Quiz - 26 - Diving Knowledge Workbook - Diving Physiology

[TMHeimer]

Actually the term "gradient" here is used slightly improperly, as the pressure gradient is the ratio between the pressure difference and the distance "d" of the two points where you did measure the pressure:
grad(p) = (pi-p2)/d
Well, for being pedantic, the gradient is obtained as the limit of the above ratio when d is reduced towards zero...
So the correct wording should have been "pressure difference", not "pressure gradient".
Perhaps this confused some people, also because the term "gradient" is not really of common use outside a physics classroom. "Difference" is much simpler, and maintains the correct meaning...

You could be right. Difference seems better to the layman like me. I DID know which answer was correct in 2009 because I had the sentence memorized....forgot which way it goes.

 

[EFX]

Actually the term "gradient" here is used slightly improperly, as the pressure gradient is the ratio between the pressure difference and the distance "d" of the two points where you did measure the pressure:
grad(p) = (p1-p2)/d
Well, for being pedantic, the gradient is obtained as the limit of the above ratio when d is reduced towards zero...
So the correct wording should have been "pressure difference", not "pressure gradient".
Perhaps this confused some people, also because the term "gradient" is not really of common use outside a physics classroom. "Difference" is much simpler, and maintains the correct meaning...

As you say pressure gradient is not the correct term and requires casual readers to read further to acquire the exact definition and how it applies in diving physiology. Coming from an electrical engineering background we used terms like voltage drop or voltage rise to describe current flow through a resistance. Pressure drop, pressure rise or pressure difference is more descriptive in explaining inert gas flow. Unfortunately, the term gradient is here to stay just as NDL (no decompression limit) is, which is another imprecise term that requires more explanation than the more accurate and simpler term NSL (no stop limit).

 

[Angelo Farina]

Furthermore, my understanding of giving oxygen to a patient with DCS was not for speeding up the Nitrogen elimination, as this question seems to suggest.
Oxygen is given for providing oxygen to tissues which are suffering of poor blood flux, caused by the embolism...
Actually oxygen is given also for other forms of embolism, not related with Nitrogen accumulation.
An MD could give us more detailed info on this topic.

 

[EFX]

The choice of answers would have been the same without the reference to DCS which in my view complicates the main idea for giving oxygen.

 

[boulderjohn]

Well, for being pedantic, the gradient is obtained as the limit of the above ratio when d is reduced towards zero...
So the correct wording should have been "pressure difference", not "pressure gradient".

Pressure drop, pressure rise or pressure difference is more descriptive in explaining inert gas flow. Unfortunately, the term gradient is here to stay just as NDL (no decompression limit)

All terms have to be considered in context and in their common usage. "Pressure gradient," for example, is a term used in weather to describe a change in pressure across a given distance. Pressure gradient has been used in scuba for decades this way, and when a word is commonly used and understood to mean a specific thing in a specific context, then that is what it means.

If you were to use the term "pressure drop," my immediate reaction would be that it refers to something like karma, where if you do something bad to someone, something bad will eventually happen to you. For context, I'll let Toots explain it:

 

[elmo]

Actually the term "gradient" here is used slightly improperly, as the pressure gradient is the ratio between the pressure difference and the distance "d" of the two points where you did measure the pressure:
grad(p) = (p1-p2)/d
Well, for being pedantic, the gradient is obtained as the limit of the above ratio when d is reduced towards zero...
So the correct wording should have been "pressure difference", not "pressure gradient".
Perhaps this confused some people, also because the term "gradient" is not really of common use outside a physics classroom. "Difference" is much simpler, and maintains the correct meaning...

You are completely correct, and this confused me no end when I started reading decompression theory years ago and was trying to work out what the 'distance' variable could be. Was it time? Was it a theoretical tissue parameter? Eventually I accepted it was just used to refer to the pressure difference.
For example, the common term gradient factor is a reduction factor applied to the "gradient" (pressure difference) between the gas pressure in a theoretical tissue and the lowest tolerable ambient pressure.
I suppose we're stuck with gradients until somebody comes up with a better model along with better terminology.

 

[EFX]

I would like to take some time to elaborate on the implication of pressure difference as it relates to diving. When changing depth there will be a pressure difference between the lungs and blood and between the blood and the tissues. For simplicity if the difference in pressure of the inspired inert gas between the lungs and the tissues is positive then ongassing will result. If the difference is negative then we will be offgassing. There must be a pressure difference to cause inert gas flow into or out of the tissues. If the pressure difference is zero there is no flow and the tissues will be saturated at the inspired inert gas pressure.

