Humans function well only within a narrow range of barometric pressures. Outside this range, they are subject to major physiologic stresses that occasionally result in disease. On land, workers are exposed to hyperbaric environments (i.e., increased barometric pressure) during tunneling projects that require the use of compressed air or when caissons are used to work in ground saturated with water. In addition, hyperbaric chamber support staff members are routinely exposed when they treat patients in hyperbaric medical treatment facilities. In the water, occupational exposures are diverse. Examples of exposures include breath-hold divers, such as the ama pearl divers of Japan, and compressed gas divers, ranging from instructors of recreational SCUBA students, who breathe compressed air, to saturation divers supporting offshore oil exploration, who dive in excess of up to 1000 ft of seawater (fsw), depending on the scope of the project, while breathing artificial gas mixtures. Divers can also be found inland. These divers are involved in such jobs as inspecting dams and reservoirs, cleaning filters, maintaining fish farms, placing underwater demolitions, and conducting police searches.

The first practical method developed for conducting useful work underwater was the diving bell, which is essentially an upside-down cone. Invented by Smeaton in 1778, the diving bell was the forerunner of the modern caisson (caisse, in French, means “box”).¹ This method is still used today in the construction of tunnels and bridge footings. In 1819 in England, Augustus Siebe invented the first practical diving dress; it consisted of a copper helmet bolted to a leather coverall. This apparatus gave humans the ability to walk and function relatively unencumbered underwater without holding their breath. In 1837, Siebe introduced an improved version of this gear, which served as the basic diving dress for deep-sea diving until the early 1980s and is still used by some divers today.² Modern surface-supplied equipment has incorporated various advances in technology, such as helmets made of lightweight composites with increased fields of vision and improved gas delivery systems that reduce breathing resistance.

Along with these advances, which have enabled divers and caisson workers to work at greater pressures or depths for longer periods of time, have come associated medical problems. The first descriptions of decompression-related disorders were made by Triger in 1841 among pressurized tunnel workers³ and by Alfonse Galin in 1872 with Greek sponge divers.⁴ By the early 1900s, it was recognized that decompression-related symptoms were due to inert gas and could be relieved by returning the individual to pressure. However, there was still no method for controlling the exposure to prevent the disease. The Royal Navy enlisted the help of the eminent physiologist J. S. Haldane to develop such a method. Haldane published his first set of decompression tables in 1908, based on experiments using sheep.³ The tables incorporated delays on ascent (called stops) to allow time for excess inert gas dissolved in body tissues to be eliminated (off-gassing). Haldanian principles form the basis for most decompression tables used today.

OCCUPATIONAL SETTING

Diving

Commercial divers are often involved in construction, inspection, and repair of pipelines, vessels, aquariums, tunnels, and marine platforms and typically spend a significant part of their time in diving operations. Scientific divers are devoted to the study of the marine environment and may dive only a few days a month as part of their job. Public safety divers become involved primarily with search and rescue operations and may only dive a few days a year. Engineers that design and oversee construction and maintenance of bridges often dive to inspect the underwater structures.

Diving operations can be classified into three basic categories: air, mixed gas, and saturation. Air diving is restricted to relatively shallow depths, due to the increasing narcotic effect of the nitrogen component of air as depth increases (the Occupational Safety and Health Administration (OSHA) restricts exposures on air to <190 fsw but will permit surface-supplied air for dives with bottom times of 30 minutes or less for depths of 220 ft). Mixed-gas diving uses a breathing mixture other than air; it is employed principally when deeper working depths are required. It generally specifies the use of helium as the inert gas constituent; however, nitrogen–oxygen mixtures (nitrox) are becoming more common for relatively shallow applications, and hydrogen has been tested for deep commercial applications. Although greater depths are permitted in standard mixed-gas diving, they are also limited by the amount of decompression required. For a constant bottom time, the required decompression time increases significantly with depth, prolonging exposure to the environment and limiting the useful work period. To overcome this problem, saturation diving methods have been developed.

