Why no N2 bubbles?

[ReefHugger]

I have never had a proper explanation for this.

The purpose of using tables and observing ascent rate and stops is to control the outgassing of nitrogen. That is, we are controlling the partial pressures differential of nitrogen. When using pure oxygen (transporting victim to med facilities; final 10' deco stop, say), this pressure differential is suddenly up to maximum. Why then are nitrogen bubbles not formed? Or are they?

 

[1_T_Submariner]

I could be wrong but, I think when breathing O2 they are not absorbing more (in the case of a Deco stop). The N2 not being present in the lungs is allowing a more effiecent exhange of N2 out of the blood an into the air you are exhaling. I'm sure you will get some better explanations that this.

 

[Steve_Dives]

You're on it Submariner!

The partial pressure of nitrogen in the breathing gas decreases with decreasing depth. The nitrogen dissolved in the blood tries to reach equilibrium with the breathing gas, and any other tissues start a trend towards the new equilibrium. The tables, ascent rates, saftey stops, etc. are intended to keep the number and size of bubbles to a minimum (which they do a good job of), however individuals and different days will have varying results.

 

[fisherdvm]

I think that at the 10 ft stop, and at the surface, the bubbles are already formed. I think the use of pure oxygen is to maximize the diffusion of nitrogen dissolved in the blood already outward, and maximimize the delivery of oxygen to the brain, the spinal cord, and nerves - which might be starved of oxygen from the micro-infarction or clogs from the nitrogen bubbles.

Using room air or pure oxygen would not affect the size or nor the numbers of nitrogen bubbles already formed, but I think perhaps we can prevent more bubbles from forming by allowing maximum diffusion down the partial pressure gradient into the lung.

Where is Dr. Compression today?

 

[H2Andy]

When using pure oxygen (transporting victim to med facilities; final 10' deco stop, say), this pressure differential is suddenly up to maximum.

nope...

the "pressure differential" remains the same; in other words, you are not creating an artificially different pressure gradient by using 100% O2

what changes is the relative percentages during the gas exchange, so someone breathing in no N2 will be able to release A TON more N2 than if he/she were breathing in 80% N2

in other words, by breathing pure O2, you are able to release tons more N2 (or any other inert gas, for that matter). it's good for you. it's what you want to happen.

(of course, this assumes you're on the surface. if you are at depth, ox tox becomes an issue. but still, breathe the highest safe O2 mix available)

 

[ReefHugger]

Hi H2Andy, Thanks for the reply.

But that's my questions. Why doesn't the 'tons of N2' come out in bubbles??? The pressure of N2 in the tissues is at max and zero in the blood!

To fisherdvm - I think there is potential for 'tons more bubbles' after the dive. Hence the requirement for surface interval.

 

[H2Andy]

The pressure of N2 in the tissues is at max and zero in the blood!

no... there is no pressure diferential between the blood and the tissue N2

they are both at ambient pressure, whatever that may be

(at the surface, that's 1 ATM)

the "bubbles" are coming out of solution because the pressure to hold them there is insufficient *all over the body*

 

[ReefHugger]

Think Dalton's Law of partial pressures.
There is no N2 in the blood (breathing pure O2), consequently partial pressure of 0.
Tissues saturated (especially DCS victim) with N2, high partial pressure. This creates the pressure gradient.

 

[H2Andy]

you are confusing the rate at which N2 comes out of solution with the rate at which N2 is expelled from the body though the respiratory system.

also, just because you are breathing 100% O2 does not mean that there is no N2 in the blood.

you've gotten two related but distinct issues jumbled up

blood, tissue, bone, it's all under the same pressure. breathing 100% O2 doesn't change things.

it does faciliate getting excess N2 out of the body (through the respiratory system)

think about it. if what you say is true, how come first thing EMS does to a bent diver is hook them up to 100% O2?

Dalton's law simply says that the total pressure of a mixture is equal to the partial pressure of all gases in that mixture.

breathing in 100%02, the total pressure is x.

breathing out N2 plus 02, the total pressure is still going to be x.

the difference is that since no N2 came in, more N2 goes out due to the principles of gas exchange.

 

[dsteding]

Fick's Law:

Rate of diffusion is proportional to the gradient of partial pressures between mediums. When breathing pure O2, the partial pressure gradient is maximized between blood and the lungs, and diffusion out of the blood into the lungs of N2 is maximized.