Injuries caused by pressure changes are generally governed by the Boyle and Henry laws of physics.
The Boyle law states, "For any gas at a constant temperature, the volume of the gas will vary inversely with the pressure," or P1 X V1 = P2 X V2. Pressure rises by 1 atmosphere for every 33 ft (10 m) of seawater depth. This means that a balloon (or lungs) containing a volume of 1 cubic foot of gas at 33 ft of seawater depth will have a volume of gas of 2 cubic feet at the surface. If this air is trapped, as occurs when a person holds his or her breath during rapid ascent, it expands with great force against the walls of that space (reverse squeeze). During rapid ascent, incidents of pneumothorax and pneumomediastinum as well as sinus squeeze and inner ear injuries can occur. Sinus squeeze occurs with eustachian tube dysfunction, which may result in inner ear hemorrhage, tearing of the labyrinthine membrane, or perilymphatic fistula.
The Henry law states that the solubility of a gas in a liquid is directly proportional to the pressure exerted upon the gas and liquid. Thus, when the cap is removed from a bottle of soda pop, the soda begins to bubble as gas is released from the liquid. In addition, when nitrogen in a diver's air tank dissolves in the diver's fatty tissues or synovial fluids at depth, nitrogen will be released from those tissues as the diver ascends to a lower pressure environment. This occurs slowly and gradually if the diver ascends slowly and gradually, and the nitrogen enters the bloodstream to the lungs and is exhaled. However, should the diver ascend rapidly, nitrogen exits tissues rapidly and forms gas bubbles.
Once bubbles are formed, they can affect tissues in many ways. They can simply obstruct blood vessels leading to ischemic injury. This can be devastating when occurring in critical areas in the brain. The bubbles can also form a surface to which proteins in the bloodstream can cling, unravel, and begin a clotting/inflammatory cascade. This cascade can lead to endothelial breakdown and permanent tissue damage.
Decompression sickness
Decompression sickness (DCS) usually results from the formation of gas bubbles, which can travel to any part of the body, accounting for many disorders. A gas bubble forming in the back or joints can cause localized pain (the bends). In the spinal cord or peripheral nerve tissues, a bubble may cause paresthesias, neurapraxia, or paralysis. A bubble forming in the circulatory system can lead to pulmonary or cerebral gas emboli. [7]
Some gases are more soluble in fats. Nitrogen, for example, is 5 times more soluble in fat than in water. Approximately 40-50% of serious DCS injuries involve the central nervous system (CNS). Women may be at an increased risk of DCS because they have more fat in their bodies. DCS also may occur at high altitudes. Those who dive in mountain lakes or combine diving with subsequent flying are at increased risk as well.
DCS is classified into two types. Type I is milder, is not life threatening, and is characterized by pain in the joints and muscles and swelling in the lymph nodes. The most common symptom of DCS is joint pain, which begins mildly and worsens over time and with movement. DCS type II is serious and life threatening. Manifestations may include respiratory, circulatory, and, most commonly, peripheral nerve and/or CNS compromise. [8]
Arterial gas embolism (AGE) is the most dangerous manifestation of DCS type II. AGE occurs after a rapid ascent, when a gas bubble forms in the arterial blood supply and travels to the brain, heart, or lungs. This is immediately life threatening and can occur even after ascent from relatively shallow depths. However, AGE can also occur from iatrogenic causes.
Patients with a patent foramen ovale (up to 30% of the population) are at higher risk of gas passing from a right-to-left shunt and causing CNS injuries. [9, 10]
Medically induced barotrauma
Medically induced barotrauma most often occurs in patients receiving respiratory support via positive-pressure ventilation (PPV). [11] The age of the patient, comorbidities such as COPD, malignancy of the upper or lower airway, trauma to the airways, or surgical procedures to the thoracic cavity or upper airway increase the risk for barotrauma due to PPV. [12]
Complications of barotrauma due to PPV include the following:
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Systemic air embolism: This is uncommon, but it can result in life-threatening complications such as cardiac arrest [12] or cerebrovascular accident. [13]
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Air cardiac tamponade: This is another uncommon but serious complication of barotrauma. Golota et al, [14] document a case study of air cardiac tamponade in a patient presenting with respiratory distress due to pneumothorax. The pneumothorax was treated with a chest tube, without relief of symptoms. CT scan revealed air in the pericardium.
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Pneumothorax: This is the most common barotrauma as the result of PPV. Chang et al [15] present a comparison study that suggests pressure-regulated volume control PPV reduces the incidence of pneumothorax versus synchronized intermittent mandatory ventilation in elderly COPD patients.
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Pneumomediastinum: This is less serious than the above conditions. It may present with chest pain, mild dyspnea, dysphagia, and vocal changes. It is usually treated supportively and resolves without intervention. [16] Ocakcioglu et al [17] report a case of pneumomediastinum after tooth extraction using high-pressure air dental drills. The patient was 1 week post procedure when he presented to the local emergency department with chest pain and a sore throat.
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Otic barotrauma: This usually involves injury to the middle ear with tympanic rupture, hemotympanum, transient hearing lost, and otalgia. McCormick et al [18] report a case of otic barotrauma as a result of continuous positive airway pressure (CPAP) therapy for obstructive sleep apnea.
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Self-inflicted barotrauma: Jeddy et al [19] present a case study of pneumomediastinum and subcutaneous emphysema of the neck and upper chest self-induced by the patient’s use of nitrous oxide as a recreational drug. The abusers use balloons or canisters used for pressurizing commercial whip cream containers (known as whip-its) filled with nitrous oxide as an inhalant.
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Insufflation: Procedures requiring insufflation of carbon dioxide or other medical gases have resulted in barotrauma to the surrounding tissues. Tracheal, esophageal, and bowel perforation have occurred. [20]
Blast injury
This barotrauma occurs from an external explosive force causing atmospheric overpressure. This is primary blast injury (PBI). Blast injuries are further delineated into secondary, tertiary, and quaternary. Secondary is impact with flying debris, tertiary deals with the person being thrown by the force of the blast or injuries from structural collapse, and quaternary is all other injuries. [21, 22] PBI involves the respiratory, digestive, auditory, and nervous systems.
Respiratory PBI is the most likely to produce immediate fatal injury. Pulmonary contusion, systemic air embolism, disseminated intravascular coagulation, and acute respiratory distress syndrome frequently follow PBI of the lungs. PBI of the thorax may also cause decreased heart rate, stroke volume, and cardiac index with resulting hypotension without compensation from the systemic vascular resistance reflex. [21] . This can be seen with the use of fuel/air explosives in a combat setting.
PBI affecting the gastrointestinal system produces organ contusion and/or rupture. Hemorrhage, peritonitis, mesenteric emboli, and organ dysfunction or failure can occur. These injuries can be occult on initial examination, prompting the clinician to reevaluate any patient with exposure to significant explosive force often and with a high degree of suspicion for developing gastrointestinal injury. [21]
Auditory PBI includes tympanic rupture, ossicle fracture, or dislocation and hemotympanum. [21]
Neurological PBI produces injuries ranging from mild postconcussion syndrome to cerebral edema, hematoma, and intracranial hemorrhage. [21, 22] Reports from military observations suggest that cerebral swelling occurs much faster with PBI than with other traumatic closed head injury. [23] Aggressive treatment with early decompressive craniotomy has been shown to decrease mortality. [21, 22]
