Hyperbaric medicine

Hyperbaric medicine, also known as hyperbaric oxygen therapy (HBOT), is the medical use of oxygen at a level higher than atmospheric pressure. The equipment required consists of a pressure chamber, which may be of rigid or flexible construction, and a means of delivering 100% oxygen. Operation is performed to a predetermined schedule by trained personnel who monitor the patient and may adjust the schedule as required. HBOT found early use in the treatment of decompression sickness, and has also shown great effectiveness in treating conditions such as gas gangrene and carbon monoxide poisoning. More recent research has examined the possibility that it may also have value for other conditions such as cerebral palsy and multiple sclerosis, but no significant evidence has been found.

Medical uses
In the United States the Undersea and Hyperbaric Medical Society, known as UHMS, lists approvals for reimbursement for certain diagnoses in hospitals and clinics. The following indications are approved (for reimbursement) uses of hyperbaric oxygen therapy as defined by the UHMS Hyperbaric Oxygen Therapy Committee:


 * Air or gas embolism;
 * Carbon monoxide poisoning;
 * Carbon monoxide poisoning complicated by cyanide poisoning;
 * Clostridal myositis and myonecrosis (gas gangrene);
 * Crush injury, compartment syndrome, and other acute traumatic ischemias;
 * Decompression sickness;
 * Enhancement of healing in selected problem wounds;
 * Diabetically derived illness, such as diabetic foot, diabetic retinopathy,  diabetic nephropathy;
 * Exceptional blood loss (anemia);
 * Intracranial abscess;
 * Necrotizing soft tissue infections (necrotizing fasciitis);
 * Osteomyelitis (refractory);
 * Delayed radiation injury (soft tissue and bony necrosis);
 * Skin grafts and flaps (compromised);
 * Thermal burns.

Evidence is insufficient as of 2013 to support its use in autism, cancer, diabetes, HIV/AIDS, Alzheimer's disease, asthma, Bell's palsy, cerebral palsy, depression, heart disease, migraines, multiple sclerosis, Parkinson's disease, spinal cord injury, sports injuries, or stroke.

Sudden hearing loss, acoustic trauma, noise-induced hearing loss and tinnitus
Recent studies have indicated that HBO therapy is recommended and warranted in those patients with idiopathic sudden deafness, acoustic trauma or noise-induced hearing loss within 3 months after onset of disorder.

Chronic ulcers
HBOT in diabetic foot ulcers increasing the rate of early ulcer healing but does not appear to provide any benefit in wound healing at long term follow up. In particular, there was no difference in major amputation rate. For venous, arterial and pressure ulcers, no evidence was apparent that HBOT provides an improvement on standard treatment.

Radiation injury
There are signs that HBOT might improve outcome in late radiation tissue injury affecting bone and soft tissues of the head and neck. In general patients with radiation injuries in the head, neck or bowel showed an improvement in quality of life after HBO therapy. On the other hand, no such effect was found in neurological tissues. The use of HBOT may be justified to selected patients and tissues, but further research is required to establish the best patient selection and timing of any HBO therapy.

Neuro-rehabilitation
There is insufficient evidence to prove the effectiveness or ineffectiveness of HBOT for traumatic brain injury. In stroke HBOT shows no benefit. HBOT in multiple sclerosis has not shown benefit and routine use is not recommended.

A 2007 review of HBOT in cerebral palsy found no difference compared to the control group. Neuropsychological tests also showed no difference between HBOT and room air and based on caregiver report, those who received room air had significantly better mobility and social functioning. Children receiving HBOT were reported to experience seizures and the need for tympanostomy tubes to equalize ear pressure, though the incidence was not clear.

Contraindications
The toxicology of the treatment has recently been reviewed by Ustundag et al. and its risk management is discussed by Christian R. Mortensen, in light of the fact that most hyperbaric facilities are managed by departments of anaesthesiology and some of their patients are critically ill.