Saturation diving, a specialized extension of standard decompression diving, is based on the principle that at a constant depth tissues will on-gas until the tissue partial pressure reaches equilibrium with the ambient pressure (see Henry’s law, p. 116). Once tissue equilibrium is reached, no net uptake will occur (unless the pressure is increased), resulting in no further increase in decompression time. At this point, the bottom time may be increased without additional decompression penalty. The divers are housed at “depth” for up to a month in a dry, pressurized chamber on the surface and transported under pressure to and from the work site via a diver transport capsule (Figure 8.1). Saturation diving techniques represent a cost-effective alternative to standard diving when deep or prolonged bottom times are required, since the need for repetitive, long in-water decompressions is avoided. The other advantage of saturation diving is that only one decompression is required, thereby reducing the risk of decompression-related incidents.

Equipment

There are two basic classes of equipment employed by divers today: self-contained underwater breathing apparatus (SCUBA) and surface-supplied equipment. Choice of equipment depends primarily on the job requirements and the employer’s capabilities. Equipment generally used in high-pressure and diving environments is summarized in Table 8.1.

Class	Equipment type	Type of air supplied
SCUBA	Open circuit	Air
	Closed circuit
	Semiclosed circuit
Surface supplied	Lightweight	Air
	Deep sea	Mixed gas
	Saturation	Mixed gas
Caisson	Shirt sleeve	Pressurized air

Modern SCUBA has evolved from equipment originally developed during World War II by Cousteau and Gagnon.² The advent of SCUBA ushered in a new era of underwater mobility. SCUBA equipment is designed either to be open circuit, closed circuit, or semiclosed (a hybrid between open circuit and closed circuit). Modern open-circuit SCUBA (Figure 8.2) is the principal equipment used in recreational diving; it is also used for many commercial applications. Closed-circuit SCUBA (rebreathers) (Figure 8.3) removes the exhaled carbon dioxide from the breathing gas prior to returning the “scrubbed” gas to the diver. Oxygen is added as needed. Some closed-circuit rigs maintain a constant partial pressure of oxygen in the breathing gas, increasing depth capabilities. Closed-circuit equipment is generally more complex and expensive, but it has the advantage of less gas consumption without the generation of bubbles. The absence of bubbles is useful in marine research, marine photography, and various military applications.⁵

Many advances have been made in surface-supplied diving since the 1960s, including specialized materials for helmets and suits, hot water heating for suits, and advanced communication systems. Surface-supplied diving equipment can generally be divided into two types: lightweight and deep sea. Deep-sea equipment can be further subdivided into air, mixed gas, and saturation capable. Lightweight equipment, such as a “band mask,” is customarily used for work where surface communications are required, but significant diver protection is not necessary. Deep-sea diving equipment (Figure 8.4) affords increased protection, generally consisting of a “hard hat” offering maximum head/neck protection along with surface-to-diver communication video capability, protective gloves and boots, weights, an emergency gas supply, and varying levels of thermal protection, as complex as suits that circulate hot water supplied from the surface.

As in most occupational settings, the choice of equipment used in diving generally depends on the characteristics of the environment and the work to be completed, along with the personal preferences and the capabilities of the individual diver. The principal advantages of SCUBA diving are its ease of use, transportability, and freedom of movement. Its disadvantages include limited gas supply, depth restrictions, minimal head protection, lack of communication, and inability to function safely in strong currents. Surface-supplied diving overcomes many of the disadvantages of SCUBA by providing increased head protection, thus reducing the chance of drowning if the diver becomes unconscious; “unlimited” gas supply; better buoyancy control, thus enabling the use of heavy construction techniques; hardwired communications; and greater depth capabilities (Figures 8.5, 8.6, 8.7, and 8.8). However, surface-supplied diving is more complex and costly, because it requires significantly more surface support in both equipment and personnel. In addition, it decreases the diver’s comfort on the surface due to weight, decreases mobility in the water, carries a risk of entanglement, and can present a significant noise hazard. Essentially all of the commercial diving in the offshore environment is done with surface-supplied air or gas or in a saturation environment.

