Friends!, What the Hell is Wrong with these People?

Depth/duration.

Any dive deeper than 33 feet (10 Meters) can produce DCS although in general, the deeper the dive, the greater the risk. The longer the dive, the more gas absorbed (until saturation). The more the DCS.

Individuals.

Some people appear to be more susceptible to DCS than others. Even an individual may vary in susceptibility at different times, and DCS can develop after a dive profile which has been safely followed on many previous occasions. Others frequently get DCS despite conservative diving.

Adaptation.

Repeated dives to similar depths over a period of time reduce the incidence of DCS. This may be due to the elimination of bubble nuclei. A diver returning to these dives after a 2 week break loses the benefits of this adaptation or acclimatisation.

Age.

Older divers tend to be more predisposed to DCS. This age factor probably comes into play after the 3rd decade.

Obesity.

This appears to be a predisposing factor probably due to increased N2 solubility (4-5 : 1) in fat compared to water. This may be relevant for those with a BMI of > 25.

Debilitation.

Factors causing the diver to be unwell such as dehydration, hangover or exhaustion tend to predispose to DCS.

Injury.

DCS, particularly involving the musculo-skeletal system and joints, is more likely with recent bruises, strains or chronic injuries.

DCS.

A previous episode of DCS, especially if it was unexpected from the dive profile, or if it damaged tissue (as in neurological DCS), makes the diver predisposed to similar subsequent episodes.

Patent Foramen Ovale.

One reason for some people to have an increased susceptibility to DCS is that they have a small hole in their heart. All of us had a hole in our heart when we were a fetus. In about a third of the population some remnant of this hole remains, it is called a patent foramen ovale, or PFO. These people have an increased susceptibility to DCS, the likely reason is that bubbles that would normally be trapped in the lungs without causing symptoms pass through the hole, by-passing the lung filter, and on to other parts of the body, where they cause noticeable symptoms. However, most results suggest the risk from a PFO is not great enough for it to be appropriate to test all divers for this.

Cold.

Diving in cold conditions makes DCS more likely, especially when the diver is inadequately insulated. More precisely, coldness during the dive inhibits inert gas uptake (because of restricted circulation) but allows more N2 to dissolve in body fluids — while coldness during decompression inhibits inert gas release. Theoretically, it may be better to be cold during the dive and warm on decompression, unless bubble formation occurs. Warming will then reduce gas solubility and increase bubble growth and DCS.

The association between cold exposure and DCS is complex and contentious. During decompression and post-diving the cold environment may cause peripheral constriction of blood vessels and more bubble formation. Alternately, taking hot showers also tends to cause increased bubble formation and DCS.

Alcohol and other drugs

It has been observed that divers who over-indulge in alcohol, or perhaps take other drugs or medications, may be especially susceptible to DCS. In the case of alcohol, especially taken the night before, the effects may be due to the associated dehydration or the vascular dilatation (remember the throbbing headache and “hangover”), increasing N2 take-up.

Exercise.

Strenuous exercise during a dive is likely to increase the N2 uptake by increasing blood flow to muscles, increasing gas uptake and favoring DCS development. Gentle exercise during decompression, by promoting circulation from the tissues probably aids in N2 elimination. Strenuous exercise after the diver has returned to the surface makes the development of DCS, particularly in the musculo-skeletal system, more likely by promoting bubble formation. Strenuously exercising (shaking) a beer can, before opening it, aptly illustrates this phenomenon. During the first hour after a dive, particularly when there has been a large N2 uptake, it is best to rest quietly as this is the period of maximal N2 elimination.

Physical Fitness.

The less physically fit the diver, the more likelihood of DCS, probably because more energy is used and more blood flow is required for the same outcome – transporting more N2.

Gender.

There is some evidence that women have a higher incidence of DCS for certain dive profiles. There are subtle differences in physiology and body composition which could explain this. The decompression tables in current use only evolved after extensive testing on men alone.

Dive profile.

Deep dives (greater than 18 meters), long dives, decompression dives and any dives exceeding the limiting line (in RN based tables) all have a higher incidence of DCS.

Rapid ascents.

These allow insufficient time for N2 elimination from fast tissues, thus encouraging bubble
formation

Multiple ascents.

Multiple ascents during a dive imply multiple decompression’s and often involves rapid ascents. Bubbles in the blood (fast tissue bubbles) are likely to form during these ascents. The bubbles may not be adequately filtered by the lungs, passing along into the tissues, or may be reduced in size during the second or subsequent descent, allowing them to escape through the pulmonary filter into the tissues. DCS is then more likely.

Repetitive dives.

Each repetitive dive begins with a N2 load of some degree from the previous dive. Since bubble formation even after routine dives is common, a repetitive dive will often start with the diver carrying N2 bubbles from the previous dive. These bubbles will be supplemented by N2 taken up during subsequent dives, and make DCS more likely.

Reverse Dive Profiles.

Divers are advised to do their deep dive first, in repetitive dives, and to dive to progressively shallower depths when multi-level diving. If this order is not followed, DCS is more likely.

Flying after diving.

The jet age often finds divers flying home after a dive holiday within hours of their last (sometimes literally) dive. International airliners are pressurized to an altitude of about 2000 meters (6500ft.) above sea level. This means a pressure reduction on the diver of about 25% with a corresponding increase in the degree of N2 supersaturation as well as a corresponding increase in the size of any bubbles he may be carrying. The increase in size of critical bubbles may be sufficient to provoke symptoms.

Multi-Factorial Effect.

