Gases for Mixed Gas and Technical Diving

Commercial mixed gas diving usually involves helium-oxygen gas mixtures prepared by commercial gas supply companies or mixed on the dive site by highly qualified life support technicians and dive supervisors. The large supply cylinders used are generally prepared by mixing gases from a high-purity supply, and the final mix is analyzed multiple times to check that the composition is correct. This process is costly in time and equipment.

The gas that fills the cylinders used in military and recreational technical mixed gas diving is generally sourced from large cylinders and then transferred into the smaller cylinders used by the diver. The mixture is often blended on site as each cylinder is filled.

In the recreational diving industry where nitrox is commonly used, oxygen is generally sourced in large cylinders from a commercial supplier. The oxygen is decanted into the cylinders, where the pressure is measured, and the cylinder is ‘topped-up’ with air to a pre-determined pressure to produce the mixture required. The mixture is then analyzed for its oxygen content. This procedure is known as ‘partial pressure blending’. If the gases supplied were of sufficient purity, significant contamination should be prevented. (Purity of supplied gas can vary, especially in some countries with lower standards of oversight). It is essential that the oxygen content is measured using two analyzers to verify the final mixture. The diver should also analyze the gas to ensure there has been no error. Failure to do so has sometimes been associated with serious consequences.

Another method of mixing (known as ‘continuous flow blending’) involves the air being first passed through a membrane system that removes some nitrogen and so creates a higher-oxygen mixture. This is then compressed. To reduce the risk of fire or explosion, the mixture must have an oxygen concentration of 40 per cent or lower. An alternative method of continuous flow blending for nitrox is carried out by introducing metered amounts of oxygen into the intake airflow stream of the compressor via mixing coils. It is important that the gas is mixed thoroughly before entering the compression stage to ensure that the oxygen concentration is lower than 40 per cent. The output is sampled and adjusted until the desired mixtures are obtained. This process can be used only on suitable compressors and to a maximum of 40 per cent oxygen concentration.

Trimix can be produced by combining helium with oxygen and air or a suitable nitrox mixture. Accurate measuring equipment is required, as are suitable valves to control the flow of gases. Once the trimix is produced, it is rolled or agitated for an hour or so to allow the gases to homogenize. To confirm the final mixture the oxygen content needs to be measured, although, ideally, the helium content should also be analyzed.

In any of these systems where air is one of the source gases used, or where a mixture is blended and then compressed by a high- or low-pressure compressor, the same potential sources of contamination exist as for air. Potential problems are multiplied by the complexity of the system and the fact that lubrication oil life can be much reduced through oxidation by oxygen-rich mixtures.

As an alternative to compressors, oil-free reciprocating ‘booster pumps’ are sometimes used to avoid these risks and to avoid the risk of oxygen fire and explosion when filling cylinders with oxygen-rich mixtures.

Sinus Barotrauma: Diving Experience

The second Australian series

Comprehensive reviews of diving-related sinus barotraumas were not easy to find. Flottes4, in 1965, described sinus barotrauma in divers.

Sinus barotrauma has been described in various texts on diving medicine5, but initially without specific clinical series being documented. A reasonably large clinical series of divers with sinus barotrauma was first described in Australia by Fagan, McKenzie and Edmonds in 19766 and was quoted widely thereafter. This series described minor and acute cases and was complemented by another series of 50 more serious cases in patients who were referred for definitive treatment7.

The first Australian series

The cases in this series were equivalent to Campbell’s aviation sinus barotrauma grades 1 and 2. This series included 50 consecutive cases of sinus barotrauma as they were observed in a Navy environment, where all such cases were referred for medical opinion irrespective of severity6. It included many cases that might otherwise have not attended for treatment.

In this series, 68 per cent of the presenting symptoms developed during or on descent, and they developed in 32 per cent during or after ascent.
In the majority, the divers were undergoing their first open water diver training course. Pain was the predominant symptom in all the cases on descent and in 75 per cent of those on ascent. Pain was referred to the frontal area in 68 per cent, the ethmoid in 16 per cent and the maxillary in 6 per cent. In one case it was referred to the upper dental area.

Epistaxis was the second most common symptom, occurring in 58 per cent of cases. It was rarely more than an incidental observation, perhaps of concern to the diver but not usually of great severity. It was the sole symptom in 25 per cent of the cases of ascent barotrauma.

Even though these were inexperienced divers, 32 per cent had a history of previous sinus barotrauma, produced by scuba diving, aviation exposure or free diving. Half had a history of recent upper respiratory tract inflammation, and others gave a history of intermittent or long-term symptoms referable to the upper respiratory tract, e.g. nasal and sinus disorders, recurrent infections or hay fever.

In 48 per cent of cases, otoscopy showed evidence of middle ear barotrauma on the tympanic membrane.

Radiologically, the affected sinuses did not replicate the clinical sites and manifestations. Either mucosal thickening or a fluid level was observed in the maxillary sinus in 74 per cent of the cases, in the frontal sinus in 24 per cent and in the ethmoid in 15 per cent. These findings contrast with the clinical manifestations. A fluid level was present in 12 per cent of the maxillary sinus cases.

Most of these divers required no treatment or responded to short-term use of nasal decongestants. Antibiotics were prescribed if there was pre-existing or subsequent sinusitis. Neither sinus exploration nor surgery was required in any case. This series has inappropriately been used to imply that such intervention is never applicable in the treatment of sinus barotrauma.

