Most diving is based on the use of compressed air and other oxygen–nitrogen mixtures as a breathing gas. Commercial, military, technical and experimental diving may involve the use of other gas mixtures. For this reason, it is desirable to give the reader some salient points on the gases mentioned in this text and related literature.
Oxygen (atomic weight 16, molecular weight 32) is the essential constituent of all breathing mixtures. At high altitude people survive with less than 0.1 ATA in their inspired air. However, for diving, oxygen should be present at a partial pressure of at least 0.2 ATA to avoid hypoxia. At higher partial pressures oxygen causes oxygen toxicity. Prolonged exposure to more than 0.55 ATA causes pulmonary oxygen toxicity, and shorter exposure to more than about 1.5 ATA results in central nervous system effects. The risk of these problems may be acceptable in a recompression chamber, where oxygen may be used at partial pressures of up to 2.8 ATA. Oxygen toxicity is discussed in Chapter 17.
In the range 0.2 to 2.8 ATA, oxygen has little effect on the respiratory centre and minute volume will remain close to normal. Oxygen is vasoactive; high oxygen tensions cause vasoconstriction.
Nitrogen (atomic weight 14, molecular weight 28) is the major component of air – about 79 per cent. Nitrogen is often considered to be physiologically inert. Bubbles, composed mainly of nitrogen, can cause DCS if a diver who has been breathing air or an oxygen–nitrogen mixture ascends too rapidly. In solution, it may cause nitrogen narcosis at depth (see Chapter 15). At partial pressures of nitrogen greater than about 3 ATA, there is a demonstrable decrement in the diver’s performance. At higher partial pressures, the effect is likely to cause the diver to make mistakes. The other problem that restricts the use of nitrogen is that its density at increased pressure increases the work of breathing.
Despite these disadvantages, nitrogen is of major importance in diving, at depths less than 50 metres and as a part of more complex mixtures at greater depths.
Helium (atomic weight 4) is a light, inert gas. It is found in natural gas wells in several countries. Helium is used to dilute oxygen for dives to depths greater than 50 metres, where nitrogen should not be used alone. The two major advantages of helium are that it does not cause narcosis and, because of its lightness, helium-oxygen mixtures are easier to breathe than most alternatives. Helium-oxygen mixtures can allow a shorter decompression time (albeit often with a different profile) than an equivalent saturation dive with the diver breathing air because helium diffuses more rapidly than nitrogen.
The use of helium can cause several problems. The speech of a diver at depth may need electronic processing to make it understandable because of the distortion. A diver in a helium atmosphere is more susceptible to heat and cold because the high thermal conductivity speeds the transfer of heat to and from the diver. The other problem with the use of helium is that it is associated with a disorder called the high-pressure neurological syndrome (HPNS) (see Chapter 20).
Hydrogen (atomic weight 1, molecular weight 2) has the advantage of being readily available at low cost. Because of its lightness it is the easiest gas to breathe. These factors may lead to its use as a replacement for helium. The reluctance to use stems from fears of explosion. Explosions can be prevented if the oxygen level does not exceed 4 per cent, and such a mixture is breathable at depths in excess of 30 metres. Hypoxia can be prevented by changing to another gas near the surface. Hydrogen causes thermal and speech distortion problems similar to those encountered with helium.
Diving and exposure to high pressures change the heat transfer from a diver’s body. In air, there is some insulation from the air trapped near the body, either by the clothes or the hair and the boundary layer. In water this is lost. The water adjacent to the skin is heated, expands slightly, and causes a convection current that tends to remove the layer of warmed water. This process is accelerated by movement of the diver or the water. The net result is that a diver cools or heats up much more quickly than he or she would in air of the same temperature.
Heat loss is also increased in warming the cooler inhaled air or gas. For a diver breathing air, most of this heat is used to humidify the dry air used for diving and is not sufficient to cause concern in most circumstances. However, the heat lost in a helium dive is more significant. Helium has a greater specific heat than nitrogen. The problem is compounded because at depth, the mass of gas inhaled is increased.