Often there is more than one factor increasing the likelihood of DCS. Thus in one large Australian series over half the cases engaged in multiple dives, deep dives (greater than 30metres) and/or had ingested alcohol within 8 hours. Another 20% were precipitated by
aviation exposure. Thus many of these divers would have had at least 2 factors increasing their likelihood of DCS.

In sport divers the main gas formed in bubbles is nitrogen (N2) because these divers almost always breathe air. However, the same principles apply to other inert gases, such as helium (He), which may be breathed by deep commercial and technical divers.

GAS UPTAKE

Image via Wikipedia

When a diver breaths air from scuba equipment at depth, N2 is breathed at an increased partial pressure. Because gas diffuses from areas of high concentration (high partial pressure) to areas of lower concentration, N2 is taken up from the lungs by the blood and transported around the body and into the tissues. The greater the depth, the greater the partial pressure of N2, and therefore the amount of N2 absorbed. Early in the 20th century, John Scott Haldane applied this concept to develop the first ever decompression tables.

The speed of N2 distributing to the tissues depends on the their blood flow. Tissues with high metabolic needs such as the brain, heart, kidneys and liver receive most of the blood pumped from the heart. They will also receive most of the N2 carried in the blood and will have a rapid N2 uptake. Such tissues are termed “fast tissues” because of their fast N2 uptake.

Because blood passing through the lungs immediately equilibrates with any change in inspired N2 partial pressure, blood is the fastest tissue of all.

Other tissues such as ligaments, tendons and fat, with a relatively small blood flow, have a relatively slow N2 uptake. These tissues are termed “slow tissues”. Between the two are tissues of intermediate blood flow such as muscle. Some organs, such as the spinal cord, have both fast and slow tissue components. The rate of uptake of N2 in a tissue is exponential i.e. it varies depending on the amount of gas already taken up by the tissue. As the tissue takes on gas, the uptake slows because the partial pressure gradient decreases.

The filling of a scuba cylinder is an example of an exponential process. When an empty cylinder is connected to a high-pressure source, the cylinder initially fills quickly, but the flow slows as the pressure in the cylinder increases and approaches that of the gas source.

The uptake of gas in any tissue is initially rapid but slows with time. Accordingly, it may take a long time for a tissue to become fully saturated with gas, but fast tissues become saturated sooner than slow tissues.

Since the exponential uptake takes a long time to reach completion, even if it starts rapidly, the concept of tissue “half times” is used to compare tissues. The half time is the time taken
for a tissue to reach half its saturation level. A fast tissue may have a half time as little as a few minutes, while a slow tissue may have a half time of some hours.

GAS ELIMINATION

N2 is eliminated in a reverse of the uptake process. As the diver ascends there is a reduction in the partial pressure of N2 in the air he breathes, allowing blood to release N2 into the lungs.  The decrease in the blood level of N2 causes N2 to diffuse into the blood from the tissues. Fast tissues naturally unload N2 quicker than slow tissues.

Theoretically, tissues should lose N2 exponentially, and most decompression tables are calculated on this assumption. While large amounts of N2 are lost initially, the process slows with time and it may take 24 hours or longer for all the N2 taken up during a dive to be released. Diving again during the time of N2 elimination will mean that the diver will start his second dive with a N2 retention in some tissues. Adjustments are provided in the decompression schedule to allow for this and are incorporated as the repetitive dive tables.

If there is diminished circulation to a tissue during decompression, gas elimination will be reduced and thus bubble formation will be more likely.

In practice, even during routine conservative dives, bubbles of N2 frequently form in the blood and tissues, interfering with N2 elimination. It has been estimated that as much as 5% of N2 taken up by the body after some dives is transformed into bubbles on decompression.  These are often termed “silent bubbles” since they usually do not produce any symptoms.   They do however have a profound and unpredictable influence on the decompression requirements for repetitive diving, because it takes much longer to eliminate gas bubbles in tissues than it does gas in solution.

SATURATION

When tissues are subjected to an increased partial pressure of inert gas during a dive, they take up dissolved gas in accordance with Henry’s Law. However, there is a limit to the amount of gas which can be dissolved by a tissue exposed to any given partial pressure of gas (i.e. depth of dive). When this limit is reached the tissue is said to be saturated.

Our bodies are normally saturated with N2 at atmospheric pressure and contain about one liter of dissolved N2. If a diver were to descend to 20 meters (3 ATA) and remain there for a day or more, his body would take up the maximum amount of N2 possible at that pressure and would then be saturated at that depth. His body would now have about 3 liters of N2 dissolved in it.

Once the body is saturated with inert gas at a given depth, it will not take up more of that gas, no matter how long the diver spends at that depth. Consequently, once the diver is saturated the decompression requirement does not increase with time. This economy of time is exploited in saturation diving, when the diver is kept at depth for very long periods of time (days, weeks, months) but then needs only the same lengthy decompression.or more, his body would take up the maximum amount of N2 possible at that pressure and would then be saturated at that depth. His body would now have about 3 liters of N2 dissolved in it.

BUBBLE FORMATION

The process of bubble formation can be demonstrated easily by opening a bottle of beer (or champagne, depending on taste and income). In a carbonated beverage CO2 is dissolved in the liquid at a high pressure, which is then maintained by the lid. When the lid is opened, the pressure over the liquid becomes atmospheric and the partial pressure of CO2 in solution exceeds the critical limit for bubble formation, causing bubbles to form. This could be avoided if the pressure was reduced slowly (decompressed).