The only absolute contraindication to hyperbaric oxygen therapy is untreated tension pneumothorax. The reason is concern that it can progress to tension pneumothorax, especially during the decompression phase of therapy. The COPD patient with a large bleb represents a relative contraindication for similar reasons. Also, the treatment may raise the issue of Occupational health and safety (OHS), which has been encountered by the therapist.

Patients should not undergo HBO therapy if they are taking or have recently taken the following drugs:
 * Doxorubicin (Adriamycin) – A chemotherapeutic drug. This drug has been shown to potentiate cytotoxicity during HBO therapy.
 * Cisplatin – Also a chemotherapeutic drug.
 * Disulfiram (Antabuse) – Used in the treatment of alcoholism.
 * Mafenide acetate (Sulfamylon) – Suppresses bacterial infections in burn wounds

The following are relative contraindications -- meaning that special consideration must be made by specialist physicians before HBO treatments begin:
 * Cardiac disease
 * COPD with air trapping - can lead to pneumothorax during treatment.
 * Upper respiratory infections – These conditions can make it difficult for the patient to equalise their ears or sinuses, which can result in what is termed ear or sinus squeeze.
 * High fevers – In most cases the fever should be lowered before HBO treatment begins.
 * Emphysema with CO2 retention – This condition can lead to pneumothorax during HBO treatment.
 * History of thoracic (chest) surgery – This is rarely a problem and usually not considered a contraindication. However, there is concern that air may be trapped in lesions that were created by surgical scarring. These conditions need to be evaluated prior to considering HBO therapy.
 * Malignant disease: Cancers thrive in blood rich environments but may be suppressed by high oxygen levels. HBO treatment of individuals who have cancer presents a problem, since HBO both increases blood flow via angiogenesis and also raises oxygen levels. Taking an anti-angiogenic supplement may provide a solution. A study by Feldemier, et al. and recent NIH funded study on Stem Cells by Thom, et al., indicate that HBO is actually beneficial in producing stem/progenitor cells and the malignant process is not accelerated.
 * Middle ear barotrauma is always a consideration in treating both children and adults in a hyperbaric environment because of the necessity to equalise pressure in the ears.

Pregnancy is not a relative contraindication to hyperbaric oxygen treatments, although it may be for SCUBA diving. In cases where a pregnant woman has carbon monoxide poisoning there is evidence that lower pressure (2.0 ATA) HBOT treatments are not harmful to the fetus, and that the risk involved is outweighed by the greater risk of the untreated effects of CO on the fetus (neurologic abnormalities or death.) In pregnant patients, HBO therapy has been shown to be safe for the fetus when given at appropriate levels and “doses” (durations). In fact, pregnancy lowers the threshold for HBO treatment of carbon monoxide-exposed pregnant patients. This is due to the high affinity of fetal hemoglobin for CO.

Therapeutic principles
Several therapeutic principles are made use of in HBOT:
 * The increased overall pressure is of therapeutic value when HBOT is used in the treatment of decompression sickness and air embolism as it provides a physical means of reducing the volume of inert gas bubbles within the body;
 * For many other conditions, the therapeutic principle of HBOT lies in its ability to drastically increase partial pressure of oxygen in the tissues of the body. The oxygen partial pressures achievable using HBOT are much higher than those achievable while breathing pure oxygen at normobaric conditions (i.e. at normal atmospheric pressure);
 * A related effect is the increased oxygen transport capacity of the blood. Under normal atmospheric pressure, oxygen transport is limited by the oxygen binding capacity of hemoglobin in red blood cells and very little oxygen is transported by blood plasma. Because the hemoglobin of the red blood cells is almost saturated with oxygen under atmospheric pressure, this route of transport cannot be exploited any further. Oxygen transport by plasma, however is significantly increased using HBOT as the stimulus.
 * Recent evidence notes that exposure to hyperbaric oxygen (HBOT) mobilizes stem/progenitor cells from the bone marrow by a nitric oxide (·NO) -dependent mechanism. This mechanism may account for the patient cases that suggest recovery of damaged organs and tissues with HBOT.