The heat transfer by conduction is also increased in a helium environment. The result is that a helium diver may need external heating to maintain body warmth at a water, or gas, temperature where external warming would not be required if the diver was in an air environment.
In warm environments, it is possible for a diver to suffer heat stress. A diver who is wearing a protective suit cannot lose heat by sweating because the sweat cannot evaporate. In a pressure chamber, the atmosphere can become saturated with water, and evaporative cooling is prevented. The heat stress for a given temperature is also increased if there is helium in the mixture.
Despite wearing thermal insulation in warm tropical waters, divers can continue to lose heat over several days of repetitive diving, and ‘silent’ hypothermia can develop, somewhat insidiously.
A diver in water or a helium-rich environment can cool or heat up at a temperature that would be comfortable in an air environment.
Even in the cleanest ocean water, only about 20 per cent of the incident light reaches a depth of 10 metres and only 1 per cent reaches 85 metres. Clean water has a maximum transparency to light with a wave length of 480 millimicrometres (blue). This variation of absorption with wave length causes distortion of colours and is responsible for the blue-green hues seen at depth. Red and orange light is absorbed most. Because of the absorption of light, the deep ocean appears black, and lights are needed for observation or photography. Because of the greater absorption of reds by water, some illumination is needed to see the true colours, even at shallow depths. Part of the appeal of diving at night is that objects that have a blue-green colour in natural light have a new brightness when they are illuminated with a torch.
Coastal water, with more suspended material, has a maximum transparency in the yellow-green band, about 530 millimicrometres. Absorption and scattering of light by suspended particles restrict vision and can tend to even out illumination. This can make the light intensity the same in all directions and is an important factor in causing loss of orientation.
When the eye focusses on an object in air, most of the refraction of light rays occurs at the air–cornea interface. In water, this refractive power is lost and the eye is incapable of focussing. A face mask provides an air-cornea boundary, which restores refraction at the cornea surface to normal. Refraction also occurs at the face mask surface, mainly at the glass–air boundary. This results in an apparent size increase of about 30 per cent and this makes objects appear closer than they are. Practice and adaption of the hand–eye co-ordination system allow the diver to compensate for this distortion, except when describing the size of fish.
Masks also restrict vision by narrowing the peripheral fields, and they distort objects that subtend large visual angles. Both absorption of light by water, which reduces apparent contrast, and scattering by suspended particles reduce visual acuity. Attempts have been made to improve the diver’s vision by modification of the face mask, the use of coloured filters, ground mask lenses and contact lenses. These can be relatively successful but can also impose their own problems.
Sound in water is transmitted as waves with a longitudinal mode of vibration. The speed of sound is about 1530 metres/second in sea water and 1470 metres/second in fresh water at 15°C. Water is a better transmitter of sound than air, so sounds travel greater distances under water. Low-pitched sounds travel farther than higher-pitched sounds. Transmission of sound is enhanced by reflection from the surface. This reflection also enhances the transmission of sound in air over water but reduces the transmission of sounds from air to water and from water to air.
Both high-pressure air and helium-oxygen mixtures cause speech distortion. This is greater when breathing helium mixtures and can render speech unintelligible. Distortion in air causes the voice to become more nasal and crisp as the pressure increases.
It is often thought that divers cannot talk underwater. This is not so if the diver has an air space to speak into. Helmet divers can communicate easily by touching their helmets together and using the air-metal-air pathway. Some scuba divers have mastered the art of talking by taking their demand valve from their mouth and speaking into an air space created by cupping their hands.
Our normal idea of diving is that a diver descends from sea level, 1 ATA, and returns when the dive has finished. There is a series of variations from this situation. A diver may have to dive in a mountain lake where the pressure on the surface is less than 1 ATA. Another variation occurs when a diver starts from an environment where the pressure is greater than 1 ATA. This happens when divers operate from a pressurized compartment or underwater habitat. These conditions introduce complexities that require understanding of the physics involved.