This prospective Australian series, by its design, included relatively minor cases of sinus barotrauma. It has, by default, been used as being typical of all sinus barotrauma cases, even those that manifest in patients with recurrent or delayed symptoms, or complications, in emergency wards or ear, nose and throat consulting rooms. That extrapolation is not necessarily valid. Also, this study was done more than 4 decades ago, before computer imaging techniques became commonplace and when sphenoidal disease was not easily detected.

Sinus barotrauma and its complications remain common medical problems of div-ing. The importance has been stressed by many workers including Edmonds, Freeman, Thomas and colleagues8, Becker and Parell9, Neblett10 and Roydhouse11.

The second Australian series

The cases in this series were equivalent to Campbell’s aviation sinus barotrauma grade 3. A series of 50 more severe cases, i.e. in patients referred for medical treatment of sinus barotrauma, was reported in 19947. These patients were seen within 1 month of the latest incident.

The cases were self-selecting because the divers with repetitive or more significant problems were more likely to present for treatment. The investigations frequently involved computed tomography (CT) scans of the sinuses, sinus endoscopy and occasionally magnetic resonance imaging (MRI).

These cases were in more experienced divers – 88 per cent had in excess of 50 dives. The distribution was skewed strongly to the extremely experienced, with 70 per cent of the divers having more than 5 years of experience, and many being dive masters, dive instructors or professional divers. Because of the extreme amount of diving exposure in this group, it is presumed that the sinus ostia or ducts may have become scarred and narrowed from the repeated insults they sustained.

In 12 per cent of the patients, the presenting headache developed and progressed while at depth. It could usually be made worse with subsequent ascents or descents, but the initial development of the headache during a time in which there was no substantial change in depth did cause some confusion in the initial physician’s assessment.

From the aviation literature, it is believed that a small degree of negative pressure is sustainable within the sinuses, without symptoms12. Exceeding this pressure may be sufficient to cause a gradual effusion to develop, and the full or heavy sensation within the sinus may take some time to develop. Extrapolation would suggest that diving-related barotrauma could occur with a reduction in sinus air volume of 5 to 10 per cent, i.e. at a depth of 0.5 to 1 metre below the surface.

In 8 per cent of the patients there was a very clear-cut and dramatic sensation of a bursting or popping during depth changes. Of these, half were on descent and half on ascent. It has been described in aviation medicine as the ‘popping of a champagne cork’, a ‘gunshot’, ‘like a bee sting over the eye’ and ‘like being struck on the head with a club or bat’. It is presumed, from the observations of Campbell1,2 and of Mann and Beck12, as well as from this series, that the sensation results from a haemorrhage stripping up the mucosa of the sinus, produced by the negative intrasinus pressure with descent.

A similar sudden sensation can also occur from the rupture of an air sac or release of pressure from a distended sinus during ascent. This may be followed by a ‘hissing’ sensation of air movement, which may then relieve discomfort and pain. One of the cases involved the ethmoidal area, and the patient had a subsequent small, oval haematoma noted over the ethmoid region within hours (Figure 8.2).

 Ethmoidal haemorrhage. Following ethmoidal sinus barotrauma of descent, the sinus burst during ascent, with sudden excruciating pain, then bruising and haemorrhage into the adjoining skin, between the eyes.
Figure 8.2 Ethmoidal haemorrhage. Following ethmoidal sinus barotrauma of descent, the sinus burst during ascent, with sudden excruciating pain, then bruising and haemorrhage into the adjoining skin, between the eyes.

In 10 per cent, repetitive incidents of sinus barotrauma appeared to be provoked by inappropriate diving and equalization techniques. In these cases there would frequently be a head first descent, and/or swallowing as a method of middle ear equalization. The substitution of the feet first descent (preferably down a shot line), together with frequent positive-pressure middle ear equalization manoeuvres, appeared to rectify the situation. These are now described in medical texts used by divers13.

A similar problem developed if descents were slow, because of discomfort noted in the sinus. The blood or effusion gradually accumulating in the sinus equalizes the pressure and reduces the degree of pain and discomfort. This may be appropriate for an emergency dive, but it is not prudent if disease is to be avoided. On the contrary, divers inappropriately used this development of the disorder (e.g. blood or effusion, mucosal congestion) as a measure to replace a contracting air space in the sinus during descent, to allow the dive to continue.

Divers in these categories were advised of the correct methods of descent and to use positive pressure middle ear equalization (e.g. the Valsalva manoeuvre; see Chapter 7). This may have an effect of aerating the sinuses before major disease and haemorrhage develop.

Previous radiological descriptions included haematomas, mucous cysts, mucocoeles, polyps or polypoid masses, opacification and, most commonly, a thickening of the mucosa. The series reported by the authors was no different in the various radiological descriptions; however, the CT scans showed more identifiable and definitive pathological features (Figure 8.3). MRI using T1- and T2-weighted imaging was more diagnostic in differentiating blood from mucosal thickening14. Sphenoidal involvement was common.

The second Australian series
Figure 8.3 Sinus barotrauma affecting mastoid and sphenoidal sinuses. Computed tomography scan and magnetic resonance imaging have replaced x-ray studies in identifying sinus disorders. Left fluid level in petrous bone and loss of pneumatization of mastoid air cells.

The current use of MRI and CT scans of the sinuses made diagnosis and treatment more definitive in most of these cases. Sinus endoscopy, sinus surgery or nasal surgery was needed in 12 per cent, often with excellent results.

Diving Marine Mammals

The study of diving animals offers the scientist an ideal opportunity to study the physiological consequences and defence mechanisms required to survive extended breath-holding. It is also of great interest to diving physicians to see how diving animals avoid the perils induced by exposure to pressure and hypothermia.

