Sunday, May 07, 2006

Pressure & the Body

From a very broad point of view, nearly all diving accidents are related to the effects of increased pressure on the body at depth. This is not to understate the importance of initiating factors – such as poor physical conditioning, inadequate training, underlying medical condition inimical to diving, a panic attack or poor planning with an out of air situation. The final outcome of fatal diving accidents is often written off as drowning, but the underlying cause of the drowning is most often related to a progression of events that end with the effects of pressure, i.e., nitrogen narcosis, gas embolism, decompression illness or oxygen toxicity.

Terminology and Definitions

Compression-That part of a dive that increases pressure upon a diver. The deeper a diver goes-the more the pressure.

Decompression -That part of a dive when the diver ascends toward the surface, decreasing the pressure. In a chamber dive, that part of a dive when the pressure is being lowered.

Recompression - a return to compression after ascent to the surface on a water dive; a return to surface pressure from altitude; a term used to describe medical treatment of decompression sickness.

Hyperbaric - a word used to describe increased pressure over the pressure in one atmosphere [surface] .

Pressure Concepts

Pressure - a force acting on a unit of area; Pressure = Force divided by Area

Atmospheric Pressure - pressure exerted by the weight of the atmosphere; this varies with the altitude. [The higher the altitude – the lower the pressure].

Barometric pressure - a measurement of atmospheric pressure; one atmosphere of pressure is equal to 760 mm Hg or 1.03 kg/cm2 or 14.7 psi (pounds per square inch).

Hydrostatic pressure - the force of a column of water acting upon a body immersed in the water, equal in all directions at a specific depth. During descent, pressure increases 0.445 psi per foot of depth in salt water or one atmosphere per 33 feet of salt water (FSW).

Gauge pressure - the difference between absolute pressure and atmospheric pressure. This can be converted to absolute pressure by adding 14.7 psi or 1.03kg/cm2.

Absolute pressure - this is the sum of all pressures acting on an object. In diving this is the sum of the atmospheric pressure (14.7 psi + the hydrostatic pressure).

Some Characteristics of Gases

Oxygen, the gas that is capable of supporting life, exists in the atmosphere on the surface of the earth at a concentration of 21%. There are specific ranges of human tolerances and toxicity can occur when concentrations exceed 30%. Oxygen also supports combustion and creates hyperbaric chamber safety problems. The maximal allowable concentration in multiplace chambers is 23%.

Carbon dioxide is a direct product of metabolism and is found at a maximum level of 1.5% at the surface. It is the gas that determines our rate of ventilation and is implicated in shallow water blackout with consequent drowning. The concentration of CO2 determines ventilation schedules in multiplace chambers.

Carbon monoxide is the product of incomplete combustion of fuel, usually caused by faulty compressors. Its' toxicity is caused by its' affinity for hemoglobin and the poisonous effect on our cytochrome A3 system. The maximum allowable level is 10 ppm (0.001%), which is about what we get on a city street.

Nitrogen is an inert gas consisting about 79% of the air we breathe. It produces nitrogen narcosis in humans at a depth of 100 fsw, causes decompression sickness on ascent due to bubbles that form on reduction of pressure. It is the gas that determines our decompression schedules.

Helium - this is an inert gas present in air in very small quantity (0.0005%) and is used to prevent nitrogen narcosis. It is also used as an emergency breathing gas but results in body heat loss, creates communication difficulties and increases the chance of decompression sickness.

Gas Laws and their Physiological Significance

Boyle’s Law

At a given temperature the volume of a given mass of gas will vary inversely with the absolute pressure.

PV=K

Boyle's Law -This law of physics determines the volume of gases and accounts for the major portion of diving medical problems. Stated simply - the volume of gases are reduced when pressure is increased (diver descending) and the volume is increased on reduction of pressure (diver ascending). Bubbles are reduced in size when chamber pressures are increased and air-containing spaces (lungs, middle ears, sinuses) try to expand when chamber pressures are decreased or when the diver surfaces.

Charles' Law

At a constant pressure the volume of a mass of gas is proportional to the absolute temperature

V1/T1=V2/T2
Or T x P = V

This states that volume varies with the temperature, explaining why compressing gases increases heat and decompression cools. Tank fills need to be done under water in order to keep the tanks cool; release air from a scuba tank and you'll notice that it becomes cold.

Dalton’s Law

Total pressure exerted by a mixture of gases is the sum of the partial pressures that would be exerted by each gas alone as if it alone occupied the total volume. This is the law that explains oxygen toxicity, nitrogen narcosis and the danger of even minute quantities of contaminant gases, e.g., carbon monoxide. It's the law of partial pressures, in which all the individual partial pressures of gases are totaled into one partial pressure.

Graham's Law -This states that gases flow to areas of lesser pressure and explains oxygenation between tissue compartments and the movement of inert gases through out the body.

Henry's Law

In fluid, dissolved gas is directly proportional to the partial pressure of the gas to which the fluid is exposed.
This rule explains O2 transport, inert gas transport and the evolution of bubbles in a solution. It states that the volume of a gas is directly proportional to the pressure above a liquid.

Pascal’s Law

P = FORCE/AREA

Pressure at any point in a body or solution has that pressure transmitted equally throughout the solution. This is the reason that deep tissue compartments and internal bubbles experience the pressure changes that occur outside the body.

How gases move about

Perfusion - the flow of liquids in the body (blood or lymph), through an organ or tissue during which gases and/or chemical substances are exchanged and/or redistributed.

Absorption -The process of moving a gas into the liquid phase in the body.

Solubility -This process determines the amount of gas that will dissolve at a given pressure.

Diffusion - The movement of dissolved substances from higher concentrations to lower concentrations.

Gradient -The amount of change of one quantity of substance with respect to another; a gas in solution will diffuse across a gradient, from an area of higher concentration to one of less concentration.

Conversion Factors

Pressure Conversion Factors for One Atmosphere of Pressure

10.08 meters sea water
33 feet sea water
101.3 kilopascals/square meter
1.033 kg/square cm
1.013 BARS
760 mmHg
1034 cm H2O

1 ATM=The weight of all the air above the surface of the earth=33 feet sea water (fsw)=14.7 pounds per square inch(psi)=0.445psi/fsw=2.25 fsw/psi=0.0303 ATM/fsw
One pint=one pound