A diver operating in a high mountain lake is returning to a lower surface pressure than a diver at sea level. This decreases the pressure at which the diver is while releasing inert gas after a dive and so increases the tendency to form bubbles. Therefore, the diver may need to modify the decompression plan. Another minor correction will be required if it is a fresh water lake. Fresh water is less dense than salt water, so the diver is exposed to a slightly lower pressure change per unit depth.
In addition, this diver will have to exhale faster during ascent. A diver who ascends from 10 metres (2 ATA) to the surface (1 ATA) without exhaling would find that the volume of gas in the lungs has doubled. Most divers realize this and exhale at an adequate rate during ascent. However, they may not realize that a similar doubling in gas volume occurs during the last 5 metres of ascent to the surface, if the pressure at the surface was 0.5 ATA.
High-altitude diving may require that the depth or duration of dive and the rate of ascent be reduced to allow for the lower than normal surface pressure at the end of the dive. Tables are available for diving at higher altitudes, and many dive computers are programmed to compensate for this.
A diver living in a human-made environment where the pressure is high can operate to deeper than normal depths. This system is used in saturation diving, where the diver operates from a base at increased pressure and becomes equilibrated with it. The eventual return to the surface can take many days. The use of such environments has proved to be invaluable where deep or long dives are required (see Chapter 67).
Another pressure-related problem can occur when a diver dives and then flies or ascends into mountains. Some dives and ascents will require the diver to ensure that adequate time is spent at the surface before ascending to high altitude, to avoid DCS. This problem is encountered by a diver tourist who wants to fly home after diving or one who needs to pass over hills or mountains when returning from a dive. It is also encountered when it is necessary to transport a diver with DCS. There may be an increase in manifestations of DCS when the pressure is decreased, even by a relatively small amount.
Measurements of energy expenditure, while swimming on the surface and underwater, have been made using indirect calorimetry and by prediction from heart rate. These results show that oxygen consumption underwater of more than 3 litres/minute (lpm) is possible, and values greater than 2 lpm are quite common. The diver’s energy expenditure when inactive may be lower than found on land, presumably because the absence of gravitational effects reduces the energy required to maintain posture underwater.
Typical gas consumption and energy expenditure levels are as follows:
For a slow swim, 0.5 knots, the diver would have an air consumption of 20 lpm and an oxygen consumption of 0.8 lpm. A swim of 0.8 knots would cause an air consumption of almost 40 lpm and an oxygen consumption of 1.4 lpm. A fast swim of 1.2 knots would cause an oxygen consumption of about 2.5 lpm and an air consumption of 60 lpm (air consumption measured at the depth the diver was swimming and oxygen consumption at 1 ATA).
Increased gas density increases the work of breathing. This increases the resistance to gas flow through the diver’s airways and breathing apparatus, increases the work of breathing and reduces ventilatory capacity. A maximum breathing gas density (helium) of around 8 g/litre appears to be realistic for practical purposes, thus limiting diving to around 400 to 500 metres for useful work.
Gas density may prove to be the limiting factor for deep diving.
It may be expected that the higher oxygen partial pressures in hyperbaric environments could improve physical performance. However, chamber experiments, in which the subjects exercised while breathing oxygen at 3 ATA, showed that the maximum aerobic work performance was not significantly increased.
Archimedes’ Principle states: ‘any object, wholly or partially immersed in liquid, is buoyed up by a force equal to the weight of liquid displaced’. A diver is an object immersed in water and is therefore affected by this principle. It determines the effort the diver must make to dive. If a diver weighs less than the weight of water he or she displaces, the diver will tend to float to the surface – i.e. he or she has positive buoyancy, which makes descent difficult. If the diver weighs more than the weight of water he or she displaces, the diver has negative buoyancy, which will assist descent and make ascent more difficult.