MEASUREMENT ISSUES AND PHYSICS OF PRESSURE

Pressure is a measurement of force per unit area. Air pressure is the force exerted by the column of atmosphere above a particular point. The height of the column determines the pressure. Water pressure, measured by a gauge, is defined as the force exerted by the column of water above the submerged object. Pressure increases linearly with depth. Absolute pressure measures the force per unit area of the combined air and water column. The absolute pressure at 33 fsw is 2 ata (atmospheres absolute) since 1 atm of pressure is contributed by both the air column (weight of atmosphere) and the water column (weight of water). Table 8.2 defines some useful measurements of pressure in diving.

An understanding of the physiologic effects of pressure requires knowledge of basic physics. The relationship between pressure, volume, and temperature is defined by the ideal gas law, which states

where P is absolute pressure, V is volume, n is the number of moles of the gas, R is the universal gas constant, and T is the absolute temperature.

Boyle’s law states that at constant temperature, the volume is inversely proportional to the absolute pressure—that is, no gas enters or exits the system (P₁V₁ = P₂V₂).⁸ For example, if a balloon is filled with 2.0 L of compressed gas at a depth of 99 ft (4.0 ata) and brought to 33 ft (2.0 ata), the gas volume will have expanded to 4.0 L (V₂ = P₁V₁/P₂ = 4 × 2/2 = 4). If taken to the surface (1.0 atm), the volume will have expanded to 8.0 L (V_S = P₁V₁/P_S = 4 × 2/1 = 8). It is important to note that the proportional change in volume per depth change increases toward the surface.

The law of partial pressures (Dalton’s law) states that, in a mixture of gases, the total gas pressure is the same as the sum of the partial pressures of the individual gases in the mixture (P = p₁ + p₂⋯p_n). As a result, the partial pressure of the gas can be calculated by knowing the total pressure of the mixture and the percentage makeup of a particular component. As the total pressure increases, the volume of the individual gases becomes more significant, and eventually Dalton’s law no longer applies.⁸

Henry’s law states that the amount of gas that will dissolve into a solution is directly proportional to the partial pressure of that gas and inversely proportional to the absolute temperature.⁸ Therefore, as the pressure increases (depth increases), the amount of gas dissolved in tissues will increase. Once at a constant depth, gas will continue to increase in the tissues until the equilibrium or saturation is attained (i.e., when the partial pressure of the gas in tissue equals the ambient pressure). Gases such as oxygen may be metabolized, whereas inert gases will not be. Subsequently, when the ambient pressure is decreased, inert gases will come out of solution. This law explains why nitrogen bubbles may form when you surface from an air dive and why a can of carbonated beverage fizzes when it is opened.

Archimedes’ principle, which describes buoyancy, states that an object immersed in a fluid is buoyed up by a force equal to the weight of the volume of fluid that the object displaces. The effect of this principle is seen throughout diving. For example, inhaling a large breath will increase the diver’s buoyancy (due to greater displacement of fluid).

EXPOSURE GUIDELINES

Commercial diving in the United States is covered by various regulations, including OSHA Standard Title 29 Code of Federal Regulations 1910 Subpart T, Commercial Diving Operations, and Title 46 Code of Federal Regulations, Subchapter V, Marine Occupational Safety and Health Standards, Subpart B, Commercial Diving Operations. Diving contractors and operators generally have developed local standard operating procedures that define procedures and safety guidelines in more detail.

Caisson work, and other high-pressure construction work, is covered by OSHA Standard Title 29 CFR 1926 Subpart S—Underground Construction, Caisson, Cofferdams, and Compressed Air.

SPECIAL UNDERWATER STRESSORS

The worker in the undersea environment is exposed to a number of unique environmental stressors that may be enhanced by pressure. These environmental factors include sensory input modifications (sound, visual, proprioceptive), thermal challenges, and gas effects. Professionals generally do not have the luxury of waiting for ideal environmental conditions. As a result, most operations are conducted when at least one element is not ideal. Medical planning for diving operations should take this into consideration.

