Management of Decompression Illness

Decompression illness (DCI) occurs due to absorbed extracorporal gas resulting from a decrease in environmental pressure during decompression. This term includes both arterial gas embolism (AGE), where alveolar or venous gas emboli enter the arterial circulation, and decompression sickness (DCS), which may result from in-situ bubble formation due to dissolved inert gas. AGE commonly presents with stroke-like symptoms affecting the brain, while DCS can impact various organs including the brain, spinal cord, inner ear, musculoskeletal tissue, cardiopulmonary system, and skin.

DCI symptoms vary widely in nature and severity, ranging from itching and minor pain to severe neurological symptoms, cardiac collapse, and death. Symptoms are mostly non-specific; therefore, the diagnosis of DCI is based on careful evaluation of the circumstances of the dive, known risk factors, and the post-dive latency and nature of the manifestations.

Treatment of choice is the on-site administration of 100% oxygen as soon as possible after onset of symptoms. First aid also includes supportive general care, immobilizing the injured patient, and giving isotonic fluids. Adjunct pharmacotherapy is usually not recommended; NSAIDs (Non steroidal anti-inflammatory drugs) can be used if there are no contraindications. Hyperbaric oxygen therapy is the primary treatment for decompression illness due to its ability to reduce bubble volume, enhance tissue oxygenation, and ameliorate inflammatory responses that contribute to tissue injury. Most DCS cases respond well to a single hyperbaric session, but repetitive treatments may be needed based on initial response. Delayed intervention is associated with increased morbidity and residual sequelae. Adhering to conservative dive profiles and screening for medical risk factors can reduce risk of decompression illness. 
This clinical review summarizes current knowledge on the pathophysiology, clinical presentation, diagnostic approach, treatment, and prevention of dive-related DCI.

Key Words: Arterial Gas Embolism, AGE, DCS, Diving, Treatment, Pathophysiology, Diagnostic, Prevention

Underwater sports activities necessitate that individuals either dive while holding their breath or use breathing apparatus to sustain prolonged exposure at significant depths. Breathing underwater is enabled by utilizing a self-contained underwater breathing apparatus (SCUBA) or supply systems that operate from surface or submerged habitats. These systems deliver the breathing gas to the diver‘s helmet or mask via pressure hoses. SCUBA diving is, by far, the most common underwater diving activity with an estimated 3.5 million European divers out of a global total of 6 to 9 million (4).

Divers face a variety of physiological challenges imposed by immersion in water and physical effects elicited on gases by increased ambient pressure. Although generally considered a safe sport, decompression illness (DCI) is a common and potentially life-threatening complication of SCUBA diving with an estimated incidence of 1 in 10,000 dives (5, 21). DCI is an umbrella term for injuries caused by pathological effects of excess gas in blood or tissue during or after a reduction in ambient pressure. It encompasses two distinct conditions: decompression sickness (DCS) and arterial gas embolism (AGE). DCS results from gas formation in tissues and/or venous blood due to dissolved nitrogen (N₂) absorbed during a dive. AGE occurs when gas bubbles enter the arterial circulation, leading to embolic occlusion of vessels, either through pulmonary barotrauma with air leakage into the pulmonary vasculature or paradoxical embolism via shunting of venous gas bubbles (figure 1). Because clinical presentations often overlap and differentiation can be difficult, and since treatment protocols are identical, the collective term DCI is used. However, few physicians are trained to recognize or manage DCI, especially outside of specialized diving medicine centers, and permanent injury may result if it is unrecognized or inadequately treated.

This review provides an overview of the underlying pathophysiology and clinical presentation of DCI. Recommendations are given on current state of the art management algorithms, with an emphasis on the pre-hospital setting, including strategies for prevention of DCI.