During ascent, the pressure surrounding the diver (the environmental pressure) is reduced.  Eventually, the pressure of N2 dissolved in the tissues may become greater than the environmental pressure. The tissue is then said to be supersaturated.

The tissues are able to tolerate a certain degree of gas supersaturation. Nevertheless, Haldane explained that if the pressure of N2 in the tissues exceeds the environmental pressure by a critical amount, then bubble formation is likely. The pressure differential needed to cause this varies between tissues but with most scuba diving it equals or exceeds 2 : 1 (i.e. the partial pressure of inert gas in the tissues should not be more than twice the environmental pressure).  This explains why DCS under recreational diving conditions is unlikely after an isolated dive to less than 10 meters — the pressure at 10 meters is 2 ATA, while the pressure at the surface is 1ATA – a 2:1 ratio.

Gas bubbles in the tissue and blood are the cause of DCS. The exact mechanism of bubble formation is complex. It is likely that microscopic gas spaces (bubble nuclei) exist in all body fluids and that these form a nucleus for bubble formation during decompression.

Bubbles can form in any tissue in the body including blood. The pressure in each bubble will be the same as the environmental pressure (if it was not, the bubble would expand or contract until it was) and the bubble size is governed by Boyle’s Law as the pressure changes.

At the onset of DCS, the pressure of N2 in the tissues is supersaturated (greater than the environmental pressure) so there is an immediate diffusion (pressure) gradient of N2 which then diffuses into any bubbles (or nuclei) present, causing them to expand.

A bubble of DCS contains mainly N2 if the diver has been breathing air, but the other gases present in the tissues, such as carbon dioxide (CO2), oxygen (O2) and water vapor, also diffuse into it.

Once a bubble has formed its behavior depends on several factors. Any increase in pressure such as diving or re-compression will reduce its size while any decrease in pressure such as ascent in the water, over mountains or in aircraft, will expand it. The bubble will continue to grow in any tissue until the N2 excess in that tissue has been eliminated. Once this has occurred (which may take hours or days) the bubble will begin to decrease in size but it may take hours, days or weeks to disappear. In the meantime the bubble can damage the tissues around it.

There is good evidence that bubbles frequently form in tissues and blood of recreational divers after routine no-decompression dives, even when the tables have been faithfully followed. These bubbles do not usually cause symptoms but certainly cause doubt about the validity of the tables.

Tissue damage by a bubble results from several factors. Bubbles in the blood obstruct blood vessels in vital organs such as the brain, while bubbles forming in the tissues may press on blood vessels and capillaries obstructing their blood flow. Bubbles in the blood can also stimulate the clotting process causing the blood to clot in the blood vessels, obstructing blood flow to vital organs, and reducing the ability of the remainder of the blood to clot adequately.  In the brain, spinal cord and other tissues, bubble pressure in or on nerves may interfere with nervous system function.

DIVE PROFILES

The type of dive has a significant bearing on where and when bubble formation takes place. Short deep dives (i.e. deeper than 30 meters) tend to cause bubbles in the fast tissues (blood, brain and spinal cord) while long shallow dives tend to produce bubbles in the slow tissues (like the joints). Long deep dives cause bubbles everywhere.

This distribution occurs because:

• in short dives, only the fast tissues take up enough N2 to form bubbles on ascent and
• after shallow dives, fast tissues eliminate their relatively modest N2 excess before a critical pressure differential develops.

It can thus be seen why it is important to ascend slowly. The slower the ascent, the longer the time for fast tissues to eliminate N2 through the lungs, before a critical N2 pressure differential develops.

Diving folklore contains a myth that a diver using a single 2000 liters (72 cu. ft) tank cannot develop DCS. The air supply available was said to limit the diver to safe dive profiles. This is only true at very shallow depths and even then only partly so, e.g. for a dive to 20 meters, the average endurance may be about 30 minutes, which is within the no-decompression time given by most tables. Remember though, as mentioned previously, that any dive in excess of 10 meters can produce DCS.

The myth may become more apparent for deeper dives. For example, a single 2000 liter tank will give around 10 minutes duration for a 50 meter dive. According to most decompression tables, a 10 minute dive to 50 meters will require 10 minutes of decompression — but there will be no air remaining to complete these stops. Even if there was sufficient air, dives to this depth have a significant risk of DCS despite the tables being followed correctly.

Related articles by Scuba Diving Sarasota

• Lung function and physiology

METABOLISM

The Need for Energy

Energy is a fundamental requirement for all life processes. It is needed for growth, repair, movement and all the active functions of the body. The fuel for this energy comes from carbon compounds, which are incorporated in complex molecules in the food we eat. This is biochemically dismantled in the digestive tract into simple chemical compounds which are absorbed and carried by the blood stream to the cells. Here they undergo further biochemical processing until ultimately the carbon is combined with oxygen (O2), forming carbon dioxide (CO2) and releasing energy.

This is similar to the energy formation which takes place in an automobile engine or a fire, where carbon in fuel or wood is combined with O2 to produce energy. The body processes will only function under strict conditions of O2 availability, temperature and acidity.

The body needs a means of transferring food products to the cells, together with delivery of O2 and removal of CO2. This is performed by the blood, in the vascular system. It comprises arteries which take blood to the tissues, a vast network of microscopic capillaries that bring the blood into contact with all the cells of the body, and veins which return blood to the heart.

The blood is circulated through the blood vessels by a muscular pump – the heart, and the whole system is called the cardiovascular system. It brings O2 from the lungs to the cells and eliminates CO2 through the respiratory system.