OTHER HAZARDS IN THE DIVING ENVIRONMENT

The elements of the professional diving environment are varied and diverse, but its hazards are similar to the occupational hazards commonly encountered on land. These hazards are summarized in Table 8.4, and most of them are covered elsewhere in the book.

In addition, it should be remembered that many divers do not dive full time. They often have primary jobs that expose them to more traditional hazards, such as welding, degreasing, paint removal, or other chemical, physical, or biological hazards.

PATHOPHYSIOLOGY OF DIRECT PRESSURE INJURY

The health effects of direct pressure injury can be divided into two major groups: barotrauma and decompression illness. Barotrauma results from the expansion or contraction of gases in anatomic spaces, causing trauma. Decompression illness results when bubbles of inert gas form in body tissues. Body fluids become supersaturated with inert gases at high pressures, and this gas comes out of solution when the ambient pressure is decreased. A summary of direct pressure effects can be found in Table 8.5.

LONG-TERM HEALTH EFFECTS

Dysbaric osteonecrosis, or aseptic bone necrosis, is a well-recognized, relatively uncommon long-term occupational hazard associated with compressed air work. The bone necrosis lesions occur principally in the femur, tibia, and humerus.⁶⁶ As a rule, shaft lesions and lesions that occur away from articular joints do not produce clinical symptoms. Juxta-articular lesions, on the other hand, can progress and produce debilitating disease. Juxta-articular lesions are more commonly found in compressed air workers than in divers. Necrosis is associated with increasing depth and duration of exposure, increasing age (although age may only be a surrogate measure of exposure), and a history of DCI, though not related to the site of DCI.⁶⁶ The lesions of dysbaric osteonecrosis are indistinguishable from other causes of aseptic necrosis. However, as McCallum and Harrison note, “in men with a history of work in compressed air or diving, the probability of bone necrosis being due to compressed air is very high”.⁶⁶ The pathogenesis of the dysbaric form is not well understood. An excellent review of dysbaric osteonecrosis was completed by Jones and Neuman.⁶⁶

Divers are at risk for sensorineural hearing loss secondary due to barotrauma, DCI, and potentially long-term noise exposure. Conductive hearing loss is also a possibility with middle-ear barotrauma and exposure to underwater explosions. Although the hearing threshold is increased underwater, a number of studies have shown that measured sound intensity levels of some diving helmets when pressurized are as high as 90–120 dB(A), depending on the application.^67,68 In addition, hyperbaric chambers during compression and some chambers during ventilation have high noise levels. Some investigators have documented standard threshold shifts with “normal” dive profiles.^68–71 The principal cause of the high noise level is the volume of gas flow required to sufficiently ventilate helmets to prevent CO₂ buildup. The frequency range of noise is 800–3000 Hz, which is within normal communication ranges. The redesign has significantly improved the noise levels of modern helmets.⁶⁹ Some epidemiologic studies have shown little difference in hearing acuity between professional divers and matched controls when corrected for age,⁷⁰ whereas others^71–74 have noted significant differences in divers. One recent age-adjusted prospective study in SCUBA divers demonstrated increased hearing loss in low frequencies only, suggesting a compression/decompression effect.⁷⁴ Although some of the high-frequency hearing loss seen in divers is probably due to other exposures, the studies imply that some of the losses may be due to workplace noise exposure.⁷⁵ Therefore, engineering controls and hearing protection should be instituted wherever possible.

Sequelae from episodes of acute DCI, particularly neurologic ones, are well recognized. However, there may be less obvious but potentially serious long-term health effects from diving, based primarily on descriptive and anecdotal evidence. This possibility has generated a number of hypotheses, which currently lack good epidemiologic support. Subsequent studies have frequently not supported the hypothesized findings. These effects include neuropsychiatric deficits, such as short-term memory deficits and emotional lability,^76–79 electroencephalographic abnormalities^80,81(principally slow waves and spikes), and retinoangiography aberrations, including pigment and capillary changes in the retina.⁸² Pulmonary function changes have been documented in groups of divers; these include increased vital capacities, which may be adaptive.⁸³ The clinical relevance and validity of these findings are still being investigated. An excellent review of the relevant literature can be found in Elliott and Moon.⁸⁴ This review concludes that “in the absence of a history of acute decompression illness, the possibility of a clinical syndrome among divers or ex-divers remains unproven. If it exists, the prevalence is unknown, and probably low.”