The northern elephant seal and the sperm whale can dive to 1500 metres. The southern elephant seal can stay submerged for 2 hours, although usual dives are 20 to 30 minutes in duration. The Weddell seal regularly dives for food to greater than 100 metres and can remain submerged for up to 60 minutes. Typical humans, with some practice, can breath-hold underwater for 1 to 2 minutes and descend to 10 to 15 metres.

How are marine mammals able to achieve these remarkable underwater depth and/or duration exposures that appear to defy conventional wisdom with respect to limits of hypoxia? How also do they achieve these feats without developing some of the disorders (e.g. hypoxic blackouts, barotrauma, decompression sickness, nitrogen narcosis, O2 toxicity or high-pressure neurological syndrome) that are the subjects of subsequent chapters in this book?

Obvious anatomical adaptations include a streamlined shape, low-friction body surface (skin or fur) and the development of flippers or fins. Dolphins can reach speeds of 20 knots with remarkably low energy consumption. A dorsal blowhole in whales and dolphins also aids energy efficient respiration. Of more interest to the diving physician and physiologist are the mechanisms to cope with prolonged apnoea. The adaptations that allow diving animals to achieve long periods underwater are both physiological and biochemical.

Oxygen stores

All diving mammals have an increased total body O2 store. The relative contribution of the lungs, blood and muscles storage areas depends on the diving pattern of the animal.

Deep diving mammals do not dive at full lung capacity and may exhibit reduced lung perfusion during dives for reasons discussed later, so the bulk of O2 is stored in blood and muscle. Such animals have increased blood volume (~15 per cent of body mass versus ~5 to 7 per cent for humans), and the blood has a higher haemoglobin concentration. About 70 per cent of the total O2 store is found in the blood. They also have a markedly increased myoglobin concentration (5 to 12 times that found in a human), especially in the swim muscles, and this myoglobin increase is proportional to the diving capacity of the animal. Myoglobin carries approximately 25 per cent of the total O2 sore. Only a tiny proportion (~5 per cent) is found in the lungs (versus ~25 per cent in humans).

An intriguing and controversial mechanism for augmenting O2 storage and delivery during a dive is the pre-dive sequestration of oxygenated red cells in the spleen followed by the release of these cells by splenic contracture during a dive. The time course of release into the systemic circulation may be further regulated by a valve-like sphincter in the vena cava. The fact that this occurs is not disputed, but its role in marine mammal diving adaptation is uncertain. It has been noted that re-sequestration after release on one dive typically takes far longer than the typical surface interval between subsequent dives during a dive series. Thus, any benefit may be restricted to the initial dive. It is possible that this adaptation is more important for keeping blood haematocrit (and viscosity) at optimal levels when the animal is not diving than for improving oxygenation during dives.

Oxygen consumption and the diving response

The increases in blood volume, haemoglobin and myoglobin described earlier all contribute to the seal’s impressive O2 supply, but O2 still needs to be conserved. Indeed, it can be readily calculated that if the submerged seal continued to metabolize at the same rate as before diving, its O2 stores would not be sufficient during long dives. Not surprisingly, these animals exhibit multiple strategies aimed at conserving O2 and ensuring that it is supplied preferentially to vital organs during the period of a dive.

The term diving response refers to a sequence of physiological events, including apnoea, bradycardia and redistribution of cardiac output, which are under the control of multiple reflexes. O2 conservation is thus partly accomplished by selective redistribution of circulating blood. Blood may be preferentially distributed to swimming rather than non-swimming muscles. Studies indicate that pinniped skeletal muscles have an enhanced oxidative capacity to maintain aerobic metabolism under the relatively hypoxic conditions associated with diving and that these adaptations are more pronounced in swimming than in non-swimming muscles. Other tissues that are most critical for survival (e.g. retina, brain, spinal cord, adrenal glands and, in pregnant seals, the placenta) are also selectively perfused. The seal essentially shuts off the flow of blood to non-essential tissues and organs, such as the kidneys, until it resurfaces.

Rapid onset of bradycardia (to as low as 10 per cent of baseline rate) at the start of a dive may be seen in diving species. This reduces cardiac work and O2 consumption. A substantial reduction in cardiac output has been shown in Weddell seals. Because stroke volume falls by only about 30 per cent, the predominant effector of this reduction is the bradycardia.

Arterial blood pressure is reasonably well preserved despite this reduction in cardiac output, and this is important to maintain perfusion of vital organs. Maintenance of arterial pressure is facilitated by the stretching of the elastic walls of large arteries during systole and their recoil during diastole. This function is augmented in many species of marine mammals by a bulbous enlargement of the root of the aorta, the aortic bulb. The aortic bulb approximately doubles the diameter of the ascending aorta in harbour and Weddell seals, thus providing an elastic capacitance for maintaining pressure and flow into the constricted arterial tree during the long diastolic intervals characteristic of diving. The entire human aorta contains less volume than the aortic bulb alone in seals of a similar body weight. The increase in left ventricular afterload that would be expected as a consequence of elevated peripheral resistance and decreased large artery compliance is reduced by this unique anatomy. The net result is a diminished peak systolic pressure, which reduces cardiac work and O2 consumption while at the same time maintaining stroke volume.

The electrocardiogram of the diving animal shows some progressive changes during prolonged apnoeic dives. In addition to bradycardia, these changes may include the gradual diminution or even abolition of the P wave. Cardiac rhythm is then apparently set independently of the sino-atrial node by a ventricular pacemaker site. Other cardiac dysrhythmias occasionally appear.

