Hyperbaric Medical Considerations for Occupational Exposure to Compressed Gas Environments

Anatomy and Physiology

Decompression sickness occurs when rapid decompression forms inert gas bubbles in the venous circulation and extravascular spaces. These bubbles can also enter arterial circulation, resulting in arterial gas embolism. Decompression sickness manifests with musculoskeletal symptoms such as arthralgias and myalgias, along with neurological manifestations such as paresthesias and numbness. Serious effects such as ataxia, visual changes, altered mental status, speech difficulty, and paralysis are rare. Other symptoms include audiovestibular disturbances, cutaneous manifestations, and occasionally, cardiopulmonary distress or gastrointestinal problems. The presence of a patent foramen ovale may contribute to transient visual and cognitive symptoms due to venous bubbles crossing into the arterial cerebral circulation.[3]

Indications

The increasing demand for underground passages, driven by urbanization and infrastructure requirements, highlights the vital role of compressed air workers in modern construction projects. Despite advancements such as tunnel-boring machines, compressed air workers remain essential for tasks requiring work under extreme atmospheric pressure.[4] These workers play a crucial role in ensuring the successful excavation and maintenance of underground passages, vital for facilitating transportation, utilities, and other essential services in densely populated areas where surface space is limited.

Compressed air workers play a critical role in tunnel-boring machine operations, particularly in accessing and maintaining the face of the tunnel-boring machine, where excavation occurs under increased atmospheric pressure. To access the face, compressed air workers undergo compression into hyperbaric chambers before entering. Once inside, the face is filled with bentonite to provide stability and prevent the escape of compressed air. This process, known as hyperbaric intervention, involves maintaining stable air pressure, conducting necessary tasks under pressure, and safely decompressing workers afterward. Hyperbaric interventions are essential for routine maintenance and inspections and responding to emergencies such as toxic gases or equipment malfunctions.[5]

Saturation and bounce diving are specialized techniques used in underwater operations performed by compressed air workers, each with its indications. Saturation diving involves prolonged stays at depth, allowing divers to work efficiently for extended periods without requiring repetitive compression and decompression cycles. Typically employed for deep-sea exploration, offshore construction, and oil rig maintenance, saturation diving is particularly useful when working at depths greater than 100 meters of sea water (msw), typically ranging from 100 to 300 msw, where traditional diving methods become impractical due to the long decompression times.[6][7] On the other hand, bounce diving, characterized by rapid descents and ascents, is suitable for short-duration tasks requiring frequent dives and resurfacing. Because compressed air workers can access the tunnel face quickly and perform tasks efficiently, it is commonly used in tunnel construction. The choice between saturation and bounce diving depends on various factors, including the depth of the operation, the duration of tasks, and the available resources.

Contraindications

The physical and mental contraindications for working in compressed air environments align closely with commercial diving. To ensure safe compressed air work practices, a comprehensive evaluation of medical history, medication use, and psychological readiness must be conducted. Any medications or other treatment regimens the worker may be on must also be elucidated. Understanding medications, underlying medical conditions, and their interaction with the hyperbaric environment is crucial. Any significant symptoms or adverse effects from medications under normal conditions may warrant rejection.

Medical evaluations for compressed air workers should be conducted by undersea and hyperbaric medicine physicians, preferably certified by organizations such as the Association of Diving Contractors International (ADCI) and the Dive Medical Advisory Committee (DMAC), having completed a course called Medical Examiner of Divers. These experts follow established protocols and guidelines to ensure the safety and health of compressed air workers, with comprehensive lists of contraindications available on their respective websites.

Compressed air work conditions can vary widely, posing challenges such as environmental changes, technical malfunctions, or emergencies. Before commencing work, each compressed air worker must assess their physical readiness, mental preparedness, and skill level. In addition to obtaining medical clearance, each individual should address the following critical questions undertaking any task:

• Is my physical condition sufficient to cope with the required strain?
• Am I mentally prepared to effectively manage the demanding situation?
• Do I possess the necessary skills and proficiency required for this specific activity?

Thus, each compressed air worker requires a tailored assessment consisting of more than medical conditions and medications to discover any possible contraindications that they may have to compressed air work.[8] 

Equipment

Modern tunnel-boring machines are employed in a range of diameters and depths, necessitating the involvement of compressed air workers to oversee their efficient functioning. The selection of equipment depends on several factors, including geological composition (rock or sand), face depth, groundwater levels, and the likelihood of encountering hazardous substances such as methane gas. Although compressed air workers no longer engage in manual tunnel excavation, their responsibilities encompass inspecting and replacing worn-out or ineffective tools, such as rippers and cutters, at the tunnel-boring machine's face to ensure uninterrupted progress and operational safety.

