The pathophysiology of decompression sickness is related to the obstructive and inflammatory effects of inert gas bubbles in tissues and the vascular system.4 Decompression sickness may occur in divers breathing compressed air, caisson workers, high-altitude pilots, or astronauts. Bubbles may form when a body with additional inert gas in solution experiences a decrease in ambient pressure that causes liberation of the gas. Uptake of inert gas occurs at different rates in different tissues.
The U.S. Navy publishes dive tables to provide the limits to a dive (measured by bottom depth and time) that can be undertaken without a decompression stop ("no decompression" or "no stop" dives). Other Navy tables provide a variety of decompression schedules for longer dives. A multitude of dive computers, often using proprietary mathematical models, provide divers with relatively safe diving limits. Decompression sickness is unlikely to occur if the limits of the dive tables or dive computer are followed, but compliance with dive table limits or a dive computer does not completely eliminate risk.
Bubbles are necessary but not sufficient by themselves to cause decompression sickness; bubbling occurs after many dive profiles that do not lead to decompression sickness. Obviously, there must be a threshold at which the bubble load causes symptoms. The exact mechanism of bubble formation is not known, although preexisting gas micronuclei in the circulation likely form a nidus for gas accumulation. This is inferred, because the energy required to form bubbles de novo is much higher than the energy state caused by the saturation of inert gas in tissue.5 Bubbles may form directly in tissues or the circulation (usually the low-pressure venous circulation). Classically, it is thought that bubbles directly obstruct blood flow, leading to direct ischemia. Also, the air–blood and air–endothelial interfaces initiate a variety of inflammatory and thrombotic processes; activate the endothelium, leading to neutrophil adhesion and activation; and change the permeability of the endothelium, resulting in third spacing of fluid. In addition, decompression stress induces the production of microparticles, which are lipid bilayer–enclosed membranous vesicles extruded from vascular endothelial and other cells. Injection of these microparticles in animal models creates a clinical condition consistent with decompression sickness.6
There are no current definitive diagnostic criteria for decompression sickness. The San Diego Diving and Hyperbaric Organizations criteria use a point system to identify dive injuries resulting in decompression sickness with a high degree of specificity.7 This is helpful to create databases of divers with decompression sickness to study outcomes and allow study of adjunctive therapies. Unfortunately, this system has relatively low sensitivity. Studies of therapies for decompression sickness often lack an acceptable case definition of decompression sickness.
The most commonly used classification divides decompression sickness into two (or sometimes three) main groups (Table 214-2). We focus on type I and II for clarity. Type I is also called "pain-only" decompression sickness and involves the joints, extremities, and skin ("cutis marmorata"). Lymphatic obstruction can occur in type I, causing lymphedema, which usually takes days to resolve despite recompression therapy. Type II involves the CNS (mainly the spinal cord in compressed air divers and the brain in high-altitude decompressions), vestibular symptoms ("staggers"), and cardiopulmonary symptoms ("chokes"). To further complicate the nomenclature and classification of decompression sickness, it can also occur when an arterial gas embolism (see below) causes inert gas to come out of solution after a dive profile that would otherwise not be expected to cause decompression sickness (called type III).8 Some advocate the use of the alternate term decompression illness, instead of differentiating between decompression sickness and cerebral arterial gas embolism, to encompass all pathologic syndromes following a reduction in ambient pressure.1
TABLE 214-2Classification of Decompression Sickness (DCS) ||Download (.pdf) TABLE 214-2 Classification of Decompression Sickness (DCS)
|Classification ||Clinical Features ||Comments |
Type I: "pain-only" DCS
Deep pain in joints and extremities, unrelieved but not worsened with movement
Skin changes—mottling, pruritus, and color changes
Usually single joint, most commonly knees and shoulders
Lymphatic obstruction can occur and takes days to resolve despite recompression therapy
Type II: "serious" DCS
Pulmonary ("chokes")—cough, hemoptysis, dyspnea, and substernal chest pain
Cardiovascular collapse can occur
Neurologic—sensation of truncal constriction® ascending paralysis, usually rapid in onset
Vestibular ("staggers")—vertigo, hearing loss, tinnitus, and disequilibrium
Has a tendency to affect the lower cervical and thoracic regions may see scattered lesions
Autonomic dysfunction seen
Usually occurs after deep, long dives
Type III: combination of DCS and arterial gas embolism
