Calculating deco and mix

[Bismark]

Oh drahk, I just read your post about Isobaric Counterdiffusion........'gonna be a long weekend.

 

[TSandM]

Pulmonary vasoconstriction is going to affect the A-a gradient (alveolar to arterial) because of what's called V/Q mismatch, where more ventilation is going to areas that aren't well perfused. The degree to which breathing high O2 mixes affects this is not something I know; in land-based medicine, we ignore the effect of 100% O2 on V/Q mismatch, because it will reliably maximize arterial ppO2 compared with any other gas mix we have available to breathe.

Another thing I've long viewed with suspicion is the statement that it takes about 3 minutes for a blood volume to circulate. In fact, the blood volume of an average adult is about 70 to 80 cc/kg body weight. This correlates with about 5 liters in the "average" 70 kg male. Interestingly, normal cardiac output in the same person is likely to be about 5 l/min. This would suggest to me that the circulation time is more in the neighborhood of one minute, although obviously there are better and more poorly perfused regions, and if you want to get a complete turnover of blood in all tissues, you may well require more than one circulation time. But I still wonder where that number comes from.

 

[Bismark]

Another thing I've long viewed with suspicion is the statement that it takes about 3 minutes for a blood volume to circulate. In fact, the blood volume of an average adult is about 70 to 80 cc/kg body weight. This correlates with about 5 liters in the "average" 70 kg male. Interestingly, normal cardiac output in the same person is likely to be about 5 l/min. This would suggest to me that the circulation time is more in the neighborhood of one minute, although obviously there are better and more poorly perfused regions, and if you want to get a complete turnover of blood in all tissues, you may well require more than one circulation time. But I still wonder where that number comes from.

I will add this to the list to find out where the number comes from. I have heard anything between 3 minutes and 5 minutes. Amazing how much of this stuff we "hear" but when you start looking into it further it becomes a "heard it from someone" type of thing. Call it my grad school training but I prefer primary sources.....

 

[limeyx]

If Charlie reads this post perhaps he has a simple explanation as his posts seem to be the clearest on all topics regarding decompression.

Oh yes, I will remember to run all my profiles by Charlie for approval from now on

 

[Bismark]

Nick, do you limit your time at 70' to 3 minute as per GUE or do you add an extra minute or so at that depth if the 70'-30' curve becomes more "elegant" if you do so?

 

[limeyx]

Nick, do you limit your time at 70' to 3 minute as per GUE or do you add an extra minute or so at that depth if the 70'-30' curve becomes more "elegant" if you do so?

never heard anything about 3 mins max, we often do more than that on a T1 dive and on a T1.5 dive you can have way more than 3 mins there.

sometimes we add the 1 min deep stop at 80 feet in to the 70 stop as well, as its better to do it on the 50% than on back gas.

30 mins deco @ T1 = 4 mins at 70,60 for 60 mins deco, it's more like 9,9 so it definitely depends.

I am going to have to run all this by Charlie first though, so take that with a pinch of salt!

 

[Bismark]

never heard anything about 3 mins max, we often do more than that on a T1 dive and on a T1.5 dive you can have way more than 3 mins there.

sometimes we add the 1 min deep stop at 80 feet in to the 70 stop as well, as its better to do it on the 50% than on back gas.

30 mins deco @ T1 = 4 mins at 70,60 for 60 mins deco, it's more like 9,9 so it definitely depends.

I am going to have to run all this by Charlie first though, so take that with a pinch of salt!

Too Funny...that last bit.

My understanding is the latest GUE take on the 70' stop is that vasoconstriction significantly reduces the effectiveness of the 02 window after 3 minutes and at that point you benefit more from an increased gradient. I am pretty sure this might just apply to Tech I level stuff because otherwise if you need longer times at 70' to shape the curve, how do you justify cutting it short? I have some feelers out for some more on this and I am starting to go cross-eyed with the 02 window stuff. We regularly add to the 70' stop in the tech I or tech II range regardless of whether the window is "closed" or "wide open", but I suspect this has more to do with shaping the proper curve than it does with the 02 window. I definitely know we don't like making too big of jumps from one stop to the next in terms of adding "hard turns" to the curve. As you said, this often necessitates longer times at 70'.

 

[JeffG]

My understanding is the latest GUE take on the 70' stop is that vasoconstriction significantly reduces the effectiveness of the 02 window after 3 minutes

after 3 mins? sounds like BS to me.


gee...why do we do 12/6 with 100% @ 20 ft then.

 

[wedivebc]

the tech I or tech II range regardless of whether the window is "closed" or "wide open", but I suspect this has more to do with shaping the proper curve than it does with the 02 window. .

Please explain this window open, closed thing. Is it sometimes open then suddenly shuts?

 

[JeffG]

Please explain this window open, closed thing. Is it sometimes open then suddenly shuts?

gotta watch your fingers too.