Anaerobic metabolism

With prolonged dives certain tissues switch to anaerobic metabolism, which produces lactic acid as a by-product. There is an increased tolerance to lactic acid in the muscles through increased buffering capacity. High levels of lactic acid, however, lower the pH of the blood and can lead to acidosis, causing a weakening of the heart’s ability to contract. Acidosis is avoided by confining anaerobic metabolism to the skeletal muscles and other tissues isolated from the blood supply. When the animals resurface, these tissues release the lactic acid into the blood for metabolism by the liver.

Diving technique

Modified diving behaviour to limit muscle activity and thus O2 consumption has been demonstrated in Weddell seals. Prolonged downward gliding, with minimal muscular effort, as a result of reducing buoyancy with lung compression at depth can result in up to a 60 per cent reduction in energy costs. Gliding is used during dives exceeding 18 metres in depth and occupies approximately 75 per cent of the descent.

Pressure changes

Structural adaptations to accommodate thoracic compression during deep dives include a flexible rib cage, stiffened alveolar ducts and attachments of the diaphragm such as to permit some shifting of abdominal contents into the thorax. These changes

Humans as Free Divers

You’re running on reserve tank and there’s no warning before you hit empty!

Record-holding free diver

There are two principal (and somewhat inter-related) challenges in free diving:

  1. The challenge of increasing depth, with its attendant risk of pressure-related injury to gas-containing spaces.
  2. The challenge of increasing duration, with its attendant risk of exhaustion of oxygen (O2) stores.

A third challenge that is most relevant to the more extreme exponents of free diving is the related exposure to markedly elevated gas partial pressures with related risks such gas toxicities and decompression sickness.

The challenge of increasing depth

Any anatomical or equipment gas spaces are subject to compression during descent, and their volumes may need to be compensated if barotrauma is to be avoided. Obvious examples, which are discussed elsewhere in this text, include the middle ear (see Chapter 7), sinuses (see Chapter 8) and mask. The lung is of particular relevance to free divers because, unlike divers using underwater breathing apparatus who compensate intrapulmonary pressure and volume with each breath of compressed gas, the lung volume of a free diver is progressively compressed as depth increases.

It was long believed that the limiting factor on depth in free diving would be the point at which lung volume was compressed to residual volume because compression to smaller volumes could, logically, result in trauma to the chest wall or lung itself. Thus, a diver with a total lung capacity of 6 litres and a residual volume of 1.5 litres should theoretically be able to breath-hold dive to 30 metres (4 ATA) where the total lung volume would be compressed to the residual volume (1.5 litres), a simple application of Boyle’s Law. A corollary was that divers with a larger total lung capacity and/or a smaller residual volume would be capable of greater depths before injury occurred.

The fallacy of the ‘residual volume limit’ is immediately clear when it is considered that a human has descended to 214 metres (22.4 atmospheres absolute [ATA]) without suffering obvious lung barotrauma and that free divers regularly descend to depths greater than a theoretical maximum calculated in this way. The factors that were missing from these early attempts to predict maximum depth were the distensibility of the pulmonary vasculature and the concomitant potential for intrapulmonary blood pooling to compensate for compression of lung volume, effectively allowing for compressions below predicted residual volume. The beginnings of such compensation can be seen with simple head-out immersion in an upright subject. The negative transthoracic pressure generated by having the airway open to a pressure of 1 ATA while the thorax is exposed to greater pressure (because of the surrounding water pressure) results in a shift of about 0.7 litre of blood into the thorax. A greater engorgement of the pulmonary circulation is likely if the transthoracic pressure increases further.

Notwithstanding this remarkable and fortunate mechanism for compensation, there will nevertheless come a point where pulmonary vascular capacitance is maximized and further descent will cause the lung’s remaining gas volume to develop an increasingly negative pressure relative to the environment and surrounding tissue. If this becomes excessive, then both fluid extravasation from capillaries to the alveolar space and frank haemorrhage are possible, and there is evidence from competitive free diving that both occur. This problem is referred to as pulmonary barotrauma of descent or ‘lung squeeze’. Although it is interesting and potentially of increasing importance as free diving depths are extended, this is currently a minor contributor to free diving accidents in comparison with the challenges of increasing duration underwater.

The challenge of increasing duration

It is self-evident that oxygenation is maintained from steadily dwindling O2 stores during a free dive. In contrast to marine mammals, a human’s stores are relatively small. The total O2 stores in a 70-kilogram man at resting lung volume (functional residual capacity) have been calculated to be approximately 1.5 litres. This store would be increased at total lung capacity whose value is variable among individuals. If nearly all this O2 can be extracted, one could predict that a resting man who has an O2 consumption of 300 mL per minute would completely deplete his O2 stores in 5 minutes. In reality, most untrained humans can only breath-hold for approximately 1 minute because the drive to breathe is dependent largely on rising pressures of carbon dioxide (CO2) rather than falling levels of O2 (although the two are synergistic). This inherent inability to breath-hold voluntarily to the point of critical hypoxia (an arterial partial pressure of O2 [Po2] above approximately 25 mm Hg must be maintained to avoid loss of consciousness) is clearly protective in free diving. However, it can be confounded in two important ways: by the use of hyperventilation before breath-holding and through the effects of changing ambient pressure during descent and ascent from a free dive.