Symptoms of DCS II noted above plus a variety of stroke syndromes, symptoms, and signs
Symptoms occur on ascent or immediately upon surfacing
Symptoms of arterial gas embolism may spontaneously resolve
Generally, the symptoms of decompression sickness occur minutes to several hours after surfacing, but in rare cases, symptoms can occur days after diving. Symptoms occurring between dives may improve during a subsequent dive (as recompression has occurred) but get worse upon resurfacing (as the inert gas load has increased and ambient pressure has decreased). Flying with the resultant decrease in ambient pressure may precipitate or worsen symptoms. For this reason, divers are generally advised to refrain from flying for at least 12 to 24 hours after the last dive depending on the nature of the diving exposure.9
Divers with type I decompression sickness typically describe a deep pain, unrelieved but not worsened with movement. This pain can be attributed to or confused with pain caused by injury, potentially making accurate diagnosis difficult. Pain is thought to be due to distention from bubbles in ligaments or fascia, intramedullary bubbles at the ends of long bones, or the activation of stretch receptors caused by bubbles in tendons. The mechanism of simple distention of tissues is supported by the rapid improvement of symptoms with recompression. Common pain locations are knees and shoulders, and most often, only a single joint is involved. Decompression sickness in commercial and military divers, caisson workers, and aviators tends to manifest most often as joint pain. Sport divers, who usually perform multiple dives, often over a period of days, are more prone to spinal cord effects.
Poorly localized and difficult-to-describe back or abdominal pain may herald the more serious signs of spinal cord involvement.
Pulmonary symptoms, generally seen usually only after more prolonged exposures, are caused by large numbers of pulmonary artery bubbles and include symptoms of cough, hemoptysis, dyspnea, and substernal chest pain. Cardiovascular collapse can occur.
The classic description of divers with neurologic decompression sickness (type II) can begin with a sensation of truncal constriction or girdle-like pain. Often a wooly feeling begins in the feet, developing into an ascending paralysis, producing symptoms of transverse myelitis. This form is usually rapid in onset and has a tendency to affect the lower cervical and thoracic regions. However, in type II decompression sickness, neurologic deficits do not necessarily cause distinct spinal cord syndromes (i.e., an anterior or posterior spinal artery syndrome), nor will a definitive level necessarily be found, as lesions may be scattered throughout the spinal cord.10 Autonomic involvement, with resulting incontinence and sexual dysfunction, is not uncommon. The pathophysiology of spinal cord decompression sickness seems to be initial bubbling in the low-pressure venous plexus system that first impedes and then obstructs venous outflow from the cord. Decreasing venous blood flow prevents dissolved nitrogen in spinal cord tissues from egressing, and in situ bubbles within the spinal cord develop (called autochthonous bubbles).
Vestibular decompression sickness usually occurs after deep, long dives, although it has been reported in sport divers. Signs are vertigo, hearing loss, tinnitus, and disequilibrium. The vestibular syndrome can be differentiated from inner ear barotrauma mainly by the history, because patients with inner ear barotrauma develop symptoms in the water and, generally, immediately after a forced Valsalva maneuver to equalize the middle ear pressure.11
Other, nonspecific symptoms such as headache, nausea, dizziness, or unusual fatigue are also reported. It may be difficult to differentiate fatigue from decompression sickness from the expected fatigue from the exertion of diving.
The association between decompression sickness and patent foramen ovale is unclear. There appears to be an increased prevalence in patients with inner ear and cutaneous decompression sickness. It is reasonable to screen divers with recurrent, unexplained decompression sickness for a patent foramen ovale. Closure of a large defect will reduce arterialization of venous gas emboli, although it has yet to be shown if such closure will reduce the incidence of subsequent decompression sickness.12
Arterial gas embolism occurs when air enters the left side of the vascular system. In the setting of diving, this most often results from pulmonary barotrauma. Arterial gas embolism can also occur as a complication of certain medical procedures, such as central vascular catheterization and cardiac bypass. Air inadvertently introduced into the venous circulation can cross from the right side of the circulation from intracardiac or pulmonary arteriovenous shunts. Air bubbles may also arterialize through these same shunts, sometimes making the source of arterial bubbles difficult to determine.13 Whatever the source, when air embolizes systemically, distribution depends mainly on blood flow and not gravity.