TREATMENT OF BUBBLE-RELATED DISEASE

Because symptoms and signs can progress, a diagnosis of DCI should be treated urgently. The patient should be placed on oxygen at as high a partial pressure as reasonably feasible. Additionally, fluids (generally oral, but intravenous or intraosseous fluids may be administered as appropriate for the condition of the patient) are pushed, and the individual is placed in a recumbent position. A hyperbaric chamber should be found and the patient transferred for recompression therapy (Figure 8.9). An excellent source of emergency information and referral is the Divers Alert Network.^a

Many recompression tables exist; however, the US Navy Treatment Table 6 (Figure 8.10 and Table 8.7) is the principal therapeutic table used for the treatment of decompression illness. The patient is given oxygen at depth (60 fsw) initially, with intermittent air breaks to reduce the incidence of oxygen toxicity. Response to therapy is generally excellent if recompression therapy is instituted promptly. Depending on the response to therapy, table modifications and follow-on hyperbaric treatments may be required if manifestations do not resolve or recur. Deeper tables include the Comex 30 Treatment Table using 50% oxygen and nitrogen mixture (100 fsw), US Navy Treatment Table 6A using 50% oxygen and nitrogen mixture (165 fsw), and the Lamberston/SOSI Treatment Table 7A to depths greater than 165 fsw. Rarely saturation tables such as US Navy Treatment Table 7 will be used if the diver does not respond to treatment based on the other treatment tables.

MEDICAL SURVEILLANCE

Healthcare personnel must understand that few other workers experience the magnitude of physiologic stresses imposed routinely on divers and caisson workers. Medical surveillance requires a thorough understanding of the physiologic aspects of diving, along with the specific workplace hazards that may be encountered by the worker. In general, different classes of workers are exposed to varying levels or types of hazards, requiring a tailored approach. Most professional divers, including military, commercial, and scientific divers, as well as caisson workers and hyperbaric chamber attendants are provided with specific guidance and standards by their employer. For example, the Association of Diving Contractors International periodically publishes medical requirements for their members. Title 29 CFR 1910 Subpart T Appendix A provides examples of conditions that restrict or limit exposure to hyperbaric conditions.¹⁹ Other sources provide more detailed guidance.^57,85–89 Examination frequency varies depending on type of exposure and the regulations being followed. Reexamination must be completed after any significant illness or injury, particularly exposure-related injuries, diving with any neurological involvement, gas embolus, or pulmonary or ear barotrauma.

Healthcare providers who conduct medical surveillance on divers should have formal instruction in diving/hyperbaric medicine. This training enables them to correlate various medical conditions to the unique hyperbaric environment. One source for information on diving medicine courses, scientific meetings, general information on diving or hyperbaric medicine, or addresses of practitioners with an interest in diving or hyperbaric medicine is the Undersea and Hyperbaric Medical Society.^b

PREVENTION

The only certain method to prevent hyperbaric injuries is to simply avoid all high-pressure and diving work. Thanks to modern engineering techniques, the need for caisson work has been significantly decreased. However, preventing diving injuries by avoiding diving altogether is generally regarded as unfeasible. Therefore, the principal methods used to prevent diving injuries are (i) extensive training, including both an academic understanding of principles and job-specific practical training; (ii) dive planning, to include emergency procedures and appropriate use of tables; (iii) maintenance of a high level of fitness; (iv) meticulous care of equipment, along with ongoing improvement in equipment design; and (v) a healthy respect for the indigenous hazards of the profession. These same principles also apply to caisson work. Current research is directed toward the refinement of decompression models and tables, improving equipment design, enhancing treatment methods, and understanding the pathophysiology and associated risk factors of diving disorders.

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