Saturday, May 16, 2009

Decompression & Theoretical Tissue Compartments

TurtlePhoto by Erika Antoniazzo - Decorated by Me

Today I had an opportunity to teach a handful of PADI divemaster candidates Decompression Theory and the eRDPML. I thought I would share a little information about theoretical tissue compartments and calculating tissue pressure.

Background Information

n the early 1900's the British Navy found it too expensive to train scuba divers only to have them retire early because they were experiencing the bends frequently. In 1905 they contracted Dr. John Scott Haldane to research the causes and possible methods of preventing decompression sickness (DCS). After experimenting on goats, Haldane concluded that when descending, nitrogen pressure in compressed air is higher than in a divers body, so body tissue absorbs dissolved nitrogen and eventually body tissue will saturate and absorb no additional nitrogen. Once a divers begins to ascend, the nitrogen in the body tissue is higher than the surrounding pressure and the body tissue releases the nitrogen. Pressure gradient is the difference between the dissolved nitrogen pressure and the surrounding pressure. If a divers ascends correctly, the body can handle the gradient pressure without DCS, however, if the gradient pressure exceeds acceptable limits, bubbles form and cause DCS. Haldane completed his experiments in 1906 and published his theoretical model and no-decompression tables in 1908. Today, virtually all dive tables and dive computers calculate their no decompression limits based on his theoretical decompression model.

Tissue Compartments, Halftimes, & M-Values

Haldane built his theoretical model based on the fact that different parts of the body absorb and release dissolved nitrogen at different rates, therefore his model is based on multiple theoretical tissue compartments. His original model included five tissue compartments, today's versions have 14 or more compartments. Each theoretical compartment has a halftime which represents the time in minutes it takes to absorb and release nitrogen and is expressed in meters/feet sea water (msw/fsw). Each compartment takes six halftimes to fully saturate or empty. Original Haldane models ranged from 5 to 75 minutes. Today's models range from 3 to more than 600 minutes. In the 1960's, Dr. Robert Workman reviewed Haldane's model and revised it noting that each tissue compartment could tolerate different levels of over-pressurization and was depth dependent. His revision introduced the M-value which describes the amount of over-pressurization each compartment could tolerate at any depth.

Calculating Tissue Pressure

Ok, now we get to the problem or the problems. The problem is that students sometimes have trouble with this part of the lesson. They have trouble understanding how to calculate tissue pressure. Here are a couple of examples that break the calculations down into their simplest terms.

QUESTION: A 5 minute halftime compartment will have how much tissue pressure 5 minutes after its taken from the surface to 90 feet in seawater?

SOLUTION:

HALFTIMES MINUTES DEPTH TISSUE PRESSURE
1 5 90.00 fsw ÷ 2 = 45.00 fsw 45.00 fsw

- After 5 minutes, a 5 minute compartment will saturate 1 halftime.
- 1 halftime is 1/2 the total depth: 90.00 ÷ 2 = 45.00

QUESTION: A 5 minute halftime compartment will have how much tissue pressure 20 minutes after its taken from the surface to 90 feet in seawater?

SOLUTION:

HALFTIMES MINUTES DEPTH TISSUE PRESSURE
1 5 90.00 fsw ÷ 2 = 45.00 fsw 45.00 fsw
2 10 45.00 fsw ÷ 2 = 22.50 fsw 45.00 fsw + 22.50 fsw = 67.50 fsw
3 15 22.50 fsw ÷ 2 = 11.25 fsw 67.50 fsw + 11.25 fsw = 78.75 fsw
4 20 11.25 fsw ÷ 2 = 5.63 fsw 78.75 fsw + 5.63 fsw = 84.38 fsw


QUESTION: A 5 minute halftime compartment will have how much tissue pressure 40 minutes after its taken from the surface to 90 feet in seawater?

SOLUTION:

HALFTIMES MINUTES DEPTH TISSUE PRESSURE
1 5 90.00 fsw ÷ 2 = 45.00 fsw 45.00 fsw
2 10 45.00 fsw ÷ 2 = 22.50 fsw 45.00 fsw + 22.50 fsw = 67.50 fsw
3 15 22.50 fsw ÷ 2 = 11.25 fsw 67.50 fsw + 11.25 fsw = 78.75 fsw
4 20 11.25 fsw ÷ 2 = 5.63 fsw 78.75 fsw + 5.63 fsw = 84.38 fsw
5 25 5.63 fsw ÷ 2 = 2.81 fsw 84.38 fsw + 2.81 fsw = 87.19 fsw
6 30 2.81 fsw ÷ 2 = 1.41 fsw 87.19 fsw + 1.41 fsw = 88.59 fsw


Since each compartment takes six halftimes to fully saturate or empty, after 30 minutes, a 5 minute compartment is fully saturated. Therefore, after 40 minutes, the compartment is still fully saturated at 88.59 fsw.

QUESTION: How long would it take a 20 minute compartment to saturate to a given depth?

SOLUTION: 20 minutes per halftime * 6 halftimes = 120 minutes

Well, I hope this helps anyone out there trying to understand Theoretical Tissue Compartments, Halftimes, & M-Values. I love working these problems. If you would like to see these same problems worked in metric or you have a different problem, feel free to contact me with a question. I'll post the question and solution.