Hyperventilation refers to taking a series of rapid deep breaths before breath-holding. This is often done in the mistaken belief that it significantly enhances O2 stores. Although hyperventilation does increase the alveolar O2 content to a small extent, the volume of O2 involved is effectively inconsequential. What hyperventilation can achieve is a marked lowering of arterial CO2 levels. Competitive breath-hold divers have had end-tidal CO2 pressures as low as 20 mm Hg measured at the end of their typical pre-apnoea routine. This has the effect of prolonging the breath-hold duration before the onset of a strong urge to breathe.

The obvious danger associated with hyperventilation is that it will extend the breath-hold duration closer to the point where the arterial Po2 falls below that required to maintain consciousness. There is little doubt that hyperventilation has been a contributory factor in many free diving deaths. There is also some evidence that well-practised free divers can induce a decrease in sensitivity of the medullary respiratory control centre to CO2, or they can learn to resist the uncomfortable urges to breathe that CO2 generates as its arterial pressure rises, or both. Interestingly, however, although competitors in static apnoea events (effectively breath-holding competitions without pressure change) aggressively employ hyperventilation and are highly motivated not to breathe for as long as possible, symptomatic hypoxia is not frequent as would be expected. This brings the discussion to changing ambient pressure during a free dive as an added and significant risk factor for critical hypoxia.

Arterial gas tensions during breath-hold dives change with the partial pressure of the gases in the lungs. When the breath-hold diver descends, the partial pressures of the gases in the lungs increase as their volume is decreased and gas inside is compressed. The reverse takes place during ascent back toward the surface. This leads to concomitant rises and falls in alveolar and arterial Po2.

Figure 3.1 shows alveolar pressures of the metabolic gases during (a) a breath-hold period without ambient pressure change, (b) a breath-hold dive to 10 metres and (c) a breath-hold dive to 10 metres with prior hyperventilation. In Figure 3.1 (b) and 3.1 (c), ambient and thus alveolar gas partial pressures rise during descent according to Boyle’s Law. The rise in O2 is somewhat reduced because of continued consumption. Because of the high alveolar Po2 at depth, there is a sufficient alveolar-arterial gradient to allow continuing O2 uptake for a considerable time.

In contrast, during ascent there is a rapid fall in alveolar Po2 as the lung re-expands and the volume of the alveolar gas increases. This is greater than expected from gas laws alone, thus reflecting ongoing oxygen metabolism. The dive with prior hyperventilation depicted in Figure 3.1 (c) had a longer bottom time as would be expected when prior lowering of the arterial CO2 makes the diver more comfortable remaining at depth for longer. It can be seen that a lower alveolar partial pressure of O2 develops by the time the diver reaches the surface, and such falls in alveolar and arterial Po2 during ascent would be even more dramatic on deeper dives. The obvious risk is that the diver could experience critical arterial hypoxaemia as the alveolar Po2 is rapidly falling in the latter stages of the ascent. Indeed, loss of consciousness during either the final phase of ascent or on arrival at the surface is a recurring event at free diving competitions. The dangers of breath-hold diving and hyperventilation are discussed further in Chapter 16.

In addition to hyperventilation, there are two other strategies, both controversial, that elite free divers use or manipulate in order to extend their duration underwater.

The first of these is an attempt to expedite the so-called diving reflex that can be observed in all air breathing vertebrates but that is highly developed in marine mammals (see later). This reflex is initiated by apnoea and also by facial cooling. Its principal effector arm is a marked sympathetically mediated increase in peripheral vascular resistance that increases blood pressure and in turn elicits a vagally mediated bradycardia. At the same time, there is some evidence that the sympathetic activation induces splenic contraction, increasing circulating red blood cells. Peripheral vasoconstriction has the effect of reducing the circulation of blood to the peripheries, and the bradycardia reduces O2 consumption by the heart. Central redistribution of blood makes more O2 available to vital organs. A concurrent and unwanted side effect of these processes is a predisposition to arrhythmias. This probably arises from vagal inhibition of nodal conduction combined with sympathetic sensitization of ectopic pacemakers. Not surprisingly, ventricular ectopic beats are common.

Although these are autonomically mediated phenomena, there is a strong belief among free divers that they can manipulate the process through conditioning, relaxation techniques and practice. Given that there is considerable inter-subject variability in the potency of the diving reflex, and that it tends to wane with age, it does seem plausible that it is ‘open’ to manipulation by skilled divers. In a 2014 interview William Trubridge, holder of the constant ballast no fins world depth record of 101 metres, articulated it thus:

The training I do is targeted at creating a physiology that conserves oxygen as much as possible. Whereas someone who is extremely fit would be able to supply a high amount of oxygen to their muscles very quickly, I need to shut down that oxygen flow to the muscles so that they can work anaerobically and that conserves the oxygen for the heart and the brain. Physiology for freediving is such a different set of effects to what is found in any other sport that we’re still discovering exactly what they consist of.

New Zealand Listener Magazine,
4 January 2014

Similarly, on his website Francesco “Pippin” Ferreras, a previous world record holder, described his approach in more detail:

My heart, under direct control of the Central Nervous System, begins a rapid slowdown. This diminution of my cardiac output is a result of the body’s decreasing needs for oxygen and energy consumption. This efficiency in energy conservation is of vital importance for survival in the undersea environment while in a state of apnea. As an example, when I begin my pre-immersion preparations my resting heart rate is 75 bpm, 10 minutes after entering a stare of deep relaxation it drops, to 55 bpm. As I begin my descent, in a matter of seconds it has slowed to 30 bpm. My cardiovascular performance is influenced by other factors, foremost being my physical conditioning, and mental preparation…. Once I have reached a depth of 110 m., I institute one last command to my heart to slow down. At this point my heart is down to a mere 10 to 14 bpm. On several immersions when all of the above mentioned factors are ideal I have obtained readings of an incredible 7 bpm! Obviously these findings are augmented by the power of mind over body that I have developed over the years, through the study and practice of Yoga.