Hyperbaric interventions, often conducted at depths conducive to repetitive bounce diving, present a minimal risk when overseen by commercial diving companies, particularly for depths exceeding 2.5 bar (25 msw). A dedicated medical lock hyperbaric chamber near the excavation site ensures prompt medical treatment for compressed air workers experiencing decompression sickness. Before participation in hyperbaric interventions, compressed air workers undergo a thorough checkout dive to assess their physiological and psychological responses to heightened atmospheric pressure, thus confirming their readiness for such activities.

In saturation diving, compressed air workers reside in a hyperbaric chamber known as the habitat on the surface, which serves as their living quarters when they are not working. Compressed air worker teams are simultaneously compressed into a saturation chamber. This chamber is utilized as their base during their work. To travel to and from the work site, which is usually located at the tunnel-boring machine face underwater, they use a specialized vehicle called a hyperbaric shuttle or transfer capsule. Once they arrive at the tunnel-boring machine, the shuttle is connected to its hyperbaric chamber through a universal collar, allowing workers to pass through the man-lock into its face. Shifts typically last 6 to 8 hours, with workers returning to the habitat through the shuttle for replacement by the next team. This saturation method involves 1 compression at the start and 1 decompression at the end, with various gas mixtures used to mitigate the risk of breathing gas toxicity.[5][9]

Personnel

Compressed air workers encompass individuals in commercial diving and tunnel construction industries; however, notable differences exist in their training, education, and medical fitness prerequisites for working in compressed air environments. Although both groups face the challenges associated with increased atmospheric pressure, commercial divers adhere to strict medical health standards set by organizations such as the Association of Diving Contractors International and undergo specialized training in diving medicine. In contrast, compressed air workers in tunnel construction often receive training specific to their roles, focusing on the unique demands of working with tunnel-boring machines and hyperbaric interventions, with medical evaluations tailored to these requirements.

Commercial dive companies play a vital role in ensuring the safety and well-being of compressed air workers by providing specialized hyperbaric chamber operators and essential support equipment for hyperbaric interventions. These teams collaborate closely with onsite hyperbaric tunnel medicine teams to deliver preintervention education, verify compression and decompression schedules, conduct postintervention examinations, and administer emergency medical care when necessary. Onsite medical teams, including physicians, must be ready to enter hyperbaric chambers to provide immediate care for compressed air workers, particularly in cases of traumatic injuries. Adherence to regulations established by organizations such as the National Institute for Occupational Safety and Health and OSHA is essential to prioritize employee health and safety. However, updates to federal regulations and state-specific applications may be necessary for each project's compliance.[10] 

Preparation

Before any hyperbaric intervention, workers receive comprehensive orientation covering site-specific tasks, potential hazards, compression and decompression procedures, and possible adverse events related to working under pressure. This orientation, conducted in a semiformal, classroom-style setting, precedes physical examinations and health screenings conducted by the hyperbaric physician and tunnel or dive medical team. Emphasis is placed on strict adherence to the intervention plan, as deviations can have legal and health-related consequences.

Notably, hyperbaric interventions in the United States often require state-approved variances due to outdated OSHA regulations concerning compressed air occupational health and safety and compression and decompression protocols. These regulations require revision to align with modern practices. Despite the efficacy of oxygen decompression schedules for compressed air workers since the 1960s, modern air or oxygen decompression schedules are not included in OSHA's outdated policies. Dr Eric Kindwall has previously highlighted this issue; however, OSHA has yet to implement updated oxygen decompression schedules.[10]

Technique or Treatment

The technique known as bounce diving is commonly employed in standard commercial wet diving and dry diving within the tunneling industry when hyperbaric interventions are necessary. During bounce diving, multiple teams of compressed air workers work consecutively over 24 hours. Bounce diving involves rapid compression to the working depth, a short working period, and extended decompression back to the surface. Compressed air workers in dry hyperbaric conditions are compressed into the face of the tunnel-boring machines using hyperbaric chambers integrated into the front during construction. The duration of their work at the tunnel-boring machine's face corresponds to factors such as depth, time, and the decompression tables utilized. In wet conditions, oxygen is commonly used during decompression, especially beyond a certain depth, to reduce the risk of decompression sickness.[11] The pressure required for hyperbaric interventions is predetermined and adapted according to site conditions. A diving plan is formulated, outlining compression rates, working durations, and the utilization of diverse decompression tables and gas mixtures, with oxygen often favored for its safety advantages.[10] After completing their shift, compressed air workers undergo decompression in the hyperbaric chamber under the observation of skilled hyperbaric chamber operators and onsite medical teams.