Respiration

Anatomical Structure

The respiratory tract begins at the mouth and nose and ends in the microscopic air sacs called the alveoli, in the lungs.

The nose, apart from its decorative function, warms and humidifies the air that we breathe. It also filters large particles which might otherwise be inhaled. If the nose is bypassed by breathing through the mouth, a snorkel or scuba regulator, the lung then has to cope with drier, colder, unfiltered air.

After passing through the mouth or nose, the air then enters the throat where the larynx (or voice box) is situated. This is recognised as the “Adams Apple”. The larynx produces the sounds of speech as well as helping to protect the lungs from inhalation of foreign material.

A structure called the epiglottis closes over the opening and the vocal cords shut to prevent the foreign material from entering the lungs. If any material passes these structures, the cough reflex, activated by foreign material touching the inside of the air passages, may cause a coughing reaction which tends to expel whatever has been inhaled.

Below the larynx the air passes through a tube called the trachea. This is about as thick as the average snorkel and branches inside the chest into two tubes, the bronchi, which lead to the lungs. Those air passages are lined with cells covered with microscopic hairs (cilia) which move a sheet of secreted mucous slowly upwards towards the larynx. Small pieces of foreign material such as dust eventually find their way to the larynx, along with this  mucous sheet. It is then either coughed-up or swallowed. The cilia may be damaged by smoking or infection, causing retention of mucous and inhaled material which may eventually obstruct the air passages.

The bronchi divide repeatedly into progressively smaller passages rather like the branches of a tree. These passages have encircling muscles in their walls which, by contraction or relaxation, can vary the diameter of the air passage.

In asthma the muscles of the small bronchi become oversensitive and overactive, causing excessive narrowing and obstruction of these air passages. This can occur in response to exercise, allergy, cold, infection, anxiety, smoking or other inhalants such as sea water. At the same time, the cells lining these passages produce excessive and thickened mucous. The combination of these factors causes airway narrowing which has serious repercussions for a diver.

The smallest branches of the bronchi end in bunches of microscopic air sacs called alveoli. The vast number of alveoli are packed together into the two sponge like organs, the lungs. There are about 300 million alveoli in the lungs and the combined surface area of all the alveoli in the lungs is equal to about half a tennis court. The alveoli are lined by a thin layer of fluid containing a detergent-like substance called surfactant. This acts as a wetting agent to prevent the alveoli from collapsing from surface tension.

The surfactant lining of the alveoli can be damaged in disease or by inhalation of water, leading to collapse of the lungs and serious respiratory difficulty.

Each alveolus is surrounded by a network of blood capillaries. These bring the blood into close contact with the air in the alveolus, with only the microscopically thin walls of the alveolus and capillary separating the two.

If the wall of an alveolus is ruptured, as it may be in pulmonary barotrauma (“burst lung”), then air from the alveolus is able to enter the blood stream where it may cause blockage of distant vessels such as those in the brain. This is called an air embolism.

The lungs occupy a cavity about the size of a football on each side of the chest. The lung is covered by a thin membrane coating, called the pleura, and the inside of the chest wall is lined by a similar membrane. Between the two pleural layers is a narrow space which contains a small amount of lubricating fluid to minimise friction as the lungs expand and contract during breathing. If the outer surface of the lung tears, as it may in pulmonary barotrauma, then air can enter this pleural space causing the lung to collapse. This disorder is called pneumothorax.

The chest wall which encloses the lungs is made up of ribs with muscles between them – known as intercostal muscles. At the base of the chest cavity lies a large thin dome shaped muscle called the diaphragm. When the diaphragm contracts, it flattens and has a piston like effect, reducing the pressure in the chest cavity and increasing the volume of the lungs. The reduced pressure draws air into the lungs through the air passages.

Contraction of the diaphragm is the main method of inhalation in the resting state. It is assisted by contraction of the muscles between the ribs which rotate the rib cage upwards and outwards, enlarging the chest cavity and reducing the pressure in the chest. A group of neck muscles which are attached to the rib cage can also assist respiration when maximal breathing is required.

At the end of inhalation the elasticity of lungs and rib cage causes the lungs and chest wall to contract and exhalation takes place. With quiet breathing, this does nor require muscular effort. With heavy breathing, exhalation can be assisted by the abdominal and chest muscles.

Respiratory Function

During quiet respiration in adult males, about 500 ml of air is moved in and out of the respiratory tract with each breath. The volume per breath is termed “tidal volume”. During extremely heavy exercise, the tidal volume can increase 10 fold, up to about 5 litres.

The total amount of air that can be held in the lungs (total lung capacity or TLC) in adult males is approximately 6 litres. Only about 10% of the air in the chest is exchanged with each breath during quiet respiration. The vital capacity (VC) is the maximum volume that can be exhaled in one breath, and the forced expiratory volume (FEV1.0) is the maximum volume that can be exhaled in one second.

The flow of air through the respiratory passages varies at different stages of respiration. It reaches a peak about midway through inspiration — and during quiet breathing this peak flow rate is approximates 30 litres per minute. This value increases during exercise to 600–700 litres per minute.

Any breathing system (such as a snorkel or demand valve) which the diver is using, should be capable of handling these large air flows without significant resistance. If this does not occur, then the diver must exert extra effort during respiration in order to overcome this resistance. This problem is compounded when the diver is breathing compressed air at depth because the increased density of the gas will further increase the resistance to airflow in both the equipment and the lungs.