Construction
The traditional type of hyperbaric chamber used for HBOT is a hard shelled pressure vessel. Such chambers can be run at absolute pressures as much as 6 bar, 6 bar. Navies, diving organizations, hospitals, and dedicated recompression facilities typically operate these. They range in size from semi-portable, one-patient units to room-sized units that can treat eight or more patients. Recent advances in materials technology have resulted in the manufacture of portable, "soft" chambers that can operate at between 0.3 and 0.5 bar above atmospheric pressure. Hard chambers and soft chambers should not be considered equivalent in regards to efficacy and safety as they are different in many aspects.

A hard chamber may consist of
 * a pressure vessel that is generally made of steel, aluminium with the view ports (windows) made of acrylic;
 * one or more human entry hatches—small and circular or wheel-in type hatches for patients on gurneys;
 * the airlock that allows human entry—a separate chamber with two hatches, one to the outside and one to the main chamber, which can be independently pressurized to allow patients to enter or exit the main chamber while it is still pressurized and a small airlock for medicines, instruments, and food;
 * glass ports or closed-circuit television that allows technicians and medical staff outside the chamber to monitor the patient inside the chamber;
 * an intercom or walkie-talkie allowing two-way communication;
 * a carbon dioxide scrubber—consisting of a fan that passes the gas inside the chamber through a soda lime canister;
 * a control panel outside the chamber to open and close valves that control air flow to and from the chamber, and regulate oxygen to helmets or masks.

A soft chamber may consist of
 * a urethane-coated, nylon-bonded flexible acrylic pressure vessel with steel-weld technology;
 * a full-length dual zipper-sealed opening;
 * an over-pressure valve, if oxygen is fed into a small mask and expired gas has to be circulated toward the end of the chamber and out through the pressure regulators.

Oxygen supply
In today's larger multiplace chambers, both patients and medical staff inside the chamber breathe from either "oxygen hoods" – flexible, transparent soft plastic hoods with a seal around the neck similar to a space suit helmet – or tightly fitting oxygen masks, which supply pure oxygen and may be designed to directly exhaust the exhaled gas from the chamber. During treatment patients breathe 100% oxygen most of the time to maximise the effectiveness of their treatment, but have periodic "air breaks" during which they breathe room air (21% oxygen) to minimize the risk of oxygen toxicity. The exhaled gas must be removed from the chamber to prevent the build up of oxygen, which could present a fire risk. Attendants may also breathe oxygen to reduce their risk of decompression sickness. The pressure inside a hard chamber is increased by opening valves allowing high-pressure air to enter from storage cylinders, which are filled by an air compressor. A soft chamber may be pressurised directly from a compressor.

Smaller "monoplace" chambers can only accommodate the patient, and no medical staff can enter. The chamber may be pressurised with pure oxygen or compressed air. If pure oxygen is used, no oxygen breathing mask or helmet is needed, but the cost of using pure oxygen is much higher than that of using compressed air. If compressed air is used, then an oxygen mask or hood is needed as in a multiplace hard chamber. In monoplace chambers that are compressed with pure oxygen, a mask is needed to provide the patient with "air breaks", or periods of breathing normal air (21% oxygen). This is in order to reduce the risk of hyperoxic seizures. In soft chambers, using compressed air and a mask supplying 96% oxygen, no air breaks are necessary, because the risk of oxygen toxicity is negligible because of the lower oxygen partial pressures used (usually 1.3 ATA), and short durations of treatment.

Treatments
Initially, HBOT was developed as a treatment for diving disorders involving bubbles of gas in the tissues, such as decompression sickness and gas embolism. The chamber cures decompression sickness and gas embolism by increasing pressure, reducing the size of the gas bubbles and improving the transport of blood to downstream tissues. The high concentrations of oxygen in the tissues are beneficial in keeping oxygen-starved tissues alive, and have the effect of removing the nitrogen from the bubble, making it smaller until it consists only of oxygen, which is re-absorbed into the body. After elimination of bubbles, the pressure is gradually reduced back to atmospheric levels. Hyperbaric chambers are also used for animals, especially race horses where a recovery is worth a great deal to their owners. It is also used to treat dogs and cats in pre- and post-surgery treatment to strengthen their systems prior to surgery and then accelerate healing post surgery.

