Hyperbaric medicine involves using high concentrations of oxygen (100%) at pressures greater than the surrounding ambient atmospheric pressure. This necessitates using a pressurized chamber where a single patient or several patients can be placed to facilitate treatment. The first documented usage of hyperbaric medical therapy was in 1662. A British physician created the "domicillium" which consisted of a pressurized airtight chamber in which pressure could be increased using a bellows. Numerous afflictions were treated with unknown results. It is interesting to note that this was done before the actual discovery of oxygen in 1774 by Joseph Priestly. In 1872 Paul Bert researched and wrote about the physiological effects of pressurized air on the human body. The first actual hyperbaric chamber was built in New York in 1891 by Dr. J. L. Corning. A Kansas anesthesiologist. Dr. O. Cunningham did extensive hyperbaric research looking at its treatment feasibility with many conditions such as syphilis, arthritis, diabetes, and other afflictions. In 1928 he built a "hyperbaric hotel" in Cleveland Ohio that could accommodate more than 70 guests. It had all of the creature comforts including a smoking lounge. This endeavor did not survive due to increasing scrutiny from the American Medical Association, as well as a lack of documented tangible evidence of effectiveness. Soon afterward the usage of hyperbaric medicine became more accepted and mainstream. World War II prompted the need for acceptable treatment modalities for treating Navy divers who suffered decompression sickness or the Bends. Detailed Navy dive charts and hyperbaric treatment tables were formulated for various diving and decompression scenarios. These charts are still used today in determining the most appropriate regimen for hyperbaric medical therapy for various situations. In the 1950s a researcher named Ite Boerema showed that he could keep swine alive after draining all of their red blood cells and hyper oxygenating their remaining plasma with hyperbaric oxygen. The actual mechanism of action of hyperbaric therapy has been extensively studied and is indicated for numerous conditions. There are 14 approved conditions by the Undersea and Hyperbaric Medical Society and numerous more are non-approved, off-label conditions where hyperbaric medicine may be utilized. New centers of hyperbaric medicine are opening yearly throughout the country. Many of these centers are now affiliated with wound care centers, but there are also centers based at military facilities, on deep-sea diving vessels, in veterinary hospitals, and trauma and burn centers. These facilities are regulated by the United States Food and Drug Administration (FDA) and require a prescription for treatment. A specially trained, accredited physician must oversee and be physically present. There are two main types of hyperbaric chambers, a single place, and a multi-place chamber. The single place chambers accommodate only a single patient at a time. The patient is closely observed during the dive through the acrylic chamber. Interaction with the patient is limited to verbal communication through an approved audio system. Multiplace chambers accommodate several patients at a time with the overseeing physician present in the chamber. This allows more flexibility in who can undergo treatments and allows direct access to the patients. Each dive follows a specific dive protocol outlined by the Navy treatment tables depending on the reason for the dive.
The mechanism of hyperbaric oxygen therapy is to subject a patient to 100% oxygen at increased pressures, usually around 2.4 atmospheres. The normal ambient atmospheric pressure is measured as 1. Each additional increment of pressure over 1 is equal to 33 feet of depth of seawater. The mechanism of action of hyperbaric medicine follows various gas laws. The most relevant of these laws is Boyle's Law. This states that the size of a gas bubble in a liquid will decrease with increased pressure. This is the basis for using hyperbaric medicine in the treatment of decompression sickness and arterial air emboli. Henry's Law states that more gas will be dissolved in solution under higher pressures. This describes the effect of delivering more oxygen to tissues with hyperbaric medicine and is useful in many situations such as wound healing, burns, carbon monoxide poisoning, acute anemia, peripheral limb ischemia and crush injuries. Other laws such as Charles' Law describes the effect of temperature on saturation and pressures, and the Ideal Gas Law that deals with the overall relationship between gases, temperatures, and pressures. It is essential to understand the 14 conditions that are approved for hyperbaric medical therapy because many conditions rely on different aspects of hyperbaric medicine to be effective. The currently approved conditions include:
The complete explanation of the functions of hyperbaric medicine is complex. It decreases the size of gas bubbles in liquid (blood). It increases the oxygen-carrying capacity of blood through increasing the oxygen concentration of the plasma to around 7%. It is bacteriostatic and bacteriocidal at higher pressures and oxygenation. Hyperbaric oxygen will promote arterial neovascularization and decrease tissue edema. It will inhibit various exotoxins such as alpha and beta toxins associated with necrotizing infections. Hyperbaric medicine will promote further diffusion of oxygen in tissues across a distance of approximately four times the normal perfusion distance. It will also promote diffusion of oxygen from an oxygen richer environment to a poor oxygen environment such as with ischemic wounds and limbs.