Decompression Sickness
Decompression sickness refers to signs and symptoms caused by the generation of intracorporeal inert gas bubbles. Henry’s law states a proportional relationship between the solubility of a gas in a liquid and the partial pressure of that gas above the liquid. When breathing air from SCUBA at increased pressure, oxygen is metabolized, whereas nitrogen, being chemically inert, dissolves in blood and body tissues. Since the pressure of a mixed gas is always the sum of the partial pressures of its components, nitrogen partial pressure (pN2) will rise with ambient air pressure. N2 will eventually dissolve in blood and tissues until equilibrium with the gas phase is achieved, i.e., tissue N2 saturation. The saturation kinetics follow a first-order exponential function where N2 uptake in the blood and other tissues is mainly dependent on the magnitude of pressure elevation, time, temperature, and tissue characteristics (e.g., perfusion, solubility). During ascent from depth tissue pN2 may exceed the ambient pressure, resulting in tissue N2 supersaturation. This will enable the formation of a gas phase and N2 gas microbubbles may grow from preformed gas nuclei in tissues and venous blood (1). These bubbles usually return to the central veins from the periphery and are carried to the pulmonary circulation, where they are filtered in the pulmonary capillary bed of the lung (3). While the presence of venous gas emboli poorly predicts DCS, inert gas emboli may exert pathological effects through mechanical tissue compression or embolization of venous blood vessels. Direct obstruction of blood vessels or local tissue compression by expanding bubble volume causes tissue ischemia and edema. Subsequently, interactions between the blood-gas interface and the endothelium may result in further tissue damage. A variety of secondary cellular and humoral changes include activation of complement, platelets and neutrophils (14). These secondary effects may promote and maintain inflammatory cascades that eventually lead to damage of the capillary endothelium with capillary leakage, fluid loss from the intravascular space, and hemoconcentration.

Arterial Gas Embolism
Arterial gas embolism is caused by the entry of gas into the pulmonary veins or directly into the arteries of the systemic circulation. When breathing from SCUBA at depth, lungs are filled with compressed air. During ascent from depth, ambient pressure decreases, and the volume of an enclosed gas will increase in accordance with Boyle’s law. Thus, expanding intrapulmonary gas during decompression needs to be continuously exhaled. When ambient pressure decreases more rapidly than expanding intrapulmonary gas can escape through the airways, e.g., by airflow obstruction through voluntary breath holding and/or pulmonary pathology, overdistension of lung tissue and consecutive rupture may occur (20). Expanding air may escape along the perivascular bed of the pulmonary arterioles and enter such an arteriole or escape during exhalation towards the pulmonary hilus and enter the anatomically thin-walled pulmonary veins. In consequence air may rapidly enter the systemic circulation and cause AGE. Transpulmonary pressures of 95-110 cmH2O have been demonstrated sufficient to disrupt pulmonary parenchyma and force gas into the lung interstitial space. The entry of gas into the left atrium allows for distribution of gas bubbles into nearly all organs. However, diver’s upright position during ascent and buoyancy of the gas bubbles may precipitate cerebral arterial gas embolism, which usually is associated with severe morbidity or death. The air emboli cause pathologic changes through a reduction in perfusion distal to the embolic obstruction and a consecutive inflammatory response elicited by the endothelial – bubble surface interaction.

Paradoxical gas embolism may occur when gas that has entered the venous side of the circulation passes to the systemic arterial circulation. Routes for the trans-pulmonary passage of venous gas bubbles include the pulmonary capillaries and intra- or extrapulmonary right-to-left shunts. The pulmonary circulation is an effective filter for gas bubbles although gas bubbles of less than 14-22 γm may pass through the lungs directly; larger bubbles spill over when a certain overload threshold value has been reached. Extra-pulmonary right-to-left shunts such as patent foramen ovale, have been found to enable gas bubble passage after SCUBA dives and have been demonstrated to be related to DCI in divers (22).

The clinical manifestation of DCI can impact various organ systems, ranging from mild symptoms like fatigue to severe neurological injury and cardiopulmonary arrest. Presentation may be monosymptomatic or indicate single organ involvement (e.g., skin rash, musculoskeletal pain, vestibular symptoms). Most DCI symptoms are non-specific and, thus, create potential for misdiagnosis (6, 15).