Gas Uptake and Loss

Air, which contains approximately 21% oxygen (O2) and 78% nitrogen (N2), is inhaled into the alveoli where it is brought into contact with the blood in the capillaries. This blood contains a lower partial pressure of O2 than the air in the alveolus and a higher partial pressure of CO2, since it has just returned from the body, which has been using O2 and generating CO2. Consequently, there is a pressure gradient causing O2 to diffuse from the alveoli to the blood, and CO2 to diffuse from the blood to the alveoli, where it is then exhaled. There is no net movement of N2 since the N2 in the alveoli and in the blood is in equilibrium, except when diving, altitude exposure or breathing different gases.

If the diver breathes air (78% N2) or another inert gas such as helium, while descending or remaining underwater, this inert gas will pass from the alveoli to the blood because the partial pressure of the gas in the lungs is increasing as the diver goes deeper.

On ascent, the partial pressure of inert gas in the lungs will reduce, and this allows inert gas to move from the blood (returning from the tissues) to the alveoli, and be exhaled.

Respiratory Control

The partial pressures of CO2 and O2 in the blood are kept within very strict limits by a sensitive control system. There are sensors in the brain which detect small changes in the blood CO2. If this increases, then the sensor causes stimulation of the respiratory centre within the brain, leading to faster and deeper respiration to eliminate more CO2.

When a snorkel diver holds his breath, the CO2 level in his blood increases. This produces respiratory stimulation which compels the diver to take a breath — hopefully after he has had time to return to the surface.

The sensors for blood O2 pressure are in the carotid arteries which supply the brain. A reduction in the blood O2 level also leads to respiratory stimulation, but this effect is not as powerful as that caused by CO2 changes.

Smoking

The ingenious habit of rolling tobacco into a tube of paper, setting fire to it and inhaling the smoke, sabotages the complex respiratory and circulatory process at several points.

As well as predisposing to lung cancer and emphysema, noxious tars in the smoke precipitate out in the bronchi producing chronic irritation, narrowing of the bronchi and cause a persistent outpouring of mucous. This ultimately results in chronic bronchitis. The tar also poisons the cilia, which conduct the mucous up the airway to the larynx, resulting in retention of old mucous in the lungs (smell the breath!).

Various toxins in the smoke ultimately cause destruction of the alveolar walls producing cavities in the lungs and destruction of the lung architecture, resulting in the disease called emphysema.  This, combined with obstruction of the air passages, makes the smoking diver less physically fit and more liable to air trapping in the lungs and pulmonary barotrauma.

The carbon monoxide content of the smoke reduces the capacity of the blood to carry O2, thereby reducing oxygenation of the tissues.

Some of the chemical constituents of the smoke are absorbed into the blood stream producing changes in the walls of the blood vessels supplying the heart, brain and limbs. Ultimately these become obstructed. In later life this can cause heart attacks, strokes and peripheral vascular disease (gangrene).

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• Levels In Respiration (slideshare.net)

Air consists of a mixture of O2 + N2 + a trace of carbon dioxide (CO2), and minute amounts of rare gases. Rare gases such as Neon (Ne), Argon (Ar) and Xenon (Xe), and Hydrogen (H2) exist in trace amounts only.

The approximate composition of air is:

Some less reputable suppliers of air fills for scuba tanks provide free additives to the compressed air, such as dust, oil, hydrocarbons, rust, water vapor and carbon monoxide (CO).

Oxygen– O2

This is a colorless, odorless, tasteless gas which is indistinguishable from air to breathe.  It is essential for metabolism and maintenance of life yet in quantities exceeding those in air it is toxic to man. Its proportion in air (21% or more specifically, a partial pressure of 0.21 ATA at sea level) is critical. A little more than this causes O2 toxicity, a little less will not support human life. For this reason most gas mixtures breathed by deep divers contain an inert gas – usually either N2 or helium (He), mixed with O2 to ensure that the O2 composition is maintained at a partial pressure close to 0.2 ATA (0.16 – 0.40 ATA).

O2 supports combustion vigorously and can cause normally non-flammable substances (such as the occupants of a recompression chamber) to burn brilliantly if it is present at a sufficiently high partial pressure.  Divers should be aware of the potentially explosive and combustible properties of oxygen, as
they may require to use it in first-aid, or be inadvisably enticed into diving with high oxygen mixtures.

Nitrogen – N2

This gas, which is the major constituent of air, is also colorless, odorless and tasteless. N2 dissolves well in body fluids and tissues, causing narcosis at depth and decompression sickness when it bubbles out of solution, after ascent.

It is termed an “inert gas” because it does not take part in human biochemical processes. The Creator appears to have included this gas in air to prevent us from developing O2 toxicity, and to reduce the fire hazard.

Divers vary this N2/O2 ratio (in Nitrox, oxygen enriched air or mixed gas diving) in an attempt to improve on nature, extend diving durations, and reduce narcosis.

Carbon Dioxide – CO2

This gas is also colorless, odorless and is said to be tasteless. However if a diver inhales a mouthful of CO2 from a buoyancy vest inflated from a CO2 cartridge it will be found to taste very nasty, due to its formation of carbonic acid in water.

CO2 is a by-product of cellular metabolism and we exhale approximately 5% of CO2 in our breath. If a diver re-breathes some of his exhaled gas by using faulty breathing equipment or an excessively long snorkel the CO2 will accumulate in the body leading to toxicity.

These effects are discussed further here.

Carbon Monoxide – CO

This gas is colorless, odorless and tasteless. It cannot be detected by a diver and even in trace amounts can cause loss of consciousness or death.

It is usually produced as a product of incomplete combustion of carbon containing compounds and is a constituent of internal combustion engine exhausts and cigarette smoke.