Protocol
The slang term, at some facilities, for a cycle of pressurization inside the HBOT chamber is "a dive". An HBOT treatment for longer-term conditions is often a series of 20 to 40 dives, or compressions. Again, these dives last for about an hour and can be administered via a hard, high-pressure chamber or a soft, low-pressure chamber—the major difference being per-dive "dose" of oxygen. Many conditions do quite well with the lower dose, lower cost-per-hour, soft chambers.

Emergency HBOT for decompression illness follows treatment schedules laid out in treatment tables. Most cases employ a recompression to 2.8 bar absolute, the equivalent of 18 m of water, for 4.5 to 5.5 hours with the casualty breathing pure oxygen, but taking air breaks every 20 minutes to reduce oxygen toxicity. For extremely serious cases resulting from very deep dives, the treatment may require a chamber capable of a maximum pressure of 8 bar, the equivalent of 70 m of water, and the ability to supply heliox as a breathing gas.

U.S. Navy treatment charts are used in Canada and the United States to determine the duration, pressure, and breathing gas of the therapy. The most frequently used tables are Table 5 and Table 6. In the UK the Royal Navy 62 and 67 tables are used.

The Undersea and Hyperbaric Medical Society (UHMS) publishes a report that compiles the latest research findings and contains information regarding the recommended duration and pressure of the longer-term conditions.

Home and out-patient clinic treatment
There are several sizes of portable chambers, which are used for home treatment. These are usually referred to as "mild personal hyperbaric chambers", which is a reference to the lower pressure (compared to hard chambers) of soft-sided chambers. Food and Drug Administration (FDA) approved chambers for use with room air are available in the USA and may go up to 4.4 pounds per square inch (psi) above atmospheric pressure, which equals 1.3 atmospheres absolute (ATA), equivalent to a depth of 10 feet of sea water. In the US, these "mild personal hyperbaric chambers" are categorized by the FDA as CLASS II medical devices and requires a prescription in order to purchase one or take treatments. Personal hyperbaric chambers are only FDA approved to reach 1.3 ATA. While hyperbaric chamber distributors and manufacturers cannot supply a chamber in the US with any form of elevated oxygen delivery system, a physician can write a prescription to combine the two modalities, as long as there is a prescription for both hyperbarics and oxygen. The most common option (but not approved by FDA) some patients choose is to acquire an oxygen concentrator which typically delivers 85–96% oxygen as the breathing gas. Because of the high circulation of air through the chamber, the total concentration of oxygen in the chamber never exceeds 25% as this can increase the risk of fire. Oxygen is never fed directly into soft chambers but is rather introduced via a line and mask directly to the patient. FDA approved oxygen concentrators for human consumption in confined areas used for HBOT are regularly monitored for purity (+/- 1%) and flow (10 to 15 liters per minute outflow pressure). An audible alarm will sound if the purity ever drops below 80%. Personal hyperbaric chambers use 120 volt or 220 volt outlets. Ranging in size from 21 inches up to 40 inches in diameter these chambers measure between 84 in (7 ft) to 120 in (10 ft) in length. The soft chambers are approved by the FDA for the treatment of altitude sickness, but are commonly used for other "off-label" purposes.

Possible complications and concerns
There are risks associated with HBOT, similar to some diving disorders. Pressure changes can cause a "squeeze" or barotrauma in the tissues surrounding trapped air inside the body, such as the lungs, behind the eardrum, inside paranasal sinuses, or trapped underneath dental fillings. Breathing high-pressure oxygen may cause oxygen toxicity. Temporarily blurred vision can be caused by swelling of the lens, which usually resolves in two to four weeks.

There are reports that cataract may progress following HBOT. Also a rare side effect has been blindness secondary to optic neuritis (inflammation of the optic nerve).