Traditionally, DCS was classified into a mild Type I category (joint or muscle pain, swelling, skin itching or rash) and a more severe Type II category with neurological or pulmonary symptoms, to guide recompression treatment protocols. More recently, categorization into mild vs. severe DCI symptoms has evolved to guide diagnosis and treatment (13). Mild symptoms often result from mechanical irritation of tendon sheaths or joint capsules by N₂ gas bubbles within poorly perfused tissues. Gas bubbles may also migrate to or grow within bone marrow, increasing intramedullary pressure, as suggested by the rapid resolution of limb/joint or muscle pain („the bends“) with recompression. DCS can also present with pruritus and rash due to endothelial damage and blood extravasation from cutaneous vessels (9). Livedo racemosa strongly supports a DCS diagnosis, though skin findings are not always present. Severe symptoms are usually neurological, reflecting the N₂ kinetics of nervous tissues and their short ischemia tolerance. Neurological injury may result from venous gas bubbles causing venous stasis and spinal cord ischemia. Symptoms typically progress from mild paresthesia to regional numbness, weakness, and eventually paresis (14, 23). Respiratory symptoms (tachypnea, cough, cyanosis, thoracic pain) may indicate heavy pulmonary gas embolism. Vestibular symptoms may arise from bubble formation in the labyrinthine artery or membranous labyrinth (inner ear DCS). Symptoms of DCS generally appear within three hours after diving in most instances, although they can also occur shortly after the dive in some cases (23). Also, mild symptoms can progress over time to more severe symptoms, or onset of severe (neurological) symptoms may be delayed even when mild symptoms do not progress.

AGE predominantly presents with neurological symptoms, typically appearing during ascent or within 5 minutes after surfacing (12, 20). However, onset can be delayed, as in DCS. Symptoms often suggest cerebral involvement, with multifocal deficits. While a stroke-like syndrome with unilateral symptoms is classically described, loss of consciousness is the most common initial manifestation, followed by confusion, dizziness, presyncope, hemiplegia, visual disturbances, headache, dysphasia, and seizures (12). Due to pulmonary barotrauma, neurological symptoms may be accompanied by chest pain, hemoptysis, and dyspnea.

AGE and DCS can be clinically indistinguishable due to overlapping presentation and timing. Both conditions may occur simultaneously. Initial spontaneous symptom improvement may occur due to partial clearance of emboli, but relapses with worsening severity are common. Table 1 provides most frequently reported symptoms of DCI according to organ system.

As outlined above, DCI encompasses diseases differing in pathophysiology but unified by pathology from excess intracorporeal gas. Clinical signs and symptoms often overlap, complicating diagnosis. Experimental animal models and human experience show that identical treatment algorithms apply to DCI, regardless of gas entry mechanism. Therefore, treatment is based on the diagnosis of dive-related decompression illness, without requiring immediate differentiation of DCS from AGE. As treatment delays worsen outcomes (14, 21), time-consuming diagnostic procedures should be deferred after therapy initiation.

DCI diagnosis relies on thorough clinical examination and detailed medical history, including dive profiles and the temporal relationship between decompression and symptom onset. Differential diagnoses, including cardiovascular and cerebrovascular events, must be considered (table 2). Detailed neurological examination is essential for all divers with suspected DCI to assess initial neurological status and set a baseline for follow-up investigations. Lymphedema and rash on the trunk can be seminal findings in support of DCS. Laboratory evaluation should only be applied initially if readily available as there are no blood biomarkers supporting diagnosis of DCI. It may be useful to assess hemoconcentration and dehydration, and an elevated serum creatine kinase has been shown to be related to size and severity of arterial gas embolism (19). A chest x-ray may be helpful in evaluating the presence of a pneumothorax, which should be drained before recompression therapy. Costly radiography such as computed tomography of the chest or magnetic resonance imaging (MRI) should be postponed until after initial treatment. MRI has been proven useful in further evaluation of neurological DCI but may be insensitive in some cases (16).

The cornerstones of current treatment for DCI are the application of 100% oxygen, recompression with hyperbaric oxygen treatment, and adjunctive therapy (11, 13, 14). The assessment of effective treatment methods for DCI is based on empirical evidence and data from animal studies as there are no randomized placebo-controlled clinical studies in humans.

Treatment of choice is the on-site administration of 100% oxygen as soon as possible after onset of symptoms. Supportive first aid includes general care, immobilizing the injured patient, and giving non-carbonated, non-caffeinated, non-alcoholic, ideally isotonic fluids (table 3). The use of NSAIDs (Non steroidal anti-inflammatory drugs) (e.g., oxicam analogues) may improve outcome, provided that there are no contraindications (14). Consultation with a diving medicine facility capable of performing recompression is recommended as soon as possible. If a local facility is unavailable, guidance can be obtained from global 24-hour specialist advisory services. Divers with severe symptoms should be transferred for recompression treatment as soon as feasible. Transportation to a hyperbaric facility will depend on available local resources, weather conditions, and the clinical status of the patient. Oxygen administration should be maintained throughout, and the altitude during unpressurized aeromedical evacuations should be kept as low as possible.