A diver can change buoyancy in several ways. If the diver wears a weight belt, he or she increases weight by a significant amount and displaces only a little more water and, as a result, will decrease buoyancy. If the diver displaces more water, he or she will increase buoyancy. This can be achieved by retaining more air in the lungs. It can also be achieved by inflating the diver’s buoyancy compensator device (BCD) – a device used to control buoyancy. It has an air space that the diver can inflate or deflate to make him positively, negatively or neutrally buoyant, as needed.
An interesting combination of the effects of Boyle’s Law and Archimedes’ Principle is shown by the changes in buoyancy experienced by a diver wearing BCD or a compressible suit. If slightly positively buoyant at the surface with air in the BCD, the diver will experience some difficulty in descending. As the diver descends he or she will pass through a zone where he or she is neutrally buoyant and, if the diver descends further, he or she will become negatively buoyant. The increased pressure reduces the volume of gas in the BCD or suit, the volume of fluid displaced and, consequently, the diver’s buoyancy.
The weight of a scuba cylinder decreases as gas is consumed from it, and this will lead to an increase in buoyancy. An empty cylinder can weigh 1 to 2 kg less than a full one, depending on the initial pressure and the size and type of the cylinder (e.g. steel, alloy).
Immersion creates a condition resembling the gravity-free state experienced by astronauts. In air, a standing person has a pressure gradient in the circulation where the hydrostatic pressure is greatest at the feet and least at the head. For an immersed diver, the hydrostatic gradients in the circulatory system are almost exactly counterbalanced by the ambient water pressure. This reduces the volume of pooled blood in the leg veins. In addition, peripheral vasoconstriction will occur in response to any cold stress. These changes result in an increase in central blood volume, leading to diuresis and subsequent haemoconcentration and decreased plasma volume.
The effect of haemoconcentration on normal dives is not major except that it gives divers a physiological excuse for well-developed thirst and sometimes the need to urinate. Urine production rates of more than 300 mL/hour cause problems for divers trying to keep their dry suit dry, unless it is fitted with a relief outlet.
The other effect of increased central blood volume is on cardiac performance. There is an increase in cardiac output as a result of increased stroke volume. Immersion alone, or in combination with various other factors associated with the diving environment, can precipitate cardiovascular dysfunction in susceptible individuals. This is discussed in Chapter 39.
On the surface of the Earth, we are exposed to the pressure exerted by the atmosphere. This is called the atmospheric or barometric pressure. Most people regard this pressure as caused by the mass of the atmosphere pressing down on them. A flaw in this argument is that the pressure remains in a bottle after it is sealed, although its contents are contained and are no longer exposed to the column of air above. The physically correct explanation is that atmospheric pressure is generated by collisions of the molecules of gas in accordance with the kinetic theory of gases. Either explanation is acceptable for the following discussion.
The pressure decreases as we move upward through the atmosphere and increases as we move down into a mine or into the sea. At the top of Mount Everest the atmospheric pressure is about 40 per cent of that at sea level. Because water is much heavier than air, the pressure changes experienced by divers over a particular depth change are much greater than those encountered by climbers or aviators as they change altitude.
Pressure is measured in a variety of units from either of two reference points. It can be expressed with respect to a vacuum, i.e. zero pressure. This reading is called an absolute pressure. The second method measures pressures above or below local pressure. These readings are called gauge pressures. At sea level, the absolute pressure is 1 atmosphere (1 ATA) and the gauge pressure is 0. These units are commonly abbreviated to ATA and ATG.
Common examples are the barometric pressure used by weather forecasters, which is an absolute pressure, and the blood pressure, which is a gauge pressure reading.
With descent in water, pressure increases. For each 10 metres of depth in sea water, the pressure increases by 1 atmosphere, starting from 1 ATA or 0 ATG at the surface. The gauge pressure remains 1 atmosphere less than the absolute pressure. For example, at 10 metres, the pressure is 2 ATA and 1 ATG. At 90 metres, the pressure is 10 ATA and 9 ATG.