On the other hand, Although more expensive, saturation diving presents potential advantages such as enhanced work efficiency and decreased susceptibility to decompression sickness by minimizing the necessity for daily compressions and decompressions. Saturation dives entail prolonged stays at the working depth, lasting up to 28 days, and require only 1 compression and decompression cycle at the beginning and end of the project. Helium is frequently incorporated into breathing gases to mitigate the risks of oxygen toxicity and nitrogen narcosis, thereby safeguarding the health and welfare of compressed air workers during hyperbaric interventions.[11][12]

Construction companies sometimes employ dive medical technologists to fulfill various roles, including chamber operation and onsite medical assistance. However, relying solely on dive medical technologists to simultaneously operate the chamber and attend to medical emergencies is considered unsafe. Instead, it is recommended to have a dedicated onsite medical team comprising emergency medical technologists with hyperbaric or dive medic certification, emergency trained nurses with hyperbaric certification, and an experienced dive medicine physician. These professionals ensure the proper delivery of health care to the compressed air worker teams, although at a higher cost.

Complications

There is a lack of specific studies on the epidemiology, incidence, and prevalence of adverse effects experienced by compressed air workers. Nevertheless, these adverse effects can be diverse and multifaceted, stemming from the construction work and the atmospheric conditions where compressed air workers operate. Environmental concerns include air quality, potential contaminants, and the effects of breathing compressed air and mixed gases. Construction-related injuries range from minor cuts and abrasions to severe traumas such as amputations, burns, ergonomic injuries, and multiple traumas from falls.[13][14] Compressed air workers may inhale air containing contaminants such as methane, other gases, and dust, leading to acute and chronic respiratory issues.[15][16][17] Adverse effects can be further classified into mechanical, physiological, and pharmacological categories, occurring during compression and decompression in the hyperbaric chamber. In this section, the adverse effects of the compressed air environment and the act of breathing compressed air are discussed.

Common Mechanical Adverse Effects of Compression and Decompression

The mechanical effects stemming from Boyle's law primarily impact the air-filled spaces within the body, including the eustachian tube, middle ear space, sinuses, respiratory tree, lungs, and gastrointestinal tract. As per Boyle's law, increased pressure decreases the volume and vice versa, affecting these air-filled cavities. However, hyperbaric pressure does not directly influence body tissues, plasma, blood, or other substances. Nonetheless, it may indirectly affect iatrogenically created air spaces, such as those surrounding dental work or resulting from a tooth abscess. Surgical procedures such as cataract extraction or intraocular injections may also introduce air into the eye globe, subjecting it to similar mechanical effects.

During descent or compression in a hyperbaric chamber, eustachian tube dysfunction and middle ear barotrauma are common adverse effects due to increased pressure in the external ear canal and subsequent negative pressure in the middle ear space when ventilation through the eustachian tube is impaired. Although these occurrences are unpredictable during dry hyperbaric compression, preventive measures such as medication use, adjustments to compression rates, or devices such as the modified Politzer device, which delivers high-pressure air into the nares while swallowing, have been shown to reduce their incidence.

Ear pain, the most common symptom, can often be alleviated by halting compression and allowing for equalization or ascending slightly in the chamber. Although less common, sinus barotrauma may also occur, presenting as sinus discomfort, facial pain, or nasal bleeding in less than 1% of divers.[18][19][20] However, it does not seem to be a significant concern in the compressed air industry and is notably absent in the tunneling literature. Air spaces left after dental work are also subject to gas compression and expansion and can cause significant tooth or mouth discomfort. On ascent or decompression, pulmonary barotrauma can result from breath-holding or trapped gas expanding during decompression, potentially leading to anatomical disruptions and, in severe cases, air gas embolism.[21][22]

Physiological Adverse Effects

Nitrogen narcosis typically manifests at depths exceeding 4 atmospheres, about 40 msw, and resembles alcohol intoxication, impairing judgment and slowing response times, thus hindering compressed air workers' performance.[23] When utilizing mixed gases, caution is warranted, particularly with helium, which is associated with high-pressure nervous syndrome and commonly observed in wet saturation diving at depths beyond typical tunneling depths.[24][25] However, despite its challenges, saturation diving is increasingly recognized as a viable alternative to bounce diving, especially in deeper tunnels.[5][9] As saturation diving becomes more prevalent, proactive management of potential adverse events by compressed air medical teams and life support technologists becomes essential. Such management includes addressing inert gas counterdiffusion, which can lead to decompression sickness despite minimal depth or ambient pressure changes. Decompression sickness is managed following standard protocols in such scenarios.

