WOB questions

[nohappy]

I have some questions about WOB chart.
1. Is the X-axis unit Volume (liter)? If yes, what volume it is referring to? Lung? According to the chart, X value is decreasing during inhale, so I guess it's not the volume of lung?

2. I assume Y-axis is not absolute pressure value but difference pressure between 2nd stage and the environment. Is that correct?

3. If it's a P-V diagram, I guess I can say that the color area is the "work" of breathing. But if it's "work", the unit should Joule. How does is transform to Joule/liter?

4. If I google "Work of breathing", I could find some more articles that is discussing P-V diagram and respiratory work. However, the figure they described is more like the picture below. The shape goes diagonal and it passes the origin point. Are they talking the same thing?

 

[rsingler]

1. Yes

2. Yes. Your posted WOB loop looks to be calibrated in mm H2O.

3. One joule is the energy needed to move 1 l of gas through a 10-cmH2O pressure gradient. The work per liter of ventilation (J/l) is the work per cycle divided by the tidal volume (expressed in liters). In a healthy subject the normal value is around 0.35 J/l.

4. The regulator diagram is a mechanical lung which gets up to maximum inspiratory flow pretty quickly, holds that flow for a predetermined volume, then decelerates quickly and transitions to the expiratory loop.
The human diagram you posted has a slower acceleration (though a fairly rapid fall-off in inspiratory flow), and then a deliberate muscular exhalation with a slower fall-off as the lung empties.
The mechanical lung is designed to test regulator flow performance under (briefly) stable dynamic conditions (inspiratory flow stable for ~1 sec). As you see, the turbulence inside that particular regulator creates variable Venturi effects, with a "wobbly" effect on work of breathing. An "ideal" WOB loop would look more like this:

As the diver "sucks", negative pressure increases until the valve opens (cracking effort). Then, ideally, Venturi effects assist the diver by decreasing effort required to keep the valve open, until decelerating respiratory effort causes Venturi assist to fall off, and the valve then closes at the same negative pressure as before (it's usually slightly less). Then during exhalation, the curve should be closer to rectangular for the mechanical lung, with expiratory pressure solely a function of exhalation valve resistance.

 

[Dr Simon Mitchell]

Hello,

rsingler has answered the questions. Here is my take with some nuances not included in his answer.

View attachment 563882
I have some questions about WOB chart.
1. Is the X-axis unit Volume (liter)? If yes, what volume it is referring to? Lung? According to the chart, X value is decreasing during inhale, so I guess it's not the volume of lung?

Yes it is volume, but no, it is not lung volume. It is the volume of gas moved in a test machine. That volume is not specified in your diagram but we are told that the test was at 75 RMV (respiratory minute volume). This could be (for example) 37 breaths at 2 L volume per breath.

2. I assume Y-axis is not absolute pressure value but difference pressure between 2nd stage and the environment. Is that correct?

Correct. The scale is millibars which is essentially the same as cm (not mm) of water. cmH2O is the unit we are most familiar with in medicine when ventilating patients.

3. If it's a P-V diagram, I guess I can say that the color area is the "work" of breathing. But if it's "work", the unit should Joule. How does is transform to Joule/liter?

Answered by rsingler.

4. If I google "Work of breathing", I could find some more articles that is discussing P-V diagram and respiratory work. However, the figure they described is more like the picture below. The shape goes diagonal and it passes the origin point. Are they talking the same thing?

I would not waste any time trying to relate the pressure-volume curve of the lung (sometimes referred to as a dynamic compliance curve) to your regulator test curve. Although both use areas under a pressure-volume curve to infer work, the two have important differences. But if you want to understand it a little better, read on.

The lung is an elastic organ, and so the pressures you measure at the mouth when it is at functional residual capacity (which for our purposes can be interpreted as "relaxed") will be lower than the pressure measured when it contains a greater volume of gas after inspiring a tidal breath (because it will be stretched). Think of it like measuring the pressure at the inflation point of a balloon which is deflated then repeating the measurement when it is inflated. This is why the plot is sloped. That does not apply to the test curve for a regulator that you have posted.

For completeness, there are some weird things about that lung curve which differ from the classic physiologic depictions. The volume scale is wrong - showing only tiny changes as it does. Tidal volumes are measured in 100s of mls, not 10s as depicted. The x axis is usually depicted as intrapleural pressure which becomes increasingly negative as the lung volume increases. You can depict it as positive pressure change as in that graph (implying that you are inflating the lung with positive pressure in the airway) but this is less typical, and the numbers are all wrong because of the incorrect units on the Y axis. Also, minor point, but in my opinion, the figure does not definitively depict "a deliberate muscular exhalation" as suggested by rsingler. Curves with this appearance are typically the result of passive exhalation in which the elasticity of the lung drives gas flow outward through the airways without any help from the respiratory muscles.

Simon M

 

[W W Meixner]

Hello,

rsingler has answered the questions. Here is my take with some nuances not included in his answer.



