Basic Decompression Principles

Question 1

Approximately, what is the composition of air?

Answer 1

78% Nitrogen (N2)
21% Oxygen (O2)
1% Argon (Ar)

Practically for scuba applications, we can say:

79% N2
21% O2

Question 2

What is meant by an ‘inert’ gas?

Answer 2

One our bodies neither use nor react with, e.g. N2.

Question 3

What is atmospheric pressure?

Answer 3

The force per unit area exerted by the atmosphere.

1 atmosphere (1 atm) at sea level
Aka 1 bar (1013mb, 1.013 bar)

Question 4

What is meant by ATA?

Answer 4

Atmospheric Absolute

The pressure at sea level - 1 bar

Question 5

How do you work out the partial pressure of a gas in a mix?

Answer 5

Percentage of that gas x Pressure of the mix

E.g. at 1 bar, with 21% O2 and 79% N2:

0.21 x 1 = ppO2 = 0.21 bar
0.79 x 1 = ppN2 = 0.79 bar

Question 6

What is Dalton’s Law?

Answer 6

The total pressure exerted by a mixture of gases (not in a state of reaction) is equal to the sum of the partial pressures of the individual gases composing this mixture

Question 7

Approximately speaking, how does pressure increase with depth in water?

Answer 7

1 bar every 10 meters (33 feet)

E.g. 1 bar at the surface, 2 bar at 10 meters, 3 bar at 20 meters, etc

Question 8

What is the ambient pressure at 40m?

Answer 8

Atmospheric pressure (1 bar) + (4 x 1) = 5 bar

Ambient pressure is the pressure of the surrounding medium in contact with an object.

Question 9

Applying Dalton’s law, what is the partial pressure of O2 and N2 in air at 10m depth?

Answer 9

10m, therefore ambient pressure is 2 bar.
21% O2
79% N2

ppO2 = 0.21 x 2 = 0.42 bar
Therefore, because the total pressure is the sum of the partial pressures:

ppN2 = 2 - 0.42 = 1.58 bar

Question 10

What is Boyle’s law?

Answer 10

When the temperature of a given mass of confined gas is constant, the product of its pressure and volume are also constant.

I.e. P1V1 = P2V2

E.g. at 10 meters, absolute pressure is doubled (2 bar), therefore the volume is halved.

Question 11

Explain why body tissue begins to absorb nitrogen as the depth of a dive increases

Answer 11

At the surface, body tissue is at equilibrium with the gases it does not metabolise (e.g. N2). I.e. there is no gradient between the amount of nitrogen in our blood and in our tissue, hence no gas exchange takes place.

A depth increases, pressure increases and hence the ppN2 in a given gas mix also increases.

Therefore, a gradient appears and the body’s tissues try to attain equilibrium by absorbing the excess nitrogen. This is known as ongassing.

Question 12

Explain what is meant by ‘offgassing’

Answer 12

When ascending (i.e. pressure is decreasing), the ppN2 in the tissues is now greater than that in the inspired gas.

Therefore, the body’s tissues start releasing the excess nitrogen.

Question 13

What is Henry’s Law?

Answer 13

At constant temperature, where no chemical reaction is taking place, the quantity of a gas that dissolves in a liquid is directly proportional to its partial pressure in the gas phase.
(corresponds with the ongassing phase)

It also states that:

When the partial pressure of the gas is reduced, a proportional amount of that gas will emerge from solution and may form bubbles in the liquid phase.
(corresponds with the offgassing phase)

Question 14

What are the principle factors governing the rate of nitrogen absorption by a particular tissue?

Answer 14

Perfusion - Amount of blood flowed to the tissue

Solubility - Of dissolved nitrogen in the tissue. The ability of one substance to form a solution with another substance, i.e. how much solvent can dissolve in a solute at a given temperature. Higher solubility = able to absorb more N2.

Question 15

What is the name of the process by which tissues uptake of eliminate nitrogen?

Answer 15

Diffusion

This is how the dissolved N2 moves from an area of high concentration to an area of low concentration

Question 16

What is the relative perfusion and solubility of N2 in fatty and muscular tissue respectively?

Answer 16

Fatty tissue - High nitrogen solubility, poor perfusion compared to muscle.
I.e. absorb more nitrogen, but on and offgas slower.

Muscular tissue - Lower nitrogen solubility, better perfusion than fatty tissue.
I.e. absorb less N2 before becoming saturated, and on and offgas quicker than fatty tissue.

Question 17

What is ‘halftime’?

Answer 17

To handle the on and offgassing of different tissues, Haldane assumed the body was made of tissues with N2 uptake and elimination expressed as an exponential function.

The tissue was half filled or half emptied in a CONSTANT time known as the halftime.

Halftime allowed the perfusion, diffusion and solubility of a particular tissue to be expressed as a single parameter, halftime.

Question 18

The brain’s white matter has a halftime of 5 minutes, what does this mean for its progress towards being ‘saturated’?

Answer 18

It will reach 50% saturation in 5 minutes.

In another 5 minutes it will move halfway towards saturation (75% saturation).

Theoretically, it will never be 100% saturated, but in 6 cycles (30 minutes) it will reach 98.4375% saturation, hence would be considered fully saturated in 30 minutes.

This means that if a diver were to make an immediate descent to 40 meters and stayed at this depth for 30 minutes, the 5 minute half time tissue would be fully saturated.

Question 19

What is meant by alveolar ppN2?

Answer 19

Inside the lungs, gas transport takes place through diffusion in the alveoli. The alveoli need to be humidified by water vapour for gas transport to take place.

The result is that, if ppN2 is 0.79, the inspired ppN2 is reduced to about 0.76 bar. This 0.76 ppN2 is typically referred to as alveolar ppN2.

Question 20

What is tissue tension?

Answer 20

The pressure of a gas in the tissue.

E.g. at the surface, alveolar ppN2 from air is approx. 0.76 bar, therefore the tissue tension at equilibrium would be 0.76 bar.

Question 21

What would be the tissue tension for a FULLY SATURATED tissue at 40m breathing air?

Answer 21

Alveolar ppN2 = 0.76 bar

5 bar ambient pressure at 40m

Tissue tension = 5 x 0.76 = 3.8 bar

3.8 bar when fully saturated, e.g. the brain’s white matter (halftime 5 minutes) would have a tissue tension of 3.8 bar after 30 minutes at 40m (fully saturated)

Question 22

What is ‘nitrogen gradient’?

Answer 22

The difference between the tissue tension and the ambient pressure.

Question 22

What is meant by a state of ‘supersaturation’?

Answer 22

The tissue tension exceeds the ambient pressure.

To attain equilibrium, the tissue will tend to release the excess dissolved N2 in to the venous blood, which will transport it to the lungs to be discharged from the body through exhalation.

Question 23

What does it mean when a tissue is fully saturated?

Answer 23

The tissue tension is the same as the partial pressure of an inspired gas. Equilibrium is achieved and until there is a change in ambient pressure, no on or offgassing will take place.

Question 24

What is NDL?

Answer 24

The ‘No Decompression Limit’.

Maximum time a diver can spend at a certain depth before the critical supersaturation threshold in their tissue is exceeded and they are obligated to make decompression stops during the ascent to offgas.

Question 25

Why are ascent rates to decompression stops important in dive planning?

Answer 25

Because if you ascend too slowly, whilst tissue with a low half time will be offgassing because it was fully saturated, tissue with a long halftime will still be ongassing because it was nowhere near the same saturation level, and hence the tissue tension will still be lower than the ambient pressure.

Longer stops made at shallower depths for the offgassing of these tissues.