Decompression using oxygen has notably reduced the incidence of decompression sickness and shortened overall decompression time, enhancing safety and efficiency in compressed air work.[10][26] However, using oxygen at elevated pressures carries the risk of oxygen toxicity, particularly in mixed gas diving and at extreme depths, necessitating careful management.[27][28] In addition, dust inhalation during construction activities, including hyperbaric tunneling interventions, can result in various clinical issues ranging from minor irritations to serious respiratory conditions.[29][30] Chronic exposure to dust leads to conditions such as asthma, pulmonary fibrosis, and decreased pulmonary function over time, highlighting the importance of proper respiratory protection and hazard mitigation measures for compressed air workers.[29]

Dysbaric osteonecrosis, a form of avascular bone necrosis, primarily affects undersea divers and workers exposed to compressed air or gas and is commonly considered a long-term manifestation of subclinical decompression sickness.[31] This condition commonly affects the hip joint, specifically the proximal femur, leading to an elevated risk of fractures and joint replacement. Incidence varies, with Japanese studies reporting rates as high as 50% among commercial divers, contrasting with a 2.5% incidence in United States navy divers.[32] Increased exposure, duration, and frequency to compressed gases heighten the risk of developing dysbaric osteonecrosis, which is more prevalent among males aged 30 to 50. Multifocal disease is common, underscoring the need for comprehensive screening in at-risk populations.

Clinical Significance

Continuous medical oversight is essential for compressed air workers to ensure their fitness for duty; monitor chronic conditions such as dysbaric osteonecrosis, barotrauma, and silicosis; and mitigate environmental and gas-related effects, including risks associated with nitrogen narcosis, oxygen toxicity, and high-pressure nervous syndrome. Regular medical evaluations and surveillance play a crucial role in maintaining the health and safety of these workers in their challenging operating environments. However, the health care provided to compressed air workers lacks consistency and standardization, with occupational medicine physicians, undersea and hyperbaric medicine physicians, and primary care practitioners typically conducting health evaluations.

Current OSHA guidelines require physicians caring for compressed air workers to have experience with individuals exposed to extreme atmospheric pressure. This requirement highlights the need for significant revision to ensure proper training, qualifications, and experience comparable to any other medical specialty.[10] Undersea and hyperbaric medicine physicians undergo rigorous 1-year fellowship training to attain board certification, enabling them to provide comprehensive care for compressed air workers at the expected standard of any medical specialty. Although some occupational medicine physicians and primary care providers may have relevant experience, particularly in regions with commercial diving activities, maintaining accurate health records is crucial, considering compressed air workers often relocate.

Access to these records ensures responsible and consistent care during periodic health evaluations and follow-ups, especially for organ systems impacting their careers. The days of only monitoring health issues such as myopia, decompression sickness, and silica exposure in caisson workers have passed.[17][33][34][35] However, despite the pressing need for new clinical trials to assess risks to compressed air workers, ongoing studies are scarce, relying mainly on limited communications and self-reporting from divers.[36] Substantial policy changes are warranted to address immediate health concerns and long-term healthcare needs for compressed air workers.

Enhancing Healthcare Team Outcomes

Effective management of hyperbaric medical considerations for occupational exposure to compressed gas environments requires a collaborative approach among various healthcare professionals to ensure patient-centered care, optimize outcomes, enhance patient safety, and improve team performance. Physicians, advanced practitioners, nurses, pharmacists, and other healthcare professionals play critical roles in this interprofessional team. Physicians and advanced practitioners are responsible for conducting comprehensive assessments, diagnosing conditions related to hyperbaric exposure, and developing individualized treatment plans. Nurses are crucial for patient monitoring, administering medications, and educating patients on preventive measures and self-care strategies. Pharmacists ensure the safe and appropriate use of drugs, including those used to manage symptoms and prevent complications associated with hyperbaric exposure.

Interprofessional communication is vital for effective care coordination and facilitates the exchange of essential information, such as patient assessments, treatment plans, and intervention responses. Regular team meetings enable healthcare professionals to share insights, discuss patient progress, and address concerns collaboratively. In addition, ongoing education and training programs ensure that healthcare professionals stay updated on the latest evidence-based practices and guidelines in hyperbaric medicine. By leveraging their respective skills, expertise, and knowledge, healthcare professionals can enhance patient-centered care, optimize outcomes, promote patient safety, and improve team performance.