The most dramatic effect of arterial gas embolism is on the brain, resulting in a variety of stroke syndromes, symptoms, and signs, depending on the part of the brain affected. Rarely, diving-related arterial gas embolism from pulmonary barotrauma causes immediate apnea and cardiac arrest. The mechanism of cardiovascular collapse appears to be air in the entirety of the large arteries and veins of the central vascular bed.14 The effects of arterial gas embolism secondary to pulmonary barotrauma usually occur on ascent or immediately upon surfacing. If the victim does not die immediately, the symptoms of cerebral arterial gas embolism often include loss of consciousness, seizure, blindness, disorientation, or hemiplegia. Symptoms may spontaneously improve as the gas enters the venous cerebral circulation after a spike in blood pressure. Sometimes, by the time the patient reaches the clinician, the only signs that remain are subtle defects. In particular, parietal lobe signs and symptoms are easily overlooked. A cascade of inflammatory processes also occurs in air embolism, just as in decompression sickness.4
The hematocrit may elevate from hemoconcentration and third spacing of fluids. The creatine phosphokinase (and other enzymes such as lactate dehydrogenase, alanine aminotransferase, and aspartate aminotransferase) will become elevated secondary to the systemic distribution of bubbles. The degree of elevation of creatine phosphokinase corresponds to the embolism severity. Cardiac troponins may also be elevated and most likely do not represent occlusive coronary artery disease.4
Treatment of Decompression Sickness and Arterial Gas Embolism
The treatment includes administering 100% oxygen, increasing tissue perfusion with IV fluids, and rapid recompression. Some advocate placing patients with air embolism in the Trendelenburg position or in the left lateral decubitus position to "trap" air in the left ventricle. By the time the victim is brought onto the dive boat or the ambulance arrives, the air has usually been distributed, and the Trendelenburg position merely increases intracranial pressure, decreases cerebral perfusion, and interferes with other first aid measures. Nonetheless, some divers with arterial gas embolism have collapsed when placed in a sitting or standing position. As a result, a supine position—not Trendelenburg position—is recommended for patients with arterial gas embolism. Vomiting patients should be placed in the lateral decubitus position to prevent aspiration.
Recompression therapy with hyperbaric oxygen treats by several mechanisms. See the chapter 21, titled "Hyperbaric Oxygen Therapy" for detailed discussion. The administered pressure decreases the size of bubbles, and the high partial pressure of oxygen in solution increases inert gas washout from bubbles and tissue. Mass action dictates a gas will travel down pressure gradients; therefore, nitrogen will move from bubbles with a high partial pressure of nitrogen into plasma, where it will travel to the lungs and be exhaled. Conversely, oxygen from plasma with a high partial pressure of oxygen will enter bubbles, but ultimately will diffuse into cells and be metabolized, further reducing bubble size. Hyperbaric oxygen also decreases tissue edema, increases oxygen delivery to ischemic tissues, and reduces neutrophil adhesion to the endothelium and neutrophil activation.15
Recompression using the U.S. Navy Treatment Table 6 is a commonly used method of management for decompression sickness, employing a maximal treatment pressure of 2.8 ATA (60 fsw). Table 6 is also used for air embolism, although some advocate an initial pressurization to 6 ATA (165 fsw) to maximize bubble compression, then continuation at 2.8 ATA (U.S. Navy Table 6A). Different treatment tables are used in other parts of the world, and there is some experience using lower treatment pressures for decompression sickness in monoplace chambers with reportedly comparable results.16 Some patients may benefit from repeated treatments if symptoms do not fully resolve. Recompression should occur as soon as possible, and it should not be withheld in cases with delayed presentation.4 Additionally, for divers who have missed needed decompression stops because of an emergency ascent or nonadherence to appropriate diving tables, it may be appropriate for them to undergo recompression therapy even if asymptomatic. U.S. Navy Table 5 recompression would usually be adequate in such a circumstance.
The administration of IV lidocaine as a therapeutic adjunct for cerebral arterial gas embolism has been advocated, because it appears to decrease neuropsychiatric deficits when given during anesthesia for cardiac procedures requiring bypass,17,18,19 since bypass operations commonly cause the entry of air into the arterial system. Dosing of lidocaine in this setting is not standardized, although typical cardiac dosing is commonly used.20
The Divers Alert Network (telephone: 1-919-684-9111; Web site: http://www.diversalertnetwork.org) has staff available 24 hours a day to provide assistance to divers and to help clinicians treat patients with decompression sickness or arterial gas embolism. The Divers Alert Network can provide information and the location of the nearest recompression facility around the world.