Chapter 77 Diving Medicine

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Scuba diving is an exhilarating and generally safe activity for people who are healthy, well trained, physically conditioned, properly equipped, and “water wise.” It can, however, be a demanding and dangerous activity because of the intrinsic hazards of the aquatic environment.

The hazards of diving include the generic problems found in other aquatic activities, such as near drowning, hypothermia, immersion-related skin disorders, water-borne infectious diseases, and hazardous marine life, as well as several relatively unique problems related to the increased atmospheric pressure found underwater and the effects of breathing gases at elevated pressure.

There are many millions of recreational, commercial, and military divers worldwide; hundreds of thousands of new divers are trained each year. According to the 2000 U.S. Census Data, approximately 2.5 million active certified divers reside in the United States alone (http://www.allcountries.org/uscensus). Diving is conducted in every imaginable aquatic setting, including oceans, lakes, reservoirs, rivers, quarries, ponds, and aquariums. Despite the extent of diving activity, diving-related fatalities and serious injuries are rare. For recreational divers, the average death rate (based on data derived from case claims of insured divers) is 16.4 deaths (range 12.1 to 22.9) per 100,000 divers per year.⁵⁷ The chance of suffering decompression sickness (DCS) on any single dive is about 4 in 10,000 in warm water and 59 per 10,000 in cold water.¹⁸²

The best, and most readily available information about diving accidents comes from the Divers Alert Network (DAN), a private, nonprofit dive safety organization supported by clinical and academic affiliations, including the University of California at San Diego, the University of Pennsylvania, and Duke University. Established in 1980, DAN provides diving medical assistance to the diving community all over the world. Services include dissemination of medical information, evacuation support, and accident insurance. DAN is a member-supported organization with over 200,000 U.S. members and approximately 265,000 members worldwide (http://www.DiversAlertNetwork.org). DAN’s medical hotline number is 919-684-9111.

The number of diving-related accidents in North America reported by DAN has been relatively stable over the past two decades, with perhaps a small downward trend, although one has to be careful not to overinterpret these observational data because of reporting bias and the lack of complete information about the frequency and type of diving performed. Notwithstanding the apparent stable or downward trend in diving-related fatalities, concern about diving safety continues and is DAN’s core mission.

One of DAN’s key roles in the diving industry is management of dive fatality and accident data. Diving-associated fatalities peaked in the mid 1970s with annual rates as high as 150 and have been stable since that time, with an average of 84 (range 77 to 91) fatalities per year.⁵⁷ In a recent effort to further define the scope of diving fatalities and overcome the perennial problem of uncertain total numbers (denominators), DAN researchers used claims data from 2000 to 2006 from insured members. In this way, the published incidence rates were based on a known denominator (number of insured). Over that 7-year period, there were 187 death claims among 1,141,367 insured member years, for which the mean annual fatality rate (AFR) was 16.4 (range 12.1 to 22.9) deaths per 100,000 persons.⁵⁷ Fatality data from the British Sub-Aqua Club are similar for the same period, with 14.4 deaths per 100,000 divers.

DAN hosted a workshop on diving fatalities in Durham, North Carolina, in April of 2010 to address industry concerns about dive accident causation and to solicit input from industry leaders about possible intervention strategies. The proceedings of this meeting are freely available online at www.diversalertnetwork.org. Of interest to the attendees were not just fatalities but their underlying cause. Based on a paper presented by Denoble and colleagues, “The most common disabling injuries associated with death were asphyxia, arterial gas embolism (AGE), and acute cardiac related events. The most common root causes were gas supply problems, emergency ascent, cardiac health issues, entrapment/entanglement, and buoyancy trouble. The risk for death while diving increased with age, starting in the early thirties. This is likely due to the naturally increased prevalence of cardiac disease with age, but an increased association of AGE and asphyxia were also associated with aging.”⁵⁶

Since scuba diving made its debut in the United States in 1951, the nature of the diving population has substantially changed. Scuba divers of the 1950s and 1960s were generally “water people”—well-trained, vigorous athletes experienced in breath-hold diving and competitive swimming. For these individuals, donning a scuba tank and regulator was a natural extension of familiar activity. These early scuba divers seldom encountered problems that required the attention of the general medical community. However, as scuba diving equipment became more available, adventure-minded persons from all walks of life became attracted to the sport. The popularization of scuba diving in recent years has attracted participants who are poorly conditioned, have little or no experience in aquatic or other sports, are of advanced age, have significant underlying medical conditions, or are even severely disabled. Because of the hostile and unforgiving nature of the aquatic environment, such persons may be at increased risk for a diving-related injury or illness. Certain medical conditions are an absolute contraindication to diving.

In the last decade, recreational divers have increasingly sought more technically complicated diving in efforts to increase the amount of time spent underwater or the depth dived. Although these diving techniques have long been used in commercial and military diving under highly controlled conditions, there are significant concerns about the safety of “technical” diving in the typically less-controlled recreational setting.

All primary care physicians should be prepared to answer basic questions about fitness for diving and to initially manage diving-related medical emergencies. Likewise, every emergency medical treatment facility should be prepared to evaluate, provide urgent care, and if needed, arrange appropriate transport to a hyperbaric treatment facility (recompression chamber) if a diver is suspected to be suffering decompression sickness or arterial gas embolism.

This chapter focuses primarily on the pressure-related diving syndromes collectively known as dysbarism; additional conditions relevant to diving are discussed in other chapters (e.g., immersion hypothermia in Chapter 6, submersion incidents in Chapter 75, hyperbaric oxygen therapy in Chapter 78, and hazardous marine life in Chapters 79 to 81).

Historical Perspective

Human beings did not evolve for an aquatic existence and are not well adapted for functioning in the aquatic environment. Nonetheless, man has been breath-hold diving to gather food and other natural resources from the oceans for thousands of years. There is archaeologic evidence that Neanderthal man breath-hold dived for shellfish 40,000 years ago. The Ama of the Izu Peninsula of Japan and the Hae Nyeo of the South Korean Island of Cheju (in both cases women) have been breath-hold diving to collect shellfish, lobster, sea urchins, octopus, and seaweed for at least 6000 years. Women of the Yahgan, Alakaluf, and other nomadic sea-going female Fuegian Indians of southern Patagonia engaged in similar diving practices for probably 5000 years before these primitive native Americans became extinct in the late 19th and early 20th centuries. The fires that these divers built to warm themselves along the shores of what is now known as the Straits of Magellan inspired Ferdinand Magellan to name the area “Tierra del Fuego” (Land of Fire).

