,550.
1,550/2,000 = 78%
Divide by twenty if twenty 2-litre breaths are taken during the descent, however long it takes, and the average deficite is 225 mls per breath. 2,000 - 225 = 1,775. 1,775/2,000 = 88%
So, in answer to your original question Doug, in addition to the other problems, there will be a transient increase in pp CO2 unles the diver increases his respiratory rate to comensate for reduced ventilatory efficiency during a fast descent.
This may partly explain the cause the severe narcosis reported with fast decents.
I hope that explains how I see it