Air contaminated by carbon monoxide, if supplied in scuba cylinders or by surface supply to divers, may have lethal results

Helium – He

This is a colorless, odorless, tasteless gas, which is very light and very expensive. It is obtained from underground natural gas sources found in North America and elsewhere.

It is used to dilute O2 in gas mixtures breathed at great depths because it has little tendency to produce narcosis (e.g. Heliox may be 90% He + 10% O2, or any other proportion).  Due to its very low density it readily escapes through small leaks in pipes and valves making it difficult to retain. It is also a very effective conductor of heat, causing serious problems with hypothermia.

The low density of He alters the normal process of speech production causing “Donald Duck” like speech when a diver breathes this gas

Hydrogen – H2

This is a very lightweight gas that can replace N2 to reduce narcosis at depth. Unfortunately it can combine explosively with O2 and the resultant water (H2O) is not sufficient to ‘put out the diver’.

It is sometimes used with very low O2 percentages, at great depths, by skilled professional divers. It shares many problems with He.

Inert Gases:

Neon – Ne, Argon – Ar,

Radon – Rn, and Xenon – Xe

These are more biologically inert gases which are present only in trace amounts in the atmosphere. They are of no importance to recreational divers.

Oil Gases

Because of lubrication needs in the compressor, oil vapors and hydrocarbons can be produced which may then contaminate the air supply. See

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It is important for divers to understand the factors affecting buoyancy. These are:

Density

Density is defined as mass per unit volume (density = mass ÷ volume).

For our purposes, mass can be considered to be the same as weight, so density is equivalent to weight per unit volume. A substance is more dense than another if the same volume has more weight.  Try lifting a bucket of water and then a bucket of lead, to illustrate this.

Specific Gravity

Specific gravity (S.G.) is the density of a substance compared to the density of fresh water which is given a value of one.  Lead has a specific gravity of 13.5 so it is 13.5 times as dense as water.

e.g.1 liter of water will weigh 1 kg., while the same volume of lead will weigh 13.5 kg.

The concept of specific gravity is important since the specific gravity of a substance determines whether it will float or sink in water.

A substance with a specific gravity greater than 1 (i.e. denser than water) will sink. Lead, with a specific gravity of 13.5, does not float well, whereas oil, with a specific gravity of 0.8, floats easily — producing an oil slick.  The human body has a specific gravity of slightly greater than 1, depending on its content (fat has a specific gravity less than 1, and bones are greater than 1) but the air content of the lungs provides enough buoyancy to allow most people to float.

Archimedes Principle

The ancient Greek, Archimedes (apparently while reclining in his bath), discovered that when an object is immersed in a fluid, it appears to be lighter, and that the apparent loss of weight (or buoyancy) is equal to the weight of water displaced by the object.  That is – the buoyant effect will be equivalent to the weight of fluid of equal volume to the immersed object.  Depending on whether the weight of fluid displaced is greater than, equal to or less than the weight of the object, an object immersed in the fluid will either float, remain suspended or sink. Even an object which sinks will still appear to be lighter than it would out of the
fluid.  Sea water is denser than fresh water because of the salt content, so a greater weight of sea water will be displaced by an object.  Hence objects in sea water are more buoyant than in fresh water.

Air (in the abdomen, buoyancy compensator and wet suit) contributes to buoyancy.  Unfortunately air in these compartments varies in volume in response to the pressure changes with varying depth, making constant buoyancy adjustments necessary. This is usually accomplished by adding air to, or releasing it from, the diver’s buoyancy compensator.  Divers go to considerable lengths to vary their buoyancy to help them submerge, to stay at a given depth, or to ascend or stay afloat in an emergency.

Proper  Buoyancy

One of the most critical skills you must learn during your open water training is a mastery of proper buoyancy, trim, and control in the water column!

One of the biggest mistakes many instructors make during classes is to not demand a mastery of buoyancy control. Many new students learn to dive and develop skills in a pool setting, over-weighted, and sitting on their knees for the majority of skill development. Take these examples below…. do they represent a natural or desirable diving position?   NO..!!!

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If the first thing a new diver learns is to be negatively buoyant during their Open Water Training, they will have a very hard time with comfort and control. A lack of this skill produces a diver with very little confidence as well.  It is believed that this is a big reason new divers quit diving within the first year of certification. Here are a few examples of divers that were the result of improper training and skill development.

Keep in mind, the first video is of a Scuba Instructor…!!!

[pro-player width='530' height='460' type='video']http://www.youtube.com/watch?v=5af-ex96BVU[/pro-player]

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This Second video is of a young student, pushed through training way to quickly…in what we call weekend crash courses. This student is obviously not comfortable and has not had the chance to develop true skill

[pro-player width='530' height='500' type='video']http://www.youtube.com/watch?v=_-KHIytrUVo[/pro-player]

As you can see in the above pictures, a lot of students learn and perform skills while firmly planted to the bottom. This is completely backwards to what should be taught.  As a new diver it is very important to develop true neutral buoyancy while performing and practicing skills. To master your buoyancy, this is the way it should be. Here are some examples of proper buoyancy in the water column.

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Gas Laws and Scuba Diving

Gases behave in nature and in scuba diving according to several gas laws. Knowledge of these scuba diving gas laws is important to the diver because they influence the duration of the air supply and affect the gas containing spaces in the body such as the ears, sinuses and lungs. They also cause other scuba diving illnesses.