Yes it is volume, but no, it is not lung volume. It is the volume of gas moved in a test machine. It is not specified in your diagram but we are told that the test was at 75 RMV (respiratory minute volume). This could be (for example) 37 breaths at 2 L volume per breath.



Correct. The scale is millibars which is essentially the same as cm (not mm) of water. cmH2O is the unit we are most familiar with in medicine when ventilating patients.



Answered by rsingler.



I would not waste any time trying to relate the pressure-volume curve of the lung (sometimes referred to as a dynamic compliance curve) to your regulator test curve. Although both use areas under a pressure-volume curve to infer work, the two have important differences. But if you want to understand it a little better, read on.

The lung is an elastic organ, and so the pressures you measure at the mouth when it is at functional residual capacity (which for our purposes can be interpreted as "relaxed") will be lower than the pressure measured when it contains a greater volume of gas after inspiring a tidal breath (because it will be stretched). Think of it like measuring the pressure at the inflation point of a balloon which is deflated then repeating the measurement when it is inflated. This is why the plot is sloped. That does not apply to the test curve for a regulator that you have posted.

For completeness, there are some weird things about that lung curve which differ from the classic physiologic depictions. The volume scale is wrong - showing only tiny changes as it does. Tidal volumes are measured in 100s of mls, not 10s as depicted. The x axis is usually depicted as intrapleural pressure which becomes increasingly negative as the lung volume increases. You can depict it as positive pressure change as in that graph (implying that you are inflating the lung with positive pressure in the airway) but this is less typical, and the numbers are all wrong because of the incorrect units on the Y axis. Also, minor point, but in my opinion, the figure does not definitively depict "a deliberate muscular exhalation" as suggested by rsingler. Curves with this appearance are typically the result of passive exhalation in which the elasticity of the lung drives gas flow outward through the airways without any help from the respiratory muscles.

Simon M

Dr. Mitchell...

Both excellent answers...having said that...ideal cracking pressure of the second stage regulator set by magnahelic/or bucket full of water is all that is required...and the optimum ''tune'' is ''comfortable''...usually delivering far more breathing gas than one would breathe at one atmosphere...

The single most common fault when tuning for optimum WOB is failing to realize that optimum WOB lives right next door to ''free-flow''...and while it is a pat on the back to the tech letting the customer breathe the freshly rebuilt/super-tuned reg in the shop...it's entirely a different story when that optimum tuned regulator is dipped below the surface in 38/45 degree fahrenheit water and it starts to free-flow like crazy...and no amount of diver adjustment will get it to stop...

It's not that complicated...and all the math is unnecessary...

W.W...

 

[Dr Simon Mitchell]

Dr. Mitchell...

Both excellent answers...having said that...ideal cracking pressure of the second stage regulator set by magnahelic/or bucket full of water is all that is required...

Hi WW,

Maybe correct for the inhalation side of things, but not for "total" work of breathing. You are forgetting that a work of breathing test also evaluates the work required for exhalation which has little to do with cracking pressure / tune of the regulator. It is a function of the design / geometry of the second stage and the exhalation valves (although I am not a UBA engineer).

Simon

 

[JackD342]

Dr. Mitchell...

Both excellent answers...having said that...ideal cracking pressure of the second stage regulator set by magnahelic/or bucket full of water is all that is required...and the optimum ''tune'' is ''comfortable''...usually delivering far more breathing gas than one would breathe at one atmosphere...

The single most common fault when tuning for optimum WOB is failing to realize that optimum WOB lives right next door to ''free-flow''...and while it is a pat on the back to the tech letting the customer breathe the freshly rebuilt/super-tuned reg in the shop...it's entirely a different story when that optimum tuned regulator is dipped below the surface in 38/45 degree fahrenheit water and it starts to free-flow like crazy...and no amount of diver adjustment will get it to stop...

It's not that complicated...and all the math is unnecessary...

W.W...

Warren- If I have read your post correctly, then I disagree with your first point. Cracking pressure does not equate to WOB, although it is one of the determining factors. Two different regulators set to the same cracking pressure are not automatically expected (or even likely) to produce the same average WOB - there is much more to WOB performance than that.

Yes, the ANSTI measured WOB performance differences between many regulators are often so good that most recreational divers will not be able to tell the difference. (what is the difference between "really good" and "really, really good"?) My rule of thumb is that anything at or below 1.0 joules/liter (using the EN250 ANSTI testing settings) is pretty darn good, and differences might only be noticed in extreme conditions.

 

[W W Meixner]

Warren- If I have read your post correctly, then I disagree with your first point. Cracking pressure does not equate to WOB, although it is one of the determining factors. Two different regulators set to the same cracking pressure are not automatically expected (or even likely) to produce the same average WOB - there is much more to WOB performance than that.