The second controversial strategy used by elite free divers to extend both depth and duration underwater is so-called ‘lung packing’, more correctly referred to as glossopharyngeal insufflation. This technique involves using the glossopharyngeal muscles to pump air into the lungs, thus enabling an increase in the total lung capacity by up to 20 per cent. This extra volume potentially increases the depth at which lung compression becomes hazardous (as described earlier) and also represents an increase in the O2 stores. Adept exponents of lung packing can increase the volume of air carried by several litres, although this does not translate directly into an increase in lung volume because the gas is held in the lungs under positive pressure and is therefore compressed. Therein lies the potential problem with this strategy. There are sporadic reports of excessive packing leading to pulmonary barotrauma because of the high positive transpulmonary pressures that can reach 60 mm Hg or even more. There are also reports of hypotensive loss of consciousness resulting from profound reduction in venous return associated with high intrathoracic pressure during the act of packing. In view of these potential hazards the technique cannot be recommended. Nevertheless, it is unlikely that packing will be abandoned by extreme free divers looking for any possible edge.

Largely for completeness (and for curious interest value), there are some extreme free divers who have developed the technique of glossopharyngeal exsufflation, that is, packing in the opposite direction. This is used in those situations near terminal depth when the lungs are compressed at or below residual volume, and it is therefore impossible to generate a Valsalva manoeuvre to clear the ears or sinuses. An alternative approach to avoiding barotrauma under these conditions, and one that has been proven radiologically, is to let the sinuses (and to some extent the middle ears) flood with water!

The challenge of avoiding gas toxicities and decompression sickness

The combination of increasing depth and duration (particularly the former) during free diving opens up the possibility that extreme exponents will suffer gas toxicities and decompression sickness, complications usually associated with compressed gas diving. Neurological decompression sickness in breath-hold divers has been reported. Although some cases may be caused by arterial gas embolism following pulmonary barotrauma, predictions of inert gas tensions following repeated and closely spaced deep breath-hold dives do suggest that pathological bubble formation from dissolved inert gas is certainly possible.

Despite the extreme depths reached by free divers, overt effects of nitrogen narcosis are only rarely reported, although there may be a strong reporting bias operant here. It may also be that narcosis is not as likely as predicted on the basis of depth alone simply because the partial pressure of nitrogen in the relevant tissues takes time to equilibrate with the partial pressure of nitrogen in the lungs, and the short duration of the dives therefore limits any effect. Nevertheless, as extreme free divers are pushing deeper, there are increasing numbers of stories of strange sensations and ‘funny turns’ during these dives. It is impossible to know their exact cause, but potential explanations include nitrogen narcosis (see Chapter 15), high-pressure neurological syndrome (see Chapter 20) and cerebral O2 toxicity (see Chapter 17). Cerebral O2 toxicity seems an unlikely explanation given the very short exposures, the starting fraction of inspired O2 of 0.21 and the fact that O2 is being consumed from the moment apnoea begins. However, some reported events (e.g. facial or diaphragmatic twitching) are very typical of O2 toxicity. These sorts of problems are likely to become more common as record depths are pushed further.

Record diving

Trained free divers have been able to achieve remarkable underwater feats, and in certain societies these divers are accorded celebrity status.

Records are attempted for various categories of diving involving depth, duration and underwater distance. Because of the potential risks involved, dedicated competitions sanctioned by an umbrella society are run according to strict protocols. Physiologists and physicians need to be aware of these remarkable achievements. The records cited here are valid for January 2015 but may have been superseded at the time of reading. A complete list of current records is available at: http://www.aidainternational.org/competitive/worlds-records.

The purest form of depth record is referred to as constant weight apnoea without fins and involves return to the surface with the same weights carried down (if any) and, as the name implies, no use of fins. The record is currently 101 metres for male divers and 69 metres for female divers.

At the opposite end of the spectrum is so-called no limits free diving. This is the most extreme category in respect of depth and requires no swimming at all. Divers hold onto a weighted, rope-guided sled for descent. On reaching the target depth, they detach themselves from the sled and pull a pin that releases compressed air from a cylinder into a lift bag, which tows them back to the surface. The current record depths are 214 metres for male divers and 160 metres for female divers. The latter is the longest-standing free diving record at the present time, set by Tanya Streeter in 2002.

The absolute limit of these hazardous ‘experiments’ remains unknown, but it seems likely that depth record increments will become smaller and smaller as immutable physiological barriers are approached. Death may be precipitated at depth by pulmonary haemorrhage, pulmonary oedema or cardiac dysrhythmias. Cerebral hypoxia is an invariable development during the latter stages of ascent. Quite often these divers require rescue by standby divers because they become unconscious as a result of rapidly developing hypoxia as they approach the surface.

Records are also held for static apnoea, which is a motionless, energy-conserving head immersion exposure. The current records are a mind-boggling 11 minutes 35 seconds for male participants and 9 minutes 2 seconds for female participants.
Underwater breath-hold horizontal distances (dynamic apnoea with fins) of 281 metres (male swimmers) and 234 metres (female swimmers) have been achieved in 50-metre swimming pools with swimmers using fins for propulsion.

