Can you Calculate Tissue Pressure?
For those who are doing their DM course or IDC, here is a little piece of theory that might help you along the way. Many people struggle to understand the M-value, Half-times and Tissue Compartment theory; I hope this will help.
How did it all start?
As early as the 1900s, British Navy divers, and many others suffered from decompression sickness. Decompression sickness is often referred to as the bends. The British Navy became very frustrated when their trained divers often experienced ‘the bends’ and went into early retirement. Apart from the extensive cost to the navy to train divers, they found their divers became ill and did not fully understand why? In 1905, Dr. John Haldane was appointed to research this problem and try to bring about a solution to prevent Decompression Sickness (DCS).
Dr. John Haldane
First he experimented on goats under pressure. He looked at body tissue, and how they absorb dissolved gases. The conclusion was, when a diver descends, nitrogen pressure in compressed air is higher than in the divers body. Dr. J. S. Haldane realised that body tissue absorbs dissolved nitrogen, but to an extent that the tissue becomes saturated and cannot absorb anymore. When the diver begins to ascend, the nitrogen in the body tissues is higher than the surrounding pressure. When this happens, the body tissues begin to release nitrogen back into the blood stream. The difference between the pressure that surrounds you and the dissolved nitrogen pressure in the divers body is what we call the pressure gradient. If the ascent is done in a correct manner, the body will release the dissolved nitrogen slowly without DCS. If the ascent is done quickly and the pressure gradient exceeds acceptable limits, bubbles will form in the blood stream and cause DCS. Haldane published his theoretical model in 1908, along with the no-decompression tables. To this day, the tables and his theoretical model are the base of all dive computers and tables we use today.
Dr. John Haldanes theoretical model used a fact that certain body parts dissolve and release nitrogen at different rates. These different body parts are what we call theoretical tissue compartments. Today we use fourteen theoretical tissue compartments, Haldane, back in 1905 used five.
Each of the fourteen theoretical tissue compartments (TTC) has something called a half-time. This half-time is represented in the minutes it takes to absorb and release nitrogen and it is expressed in metres of sea water. Each tissue compartment has six half-times to either fully empty or saturate. Haldanes model ranged from 5 – 75 minutes, today the model ranges from 3 minutes to 600 minutes.
Dr. Robert Workman
In 1960 Dr. Robert Workman revised the model of Haldane. Workman noted that each tissue compartment could tolerate different levels of over-pressurisation and that it was depth dependant. Through this revision the M-value was created.
This describes the amount of over-pressurisation each tissue compartment can tolerate at depth.
Calculating Tissue Pressure
This is often the section where students have problems. Below are some questions and examples of how to calculate tissue pressure it in its simplest form.
A 5-minute half-time compartment will have how much tissue pressure, 5 minutes after its taken from the surface to 30m in seawater?
30.00 msw ÷ 2 = 15 msw
-msw. means meters in seawater
-After 5 minutes, a 5-minute compartment will saturate 1 half-time
– 1 half-time is 1/2 of the total depth: 30.00÷2= 15.00
A 5-minute half-time compartment will have how much tissue pressure, 20 minutes after its taken from the surface to 30 meters in seawater?
30.00 msw ÷2= 15.00 msw
15.00 msw ÷2= 07.50 msw
15.00 msw+ 07.50 msw = 22.50 msw
07.50 msw ÷2= 03.75 msw
22.50 msw+ 03.75 msw = 26.25 msw
03.75 msw ÷2 = 01.88 msw
26.25 msw + 01.88 msw = 28.13 msw
A 5-minute compartment will have how much tissue pressure 40 minutes after its taken from the surface to 30 meters in seawater?
30.00 msw÷2= 15.00 msw
15.00 msw÷2= 07.50 msw
15.00 msw+07.50 msw = 22.50 msw
07.50 msw÷2= 03.75 msw
22.50 msw+03.75 msw = 26.25 msw
03.75 msw ÷2= 01.88 msw
26.25 msw + 01.88 msw = 28.13 msw
01.88 msw ÷2= 00.94 msw
28.13 msw + 00.94 msw = 29.07 msw
00.94 msw ÷2= 00.47 msw
29.07 msw + 00.47 msw = 29.54 msw
If you remember, I said each theoretical tissue compartment has only 6 half-times to fully saturate or empty. So, looking at the table after 40 minutes the compartment cannot take on any more nitrogen as it is fully saturated after six half-times, 30 minutes to 29.54 msw.
How long would it take in minutes for a 30 minute compartment to saturate to a given depth?
Answer: 30 minutes per half-time x 6 half-times = 180 minutes
The theory of decompression sickness and dive tables is a comprehensive subject. It is a topic of interest among divers throughout the dive community.
If you are interested in how to become a Divemaster in training, why not visit our DM. internship page for more details. M-values, Half-times and Compartments will definitely be part of the theory at Dive Smart Gozo.