Written records of diving for salvage and military purposes date back to around 500 BC, when the Greek historian Herodotus recorded the feats of Scyllis and his daughter Cyana as they dived in the Mediterranean Sea for the Persian king Xerxes during the 50-year war between Greece and Persia. Many early cultures around the Mediterranean Sea made use of divers in military operations—usually to cut the anchor cables of ships, bore holes in the hulls of enemy vessels, and build harbor defenses. Aristotle described the use of diving bells (basically, upside-down buckets) to supply air to sponge fishermen in 360 BC, and Alexander the Great is reported to have gone underwater in a specially constructed glass diving bell to observe his divers removing defensive obstructions from the besieged harbor of Tyre in 332 BC.

Colorful accounts of military and salvage divers dot the history of Roman and other early cultures. By 100 BC, diving operations around the major shipping ports of the eastern Mediterranean were so well organized that there were legally binding payment schedules, which recognized that the risk to the diver increased with depth underwater.

Although written records of diving in the Americas were not discovered until after European explorers arrived in the New World in the 16th century, Peruvian artifacts dating to AD 200 show divers wearing goggles and holding fish, so it is reasonable to assume that breath-hold diving had been practiced long before the Europeans arrived. Spanish explorers are reported to have enslaved native divers and forced them to dive for pearls in the Caribbean. These explorers also made extensive use of divers to salvage galleons wrecked in the Caribbean and along the coast of Florida. In one such instance, in 1680, Sir William Phipps is believed to have recovered more than £200,000 in sterling silver from a Spanish galleon.

Human underwater exploits remained limited to breath-hold diving until about 300 years ago, when a series of technological developments began to expand human underwater activity. These developments principally involved the use of different types of external air supply to prolong submergence.

In the 17th century, primitive bells containing air were carried from the surface, allowing Swedish divers to stay underwater longer than a single breath and to salvage cannons from Stockholm’s harbor.¹⁵⁴ In 1690, Sir Edmund Halley devised a leather tube to carry surface air to barrels, which resupplied air to manned bells at a depth of 18 m sea water (msw) (60 feet sea water [fsw]). These barrels were submerged, and the air they contained was compressed.⁵³

The first practical diving suit was fabricated by Augustus Siebe in 1837.^3,^53,¹⁵⁴ Atmospheric air was supplied to the diver as compressed air from a manually powered pump on the surface. By 1841, French engineers had developed the technique of using compressed air to keep water and mud out of caissons sunk to the bottom of riverbeds for bridge footings and tunnels. Soon thereafter, it was noted that people working in a compressed-air environment sometimes suffered joint pains, paralysis, and other medical problems soon after leaving the caisson. This poorly understood condition was called “caisson’s disease” and was the first recognition of what is now known as decompression sickness.¹⁵⁸

Underwater diving remained an esoteric activity having limited commercial and military utility until the 1930s. By that time, and increasingly during World War II, the military importance of submarines and other undersea activities became evident to navies throughout the world. With the development of submarine forces came the need to train men to escape from submarines that became disabled at depth (an all-too-frequent occurrence in the early days of submarines). Given the shallow operational depths of these early boats, it was usually possible to escape by simply exiting the vessel and ascending to the surface. It was noted early that failure to exhale while ascending through the water led to pulmonary overpressurization accidents and a new and dramatic syndrome that we now know to be arterial gas embolism.

In 1865, the French engineers Rouquayrol and Denayrouse developed a device that could supply air on demand at ambient pressures different from the 1 atmosphere of pressure found at sea level. These inventors were able to supply air on demand at appropriate breathing pressure to persons underwater with a “demand regulator,” as it subsequently became known. This device originally required a surface air supply connection.³ The demand valve regulator was later modified to supply auxiliary oxygen for pilots operating at high altitude. In 1943, while working with the French resistance against Nazi Germany, Jacques-Yves Cousteau and Emile Gagnon combined a demand valve regulator with a compressed air tank, giving rise to what they called “self-contained underwater breathing apparatus,” or scuba.

The potential military usefulness of scuba was immediately recognized and led to a considerable amount of investigation during World War II. As initially configured, scuba was used in an open-circuit mode in which exhaled air was simply vented into the water. This was wasteful of the compressed air supply and had other disadvantages for military uses. Further work led to refinement of rebreather scuba devices (both closed- and semiclosed-circuit systems), such as Lambertsen’s amphibious respiratory unit.¹¹ These rebreather systems conserved the breathing gas by using a carbon dioxide scrubber and recirculating all or part of each exhalation (see Rebreather Diving, later). These specialized scuba systems were useful for military purposes because they allowed longer submergence times and could be used in clandestine operations or when disarming pressure-sensitive explosive devices. However, they had a greater frequency of mishap, so rebreather systems were not widely used until the 1990s, when there began a resurgence of interest in them by “technical” recreational divers.

After World War II, development and marketing of open-circuit scuba equipment to the general public made the underwater world accessible to growing numbers of people. In the last four decades, scuba diving has opened the underwater world to millions of divers and hundreds of millions of cinema observers. Scuba is now used as a basic tool with myriad commercial, military, scientific, and recreational applications (Box 77-1).⁷

Types of Diving and Diving Equipment

There are several general types of diving, each using different equipment and having different logistical support needs. From the least to the most sophisticated equipment used, the types of diving are breath-hold diving, open-circuit scuba diving, rebreather diving (closed- and semiclosed-circuit diving), and surface-supplied or tethered diving. Mixed gas and technical diving are also discussed later, as well as saturation diving and one-atmosphere diving.

Breath-Hold Diving

Breath-hold diving is the simplest and oldest form of underwater activity, dating back many thousands of years. In breath-hold diving, no supplemental air source or underwater breathing device is used, so submergence is limited to the length of time the diver can hold his or her breath. There are several types of breath-hold diving that are characterized by activity and equipment.

Snorkeling is the most common form of breath-hold diving. Snorkelers typically use a face mask to facilitate underwater vision, fins for propulsion, a snorkel to breathe air while swimming face down on the water’s surface, some sort of attire for environmental protection (e.g., a neoprene wetsuit or full-body spandex [Lycra] suit), and sometimes lead weights to counterbalance the positive buoyancy of a wetsuit or one’s innate positive buoyancy. Snorkelers often remain exclusively on the surface, breathing through their snorkel with their face submerged, but never actually diving under the water. Snorkeling is widely used at tropical resorts today to introduce people to the beauty of coral reefs.

Freediving generally refers to one of several types of competitive breath-hold diving. Freediving is classified as an “extreme sport,” and many consider it to be the original extreme sport. Competitive freediving dates back to at least the early 1900s, with perhaps the best recorded account involving the Greek sponge diver Haggi Statti Giorgios. In 1913, Giorgios was offered a few dollars to dive more than 61 msw (200 fsw) underwater to retrieve the anchor of the Italian ship Regina Margherita, which had become stuck in the Aegean Sea at a depth of 70 msw (230 fsw). He freed the anchor after three consecutive dives of between 1.5 and 3.5 minutes’ duration, going as deep as 80 msw (263 fsw). He did not consider this an especially taxing feat, saying that he had dived as deep as 110 msw (361 fsw) and stayed underwater for as long as 7 minutes on other occasions. Some consider Giorgios to be the “father of freediving.” However, it was not until Jacques Mayol of France dived to 101 msw (331 fsw) in 1976—which was then considered a stunning feat—that freediving really began to grow in popularity as an extreme sport. In 1988, the popular film The Big Blue portrayed the lives of Jacques Mayol and Italian Enzo Majorca as competitive freedivers and further popularized the sport.