Table 2.1 Pressure conversion factors (commonly used approximations shown in brackets)
Because diving involves facets of engineering and science, it is plagued with many units of pressure. These include absolute and gauge atmospheres, pascals and multiples such as the kilopascal, metres or feet of sea water, bars, pounds per square inch, torr and several other rarer units. Table 2.1 lists conversions for the more commonly used units.
Pressure and the diver’s body
Many people have difficulty in understanding why the pressure of the water does not crush the diver. The answer to this problem may be considered in two parts:
The solid and liquid parts of the body are virtually incompressible, so a pressure applied to them does not cause any change in volume and is transmitted through them. After immersion, the increased pressure pushes on the skin, which in turn pushes on the tissues underneath, and so the pressure is transferred through the body until the skin on the other side is pushed back against the water pressure. Therefore, the system remains in balance. This is in accordance with Pascal’s Principle, which states: ‘A pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio remains the same’.
However, the effect of pressure on the gas spaces in the diver’s body is more complex. The applied pressure does not cause any problems if the pressure in the gas space is close to that of the surrounding water. There is, for example, no physical damage to a diver’s lungs if the air space was exposed to an internal pressure of 100 metres of water, provided that this pressure is balanced by the pressure exerted by surrounding water acting on the walls of the lung to balance any tendency of the lungs to expand. If the lungs were exposed to an internal pressure sufficiently more than the surrounding atmospheric tissue, they would overexpand and burst.
Water pressure and lung inflation
Immersion up to the neck in water reduces vital capacity by about 10 per cent (Figure 2.1 shows lung volumes). This is caused in part by the hydrostatic pressure of the water compressing the thorax. With immersion, there is also a loss of gravitational effects. This reduces the volume of blood in lower, mainly leg, veins and increases thoracic blood volume. This change in turn reduces the compliance of the lungs.
When a diver is using breathing equipment, pressure at the point from which the gas is inhaled can be different from the pressure at the chest. If upright in the water, a scuba diver is inhaling air released at the pressure at the level of the mouth. A snorkel diver is inhaling air from the surface, and this is at surface pressure. In both these cases, the air is at a lower pressure than the diver’s lungs. This reduces the amount of air the diver can inhale because part of the inhalation force is used in overcoming this pressure difference.
Conversely, when descending, face-down, a diver whose air is released at mouth pressure can inhale to greater than normal vital capacity but could not exhale to the normal residual volume. This is because in this orientation, the water pressure is helping to inflate the lungs.
Pressure and volume changes
When a diver descends, the increased pressure of the surrounding water compresses gas in the gas spaces within the diver’s body. These spaces include the lungs, middle ears, sinuses and intestines.
This is one of the many aspects of diving medicine that is concerned with the relationship between pressure change and change of gas volume. The relationship between changes in volume of a gas and the pressure applied to it is described by Boyle’s Law. This states: ‘if the temperature remains constant, the volume of a given mass of gas is inversely proportional to the absolute pressure’. This means that the absolute pressure multiplied by volume has a constant value, and this constant changes with the mass of gas considered. To a mathematician, this means that P × V = K or P1 × V1 = P2 × V2, where P and V are pressure and volume. For example, 10 litres of gas at sea level pressure (1 ATA) will be compressed to:
5 litres at 2 ATA (10 metres).
2 litres at 5 ATA (40 metres).
1 litre at 10 ATA (90 metres).
During ascent into the atmosphere, the reverse happens and the gas expands. This means that the 10 litres of air would expand to 20 litres at 0.5 ATA (an altitude of about 5000 metres or 18 000 feet) and to 40 litres at 0.25 ATA (an altitude of about 10 300 metres or 33 400 feet).
Gas volumes expand when pressure decreases and contract when pressure increases.
The volume of a mass of gas in a flexible container decreases with pressure or depth increase and expands during ascent or pressure reduction (Figure 2.2). It should be noted that volume changes are greatest near the surface. Conversely, gas has to be added if the volume of a container or gas space is to remain constant as the pressure is increased. The effects of this law are important in many aspects of diving medicine.
