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Hyperbaric oxygen therapy

Hyperbaric oxygen therapy (HBOT) is the medical use of oxygen at a higher than atmospheric pressure.

Contents

Uses

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.
  • For many other conditions, the therapeutic principle of HBOT lies in a drastically increased partial pressure of oxygen in the tissues of the body. The oxygen partial pressures achievable under HBOT are much higher than those under 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 atmospheric pressure, oxygen transport is limited by the oxygen binding capacity of 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 can not be exploited any further. Oxygen transport by plasma however is significantly increased under HBOT.

The main indications for HBOT are:

  • Certain nonhealing wounds (post-surgical or diabetic)
  • Radiation soft tissue necrosis and radiation osteonecrosis
  • Necrotizing fasciitis (flesh eating bacteria)
  • Carbon monoxide poisoning
  • Decompression sickness
  • Severe infection by anaerobic bacteria (such as gas gangrene)
  • Air or gas embolism
  • Severe uncorrected anemia
  • Chronic refractory Osteomyelitis
  • Enhancement of healing in problem wounds
  • Sports injuries

HBOT is recognized by conventional medicine (in the USA) as an appropriate treatment for about 14 conditions. However, alternative healing advocates of many stripes believe it is useful for many additional conditions. Among the "off label" uses of HBOT are use as a therapy for brain healing (as in stroke, dementia, cerebral palsy), and for some infectious conditions, such as Lyme disease and Post-polio syndrome

HBOT is quite expensive, with a session costing $100 to $600 in the USA.

In the UK most chambers are financed by the National Health Service but there are non-profit HBOT chambers, such as those run by the Multiple Sclerosis Society .

The traditional chamber

The traditional type of HBOT chamber is a hard shelled pressure vessel. Such chambers can be run at absolute pressures up to 600 kilopascals, 6 atmospheres, 6 bars or 85 pounds force per square inch.

Navies, diving organisations and hospitals typically operate these. They range in size from those that are portable and capable of transporting just one patient to those that are fixed, very heavy and capable of treating eight or more patients.

The chamber may consist of:

  • a pressure vessel that is generally made of steel, [[aluminium], or plexiglass
  • one or more human entry hatches - theses could be small and circular or wheel-in type hatches for patients on trolleys
  • an airlock allowing human entry - a separate chamber with two hatches, one to the outside world and one to the main chamber, which can be independently pressurised to allow patients to enter or exit the main chamber while it is still pressurised
  • an airlock allowing medicines, instruments and food to enter the main chamber
  • glass ports or closed-circuit television allowing the technicians and medical staff outside the chamber to monitor the inside of the chamber
  • an intercom allowing two-way communications inside and outside the chamber
  • 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 is used to open and close valves allowing air to enter or leave the chamber and oxygen to be supplied to masks

In larger "multiplace" chambers, both patients and medical staff inside the chamber breathe from individual masks, which supply pure oxygen and remove the exhaled gas from the chamber. During treatment patients breathe oxygen most of the time but have periodic air breaks to minimise the risk of oxygen toxicity. The exhaled gas must be removed from the chamber to prevent the build up of oxygen, which could provoke a fire. Medical staff may also breathe oxygen to reduce the risk of decompression sickness. The masks that are used may simply cover the mouth and nose or they may be a type of flexible, transparent helmet with a seal around the neck. The pressure inside the chamber is increased by opening valves allowing high-pressure air to enter from storage cylinders, similar to diving cylinders. A gas compressor is used to fill these cylinders.

A recompression chamber for a single diving casualty
Enlarge
A recompression chamber for a single diving casualty

Smaller "monoplace" chambers can only accommodate the patient. No medical staff can enter. The chamber is flooded with pure oxygen and the patient does not wear a mask or helmet.

Patients inside the chamber will 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 maneouvre or by "jaw wiggling". As the pressure increase further, mist may form in the air inside the chamber and the air may become warm. When the patient speaks, the tone of the voice may increase to the level that they sound like cartoon characters.

To reduce the pressure, a valve is opened to allow gas out of the chamber. As the pressure falls, the patientís ears may "squeak" as the pressure inside the ear equalises with the chamber. The temperature in the chamber will cool.

Chambers for home treatment

U.S. Federal law requires that HBOT is administered only on the prescription of a physician. Home hyperbaric therapy is not accepted. There are soft sided HBOT chambers, which are sometimes used for self-prescribed home treatment. These are usually referred to as "mild chambers", which is a reference to the lower maximum pressure of soft-sided chambers. Those commercially available in the USA only go up to 1.4 ATM, 1.4 bar or 20 pounds per square inch (about 9 feet under water). These chambers are not useful for diving injuries and deliver only a slight increase in total blood oxygen content. They are of use for high altitude "mountain sickness".

Historical link to diving

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 in several ways:

  • the increase in pressure in the chamber reduces the size of the gas bubbles improving transport of blood to tissues downstream of the bubbles
  • the high concentrations of oxygen breathed by the casualty are beneficial in keeping oxygen-starved tissues alive
  • the high concentrations of oxygen in the tissues have the effect of removing the nitrogen from the bubble making it smaller until it consists only of oxygen which is re-adsorbed into the body

Bubbles are eventually eliminated by long exposure to pressure and high oxygen concentrations, allowing a gradual reduction of pressure back to atmospheric levels.

Treatments

The slang term for a cycle of pressurization inside the HBO chamber is "a dive".

Emergency HBOT for diving disorders typically follows one of these two forms:

  • for most cases, a shallow "dive" to a pressure the equivalent of 18 metres / 60 feet of water for 3 to 4.5 hours with the casualty breathing pure oxygen with air breaks every 20 minutes to reduce oxygen toxicity
  • for extremely serious cases, a deeper "dive" to a pressure the equivalent of 37 metres / 122 feet of water for 4.5 hours with the casualty breathing air.

An HBOT treatment for longer-term conditions is often a series of 20 to 40 "dives".

In Canada and the United States, the U.S. Navy Dive Charts are used 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.

Complications

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 even trapped underneath dental fillings.
  • Breathing high-pressure oxygen for long periods can causes oxygen toxicity. One of the side effects of oxygen toxicity is a seizure.

Also see : recompression chamber, decompression chamber, in-water recompression

External links

  • Undersea and Hyperbaric Medical Society http://www.uhms.org/
  • Diving Diseases Research Centre http://www.DDRC.org/
  • Diving Medicine Online http://www.scuba-doc.com/
  • HBO evidence http://www.hboevidence.com/


Last updated: 03-15-2005 03:33:33
Last updated: 05-01-2005 03:44:41