Hyperbaric oxygen therapy
Hyperbaric oxygen therapy (HBOT) is the medical use of
oxygen at a higher than
atmospheric pressure.
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A father and his son inside a small hyperbaric oxygen chamber. |
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 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 can not be exploited any further. Oxygen transport by plasma however is significantly increased under HBOT.
The main indications for HBOT are:
*Certain non-healing 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 Medicare as an appropriate treatment for 14 conditions. European medicine recognizes it as an appropriate treatment for many more [
1] Alternative healing advocates of many stripes believe it is useful for additional conditions. Among the uses of HBOT are therapy for
brain healing (as in
stroke,
dementia,
cerebral palsy and
absence seizure), and for some infectious conditions, such as
Lyme disease and
Post-polio syndrome.
HBOT is expensive, with a session costing anywhere from $100 to $1200 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 Multiple Sclerosis Therapy Centres.
The traditional type of HBOT chamber is a
hard shelled pressure vessel. Such chambers can be run at absolute pressures up to 600
kilopascals or 85
PSI (lbf/in²), nearly 6
atmospheres.
Navies, diving organizations 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 with the view ports (windows) or hull made of acrylic.
* one or more human entry hatches - these 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 pressurized to allow patients to enter or exit the main chamber while it is still pressurized
* 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
oxygen 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 minimize 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 oxygen 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.
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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 an oxygen 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 maneuver 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 patient's ears may "squeak" as the pressure inside the ear equalizes with the chamber. The temperature in the chamber will cool.
There are portable HBOT chambers, which are sometimes used for 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 go up to 4.1 PSI which is equivalent to a water depth of 11 ft. These chambers are typically not operated with 100% oxygen as the breathing gas; therefore, they are not hyperbaric oxygen therapy chambers
These chambers were originally developed and used for
altitude sickness, which is a valid application. The benefits of HBOT to a number of other off label conditions such as lyme disease, cancer, stroke, cerebral palsy, multiple sclerosis, cardiac conditions, brain injury, and other immune disorders have never been substantiated by research. Benefits of HBOT as a treatment for autism have been hypothesized, but no randomized controlled studies for this application have been published.
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-absorbed 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.
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 meters / 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 meters / 122 feet of water for 4.5 hours with the casualty breathing air.
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.
An HBOT treatment for longer-term conditions is often a series of 20 to 40 "dives".
The
Undersea and Hyperbaric Medical Society (UHMS) publishes a "Committee Report" which compiles the latest research findings and contains information regarding the recommended duration and pressure of the longer-term conditions.
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 cause oxygen toxicity. One of the side effects of oxygen toxicity is a
seizure.
* Vision changes (
myopia or nearsightedness) caused by swelling of the
lens. This is more a temporary side-affect than a complication and usually resolves in 2-4 weeks following completion of HBOT.
The only absolute
contraindication to hyperbaric oxygen therapy is untreated
pneumothorax. Relative complications include
grand mal seizure, fever, the inability to clear the ears or sinuses, and the use of certain chemotherapy agents.
*
Recompression chamber*
Decompression chamber*
In-water recompression*
Oxygen therapy*
HyperbaricMedicine.ca*
Canadian Association of Hyperbarics*
Undersea and Hyperbaric Medical Society*
Diving Diseases Research Centre*
Diving Medicine Online*
HBO evidence*
Hyperbaric Oxygen Therapy