Today, competitive freediving attracts athletes from all over the world and is regularly featured on sports television channels. The sport is governed by the Association Internationale pour le Developpement de l’Apnée—also known as the International Association for the Development of Apnea (AIDA; http://www.aida-international.org/) or the International Association for the Development of Freediving. AIDA has been the officiating body for freediving, setting standards and recognizing records since 1992. AIDA recognizes eight types of freediving and breath holding; descriptions of these disciplines and the world records for reach can be found in Table 77-1.

TABLE 77-1 AIDA Competitive Freediving World Records (as of August 2011)

The freediver typically wears a face mask and a single two-foot monofin, or a pair of fins, and some sort of body suit for environmental protection.

Medical Problems of Breath-Hold Diving

The major medical concern of breath-hold diving is development of hypoxia leading to loss of consciousness and drowning, especially if submergence is preceded by hyperventilation (see Hyperventilation and Shallow Water Blackout, later). Breath-hold divers also may become hypothermic, get entangled in underwater debris (e.g., fishing line, ropes, and cables) or vegetation, be harmed by marine animals, or be injured by boats or other watercraft. Divers are also subject to barotrauma of the ears and sinuses, as described later. Although it is very rare, breath-hold divers also can suffer from decompression sickness (see Decompression Sickness, later).

Scuba Diving

Scuba diving uses a tank fitted with a pressure regulator that supplies compressed air to the diver at a pressure equal to ambient water pressure. In recreational or sport diving, the diver’s tank usually contains 90 cubic feet of filtered, oil-free compressed air pressurized to about 3000 pounds per square inch gauge (psig); tanks of 72 and 50 cubic feet are also used. Originally, tanks were made of steel, but most tanks in use today are made of aluminum.

The regulator employs two stages. The first stage is attached to the tank and makes an initial reduction in pressure to the lower-pressure second stage, which is attached to the diver’s mouthpiece and from which the diver actually breathes.

Like the snorkeler, the scuba diver wears a face mask covering either just the eyes and nose (most often) or the entire face to allow underwater vision and equalization of pressure in the air space over the eyes, fins for propulsion, snorkel for breathing during surface swimming, weight belt, buoyancy-compensating vest to adjust buoyancy underwater and for flotation in case of an emergency, diving watch and depth gauge and/or a diving computer to track time and depth underwater, compass, and tank pressure gauge to monitor air consumption. Because of the higher thermal conductivity of water, divers typically wear neoprene wetsuits to stay warm (as well as to provide protection from jellyfish and other stinging marine life), even in relatively warm tropical oceans. These suits maintain a layer of water warmed by body heat between the skin and suit. The suits are typically 7 mm in thickness when diving in temperate waters and 2 to 3 mm when diving in warm tropical waters. In tropical waters, some scuba divers wear a body suit made of spandex (Lycra) instead of a wetsuit to protect themselves from jellyfish, hydroids, and other marine stingers. An impermeable drysuit and warm undergarments are usually worn when diving in water colder than 10° C (50° F).

In addition to the preceding basic equipment, additional equipment (e.g., diving knife, camera, spear gun, or game bags) may be needed for safety, navigation, communication, or other purposes. All of these objects increase the diver’s resistance to movement underwater, decrease efficiency of movement, and increase the work of diving. Extra drag may become a life-and-death matter in the cases of poorly conditioned individuals who have to respond to underwater exigencies.

Although the majority of recreational scuba diving is done using compressed air in an open-circuit mode, specially adapted scuba gear may be used for mixed-gas diving (see later) or for semiclosed- or closed-circuit diving. Use of enriched air nitrox by sport divers has markedly increased in recent years.

Rebreather Diving

Although the first self-contained underwater diving is usually associated with scuba, it was actually first done with the assistance of a rebreather in the 1880s. Rebreathing devices for diving were perfected over the years, but with the advent of scuba in the 1940s, rebreathers were relegated to use in only commercial and military situations. Beginning in the 1990s, however, use of rebreathers started to increase in recreational diving (especially for underwater photography and cave diving).

Conceptually, rebreathers are devices that capture and recirculate a diver’s exhaled breath, removing the carbon dioxide added by the body’s metabolism and replacing the oxygen extracted by the body before giving the air back to the diver to once again breathe. From a diving technology standpoint, rebreather diving is much more efficient than is open-circuit scuba, and it allows divers to reach greater depths and stay underwater longer than is possible with conventional open-circuit scuba. Rebreathers are also very useful when exhaled bubbles may be problematic (e.g., when studying or photographing marine life or when disarming pressure-sensitive mines). However, they can be quite hazardous if not maintained and operated properly.

All rebreathers have certain elements in common, and they fall into one of four general categories of devices:⁸

1 Closed-circuit oxygen rebreather

2 Closed-circuit mixed-gas rebreather

3 Semiclosed-circuit rebreather using premixed gas

4 Semiclosed-circuit rebreather that mixes the breathing gas while diving

The advantages and disadvantages of the different types of rebreathers are beyond the scope of this chapter. Many different rebreather designs are available, and new systems continue to be developed. Therefore a diver must be individually trained on the use of each rebreather.

Surface-Supplied or Tethered Diving

Surface-supplied or tethered diving includes several different diving technologies, all of which share the common characteristic of the diver’s breathing gas (compressed air or mixed gas) being supplied to the diver by hoses from a surface source or from a diving bell at a pressure equal to ambient water pressure.

The best known form of surface-supplied diving is classic hardhat diving, which is sometimes called “mud diving” or “blackwater diving,” because it is often done in harbors with very muddy bottoms and in dirty water with typically zero visibility. In this type of diving, which was portrayed in the 2000 movie Men of Honor, the diver wears a large bronze helmet with glass faceplates, a canvas suit, weight belt and weighted shoes, and other gear. Altogether, the traditional hardhat diver’s gear weighs 87 kg (192 lb). Although traditional hardhat gear is still used in a variety of settings, most surface-supplied diving uses more modern gear that is not as heavy or cumbersome as the traditional diver’s dress and that has communication capabilities and the ability to keep the diver warm.

Surface-supplied diving is most often used in commercial or military settings. It is frequently performed in arduous circumstances. The diver operates in total darkness in cold water, often against a current or surge, performing tasks primarily by feel.

The diving techniques of surface-supplied diving are quite different from those of scuba diving, and they are not further discussed here; however, most of the physiologic and medical problems of surface-supplied divers are identical to those encountered in scuba diving.