If recompression would be logistically difficult to access from the diver’s location, DCI cases fitting the mild DCS classification may be treated with initial treatment measures (table 3) alone (13). Mild DCS is defined as limb pain, constitutional symptoms, subjective sensory symptoms or rash, with non-progressive symptoms, and clinical stability for 24 hours or more including a normal neurological exam. Such decision should be made individually and must always involve an experienced diving medicine physician.
The definitive gold standard in treatment of DCI remains the application of oxygen at increased ambient pressure, i.e., hyperbaric oxygen therapy. It accelerates the elimination of the gas phase both by raising the ambient pressure and by creating systemic hyperoxia. Hyperbaric oxygen therapy involves placing the patient in a pressurized environment of two to three times sea level atmospheric pressure while breathing 100% oxygen (figure 2). This usually results in arterial oxygen tension in excess of 2000 mm Hg (~267 kPa) and an amount of oxygen dissolved in the blood of approximately 60 ml per liter, which meets the resting body’s cellular oxygen requirements without any contribution of oxygen bound to hemoglobin. Physiologic effects of hyperbaric oxygen such as an increase in the diffusion gradient for oxygen into the gas bubble and for nitrogen out of the bubble, an improved oxygen delivery to tissues, and hyperoxic vasoconstriction, are considered to account for the improved outcome seen in DCI. The standard treatment involves breathing oxygen at pressures equivalent to 18 meters and 9 meters depth, with a recompression time of about 4 hours and 45 minutes (7). Clinical experience suggests that recovery from DCI both in divers and aviators is inversely related to time to hyperbaric oxygen therapy. Most improvement occurs within the first minutes after onset of treatment, but improvement has still been seen when definitive treatment began hours after onset of symptoms. Repetitive recompression treatments should be used if symptoms improve, but the rate of complete relief drops significantly after four or more treatment sessions. High oxygen partial pressure above 1.6 ATA can increase the risk of acute oxygen toxicity in the brain, potentially leading to seizures. However, the incidence of oxygen-induced seizures during hyperbaric oxygen treatments ranges between 0.01% and 0.05% (17), and the consequences are generally minimal and do not prevent further treatment. The effects of chronic oxygen toxicity on the lungs, eyes, and other organs should be monitored carefully during repeated hyperbaric treatments.

Minimizing exposure to predisposing factors can significantly lower the risk of developing DCI. Preventative strategies for DCS focus on limiting supersaturation during and after ascent by use of conservative depth-time profiles and modifying extrinsic risk factors (table 4). A patent foramen ovale (PFO), has been identified as a significant risk factor for DCI, particularly in cases of cerebral or cutaneous involvement (18, 22); however, strict adherence to conservative dive profile can eliminate that risk (10). Risk of pulmonary barotrauma is unrelated to depth-time profiles but can be reduced by slowing dive ascent rates and excluding pulmonary pathology during fitness to dive medical check (2, 8).

Decompression illness is uncommon but poses a medical challenge that many physicians are not trained to handle. DCI can lead to permanent injury or death, particularly if not promptly recognized and adequately treated. Emergency medical personnel should be aware of DCI in patients who have recently been exposed to diving or ambient pressure changes. Initial treatment should involve administering 100% oxygen until recompression and hyperbaric oxygen therapy can be conducted. Diving emergency hotlines or centers may be contacted for further specialist advice.

Conflict of Interest
The authors have no conflict of interest.

Decompression illness (DCI) encompasses arterial gas embolism and decompression sickness, both resulting from gas formation during decompression and presenting with a wide range of often non-specific symptoms. 
Prompt administration of 100% oxygen and hyperbaric oxygen therapy are the treatments of choice, while prevention focuses on conservative dive profiles and risk factor screening.

This clinical review summarizes current understanding of DCI pathophysiology, diagnosis, management, and prevention in diving medicine.