Boyle’s Law

This defines the relationship between pressure and volume. It states that the volume of a given mass of gas varies inversely with the absolute pressure (if the temperature remains constant).

Stated simply, for a given amount of gas, if the pressure is increased, the volume is proportionally decreased and vice versa. This means that if the pressure is doubled, the volume is halved and vice versa.

Stated mathematically:

It follows that for a given amount of gas, the volume multiplied by the pressure always has a constant value.
i.e. P × V is constant.

So if a sample of gas has an original volume of V1 and an original pressure of P1, and either the pressure or volume are changed, the new volume V2 and the new pressure P2 will multiply out to the same value.
i.e. P1 × V1 = P2 × V2

This law can easily be demonstrated by a piston and cylinder such as a bicycle pump. If the piston is pushed into the cylinder half way, and the escape of gas prevented, the pressure in the cylinder will be found to have doubled. By this process, many liters of air can be crammed into a bicycle tire but at the cost of an increase in pressure in the tire (and hard work).

Compressors work in this way, squeezing 2000 or
more liters of air into a scuba cylinder – but at a high pressure.  Since water pressure increases with depth, the consequent reduction in gas volume becomes very important to the diver because his body has numerous air spaces.

Descent Problems: The air in the diver’s middle ear and sinuses will contract in volume as the diver descends. If these volume changes are not compensated for by adding more air (“equalization”), then pressure damage (barotrauma) to the tissues will result.

For example:

In the same way, if a breath-hold diver takes a full breath at the surface and descends to 20 meters(3 ATA), the volume of air in his lungs may be reduced from 6 liters to 2 liters. The chest and lungs cope with compression better than distension. The limit for breath-hold diving is not known, but now has been shown to exceed 150 meters in certain individuals.

Ascent Problems. An average male diver’s lungs may contain about 6 liters of gas. If a diver takes a full breath at 20 meters (3 ATA) from his scuba set and returns to the surface (1 ATA) without exhaling, the volume of gas in his lungs will increase from the 6 liter total lung capacity to 18 liters (6 × 3 liters).

This can be easily calculated this way:

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The lungs would have to expand to 18 liters to accommodate this volume – well beyond their rupturing point, causing burst lung (pulmonary barotrauma of ascent).  An important practical observation of Boyle’s Law is that the greatest volume changes take place near the surface. This means that the greatest danger from barotraumas is near the surface — and this applies with descent as well as ascent.

For example, if diver has a maximum of 4 liters of air in his lungs at 40 meters depth (5 ATA) and ascends 10 meters without exhaling (to 4 ATA), the volume in the lungs will increase to 5 liters:

Some people could possibly accommodate this expansion without lung damage.  If the same diver started at 10 meters depth (2ATA), and then ascended 10 meters to the surface (the same ascent distance as before), without exhaling, the pressure would change from 2ATA to 1ATA. The air in the lungs would expand from 4 to 8 liters. This would rupture his lungs.  Although the dives involved the same ascent distances, the volume change, and hence the danger, in response to Boyle’s Law, is much greater near the surface.  Many divers are not aware of this and have a fallacious belief that if they confine their diving to shallow depths they will minimize the risk of barotrauma.  Buoyancy compensator’s are similarly affected by depth changes in response to Boyle’s Law.  Wet suits are also affected and lose their buoyancy and insulating properties with depth.

Charles’ Law

Most divers will have noticed that bicycle pumps and air compressors become hot during use.  As the volume of gas is compressed, heat is produced. This is explained by Charles’ Law.  This Law states that if the pressure remains constant, the volume of a given mass of gas varies directly with the absolute temperature (absolute temperature is obtained by adding 273 to the temperature in degrees Celsius).  In other words, at a fixed pressure, if gas is heated it expands, and if gas is cooled its volume contracts.

Charles’ and Boyle’s laws can be combined into the General Gas Law :

For the non-mathematically minded this means that for a given amount of gas, the pressure multiplied by the volume, divided by the temperature, always comes to the same value – so if one of these factors is varied, it has an effect on the other two.

Stated in another way; if a gas is compressed, its volume decreases and it gets hotter. If the gas is heated and the volume is prevented from expanding, the pressure rises.  The consequence of this law has lead to the demolition of several perfectly good automobiles (and divers!) following the storage of full scuba cylinders in the boot (trunk) in hot weather.  Similarly, inflatable dive boats are often pressurized to the maximum and are then left in the sun. As the temperature rises, the pressure of the contained air progressively increases and then suddenly reduces – when the volume increases and when the boat explodes.  If gas is allowed to expand rapidly, it cools. Cooling from the expansion of previously compressed air, as it is breathed from a scuba cylinder, can lead to the regulator freezing up during cold water diving.

Dalton’s Law

With a mixture of gases, the total pressure exerted by the mixture, is the sum of the pressures that would be exerted by each of the gases if it alone occupied the total volume. That is, the total pressure is the sum of the partial pressures.  As the overall pressure increases (with descent underwater), so the partial pressure of each constituent gas increases. e.g. if air contains approximately 21% oxygen (O2) and 78% nitrogen (N2), then in a sample of air at a given pressure, O2 will contribute 21% of the total pressure and N2 will contribute 78%.

To calculate the partial pressure of a gas, multiply the percentage of gas by the absolute pressure.  This law is important when considering the toxic effect of gases at depth or the use of O2 for
treatment purposes.

Henry’s Law

This law describes the dissolving of gas in a liquid and states that the quantity of gas which will dissolve in a liquid at a given temperature is proportional to the partial pressure of gas in contact with the liquid. This means that if the pressure of gas exposed to a liquid increases, then more gas will dissolve in the liquid.