Mixed-Gas Diving

Diving can be done using either compressed air or mixed gas. Compressed air is most commonly used, especially with scuba, but there are a number of settings where mixed gas is needed or preferred.

Mixed-gas diving refers to diving using a breathing mixture other than compressed air (e.g., a gas mixture in which the concentrations of nitrogen and oxygen have been changed or in which a different inert gas [e.g., helium] is substituted for nitrogen). Mixed-gas diving can be used in surface-supplied, saturation, or scuba diving modes, although historically it has been used most often in surface-supplied or saturation diving operations.

Mixed-gas diving has been used for many decades, but because of the greater logistic support required and associated greater expense and hazards, it has been used primarily in commercial, scientific, and military diving operations. This has changed in the past 15 years as “technical” sport divers have sought to go deeper and stay down longer. Increasing numbers of recreational divers are now using mixed gas, especially nitrox.

Technical Diving

Beginning in the late 1980s, a small but rapidly growing number of recreational divers began to use mixed-gas, rebreathers, and other technology previously used by military and commercial divers to dive deeper and stay down longer than possible with conventional scuba. “Technical diving,” as it was dubbed in 1990 by AquaCorps (a diving periodical devoted to this then-new endeavor), has gained a large following since then. It is employed by wreck and cave divers.

Technical diving represents the leading edge of underwater discovery and challenge for the nonprofessional diver. It is at the forefront of dive technology and technique and is inherently more hazardous than is typical sport scuba diving because of the intrinsically greater hazard and diminished margin of error allowed when diving deep and because of its reliance on technology. For example, rebreathers using trimix have been used for very deep diving, and about half a dozen people have dived deeper than 240 msw (800 fsw) using this technology. Unfortunately, most of these divers are now dead secondary to diving mishaps.

Despite its hazards, technical diving has markedly increased in popularity in recent years, as witnessed by the establishment of the International Association of Nitrox and Technical Divers (IANTD) and other technical diving associations and by the convening of “tek” conferences drawing thousands of divers. This movement has been supported by former navy diving suppliers looking for new buyers and commercial diving vendors trying to cross over to the much larger recreational diver market.

Saturation Diving

Once underwater, the diver begins to absorb increased amounts of nitrogen or other inert gas, depending on the breathing medium, until a new equilibrium is established according to the pressure of the depth of submergence. In most deep-diving scenarios, the time needed to off-gas, or decompress, this inert gas on returning to normal atmospheric pressure may be much greater than the time spent at depth.

The need to minimize prolonged decompression after deep diving led to development of saturation diving. In the late 1950s, experiments by USN diving medical officers George Bond and Robert Workman coincided with those of Jacques-Yves Cousteau and Edward Link in the commercial sector, all of whom were working on ways to stay underwater at great depths long enough to perform useful work.³

The basic concept of saturation diving is that after approximately 24 hours at any given depth, the diver’s tissues establish equilibrium with the gases in the breathing mixture. From that point, the decompression obligation remains essentially the same no matter how long the diver remains at that depth. If the diver can be kept “at pressure” for a prolonged period, the decompression is the same as if he or she were there for only a short time.

Modern saturation complexes allow divers to live for days in large chambers at the pressure of a given work site and to be transported to the underwater site by locking into a personal transfer capsule (PTC) or sealed diving bell. When the desired depth is reached, the water pressure equals the gas pressure within the capsule and divers may exit the PTC into the water while breathing gas supplied by umbilical hoses. To maintain the diver’s thermal balance in the very cold water found at great depths, heated water is circulated in special hot-water suits.

Another application of saturation diving, used primarily for scientific purposes, uses underwater habitats, which are steel chambers situated at a given depth of water and pressurized with a compressed gas atmosphere at the same pressure as the surrounding water. Divers may live for days or weeks in the habitat, leaving the chamber with scuba equipment to perform studies or observe marine life in its natural state. In this specialized type of diving, rigorous precautions are taken to avoid inadvertent surfacing, thermal stress, and skin and ear infections during prolonged stays in the continuously moist environment of the habitat. Prolonged saturation decompression schedules, often taking several days, are required to return divers safely to sea level pressure on completion of the underwater mission.

One-Atmosphere Diving

In recent years it has become clear that humans cannot reliably or safely function at the greatly elevated pressure of deep water, and the human factor aspects of diving have become the limiting factors in manned exploration of the ocean depths. This has led to numerous developments in one-atmosphere diving systems. In the 15th century, Leonardo da Vinci drafted schematic drawings that look similar to modern one-atmosphere diving systems, but these systems had to await late 20th-century advances in metallurgy, engineering, and communications before they could become functional.

One-atmosphere absolute (ATA) diving systems are, in essence, small submarines with various types of propulsion systems and manipulators that allow the operator to work at great depth. The interior of the unit is maintained with environmental control systems to retain safe physiologic parameters. These systems range from one-person ATA suits in which a diver can “walk” (Figure 77-1, online) to submersibles that accommodate two or more occupants.

FIGURE 77-1 JIM diving suit. The diver remains at sea level pressure inside the suit and can work for prolonged periods of time at extreme depths underwater.

(Courtesy Kenneth W. Kizer, MD.)

Diving Physics

Divers encounter many adverse environmental conditions underwater. These include cold, changes in light transmission and sound conduction, lack of air to breathe, increased density of the surrounding environment, and increased atmospheric pressure. Not surprisingly, diverse medical problems are related to diving (Box 77-3).

Of the various environmental factors affecting divers, pressure is by far the most important because it contributes either directly or indirectly to the majority of serious diving-related medical problems. Therefore knowing the basic physics and physiologic effects of pressure is essential to understanding and treating the pressure-related disorders.

Pressure is defined as force per unit area. Atmospheric pressure is the pressure exerted by the air above the earth’s surface. Atmospheric pressure varies with altitude. At sea level, atmospheric pressure is 760 millimeters of mercury (mm Hg), or 14.7 pounds per square inch (PSI). The barometric pressure at sea level is generally referred to as 1 atmosphere (atm). Absolute pressure is the total barometric pressure at any point. With pressure gauges calibrated to read zero at sea level, gauge pressure is the amount of pressure greater than atmospheric pressure. In general, gauge pressure is 1 atm less than absolute atmospheric pressure. It is necessary to specify whether pressure is expressed in terms of gauge or absolute pressure. Except in situations requiring laboratory precision, the following units are commonly used to express water pressure:

• Feet of seawater (fsw)

• Feet of fresh water (ffw)

• Meters of seawater (msw)

• Atmospheres absolute (ATA)

• Pounds per square inch gauge (psig)

• Pounds per square inch absolute (psia)

As a diver descends under water, absolute pressure increases much faster than in air. Each foot (0.3048 m) of seawater exerts a force of 0.445 psig. Therefore, if the 14.7 psi pressure of 1 atm is divided by 0.445 psi per foot of seawater, the absolute pressure will have doubled at 33 fsw. In the ocean, each 33 feet of depth adds one additional atmosphere of pressure. The gauge pressure at 33 fsw is 14.7 psig (in excess of atmospheric pressure), and the absolute pressure is 29.4 psia.