Yes, the ANSTI measured WOB performance differences between many regulators are often so good that most recreational divers will not be able to tell the difference. (what is the difference between "really good" and "really, really good"?) My rule of thumb is that anything at or below 1.0 joules/liter (using the EN250 ANSTI testing settings) is pretty darn good, and differences might only be noticed in extreme conditions.

Hey Jack...Happy New Year...

You're right...tuned cracking pressure does not equate to WOB...regulator design does that...the primary reason for an adopted design standard...that's why WOB in some early rebreathers was not so good...it's because of numerous factors built into the design...and before any accepted design standard being adopted...

Really good...really really good...I can see that kind of difference(s) when rebreathers are being discussed...when there's so many factors affecting WOB...but a typical...good quality...second stage regulator...I've yet to run across one in over 39 years that the work of breathing was difficult...maybe with one exception...that being...diving with a vintage double hose regulator...and then only if the regulator was positioned incorrectly on my back...I had a completely rebuilt two stage DA Aquamaster...used it on an ''H'' valve with redundant 1st/2nd/SPG on the left post...I had it down to 100 + ft...breathed like magic...as long as it was positioned properly...

When current generation regulators are being built to a standard...as you put it...'''most recreational divers will not be able to tell the difference''...then who and what is there to be concerned about...and what is ''extreme''...

When some of the popular/inexpensive ''Taiwan" brands are used to technical depths with no WOB difficulties...what level of ''extreme'' are we discussing finding ourselves in...300/400/500 ft...certainly nothing I'm ever going to worry about...as well as 99.999% of everyone else...the few left in that tiny fraction will find a way...always have...always will...some unfortunately will die trying...

W.M...

 

[rsingler]

It used to be that the technician tuning a G250 could play a role in WOB by how he/she adjusted the VIVA venturi vane. This was before SP shifted to a Pre-Dive/Dive knob.
That vane plays a key role in whether the WOB curve follows its ideal path just greater than zero inspiratory effort, or whether poor vane position creates such turbulence that inspiratory effort actually rises with high respiratory flow. Conversely, a vane tuned too "loose" allows such negative pressure inside the regulator case due to air flow Venturi effects, that the pull on the diaphragm becomes greater than your inspiratory effort and shifts the regulator into freeflow. These two effects become more pronounced at depth, with "thicker" air.
Regulator case design has now become sophisticated enough that manufacturers are able to design in features that enable minimal breathing effort due to Venturi assist without crossover. The trouble is, you don't know what you may or may not be getting without testing. More important, what used to be a specific adjustment to optimize flow is now "on or off", even if a vane position at depth slightly less than "full on" might be better at preventing freeflow.
Pete Wolfinger's technique as detailed in Regulator Savvy remains within the reach of a good shop, but since there's no money in it, very few shops will do it. But for the geek, you can build a dynamic flow device that allows you to plot inspiratory pressure against flow, to see just how your reg performs.
Here's a sample plot from an old G250.

The two lines show the extremes of vane position. The squiggly line shows the prevention of crossover that occurs with a slight tilt of the vane toward the predive position.

While these flows are not normally encountered even with vigorous breathing effort, they may approximate reg performance at depth, where thicker air means higher numbers of molecules passing thru the case, somewhat similar to high flow at one atmosphere.

Scubatools offers a flow bench for $1,100 after tax and shipping. I built this one for ~$250 from individual parts off Amazon, eBay and the hardware store:

 

[nohappy]

Yes it is volume, but no, it is not lung volume. It is the volume of gas moved in a test machine. That volume is not specified in your diagram but we are told that the test was at 75 RMV (respiratory minute volume). This could be (for example) 37 breaths at 2 L volume per breath.

Hey guys, appreciate for all your respond. @Dr Simon Mitchell I'd like to add one more question.
If it's the volume that move in the machine, why is it decreasing in inhaling phase (X value decreases)? In my imaginary, the machine should suck in (increase volume) during inhaling like a lung does? Or I should think negative X value as a concept of air "direction"? i.e. If it's moving out of the regulator, it's "negative" direction.

 

[Dr Simon Mitchell]

Hey guys, appreciate for all your respond. @Dr Simon Mitchell I'd like to add one more question.
If it's the volume that move in the machine, why is it decreasing in inhaling phase (X value decreases)? In my imaginary, the machine should suck in (increase volume) during inhaling like a lung does? Or I should think negative X value as a concept of air "direction"? i.e. If it's moving out of the regulator, it's "negative" direction.

Hello nohappy,

The distance along the x axis indicates the volume moved. It is not indicating an increasing or decreasing volume per se. We see the same thing in flow-volume loops of the lung. The loop works in the clockwise direction. Interpreted your way, they would indicate that lung volume is decreasing during inspiration and increasing during expiration. In reality, the x axis just shows the volume moved between residual volume (RV - a full exhalation) and total lung capacity (TLC - a full inspiration). It is a little counter-intuitive, and sometimes people label the x axis in the opposite direction for that reason.

Anyway, try to think of it as the volume moved in either direction rather than an increasing or decreasing volume.

Simon