During descent, the increasing pressure in the water is transmitted through the body fluids to the tissue surrounding the gas spaces and to the gas spaces themselves. The pressure in any gas space in the body should increase to equal the surrounding pressure. In the lungs, during descent on breath-hold dives, this is accompanied by a decrease in lung volume. Air should enter cavities with rigid walls, such as the sinuses or the middle ear. If air entry does not take place to equalize pressures, then a pressure difference between the space and the surrounding tissue will develop, with the pressure in the gas space being less than in the surrounding tissue. The results are tissue distortion and damage, such as congestion, oedema or haemorrhage.
During ascent, as the pressure decreases, gas within body spaces will expand. Unless gas is vented from the space, the expanding gas will exert pressure on the surrounding tissue and will eventually damage it. Pressure changes in the middle ear can also result in rupture of the tympanic membrane.
The same volume changes with pressure occur in bubbles in tissue or blood. Again, the volume changes are greatest close to the surface. An injury caused by pressure change is called barotrauma.
Barotrauma is the general name for an injury caused by pressure change.
Respiration in water and under pressure
While breathing air underwater, the diver’s respiratory volume is about the same as it would be if he or she worked at the same rate on the surface. A consequence of this is that a cylinder that contains enough air for 100 minutes at 1 ATA would last about 50 minutes at 2 ATA (10 metres) or 20 minutes at 5 ATA (40 metres) for dives with the same energy expenditure. This is because the gas in the cylinder expands to a smaller volume when it is released against the ambient pressure at depth than it would if used at the surface. A cylinder that contains 5000 litres of gas if it is released at the sea surface would yield only 1000 litres of gas if it is released at 5 ATA, or 40 metres. A diving physician needs to keep this in mind when estimating the amount of gas needed for any task or therapy.
With depth, gas is compressed and there is an increase in density of the gas because there are more molecules in a given space. So, at depth, a diver must move a greater mass of gas with each breath. This requires greater effort and involves an increase in the work of breathing. In some situations, this can limit the capacity to do work.
The density of the breathing gas can be reduced by replacing nitrogen with a lighter gas such as helium. For example, the density of air at 1 ATA is about 1.3 kg/cubic metre. At 10 ATA, the density of air would be about 13 kg/cubic metre. The use of lighter gas helps to reduce density. For example, at 40 ATA, the density of a 1 per cent oxygen and helium mixture is 6.7 kg/cubic metre.
As the density of a gas increases, there is an increased tendency for the flow to become turbulent. This causes a further increase in the energy used in breathing. These factors can lead to fatigue of the inspiratory muscles and reduce maximum breathing capacity and the work output. To minimize this load, the body responds by using less gas for a given workload. This can result in the development of hypercapnia. Continued exposure to dense gas, as is encountered in deep dives, may cause an adaptive response.
Temperature and volume changes
Charles’ Law states: ‘If the pressure is constant, the volume of a mass of gas is proportional to the absolute temperature’.
The absolute temperature (A°) is always 273° more than the centigrade temperature. A more useful expression of the law is as follows:
Where V1 is the volume of a mass of gas at temperatures T1°A and V2 is its volume after the temperature has changed to T2°A.
This law has much less relevance to diving medicine than Boyle’s Law. However, it should be remembered when considering gas volumes and how they may change.
Boyle’s and Charles’ Laws may be combined and used if temperature and pressure both change – from P1 and T1 to P2 and T2 with a volume change from V1 to V2. The combined laws can be expressed as the universal gas equation:
A temperature-pressure problem that often causes discord can be used to illustrate the use of this equation. This is the effect of temperature on the pressure in a gas cylinder.
A diver may ask to have the compressed air cylinder filled to 200 ATA. The gas compressor heats the gas so the cylinder may be charged with gas at 47°C. When the diver gets in the water at 7°C, the diver may find that he or she has only 175 ATA in the cylinder. In this case V1 = V2 because the cylinder is rigid and the pressure falls as the gas cools.