About Free Diving

Free diving refers to dives made from surface to surface during voluntary apnoea on a single breath. No underwater breathing apparatus is used. Free diving (also often referred to as ‘breath-hold diving’ or ‘snorkel diving’) is regarded as the purest and most natural form of diving. Unencumbered by bulky equipment, the diver is free to move weightlessly and silently in the underwater world. Practised in some societies for thousands of years, free diving in its simplest form requires no equipment at all. The introduction of various performance-enhancing apparatus such as face masks, fins, weight belts, buoyancy vests and thermal protection suits may present new problems. For example, the addition of goggles or face masks allows for clear vision but introduces a gas space that must be ‘equalized’ to prevent barotrauma. Near the surface, wetsuits generate positive buoyancy that decreases as they are compressed during descent. If a weight belt is used to offset the initial positive buoyancy of the wetsuit, this will render the diver negatively buoyant as he or she begins the ascent. Nevertheless, recreational free divers and spearfishers often wear a mask, snorkel, fins, wetsuit and weights and carry a spear gun, knife and bag. Competitive free divers may also employ specialized devices such as weighted sleds for descent and inflatable lift bags for ascent to achieve remarkable depths. Even with such modern specialized equipment, human diving capabilities are paltry in comparison with those of marine mammals and other sea animals.

Depth penetrations of human divers and marine animals
Depth penetrations of human divers and marine animals

Altitude and Saturation Diving

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.

Diving:Energy Expenditure

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.

Pressure, Gases and Diving

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)

Note: Actual conversions from sea water depth to ATA depend on salinity and temperature. A complete conversion matrix is provided in Table 2.2.

Pressure units

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.

Figure 2.1 Lung volumes and intrapulmonary pressure. The various components of lung volumes are labelled on the left. On the right, the relationships among lung volume, airway pressure and the maximum effort that can be made for inhalation and exhalation of air are plotted. Curve 1 is the volume change during quiet breathing, and curve 2 is the volume change during a maximum inhalation starting at the residual volume. ERV, expiratory reserve volume; insp., inspiratory; IRV, inspiratory reserve volume; RV, residual volume; TV, tidal volume; VC, vital capacity. (Redrawn from Lamphier EH, Camporesi EM. Respiration and exertion. In: Bennett PB, Elliott DH. The phyisology & medicine of diving, 4th edn. London:WB Saunders Co Ltd; 1993, with permission).

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.

Figure 2.2 Effect of Boyle’s Law: While breathing 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. Because of the increase in density of this breathing gas under increased pressure, the diver must move a greater mass of gas with each breath. In some situations, this physical effect can limit the diver’s capacity to do work.

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.

Partial pressures in gas mixtures
Dalton’s Law: (a) two spaces each at 1 ATA; (b) total pressure 1 ATA, 0.5 ATA each component of the mixture; (c) total pressure 2 ATA, 1 ATA of each component of the mixture.

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.

Figure 2.4 Henry’s Law.

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.

Early Diving Equipment

The history of diving with equipment is long and complex, and in the early stages it is mixed with legend. The exploits of Jonah are described with conviction in one text, but there is a shortage of supporting evidence. Further reference is made to him later, on the technicality that he was more a submariner than a diver. Because his descent was involuntary, Jonah was at best a reluctant pioneer diver. The history of submarine escape, when the submariner may become a diver, is discussed in Chapter 64.

Some claim that Alexander the Great descended in a diving bell during the third century BC. Details of the event are vague, and some of the fish stories attributed to him were spectacular. One fish was said to have taken 3 days to swim past him! It is most unlikely that the artisans of the time could make glass as depicted in most of the illustrations of the ‘event’. This may have been a product of artistic licence or evidence that the incident is based more in fable than in fact.

Snorkels, breathing tubes made from reeds and bamboo (now plastic, rubber or silicone), were developed in many parts of the world. They allow a diver to breathe with the head underwater. Aristotle inferred that the Greeks used them. Columbus reported that the North American Indians would swim toward wild fowl while breathing through a reed and keeping their bodies submerged. They were able to capture the birds with nets, spears or even their bare hands. The Australian aborigines used a similar approach to hunt wild duck. Various people have ‘invented’ long hose snorkels. The one designed by Vegetius, dated 1511, blocked the diver’s vision and imposed impossible loads on the breathing muscles.

Some have interpreted an Assyrian drawing dated 900 BC as an early diving set. The drawing shows a man with a tube in his mouth. The tube is connected to some sort of bladder or bag. It is more likely a float or life jacket. The tube length was a metre or more and so impossible to breathe through.

Leonardo da Vinci sketched diving sets and fins. One set was really a snorkel that had the disadvantage of a large dead space. Another of his ideas was for the diver to have a ‘wine skin to contain the breath’. This was probably the first recorded design of a self-contained breathing apparatus. His drawings appear tentative, so it is probably safe to assume that there was no practical diving equipment in Europe at that time.

Another Italian, Borelli, in 1680, realized that Leonardo was in error and that the diver’s air would have to be purified before he breathed it again. Borelli suggested that the air could be purified and breathed again by passing it through a copper tube cooled by sea water. With this concept, he had the basic idea of a rebreathing set. It could also be claimed that he had the basis of the experimental cryogenic diving set in which gas is carried in liquid form and purified by freezing out carbon dioxide.

Diving bells were the first successful method of increasing endurance underwater, apart from snorkels. These consist of a weighted chamber, open at the bottom, in which one or more people could be lowered under water. The early use of bells was limited to short periods in shallow water. Later, a method of supplying fresh air was developed. The first fully documented use of diving bells dates from the sixteenth century.