An example of this law can be seen whenever a fizzy soft drink bottle is opened. During the manufacture of these drinks, carbon dioxide is dissolved in the liquid under pressure and the lid on the bottle maintains the pressure. When the bottle is opened and the pressure released, the liquid will not allow as much gas to be dissolved and so the excess gas is released from solution in the form of bubbles.   At sea level (1ATA) the human body contains approximately 1 litre of N2 dissolved in the tissues. Whenever a diver breathes compressed air at depth, more N2 will dissolve in the body because the partial pressure of N2 in the air being breathed is increased. This is the cause of nitrogen narcosis.  Under certain circumstances, when the diver returns to the surface this N2 can come out of solution in the form of bubbles.  These bubbles cause tissue injury which is the basis of decompression sickness (“bends”).

Henry’s Law 2

Henry’s Law 3

Diffusion of Gases

If a diver were to pass wind in a confined room, all the occupants of the room would soon be aware of the fact but, fortunately, not necessarily the source.  This process of distribution of gas is termed diffusion. It is caused by the rapid random movement of gas molecules to all parts of a contained space. Gas molecules, being only single or small groups of atoms, are able to easily diffuse through watertight membranes such as
blood capillaries or cell walls. This process allows O2 and other gases to pass from the lungs to the blood and tissues, and then back.

Diffusion 1

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The majority of this text was contributed via the Free and Open Works of
 Dr. Edmond's Diving Medicine- http://www.divingmedicine.info
please visit their site for a FREE Downloadable PDF of their entire works.

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Some of the major physical hazards are related to the effects of pressure. Pressure is defined as force per unit area. i.e.

PRESSURE = FORCE
AREA

If a force is spread over twice the area, the pressure is halved.  This explains why, for example, wide tires are preferable for driving on beaches. The weight of the vehicle (force) when spread over a large area causes less pressure on the sand. This vehicle is less likely to sink into the sand than one with narrow tires.

Gases exert pressure because they are made up of lots of fast moving molecules. The greater the number and the faster they move, the greater the pressure. Pressure on a Submerged Diver  The pressure acting on a submerged diver has two components:

1. The atmosphere above the water, termed atmospheric pressure,
2. The weight of the water above the diver, termed hydrostatic pressure.

Divers’ depth gauges are calibrated only to read the hydrostatic pressure (the depth of water) and so they read zero at sea level. They do not read the 1 atmosphere (1 ATA) above them. Thus the “gauge pressure” is always 1 atmosphere less than the true or “absolute” pressure.  We will now elaborate.

Atmospheric Pressure

The atmosphere above the earth is some 150 km high. Although air is very light, this amount of air has significant weight and exerts substantial pressure on the earth’s surface.

Atmospheric pressure at sea level is referred to as “one atmosphere” or “one bar”. It is the same as 101.3 kPa, 1 kg/cm2, 760mm Hg and 14.7 psi. At higher altitudes, atmospheric pressure is reduced, a factor which has a significant effect on diving in mountain lakes (see  Chapter 6).

Fig. 2.2 Atmospheric and Hydrostatic Pressures (depth)
added and thus converted to Absolute Pressure

Water is much denser than air and 10 meters (or 33 ft) of sea water exerts the same pressure (weight) as the whole 150 km of atmospheric air i.e. 1 ATA. For every additional 10 meres the diver descends, the water will exert a further pressure, equivalent to another atmosphere (1 ATA).

Common units of pressure (approximately):

Gauge Pressure

As described above, hydrostatic pressure in diving is generally measured by a pressure or depth gauge. Such a gauge is normally set to register a pressure of zero at sea level and so it ignores the pressure due to the atmosphere (1ATA).  The pressure registered by a gauge at 10 meters sea water depth would thus be one atmosphere gauge (1ATG) or equivalent units. Gauge pressure is converted to absolute pressure by adding 1 atmosphere pressure.

Partial Pressure

With a mixture of gases, the proportion of the total pressure contributed by each of the gases is termed its partial pressure (its part of the pressure). The partial pressure contributed by each gas is proportional to its percentage of the mixture. Each gas contributes the same proportion to the total pressure of the mixture, as is its proportion in the composition of the mixture. e.g. air at 1 ATA contains 21% oxygen, hence the partial pressure of oxygen is 0.21 ATA and
air at 1 ATA contains 78% nitrogen, hence the partial pressure of nitrogen is 0.78 ATA.

Absolute Pressure

The total pressure exerted on a diver at depth will be the pressure due to the atmosphere acting on the surface of the water (atmospheric pressure) plus the pressure due to the depth of the water itself (hydrostatic pressure).  The total pressure acting on the diver is termed the “absolute pressure”. It is often expressed in terms of atmospheres and is called “atmospheres absolute” or “ATA”. To calculate the absolute pressure acting on a diver at a given depth in terms of atmospheres, divide the depth in meters by 10 (since every 10 m. sea water exerts 1 atmosphere pressure) and add 1 (the pressure of the atmosphere above the water). e.g. the absolute pressure at 40 meters is [40 ÷ 10] + 1 = 5 ATA (The depth in feet, divided by 33 + 1 also calculates absolute pressure, for those in the USA, e.g. the absolute pressure at 99 ft is 99/33 +1 = 4 ATA).

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The majority of this text was contributed via the Free and Open Works of  Dr. Edmond's Diving Medicine- http://www.divingmedicine.info please visit their site for a FREE Downloadable PDF of their entire works.