Because of the weight of solutes in seawater, it is slightly heavier than is fresh water. In fresh water, 10.4 msw (34 fsw) equals one additional atmosphere of pressure.

Pressure change with increasing depth is linear, although the greatest relative change in pressure per unit of depth change occurs nearest the surface. Table 77-2 lists commonly used units of pressure measurement in seawater.

TABLE 77-2 Commonly Used Units of Pressure in the Underwater Environment

When a diver submerges, the force of the tremendous weight of the water above is exerted over the entire body. Except for air-containing spaces such as the lungs, paranasal sinuses, intestines, and middle ears, the body behaves as a liquid. The law that describes the behavior of pressure in liquids is named for the 17th-century scientist, Blaise Pascal. Pascal’s law states that a pressure applied to any part of a fluid is transmitted equally throughout the fluid. Thus, when a diver reaches 10 msw (33 fsw), the pressure on the surface of the skin and throughout the body tissues is 29.4 psia or 1520 mm Hg (Figure 77-2). The diver’s body is generally unaware of this pressure, except in the air-containing spaces of the body. The gases in these spaces obey Boyle’s law (Figure 77-3), which states that the pressure of a given quantity of gas for which temperature remains unchanged varies inversely with its volume. Thus, air in the middle ear, paranasal sinuses, lungs, and gastrointestinal tract is reduced in volume during compression or descent underwater. Inability to maintain gas pressure in these body spaces equal to the surrounding water pressure leads to various untoward mechanical effects, which are discussed later.

FIGURE 77-2 Pascal’s law. Pressure applied to any part of a fluid is transmitted equally throughout the fluid.

FIGURE 77-3 Boyle’s law. The volume of a given quantity of gas at constant temperature varies inversely with pressure: P₁V₁ = P₂V₂.

Because of the weight of the water exerting pressure over the chest wall, humans can breathe surface air through a snorkel or tube connected to the surface typically only to a depth of 1 to 2 feet. Attempts to breathe at greater depths through the tube are not only impossible but are dangerous, because the respiratory effort greatly augments the already physiologic negative-pressure breathing. In other words, when the respiratory muscles are relaxed at sea level, alveolar pressure is equal to surrounding air pressure. At a depth of 1 foot, the total water pressure on the chest wall is nearly 91 kg (200 lb). Because of the loss of normal chest expansion and the pressurization of intra-alveolar air, the diver has to use forceful negative-pressure breathing to draw surface air into the lungs through the tube. Even at a depth of 1 foot, the great respiratory effort required is rapidly fatiguing, and respiration becomes impossible at further depths of only a few inches. Forced negative-pressure breathing can ultimately result in pulmonary capillary damage, with intra-alveolar edema or hemorrhage. Symptoms include dyspnea and hemoptysis. Should this occur, there is no specific treatment; therapy is purely supportive.

Barotrauma

Gas pressure in the various air-filled spaces of the body is normally in equilibrium with the environment. However, if anything obstructs the passageways of gas exchange for these spaces and a change in ambient pressure occurs, pressure disequilibrium develops. The tissue damage resulting from such pressure imbalance is known as barotrauma and commonly referred to as a squeeze.

Overall, barotrauma is the most common medical problem in scuba diving, potentially involving any structure or combination of structures that leads to entrapment of gas in a closed space. This includes the ears, paranasal sinuses, lungs, gastrointestinal tract, portion of the face under a face mask, and skin trapped under a fold in a drysuit.

Barotrauma of Descent

Mask Barotrauma

For humans to see underwater, an air space must be present between the eyes and water. In scuba diving, this is created using a face mask consisting of tempered safety glass in a soft malleable mask that seals across the forehead, on the sides of the face, and under the nose to allow nasal exhalations into the mask space to maintain air pressure inside the mask. As a diver descends in the water, the ambient pressure increases and air must be added to the gas inside the face mask to equalize water pressure. If inexperience or inattention cause the diver to forget to maintain this balance, negative pressure in the mask can rupture capillaries, causing skin ecchymosis, subconjunctival hemorrhage, lid edema, and rarely hyphema. This unusual condition is known as mask squeeze (Figure 77-4; see also Figure 28-20).

FIGURE 77-4 Mask barotrauma. A novice diver with mask barotrauma showing subconjunctival hemorrhages.

Most divers with mask squeeze are asymptomatic. The condition usually resolves over a few days to a week without any intervention, but can be treated with cold compresses and analgesics if needed.

Face mask barotrauma is easily prevented simply by exhaling through the nose during descent. It may also be prevented by wearing a full-face face mask.

In recent years, full-face face masks, which are standard issue for commercial, military, and public safety divers, have become more popular with recreational divers as more models have become available, but they are still not commonly used. A full discussion of the advantages and disadvantages of full-face face masks is beyond the scope of this chapter, but these masks should be remembered as a potentially good alternative for persons who have trouble with face mask fit, face mask flooding, or feelings of claustrophobia.

Orbital hemorrhage from face mask barotrauma is an unusual complication and can be associated with diplopia, proptosis, and visual loss.^35,^123,¹⁶³ Although such neurologic findings after scuba diving may suggest arterial gas embolism or neurologic decompression sickness, the presence of the unmistakable stigmata of mask squeeze and a consistent history should prompt consideration of orbital hemorrhage. Under these circumstances, instead of immediate referral to a recompression facility, the diver should be referred for immediate magnetic resonance imaging (MRI) and ophthalmology consultation, because of the possibility of permanent vision loss caused by compression of the optic nerve or elevated intraocular pressure. In rare cases, surgery intervention may be necessary.¹⁶³ Recompression is contraindicated for orbital hemorrhage unless the diver also suffers from arterial gas embolism or decompression sickness.

Sinus Barotrauma

The four paired paranasal sinuses—frontal, maxillary, ethmoid, and sphenoid—have narrow connections to the nasal cavity via the sinus ostia. If there is inability to maintain the air pressure in any paranasal sinus during descent, a relative vacuum develops in the sinus cavity. This negative pressure causes congestion of the mucosal lining with subsequent edema and intramucosal bleeding and possible hematoma, hemorrhagic bullae, and bleeding into the sinus (Figure 77-5). In cases of sinus squeeze, the diver usually experiences increasingly severe pain over the affected sinus during descent. On ascent, the remaining gas in the sinus expands and may force mucus and blood into the nose and mask.

FIGURE 77-5 Frontal sinus barotrauma that first occurred 3 days earlier. Note the persistent air-fluid level.