So the reduced pressure is a result of temperature change, not a leaking valve or fraud by the air supplier.
Partial pressures in gas mixtures
Dalton’s Law states: ‘the total pressure exerted by a mixture of gases is the sum of the pressures that would be exerted by each of the gases if it alone occupied the total volume’. The pressure of each constituent in a mixture is called the partial pressure (Figure 2.3). In air, which is approximately 80 per cent nitrogen and 20 per cent oxygen, the total pressure at sea level (1 ATA) is the sum of the partial pressures of nitrogen, 0.8 ATA, and oxygen, 0.2 ATA. At 2 ATA (10 metres) these partial pressures will rise to 1.6 and 0.4 ATA, respectively.
The partial pressures of breathing gases can be manipulated to the diver’s advantage. For example, the composition of the gas breathed may be modified to reduce the chance of decompression sickness (DCS) by decreasing the percentage of inert gas in the mixture.
Undesirable effects can also occur. Air from an industrial area may contain more than 0.3 per cent carbon dioxide and 0.002 per cent carbon monoxide. If incorporated in compressed breathing gas and delivered at high partial pressures, both constituents could be toxic unless measures were taken to remove these contaminants before use.
It may be necessary to combine Boyle’s and Dalton’s Laws in calculations. For example, it may be decided that a diver should be given a mixture with a partial pressure of 0.8 ATA oxygen and 1.2 ATA nitrogen in a recompression chamber pressurized to 2 ATA. If oxygen and air are the only gases available, the gas laws can be used to calculate how to prepare a cylinder charged with the right gas mixture.
The mixture will need to be 40 per cent oxygen and 60 per cent nitrogen (Dalton’s Law). If the gas is to be prepared in a cylinder charged to 200 ATA, it should contain 120 ATA of nitrogen (60 per cent of 200). If this is to be obtained from compressed air (assumed to be 80 per cent nitrogen in this exercise), it will be necessary to put 150 ATA of compressed air into the cylinder (30 ATA of oxygen and 120 ATA of nitrogen) with 50 ATA of oxygen.
This simple mixing process cannot be used as successfully with helium mixtures. At high pressures, helium does not follow the predictions of Boyle’s Law accurately. It is less compressible than the ideal gas described by Boyle’s Law. Mixing can be conducted with allowance for this or by putting a calculated weight of each gas in the cylinder.
Solution of gases in liquids
Henry’s Law states: ‘at a constant temperature, the amount of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas over the liquid’. This law implies that an equilibrium is established with each gas passing into and out of any solution in contact with it (Figure 2.4). At sea level (1 ATA), an individual’s body tissues contain about 1 litre of gaseous nitrogen in solution. If the diver dived to 10 metres and breathed air at 2 ATA, more gas would dissolve and he or she would eventually reach equilibrium again and have twice as much nitrogen in solution in the body. The time taken for any inert gas to reach a new equilibrium depends on the solubility of the gas in the tissues and the rate of gas supplied to each tissue.
When the total pressure, or the partial pressure of a particular gas, is reduced, gas must pass out of solution. If a rapid total pressure drop occurs, a tissue may contain more gas than it can hold in solution. In this situation, bubbles may form and may cause DCS.
The physiological effects of the solubility of gases are also relevant in nitrogen narcosis and oxygen toxicity.
It should be noted that each gas has a different solubility and the amount of any gas that will dissolve in a liquid depends on the liquid. For example, carbon dioxide is very soluble in water compared with other common gases. Beer aerated with compressed air instead of carbon dioxide would have far fewer bubbles. Nitrogen is more soluble in fats and oils than in aqueous solutions.
Henry’s Law is also time dependent. It takes time for gases to enter and leave solution or form bubbles. If this was not so, champagne would go flat as soon as the cork was popped.