In 1691, Edmond Halley, the English astronomer who predicted the orbit of the comet that bears his name, patented a diving bell that was supplied with air in barrels (Figure 1.1). With this development diving bells became more widespread. They were used for salvage, treasure recovery and general construction work. Halley’s bell was supplied with air from weighted barrels, which were hauled from the surface. Dives to 20 metres for up to 1 1/2 hours were recorded. Halley also devised a method of supplying air to a diver from a hose connected to the bell. The length of hose restricted the diver to the area close to the bell. It is not known whether this was successful. Halley was one of the earliest recorded sufferers of middle ear barotrauma.

Swedish divers had devised a small bell, occupied by one person and with no air supply to it. Between 1659 and 1665, 50 bronze cannons, each weighing more than 1000 kg, were salvaged from the Vasa. This Swedish warship had sunk in 30 metres of water in Stockholm harbour.

Figure:Edmond Halley’s diving bell, 1691. The weighted barrels of air that were used to replenish the air can be clearly seen.

The guns were recovered by divers working from a bell, assisted by ropes from the surface. This task would not be easy for divers, even with the best of modern equipment.

Breath-Hold Diving

The origins of breath-hold diving are lost in time. Archaeologists claim that the Neanderthal human, an extinct primitive human, dived for food, likely in the first instance gathering shellfish by wading at low tide before diving from canoes. By 4500 BC, underwater exploration had advanced from the first timid dive to an industry that supplied the community with shells, food and pearls.

From the ancient Greek civilization until today, fishers have dived for sponges, which, in earlier days, were used by soldiers as water canteens and wound dressings, as well as for washing.

Breath-hold diving for sponges continued until the nineteenth century when helmet diving equipment was introduced, allowing the intrepid to gamble their lives in order to reach the deeper sponge beds. Greek divers still search the waters of the Mediterranean Sea as far afield as northern Africa for sponges.

The ancient Greeks laid down the first rules on the legal rights of divers in relation to salvaged goods. The diver’s share of the cargo was increased with depth. Many divers would prefer this arrangement to that offered by modern governments and diving companies.

In other parts of the world, industries involving breath-hold diving persist, to some extent, to this time. Notable examples include the Ama, or diving women of Japan and Korea, and the pearl divers of the Tuamoto Archipelago.

The Ama has existed as a group for more than 2000 years. Originally the male divers were fishermen, and the women collected shells and plants. The shells and seaweed are a prized part of Korean and Japanese cuisine. In more recent times, diving has been restricted to the women, with the men serving as tenders. Some attribute the change in pattern to better endurance of the women in cold water. Others pay homage to the folklore that diving reduces the virility of men, a point many divers seem keen to disprove.

There is a long history of the use of divers for strategic purposes. Divers were involved in operations during the Trojan Wars from 1194 to 1184 BC. They sabotaged enemy ships by boring holes in the hull or cutting the anchor ropes. Divers were also used to construct underwater defences designed to protect ports from the attacking fleets. The attackers in their turn used divers to remove the obstructions.

By Roman times, precautions were being taken against divers. Anchor cables were made of iron chain to make them difficult to cut, and special guards with diving experience were used to protect the fleet against underwater attackers.

An interesting early report indicated that some Roman divers were also involved in Mark Anthony’s attempt to capture the heart of Cleopatra. Mark Antony participated in a fishing contest held in Cleopatra’s presence and attempted to improve his standing by having his divers ensure a constant supply of fish on his line. The Queen showed her displeasure by having one of her divers fasten a salted fish to his hook.
Marco Polo and other travellers to India and Sri Lanka observed pearl diving on the Coromandel Coast. They reported that the most diving was to depths of 10 to 15 metres, but that the divers could reach 27 metres by using a weight on a rope to assist descent. They carried a net to put the oysters in and, when they wished to surface, were assisted by an attendant who hauled on a rope attached to the net. The divers were noted to hold their nose during descent.

The most skilled of the American native divers came from Margarita Island. Travellers who observed them during the sixteenth, seventeenth and eighteenth centuries reported that these divers could descend to 30 metres and remain submerged for 15 minutes. They could dive from sunrise to sunset, 7 days a week and attributed their endurance to tobacco! They also claimed to possess a secret chemical that they rubbed over their bodies to repel sharks. The Spaniards exploited these native divers for pearling, salvage and smuggling goods past customs. The demand for divers was indicated by their value on the slave market, fetching prices up to 150 gold pieces.

Free diving appears to have evolved as a modern sport in the mid-1940s, initially as a competition among Italian spearfishers. Currently the sport, which is steadily gaining popularity, encompasses a variety of disciplines. These include the following:

In ‘no limits’, a diver can use any means to travel down and up the line, as long as the line is used to measure the distance. Most divers descend down a line using a weighted sled and return to the surface aided by an inflatable balloon. Officially recorded depths in excess of 210 metres have been achieved using this method.

‘Constant weight apnoea’ diving is where descent and ascent occur along a line, although the diver is not permitted to pull on this line to assist movement. No weights can be removed during the dive. Mono-fins or bi-fins can be used.

‘Constant weight without fins’ is the same as constant weight apnoea but without the use of fins.

With ‘variable weights’, the diver again descends with the aid of a weighted sled, but this weight is limited. Ascent is achieved by finning or pulling up the cable, or both.
‘Free immersion’, which emerged in places where equipment was difficult to obtain, involves a finless diver (with optional suit, mask or weights) who pulls himself or herself down and then up a weighted line.

‘Static apnoea’ involves resting breath-holding (usually lying in a pool) with the face submerged. Officially recorded times in excess of 11 minutes have been achieved using this method.

‘Dynamic apnoea’ measures the distance covered in a pool during a single breath-hold.