(Courtesy Kenneth W. Kizer, MD.)

The frontal sinus is most commonly affected by barotrauma, followed by the maxillary sinus. With maxillary sinus involvement, the diver often experiences pain in the maxillary teeth caused by compression of the posterior superior branch of the fifth cranial nerve, which runs along the base of the maxillary sinus. Additionally, maxillary sinus barotrauma can cause compression or ischemic neuropraxia of the infraorbital branch of the fifth cranial nerve, causing tingling and numbness of the cheek and upper lip.^34,¹⁴⁸ Other complications include sinusitis from infection of the intrasinus fluid, or periorbital emphysema from air dissecting through the lamina papyracea from the ethmoid sinus into the orbits.

Treatment of sinus barotrauma involves use of systemic (e.g., pseudoephedrine) and topical (e.g., phenylephrine or oxymetazoline) vasoconstrictors, analgesics, abstinence from diving until resolved, and antihistamines if needed. Antibiotics are indicated only if signs of sinusitis are present, including fever or purulent nasal drainage. In the field, a 3- to 5-day course of corticosteroids has been sometimes used to hasten recovery and allow an otherwise healthy diver to return to diving. On rare occasions, drainage of the affected sinus by an otolaryngologist is required for persistent pain.

Sinus barotrauma usually occurs in the setting of a diver who has an upper respiratory infection or severe allergies or who has an anatomic deformity such as nasal polyps or a deviated septum. Divers with a history of sinusitis or middle-ear barotrauma may be more prone to paranasal sinus barotrauma.¹⁸⁰ Consequently, prevention of sinus barotrauma includes avoidance of diving when suffering from an upper respiratory infection, while symptomatic from allergic rhinitis, and when sinusitis, nasal polyps, or any other condition is present that impairs free flow of air from sinus cavity to nose. Significant nasal deformity may predispose to sinus barotrauma and may warrant surgical correction to allow one to continue diving.

External Auditory Canal Barotrauma

A tight-fitting wetsuit hood or drysuit hood can trap air in the external auditory canal and potentially lead to a painful external ear squeeze during descent as the volume of air is reduced according to Boyle’s law. External ear squeeze also can occur if the canal is blocked by cerumen, exostoses, or foreign objects.

Symptoms and signs of external ear canal squeeze include pain, swelling, erythema, petechiae, or hemorrhagic blebs of the ear canal wall and possible bleeding when the hood is removed. Bullae may be present in the canal and on the tympanic membrane. In very severe (and very rare) cases, the tympanic membrane can rupture from the negative pressure in the ear canal. The diver, feeling pain in the canal, may believe there is inadequate equalization of the middle ear and attempt a forceful Valsalva maneuver, which increases pressure on the tympanic membrane, leading to rupture. If this occurs, further diving is contraindicated until the tympanic membrane has healed.

Treatment of ear canal barotrauma includes washing the canal with lukewarm water. Bullae should not be incised. Antibiotic drops, such as Cortisporin otic or a fluoroquinolone preparation combined with hydrocortisone, can be used to prevent infection due to contamination with seawater. For tympanic membrane perforation, fluoroquinolone otic drops should be given.

Ear canal barotrauma can be prevented by remembering to break the seal of the wetsuit hood to allow water to fill the external ear canal before descent. Ear plugs should never be worn when scuba diving.

Middle Ear Barotrauma (Barotitis Media)

Referred to in diver parlance as ear squeeze, barotitis media is the most common medical problem in scuba diving, probably affecting more than 40% of divers at one time or another.⁹¹ The problem can be explained by a direct application of Boyle’s law (Figure 77-6), potentially compounded by the structure of the eustachian tube.

FIGURE 77-6 Middle ear barotrauma. Symptoms include fullness and pain caused by stretching of the tympanic membrane.

As previously noted, Boyle’s law describes the inverse relationship of pressure and volume in an enclosed airspace. It also explains why the greatest relative volume change for a given depth change occurs near the surface, which is why the greatest risk for middle ear squeeze occurs near the surface. As the diver descends, hydrostatic water pressure forces the tympanic membrane inward, and the volume within the middle ear cavity is reduced. The diver can add air into the middle ear through the eustachian tube, equalizing the pressure in the middle ear cavity with the external ambient pressure.

Because each foot of seawater exerts a pressure of about 23 mm Hg, a diver who descends 76 cm (2.5 feet) and does not equalize pressure in the middle ear will develop a relative vacuum in the middle ear because of contraction of air volume. Typically, the diver notices slight pain at a 60-mm Hg pressure differential between air in the middle ear and ambient water pressure. This pressure differential causes the tympanic membrane to stretch and bulge inward, causing increasing discomfort and eventually severe pain. Additionally, when the tissues lining the middle ear cavity are exposed to this vacuum, vasodilation, edema, transudation, and vascular rupture occur, causing bleeding into the mucosa and middle ear cavity.

At a depth of 1.2 msw (4 fsw), a 90-mm Hg pressure differential is generated, and the unsupported, flutter-valve medial one-third of the eustachian tube collapses and becomes obstructed. At this point, attempts to autoinflate the middle ear by Valsalva or Frenzel maneuvers may be unsuccessful. The diver must ascend to equalize middle ear pressure with ambient environmental pressure.

If a diver does not heed the initial symptoms of barotitis media and allows the pressure differential to reach 100 to 400 mm Hg (i.e., at depths of 1.3 to 5.3 msw [4.3 to 17.4 fsw]), the pressure imbalance may lead to rupture of the tympanic membrane.⁸² In such a case, the problem for the diver may be compounded by entry of cold water into the middle ear. This may cause severe vertigo.

Symptoms of middle ear squeeze include ear pain during descent, a sensation of fullness, and reduced hearing in the affected ear. There may be mild tinnitus or vertigo. With tympanic membrane rupture, the pain is relieved, but the cold caloric stimulation created by seawater entering the middle ear cavity causes severe vertigo with nausea, vomiting, and disorientation underwater. The vertigo may resolve after a few minutes or continue for hours after surfacing.

Otoscopic appearance of the tympanic membrane in cases of barotitis media varies with the severity of the injury. A commonly used grading scheme is the Teed classification, which grades severity according to the amount of hemorrhage in the tympanic membrane (Table 77-3).⁷⁴ Each higher grade tends to be more painful than the preceding one, except for grade 5, which may be relatively painless. With grade 5, the cessation of pain corresponds with the membrane tearing, which immediately equalizes the pressure in the middle ear with the external environment. Use of this grading scheme facilitates communication when describing these injuries.

TABLE 77-3 Teed Grading System for Middle Ear Barotrauma

Grade	Description
0	Symptoms without otologic findings
1	Erythema and mild retraction of the TM
2	Erythema of the TM with mild or spotty hemorrhage within the TM
3	Gross hemorrhage throughout the TM
4	Grade 3 changes plus gross hemorrhage within the middle ear (hemotympanum)
5	Free blood in middle ear plus perforation of the TM

TM, Tympanic membrane.