At depth, a diver breathing air absorbs nitrogen in accord with Henry’s Law. The amount depends on depth and time of exposure. When the diver surfaces, the excess nitrogen must pass from the body. If it is eliminated from solution through the lungs, there will not be any complications. In some cases, the nitrogen comes out of solution in the blood or tissues, thus forming bubbles that may lead to DCS.
Gas movement in body tissues
Gas transfer from the lungs to the tissues is dependent on the cardiovascular circulation, and the gas supplied to a portion of tissue depends on the blood perfusing it. In a permeable substance such as body tissues, gas molecules can migrate by diffusion. That is, gas molecules dissolve in the tissue fluids and tend to move from areas of high to low partial pressure until the partial pressure of the dissolved gas is uniform. This can take hours. It is the dissolved gas pressures that tend to equilibrate, not the number of gas molecules. If a gas is twice as soluble in one tissue compared with another, then twice as many molecules will be in the first tissue to produce the same partial pressure in the tissue. This information can be estimated from the solubility coefficients of the gas in the components of the tissue.
The rate of gas movement between two points depends on several factors. The difference in partial pressure and the distance between the two points may be combined into a concentration gradient. The other major factor is the permeability of the tissue, an expression of the ease of gas movement. A large partial pressure between two points that are close together (a steep gradient) and a greater permeability both increase the rate of gas transfer.
Metabolic gas exchange
In divers, gas exchange mechanisms are basically the same as at normal pressure. Oxygen diffuses down a concentration gradient from the lungs to the tissues. The carbon dioxide gradient is normally in the opposite direction. The exchange of inert gases becomes important and there are changes in the finer details of metabolic gas exchange.
With increasing depth, there is an increase in the partial pressures of the constituents of the breathing mixture in accordance with Dalton’s Law. This causes higher alveolar pressures and arterial pressures of the inhaled gases.
Elevated pressures of oxygen facilitate oxygen transport, but they may interfere with the elimination of carbon dioxide in two ways: first, by the depression of respiration induced by high arterial oxygen tensions; and second, by direct interference with the transport of carbon dioxide. When the inspired oxygen partial pressure is elevated, there is an increase in oxygen transport in solution in the plasma (Henry’s Law).
When one is inhaling oxygen at a partial pressure above 3 ATA, the total oxygen requirement may be carried in solution. If this happens, the haemoglobin may be still saturated with oxygen in the venous blood, and this can prevent the transport of carbon dioxide in the form of carbaminohaemoglobin.
The result is an increased tissue carbon dioxide level. In some situations, there may also be an increase in the inspired carbon dioxide pressure. Causes include contamination of the breathing gas supply, the external dead space of the equipment, inadequate ventilation or failure of the absorbent system.
There is a tendency for experienced divers to be less sensitive to elevated carbon dioxide partial pressures. This reduces the total ventilation requirement during working dives. Elevated arterial carbon dioxide levels increase susceptibility to oxygen toxicity, DCS and inert gas narcosis. For these reasons, it is desirable to control the factors that cause carbon dioxide retention.
Diving is associated with a tendency to retain carbon dioxide.
Inert gas exchange
The topic if inert gas exchange is considered in the chapters on DCS. Therefore, to avoid duplication, the topic is not considered in detail here. As indicated earlier, increased total pressure is usually accompanied by an increase in nitrogen (and/or other inert gas) pressure (Dalton’s Law). This causes gas transfer to the body tissues. When pressure is reduced at the end of the dive, the transfer is reversed. If there is an excess of gas, then it can come out of solution as bubbles. These bubbles are the cause of DCS. If bubbles do occur, they are also subject to the same physical laws. Their size decreases if the pressure is increased, and gas enters or leaves them depending on the concentration gradients of gases.
A basic knowledge of the physics and physiology of diving is essential to understand most of the medical problems encountered. Aspects of physics and physiology that have a wide application to diving are discussed in this chapter.
Some of the basic physiological implications are also mentioned, but most aspects of diving physiology and pathophysiology are relegated to the relevant chapters on specific diving disorders.