Type 2 vs. Type 1 hit

[bleeb]

Could someone confirm what the labels are saying: Average total blood flow is 5.0 L/min. Of that, 0.7 L/min (14% of total) goes to the brain. 55 mL/100 g/min is the amount of blood per unit mass of brain. Etc.

Thanks.

 

[Charlie99]

Can anyone clarify what this 2:1 decompression gradient is? I've seen it referred to in several places, but don't understand what it is.

In Haldane's work back in the early 1900's he found that a human could be taken to 33' depth for many hours and then go back directly to the surface with acceptable (for that day and age) risk of DCS. He also observed that the same held true, more or less, for an ascent from 99fsw (4 ata) to 33 fsw (2ata). Again, a roughly 2:1 pressure gradient. That observation, with several refinements is still the basis behind much of decompression planning today. We have become more risk adverse, and the "minimum bends depth" for most models today is in the 20-25fsw range rather than the 33fsw of Haldanes 2:1 ratio.

One refinement was to note that the real pressure that counts is the dissolved N2 pressure vs. the total ambient pressure. Since N2 is 79% of air, 33fsw or 2ata depth has partial pressure of N2 of 1.58 and Haldane's 2:1 pressure rule is better described as a 1.58 to 1 ratio of dissolved N2 to ambient being the allowable pressure ratio.

Haldane himself recognized that not all sections of the body ongass and offgas N2 at the same rate, and this led to the neo-Haldanian decompression model of multiple compartments of varying halftimes for N2 ongassing and offgassing. Further testing showed that the allowable N2 to ambient pressure ratio varies with the halftime of the different compartments. The faster compartments can tolerate a greater pressure ratio or overpressure than can the slower compartments. While the exact compartments and limits vary from model to model, the basic math is the same for all neo-Haldanian models such as the many models by Bulhmann, including ZHL16a/b/c; the USN/Workman model upon which the USN tables are based; and the DSAT model upon which the PADI RDP is based.

So the 2:1 ratio of air pressure to total pressure became 1.58 to 1 when just N2 was considered. And then that ratio was changed to a whole series of ratios, a different ratio (or overpressure) limit for each compartment.

While bubble models such as RGBM and VPM have brought some minor changes to decompression planning, the basic multicompartment neo-Haldanian decompression model has proven to be very useful, and a good understanding of the neo-Haldanian model is important if you want to have good understanding of decompression.

A good article to read for further information is "Understanding M-Values" by Erik Baker. (M-values is one of the terms used for the maximum allowable dissolved N2 pressure for each compartment). http://www.ddplan.com/reference/mvalues.pdf

 

[Saturation]

Could someone confirm what the labels are saying: Average total blood flow is 5.0 L/min. Of that, 0.7 L/min (14% of total) goes to the brain. 55 mL/100 g/min is the amount of blood per unit mass of brain. Etc.

Thanks.

Yes. FYI, that image maybe copyrighted by its owners, who I don't know, I found it because it was well done and available easily through google images. Its fine to use under fair use guidelines but please check google to locate its original owner if beyond fair use.

Some clarifications on the whole discussion:

The tissue flow rates are average, and can vary for different physiologic states. What is best represented by the image is that flow to tissues are different for different tissues, the actual numbers are highly variable.

Tissue compartments are theoretical constructs used in decompression calculation.

Organs and tissues are real things. There is some work suggesting a relationship of tissue compartments to tissues, but they aren't the same strictly speaking. For example, fat tends to be present in masses all by itself, such in the omentum of the abdomen, but can co-exist within muscles itself. So, a strip of say bicep muscle has 2 tissues in it, but it is dominated by muscle if you are lean and young. Tissues also have intrinsic properties in regards to gas solubility.

Treating the organs or a combination of them as compartments is just a modeling tool for physiologic work. The whole body can be considered entirely as liquid water, and is thus non compressible at least tested to ~ 2000', it is thus, just one large compartment in this case. You can thus immerse a body to great depth and it will not crush like a can or deform, but getting him out of that depth is now a problem because we are faced with the true nature of biology versus a simple model. In radiology, the whole body is treated as hydrogen in MRI scans. Its true for that purpose, but not true in reality, but it works. Why hydrogen? Because of H20, again back to the model of the body as liquid, in this case water.