In addition to having an abnormal-appearing tympanic membrane, persons with barotitis media occasionally have a small amount of bloody drainage around the nose or mouth. Audiometry usually reveals conductive hearing loss.

Middle ear squeeze should be treated with decongestants and analgesics, although most cases clear spontaneously in 3 to 7 days without complication. Antihistamines may be used if the eustachian-tube dysfunction has an allergic component. Divers should abstain from diving until the condition has resolved. Combining an oral decongestant with a long-acting topical nasal spray (such as 0.5% oxymetazoline or phenylephrine) for the first few days is usually most effective. Repeated gentle autoinflation of the middle ear by use of the Frenzel maneuver also can help to displace any collection of middle ear fluid through the eustachian tube. Antibiotics (e.g., amoxicillin-clavulanic acid) should be used when there is a tympanic membrane rupture or preexisting infection. If the tympanic membrane has ruptured, no diving should be done until it is fully healed. The majority of tympanic membrane perforations from diving heal spontaneously without complications in 1 to 3 months. Surgical repair can be considered if healing has not occurred within 1 month.

Prevention is key for barotitis media. Training must emphasize the importance of early and correct pressure equalization techniques. The diver should start equalizing immediately on leaving the surface. Without equalization, the eustachian tube may become “locked” by 1.2 msw (4 fsw), as described earlier.

There are several maneuvers for equalizing pressure in the middle ear. The Valsalva maneuver involves blowing with an open glottis against closed lips and nostrils to increase pressure in the nasopharynx to inflate the middle ear through the eustachian tube. This may force open a collapsed eustachian tube. The Toynbee maneuver is performed by swallowing with a closed glottis while the lips are closed and the nostrils are pinched. The Frenzel maneuver is performed by pinching the nose, closing the glottis, and keeping the mouth closed while moving the jaw forward and down. This moves the pharyngeal muscles, which open the eustachian tube. This move does not increase intracranial pressure. Descending feet first underwater also makes it easier to clear the middle ear.

Divers who understand the pathophysiology of barotitis media generally take steps to inflate the middle ear immediately on submerging and thereby prevent the problem as they descend in the water. If middle ear pressure is kept equal to or greater than water pressure, no problem should occur. However, if the diver forgets to inflate the middle ear or suffers from eustachian tube dysfunction (caused by mucosal congestion secondary to upper respiratory infection, allergies, smoking, mucosal polyps, excessively vigorous autoinflation maneuvers, or previous maxillofacial trauma), middle ear barotrauma may occur. This most often happens just after the diver leaves the surface, with the diver complaining of ear fullness or pain. Generally, the pain rapidly becomes so severe that the diver either corrects the problem or aborts the dive.

Topical and oral decongestants are often used before diving to facilitate clearing the ears.¹⁵³ Pseudoephedrine has been reported to reduce the incidence and severity of middle ear barotrauma in novice divers.²⁷ The use of intranasal surfactant has been suggested to improve eustachian tube function to prevent middle ear barotrauma during repetitive diving.⁶⁹ Divers are sometimes taught not to use decongestants before diving because of theoretical concern of the medication wearing off while diving and causing problems during ascent; however, no data support this concern, and the judicious use of oral or nasal decongestants can facilitate pressure equalization.

Inner Ear Barotrauma

A serious but relatively unusual form of aural barotrauma is inner ear barotrauma, causing labyrinthine window rupture. This is the most serious form of aural barotrauma because of possible injury to the cochleovestibular system, which may lead to permanent deafness or vestibular dysfunction.^72,⁸¹

Inner ear barotrauma results from rapid development of markedly different pressures between the middle and inner ear, such as may occur from an overly forceful Valsalva maneuver or an exceptionally rapid descent, during which middle ear pressure is not adequately equalized. During descent, the tympanic membrane is pressed inward, pushing the stapes against the oval window. The perilymph and endolymph are not compressible; the resulting increased pressure causes the round window to bulge outward. The diver may attempt a forceful Valsalva maneuver to equalize the middle ear; this will raise intracranial pressure, which is propagated through the perilymphatic duct to the inner ear, causing the round or oval window to rupture. Rupture of either window can also occur by a sudden increase in middle ear pressure by a forceful Valsalva. This pressure disequilibrium may cause several types of injury to the cochleovestibular apparatus, including hemorrhage within the inner ear; rupture of Reissner’s membrane, leading to mixing of endolymph and perilymph; fistulation of the oval or round window, with development of a perilymph leak; or a mixed injury involving any or all of these conditions.¹⁵¹ During ascent, the expanding middle ear gas may be forced through the perilymph fistula and enter either into the scala tympani or scala vestibule, which may damage cochlear or vestibular structures, leading to a permanent hearing loss.

The classic triad of symptoms indicating inner ear barotrauma is roaring tinnitus, vertigo, and hearing loss. In addition, a feeling of fullness or “blockage” of the affected ear, nausea, vomiting, nystagmus, pallor, diaphoresis, disorientation, or ataxia may be present in varying degrees. Symptoms of inner ear barotrauma may develop immediately after the injury or may be delayed for hours, depending on the specific damage and the diver’s activities during and after the dive. Vigorous isometric exercise after a dive may complete an incipient or partial membrane rupture. Findings on physical examination may be normal or may reveal signs of middle ear barotrauma or vestibular dysfunction. Audiometry may demonstrate mild to severe high-frequency sensorineural hearing loss or a severe loss of all frequencies.

Symptoms usually improve with time. Tinnitus tends to decline over time, and vestibular injury is centrally compensated. The diver may be left with a residual high-pitch tone and high-frequency hearing loss.

Persons with inner ear barotrauma should be treated with bed rest (with the head elevated to 30 degrees), avoidance of any strenuous activity or any straining that can lead to increased intracranial pressure, and symptomatic measures as needed. There is a good prognosis for full recovery of hearing in 3 to 12 weeks. Labyrinthine window fistulas usually heal spontaneously, and data support conservative treatment initially.¹⁵¹ Deterioration of hearing, worsening of vestibular symptoms, or persistence of significant vestibular symptoms after a few days heralds the need for detailed otolaryngologic evaluation and possible surgical exploration and fistula closure. No consensus exists as to how long to wait before surgical intervention. If a perilymph fistula is suspected, it has been recommended to explore the ear surgically as soon as possible or if symptomatic after 24 hours.²¹ Patients with a tear in Reissner’s membrane have manifestations similar to those with inner ear hemorrhage, although there will be persistent localized sensorineural hearing loss commensurate with the area of membrane tear. Management is the same.

Prevention of this condition is aimed at avoiding sudden, dramatic increases in middle ear pressure. Special emphasis during diver training and education should be placed on gentle pressure equalization. Upper respiratory tract infections and allergies reduce eustachian tube function and may be a precursor to inner ear barotrauma.