Boyle's Law is the basis for effectiveness in decompression sickness and air embolism. Surfacing too fast from a deep underwater dive will result in the precipitation of nitrogen bubbles in the blood. This will result in very painful joints, the bends, and even death. The treatment goal is to prevent the formation of nitrogen bubbles or to cause them to decrease in size and return into solution. The same holds true for the treatment of air embolism. Increased pressures provided by hyperbaric medical therapy will diminish such gas bubbles.
Carbon monoxide poisoning is caused by the displacement of oxygen from the blood's hemoglobin forming carboxyhemoglobin. This has many deleterious results. Hyperbaric oxygen will move the saturation curve to increase the oxygen saturation of red blood cells displacing the carbon monoxide molecules. This is considered an emergent indication for treatment.
Conditions such as compromised wound healing, refractory osteomyelitis, radiation injury, compromised grafts, crush injuries, and burns all benefit from the many effects of hyperbaric therapy. Increased delivery of oxygen to the area, neovascularization, decreased edema and greater perfusion distances of oxygen are all desired results of therapy shown to positively impact outcomes.
Necrotizing infections and intracranial abscesses also benefit from the above effects of hyperbaric oxygen. They are also positively affected by the bacteriostatic and bactericidal action of therapy as well as the inhibition of various exotoxins such as alpha, beta and delta toxins. Hyperbaric oxygen has also been shown to enhance the effectiveness of several antibiotics such as Quinolones and Gentamicin.
The rationale for the treatment of acute sensorineural hearing loss and acute retinal artery occlusion (not on the approved list) is that these tissues, the retina, and cochlea are highly sensitive to oxygen deprivation. Hyperbaric oxygen therapy will provide sufficient oxygen to these areas as well as promote neovascularization until there is sufficient restored innate oxygen supply to the area. 
Hyperbaric oxygen is not without pitfalls. It has been shown that animals exposed to persistently high levels of oxygen may suffer oxygen seizures. This has been documented with hyperbaric therapy and is usually self-limiting. To decrease the incidence of this, patients will take several air breaks during a dive. This involves supplying room air to breathe for periods of time and has proven to decrease episodes of oxygen seizures. Various types of barotrauma can also be seen with hyperbaric dives. Anytime there is a closed, air-filled compartment of the body; there is a greater chance of barotrauma. This is from confined air expanding with no outlet. Most common forms of barotrauma associated with dives are perforation of the eardrum or possibly pneumothorax. Caution must be maintained with the general usage of the chambers. Pressurized oxygen can be explosive. Care must be taken to avoid excessively combustible materials and ignition sources. Patients must remove any makeup, oils, deodorants, and similar products. They are grounded, and all efforts are taken to eliminate static sparks.
The only truly absolute contraindication for hyperbaric treatment is an untreated pneumothorax. Other relatively strong contraindications are if patients are on specific chemotherapeutic agents such as Adriamycin and Cisplatinum or Antabuse. Other issues of concern are ventilated patients, patients with uncontrolled hypertension, and people with diabetes. The issues with ventilated patients are the variances in air volumes and pressures and barotrauma concerns. Serum blood sugars often drop during a dive. Therefore, it is advised to make sure the serum glucose in patients with diabetes is on the high side before a dive.
It is important to be aware of emergent versus non-emergent indications for hyperbaric therapy. Emergent indications include decompression sickness, air embolism, carbon monoxide poisoning, crush injury, acute blood loss anemia, intracranial abscess, thermal burns, necrotizing fasciitis, gas gangrene, and sudden neural hearing loss. These patients need to be cared for at an acute care hyperbaric center. Often this requires an immediate transfer. Most hyperbaric centers are involved with nonemergent conditions such as poor wound healing, delayed radiation injury, chronic osteomyelitis and compromised flaps. It is essential for the treating physician to recognize emergent hyperbaric indications.
There are some off-label indications for hyperbaric therapy. They all have a theoretical basis for treatment, but most have not been scientifically proven to be effective. Autism, neurologic injury, fibromyalgia, migraine headaches, sleep disorders, cerebral palsy, and many others are currently being treated with hyperbaric medicine. New conditions will be added to the list of accepted indications as more scientific, clinical data is being produced, and effectiveness is proven. 
Accurate knowledge of indications and contradictions for the institution of hyperbaric medicine is paramount especially in first contact caregivers such as emergency physicians, primary care, and surgeons. There are several emergent/ urgent indications that must be recognized and instituted for hyperbaric therapy to provide maximum efficacy. Complications of hyperbaric therapy must also be recognized and hyperbaric care must be initiated and staffed with an appropriately trained and experienced physician.
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