Effects of pressure
Patients inside the chamber may notice discomfort inside their ears as a pressure difference develops between their middle ear and the chamber atmosphere. This can be relieved by the Valsalva maneuver or by "jaw wiggling". As the pressure increases further, mist may form in the air inside the chamber and the air may become warm. Increased pressure may also cause ear drums to rupture, resulting in severe pain.

To reduce the pressure, a valve is opened to allow air out of the chamber. As the pressure falls, the patient’s ears may "squeak" as the pressure inside the ear equalizes with the chamber. The temperature in the chamber will fall. The speed of pressurization and de-pressurization can be adjusted to each patient's needs.

Costs
HBOT is recognized by Medicare in the United States as a reimbursable treatment for 14 UHMS "approved" conditions. A 1-hour HBOT session may cost between $108 and $250 in private clinics, and over $1,000 in hospitals. U.S. physicians (either M.D., D.C. or D.O.) may lawfully prescribe HBOT for "off-label" conditions such as stroke, and migraine. Such patients are treated in outpatient clinics. In the United Kingdom most chambers are financed by the National Health Service, although some, such as those run by Multiple Sclerosis Therapy Centres, are non-profit. In Australia, HBOT is not covered by Medicare as a treatment for multiple sclerosis. The average U.S. hospital charge of $1,800.00 per 90 minute HBOT treatment. China and Russia treat more than 80 maladies, conditions and trauma with HBOT.

Research
Aspects under research include:
 * Radiation-induced hemorrhagic cystitis;
 * Inflammatory bowel disease.

Neurological
Tentative evidence shows a possible benefit in cerebrovascular diseases. The clinical experience and results so far published has promoted the use of HBO therapy in patients with cerebrovascular injury and focal cerebrovascular injuries. However, the power of clinical research is limited because of the shortage of randomized controlled trials.

Radiation wounds
Many studies indicate a positive share of HBOT after radiation injury, and HBOT is prescribed for treating chronic wounds associated with radiation exposure. However, no significant evidence was found on HBOT having either a positive or negative effect on radiation wounds. This might be explained due to the lack of experimental and clinical studies.

Hyperbaric air
The use of air at raised ambient pressure for the treatment of illness is recorded from 1662 for afflictions of the lung, by Henshaw. It is unlikely to have had any significant effect.

Junod built a chamber in France in 1834 to treat pulmonary conditions at pressures between 2 and 4 atmospheres absolute.

During the following century “pneumatic centres” were established in Europe and the USA which used hyperbaric air to treat a variety of conditions.

Orval J Cunningham, a professor of anaesthesia at the University of Kansas in the early 1900s observed that people suffering from circulatory disorders did better at sea level than at altitude and this formed the basis for his use of hyperbaric air. In 1918 he successfully treated patients suffering from the Spanish flu with hyperbaric air. In 1930 the American Medical Association forced him to stop such hyperbaric treatment, since he did not provide acceptable evidence that the treatments were effective.

Hyperbaric oxygen
The English scientist, Joseph Priestley discovered oxygen in 1775. Shortly after its discovery, there were reports of toxic effects of hyperbaric oxygen on the central nervous system and lungs, which delayed therapeutic applications until 1937, when Behnke and Shaw first used it in the treatment of decompression sickness.

In 1955 and 1956 Churchill-Davidson, in the UK, used hyperbaric oxygen to enhance the radiosensitivity of tumours, while Ite Boerema, at the University of Amsterdam, successfully used it in cardiac surgery.

In 1961 WH Brummelkamp et al. published on the use of hyperbaric oxygen in the treatment of clostridial gas gangrene.

In 1962 Smith and Sharp reported successful treatment of carbon monoxide poisoning with hyperbaric oxygen.

The Undersea Medical Society (now Undersea and Hyperbaric Medical Society) formed a Committee on Hyperbaric Oxygenation which has become recognized as the authority on indications for hyperbaric oxygen treatment.