A diagnostic dilemma exists whenever a diver complains of vertigo, tinnitus, and hearing loss after diving. These symptoms are classic for labyrinthine rupture, in which case recompression is contraindicated because of the potential for further barotrauma to worsen the injury. Conversely, these symptoms may indicate a diagnosis of inner ear decompression sickness, which requires expeditious treatment in a hyperbaric chamber. In such cases, the most important differential feature for diagnostic use on a dive boat or other diving site is a careful history as to time of onset and dive activities preceding the onset of symptoms. If symptom onset was during descent, and ear clearing was difficult or impossible, requiring forcible Valsalva maneuvers, perilymph fistula is more likely. If the onset was during or after a decompression dive, decompression sickness must be assumed and chamber treatment sought. In some cases, however, it simply is not possible to rule out decompression sickness, or rarely, air embolism, and a “trial of pressure” in the recompression chamber may be necessary.

Suit Squeeze

If an area of the diver’s skin becomes trapped under a fold or wrinkle of a drysuit, causing a closed airspace, the pressure-induced contraction of air under the fold, and the resulting partial vacuum, can cause transudation of blood through the skin. This is unusual and generally benign, although the resultant skin ecchymosis may have a dramatic appearance. Suit squeeze requires no treatment and resolves in a few days to a couple of weeks.

Dental Barotrauma

Dental barotrauma or barodontalgia (tooth squeeze) is one of the most infrequent yet dramatic types of barotrauma. This painful condition, sometimes called aerodontalgia, is caused by entrapped gas in the interior of a tooth or in the structures surrounding a tooth. The confined gas develops either positive or negative pressure relative to ambient pressure, which exerts force on the surrounding sensitive dental structures and causes pain.

Barodontalgia may be caused by an array of dental conditions, including caries, defective restorations, oral tissue lacerations, recent extractions, periodontal abscesses, pulpal or apical lesions or cysts, and endodontal (root canal) therapy.¹⁵⁷ If a pocket of trapped air remains at sea level pressure while ambient pressure increases during descent, the tooth can implode or the cavity fill with blood. Conversely, air that is forced into a tooth during descent can expand during ascent, causing the tooth to explode. To prevent barodontalgia, a diver probably should wait at least 24 hours after dental treatment (including fillings) before diving.

Other causes of tooth pain associated with pressure changes are less well understood. Pulpitis or other dental infections can produce pain during a dive. Upper tooth pain associated with pressure changes should raise a question about a pathologic condition in the maxillary sinus.

Lung Squeeze

Lung squeeze is a very unusual form of barotrauma that has been observed with breath-hold diving. Persons having this syndrome complain of shortness of breath and dyspnea after surfacing from a deep (greater than 30.5 msw [100 fsw]) breath-hold dive. The diver may cough up frothy blood, and a chest radiograph may show pulmonary edema. The condition is treated with supplemental oxygen and respiratory support as needed. Symptoms typically resolve within a few days.

The classic understanding of lung squeeze is that it occurs when a diver descends to a depth at which total lung volume (TLV) is reduced to less than residual volume (RV). At this point transpulmonic pressure exceeds intra-alveolar pressure, causing transudation of fluid or frank blood (from rupture of pulmonary capillaries) and the overt manifestations of pulmonary edema and hypoxemia.

According to this scenario, a breath-hold diver with a TLV of 6000 mL and an RV of 1200 mL could dive to only 6000/1200 or 5 ATA (equal to 40.2 msw [132 fsw]) before lung squeeze would occur. However, breath-hold divers have dived much deeper without apparent problem.

In 1968, Schaefer and associates reported that breath-hold divers pool their blood centrally, accumulating a central volume increase of as much as 1047 mL at 27.4 msw (90 fsw).¹⁶⁸ If it is assumed that this adjustment in pulmonary blood volume reduces the RV, then theoretically, it should be possible for the diver with a TLV of 6000 mL to breath-hold to 6000/(1200 − 1047 = 153), or almost 40 ATA. Although deep breath-hold dives seem to support the beneficial effect of central pooling of blood, cases of lung squeeze continue to occur at much shallower depths. The exact pathophysiology of this condition remains unclear. Fortunately, it occurs very infrequently.

Underwater Blast Injury

Barotrauma can be caused by underwater explosions. Shock waves from a blast are propagated farther in the dense medium of water than in air.⁴¹ Underwater explosions may result from ordnance or ignition of explosive gases during cutting or welding operations.

Underwater blasts can cause serious injuries to divers. Air-containing body cavities such as the lungs, intestines, ears, and sinuses are most vulnerable. Pneumothorax, pneumomediastinum, and air embolism may result from laceration of the lungs and pleura.¹⁰² There may be intestinal perforation, subserosals hemorrhage, and subsequent peritonitis. The occurrence of blast-related air embolism at depth, which worsens with ascent to the surface, requires treatment by recompression, if possible. Otherwise, management of underwater blast injuries is the same as for terrestrial blast injury.

Barotrauma of Ascent

Reverse Sinus or Ear Barotrauma (Reverse Squeeze)

The sinuses and ears are subject to barotrauma during ascent as well as descent. As the ambient pressure drops, the volume of the gas within the sinus or ear cavities expands, causing pain and tissue damage if not released. Gas pressure can exceed intravascular pressure in adjacent tissue, causing local ischemia. In the sinuses, a cyst or polyp can act as a one-way valve; air enters the sinus as the diver descends but cannot escape as the diver ascends. Fainting underwater as a result of frontal sinus pain has been reported.⁸³

Pain can develop in the ear during ascent because of expanding gas pressure in the middle ear that is not released through the eustachian tube. If a diver makes frequent descents and ascents during a dive or multiple short dives back-to-back (“bounce diving”), the mucosal lining of the eustachian tube often becomes inflamed, making equalization of pressure in the ears difficult. The eustachian tube may not allow air to escape fast enough, and the pressure rises in the middle ear during ascent. This results in pain, often tinnitus and vertigo, and may lead to an outward rupture of the tympanic membrane.

Alternobaric Vertigo

An unusual type of aural barotrauma is alternobaric vertigo (ABV). This usually occurs with ascent and is caused by sudden development of unequal middle ear pressure, which causes asymmetric vestibular stimulation and resultant pronounced vertigo.¹³⁰ Vertigo, nausea, and vomiting may occur as the diver ascends. Although usually only transient and requiring no treatment, ABV may precipitate a panic response, leading to near drowning, pulmonary barotrauma, and resultant air embolism; however, the incidence of diving injuries due to ABV is unknown. Rarely, alternobaric vertigo lasts for several hours or days, in which case it should be treated symptomatically after excluding inner ear barotrauma. Diving should be avoided when middle ear equalization is compromised.