On ascent the reduction of ambient pressure causes a reduction of inspired inert gas pressure whcih creates a positive pressure difference between the tissues and the lungs. This causes the inert gas to flow out of the tissues which we call offgassing. DCS is caused when the rate of offgassing from the tissues exceeds the safe rate at which the blood/lung system can remove the inert gas. Flow is related to the pressure difference divided by the resistance to flow between the tissues and the lungs. Since we can't measure flow we use pressure (difference) to determine a tissues maximum inert gas uptake that would exceed safe rates of offgassing on ascent.

The Buhlmann model uses the Buhlmann equation or an alternate form of it called the Schreiner equation to calculate a tissues pressure (relative to zero) during the dive. Here's the Schreiner equation which can be used for all phases of the dive (descent, ascent, and constant depth):

P = Pio + R(t - 1/k) - [Pio - Po - (R/k)]e^-kt

where:
P = compartment inert gas pressure (final)
Pio = initial inspired (alveolar) inert gas pressure (called Pi below)
(Pio = initial ambient pressure minus water vapor pressure)
Po = initial compartment inert gas pressure
R = rate of change in inspired gas pressure with change in ambient pressure
(this is simply rate of ascent/descent times the fraction of inert gas)
t = time (of exposure or interval)
k = half-time constant = ln2/half-time (same as instantaneous equation)

For the sake of argument let's assume constant depth (R = 0) which reduces the equation to:

P = Po + (Pi - Po)(1 - e^-kt)

The term (Pi - Po) is the pressure difference between the inspired (ambient) inert gas pressure (Pi) and the tissue pressure (Po). On descent to a deeper depth Pi will be greater than Po which results in ongassing. On ascent the term becomes negative which causes the tissues to offgas. The dive computer is doing this calculation every period which is set internally in the software. For modern computers it's usually a second but the period can vary depending on the manufacturer and price of the computer. For each iteration of the program a new P is calculated which is used to determine the ceiling (or NDL) at the current depth. Once the calculations are done the new value of P is saved and becomes the Po in the next iteration of the program. If we remain at depth, eventually Pi equals Po and the equation reduces to P = Po or the new tissue compartment pressure equals the last calculated TC pressure.

 

[elmo]

DCS is caused when the rate of offgassing from the tissues exceeds the safe rate at which the blood/lung system can remove the inert gas.

Not quite. The Bühlmann (and VPM) model doesn't determine a "safe rate of offgassing" but a safe pressure difference between the pressure of nitrogen (and helium) in the tissue and the ambient pressure at the diver's depth.
Yes, increasing the pressure difference (by ascending to a shallower depth) will increase that rate and exceed the tolerable pressure difference limit, but the rate of offgassing can also be increased safely without ascending by breathing a gas with less nitrogen and helium (and more oxygen). The distinction is subtle but is incredibly important when it comes to choosing deco gasses.

 

[EFX]

Not quite. The Bühlmann (and VPM) model doesn't determine a "safe rate of offgassing" but a safe pressure difference between the pressure of nitrogen (and helium) in the tissue and the ambient pressure at the diver's depth. Yes, increasing the pressure difference (by ascending to a shallower depth) will increase that rate and exceed the tolerable pressure difference limit, but the rate of offgassing can also be increased safely without ascending by breathing a gas with less nitrogen and helium (and more oxygen). The distinction is subtle but is incredibly important when it comes to choosing deco gasses.

You're connecting dots in my post that shouldn't be connected. But, thanks for the additional information.

The Buhlmann model uses the Buhlmann equation or an alternate form of it called the Schreiner equation to calculate a tissues pressure (relative to zero) during the dive.

 

[Pedro Burrito]

Spoiler: Answer

b. increases the pressure gradient between the nitrogen pressure in the tissues and the alveolar nitrogen pressure.

...the difference in the pressure within a liquid (gas tension) and the gas in contact with it is referred to as the pressure gradient. When this condition occurs, the gas within the liquid and that in contact with the liquid will attempt to equalize. A large pressure gradient is said to create a high driving force, meaning the gas exchange will take place rapidly. In DCS the tissues of the body contain a high level of nitrogen. The alveolar air also has nitrogen within it, but less than what is within the tissues. As a result, there is a pressure gradient between the nitrogen in the tissues (high) and the nitrogen within the alveolar air (low). Hence, the nitrogen begins to come out. But, if instead of air the diver breathes oxygen, there will be no - or very little - alveolar nitrogen. This will further increase the pressure gradient between the tissue nitrogen levels and the alveolar nitrogen levels. The result is a significant increase in the driving force of the tissue nitrogen, thus aiding in its elimination.