Toggling effect

[Kendall Raine]

I reread the thread about 100 v. 80/20 and no one mentioned the importance of the toggling effect directly. The toggling effect is George Irvine's term for air break. Specifically, in the WKPP deco procedures, this is not an air break, but rather a break from high PO2 to low PO2. Those divers will switch from, say 1.6 ATA or higher to backgas resulting in say .16 ATA. Irvine attributes various beneficial physiological effects including amelioration of vasoconstriction inherent in prolonged high PO2 breathing, reduction in capillary bed swelling and other tertiary high PO2 effects to this toggling. Preventing these negative reactions allows maximal deco efficiency. This advantage is so profound that Irvine stipulates that the low ox break be counted toward total deco time 1:1. Obviously, the WKPP profiles include sat exposures where both CNS and pulmonary toxicity are factors-not the case for most "tech" exposures.

In the context of the 100 v. 80/20 discussion, the toggling effect is not accounted for in any model of which I'm aware, which, if valid, makes the model indicated deco time comparison less meaningful. Seems to me the only way to really test this is with a Doppler. Naturally, that concedes the significant limitations of the Doppler in predicting a hit, but it would provide some basis for comparative deco stress assessment.

Has any controlled work been done on this?

 

[Dr Deco]

Hi Kendall:

Vasoconstriction

This is certainly a real effect in high-pressure oxygen breathing. There have been some research groups that found they could not decompress successful with pure oxygen. They found that it was necessary to insert air breaks. (This is reported in the literature, but I do not recall where I read it.) Other groups have been successful without the insertion of but a few pauses.

I suspect that one difference could be in the movement of the divers. The group that absolutely needed the air breaks instructed their divers to avoid all unnecessary movement.
They believed that the movement would lead to DCS. While I believe there is truth to this, especially if the diver is standing and maybe even walking around in the decompression chamber, movement of the arms and legs is not a primary nucleating event. It will, however, foster the formation of carbon dioxide, which is a vasodilator. Carbon dioxide formation (in moderate amounts) is valuable to counteract capillary closure.

Physical Activity

I suspect that vasoconstriction is reverse with physical activity. I have seen this in my studies at NASA, and, in conjunction with data from other laboratories (Duke University and Armstrong laboratory) it has been very useful for us.

Utility

I believe that much of the rapid decompression for deep diving can be attributed to the reduction in nuclei formation by being adynamic (weightless) in the water. If combined with some activity, it can probably be very useful in accelerating decompression. If the diver is too quiescent during the oxygen breathing cycles, however, the vasoconstriction will be a limitation.

The bottom line is that I do not believe that “toggling” would be necessary with some arm and leg movement. I do not have any laboratory data to substantiate this in divers, however.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm