Hyperbaric chamber encyclopedia topics

Basic types (pressure control)

There are two basic types of diving chamber differentiated by the way in which the pressure in the diving chamber is produced and controlled.

Diving bell type

The historically older open diving chamber is in effect a large diving bell, utilising the equivalent of a moon pool to equalise internal air pressure and external water pressure automatically without the need, necessarily, to measure and control it. An air compressor or bottled compressed air is required to maintain the volume of the air as it becomes compressed with increasing depth, or to make up for oxygen depleted by the occupants' breathing and for carbon dioxide removed from exhaled air by a carbon dioxide scrubber system. This type of diving chamber can only be used underwater, as the internal air pressure is directly proportional to the depth underwater and raising or lowering the chamber is the only way to adjust the pressure.

Hyperbaric chamber

A sealable diving chamber is a pressure vessel with hatches large enough for people to enter and exit, and an air compressor to raise the internal air pressure. This type is called a hyperbaric chamber whether used underwater or at the water surface or on land to produce underwater pressures, though some use submersible chamber to mean those used underwater and hyperbaric chamber to mean those used out of water. There are two related terms which reflect particular usages rather than technically-different types:

Decompression chamber: a hyperbaric chamber used by surface-supplied divers to make their decompression stops
Recompression chamber: a hyperbaric chamber used to treat or prevent decompression sickness.

When used underwater there are two ways to prevent water flooding in when the submersible hyperbaric chamber's hatch is opened. The hatch could open into a moon pool chamber, and then its internal pressure must first be equalised to that of the moon pool chamber. More commonly the hatch opens into an underwater airlock, in which case the main chamber's pressure can stay constant, while it is the airlock pressure which changes. This common design is called a lock-out chamber, and is used in submarines, submersibles, and underwater habitats as well as diving chambers.

Another arrangement utilises a dry airlock between a sealable hyperbaric compartment and an open 'diving bell' compartment (so that effectively the whole structure is a mixture of the two types of diving chamber).

When used underwater all types of diving chamber are attached to a diving support vessel by a strong cable for raising and lowering and an umbilical cable delivering compressed air, power and communications, and all need weights attached or built in to overcome their buoyancy. The greatest depth reached using a cable-suspended chamber is about 1500 m, beyond this the cable becomes unmanageable.

Related equipment

In addition to the diving bell, related diving equipment includes the following.

Submersibles and submarines differ in being able to move under their own power.
Bathysphere (vessel): name given to an experimental deep-sea diving chamber of the 1920s and 1930s.
Benthoscope: a successor to the bathysphere built to go to greater depths.
Bathyscape: a self-propelled submersible vessel able to adjust its own buoyancy for exploring extreme depths.
Underwater habitat: consists of compartments operating under the same principles as diving bells and diving chambers, but fixed to the sea floor for long-term use.

Diving chambers in use underwater

As well as transporting divers, a diving chamber carries tools and equipment, breathing gas cylinders to replenish scuba tanks, and communications and emergency equipment. It provides a temporary dry air environment during extended dives for rest, eating meals, carrying out tasks which can't be done underwater, and for emergencies. Diving chambers also act as an underwater base for surface supplied diving operations, with the divers' umbilicals (air supply, etc) attached to the diving chamber rather than to the diving support vessel.

Use of open diving bell type

Diving bells and open diving chambers of the same principle were more common in the past owing to their simplicity, since they do not necessarily need to monitor, control and mechanically adjust the internal pressure. Secondly since internal air pressure and external water pressure on the bell wall are almost balanced, the chamber does not have to be as strong as a sealable diving chamber. (Actually if h is the distance between a point on the side of the bell and the air/water interface at the bottom, the air pressure at that point is higher than the water on the other side by a water head pressure equivalent to h).

The diving bell or open diving chamber must be raised slowly to the surface with stops every 10 m so that divers can follow decompression procedures and avoid decompression sickness. This may take hours, and so limits its use.

Use of hyperbaric chambers

Submersible hyperbaric chambers can be brought to the surface without delay to allow divers to decompress since they can maintain the same pressure at which the divers were working. The divers can stay in the chamber on the support vessel to decompress. This flexibility makes them safer to use and more useful in an accident or emergency, including problems affecting the dive support vessel, such as sudden bad weather. They are used to support saturation diving for which the decompression times are very long.

A diving chamber based on a pressure vessel is more expensive to construct since it has to withstand very high pressure differentials. These may be both crushing pressures when the chamber is lowered into the sea and the internal pressure is kept less than ambient water pressure, or it may be an outwards pressure when it is out of the water and its internal pressure is set the same as water pressure at a certain depth.

Hyperbaric chambers also require more sophisticated systems to set and control internal gas pressure. However modern manufacturing techniques and control systems have reduced the cost and this type of diving chamber is now more common than the older dive bell type.

Hyperbaric lifeboats are specialized diving chambers or submersibles able to retrieve divers or occupants of diving chambers or underwater habitats in an emergency and to keep them in the required decompression phase. They have airlocks for underwater entry or to form a watertight seal with hatches on the target structure to effect a dry transfer of personnel. Rescuing occupants of submarines or submersibles with internal air pressure of one atmosphere requires being able to withstand the huge pressure differential to effect a dry transfer, and has the advantage of not requiring decompression measures on returning to the surface.

Diving chambers in use on land

Hyperbaric chambers are also used on land and at the ocean surface:

to treat divers for decompression sickness (recompression chambers)
to take surface supplied divers who have been brought up from underwater through their decompression stops
to train and test divers to adapt to hyperbaric conditions and decompression routines
to treat people using raised oxygen pressure in hyperbaric oxygen therapy, HBOT
in scientific research requiring elevated gas pressures.

Hyperbaric chambers designed only for use out of water do not have to resist inwards crushing forces, only outwards expansion forces. Those for medical applications typically only operate up to two or three atmospheres, while those for diving applications have to go to six atmospheres and above.

Lightweight portable hyperbaric chambers which can be lifted by helicopter are used by commercial diving operators and rescue services to carry one or more divers requiring hospitalisation.

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 and air embolism.
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 United States, the Undersea and Hyperbaric Medical Society, known as UHMS, approved for reimbursement diagnoses for application of HBOT in hospitals. The following approved indications are approved uses of hyperbaric oxygen therapy as defined by the UHMS Hyperbaric Oxygen Therapy Committee. The Committee Report can be purchased directly through the UHMS

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
Exceptional Blood Loss (Anemia)
Intracranial Abscess
Necrotizing Soft Tissue Infections (Necrotizing fasciitis)
Osteomyelitis (Refractory)
Delayed Radiation Injury (Soft Tissue and Bony Necrosis)
Skin Grafts & Flaps (Compromised)
Thermal Burns

In the United States, HBOT is recognized by Medicare as a reimbursable treatment for 14 UHMS "approved" conditions. An HBOT session costs anywhere from $100 to $200 in private clinics, to over $1,000 in hospitals. More U.S. physicians are lawfully prescribing HBOT for "off label" conditions such as Lyme Disease and stroke and also in Autism and related disorders like ADHD . Such patients are treated in outpatient clinics, however it is unlikely that their medical insurance will pay for off label treatments. 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.

HBOT is controversial and health policy regarding its uses is politically charged. Both sides of the controversy on the effectiveness of HBOT is available in the form of PubMed and the Cochrane reviews and a discussion of "Medical Polemics, a discussion of Multiple Sclerosis in particular.

The chamber

Traditional

The traditional type of hyperbaric chamber used for HBOT is a hard shelled pressure vessel. Such chambers can be run at absolute pressures up to 600 kilopascals or 85 PSI (lbf/in²), nearly six atmospheres.

Navies, diving organizations and hospitals typically operate these. They range in size from those which are portable and capable of treating just one patient to those which 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 and 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

Oxygen Breathing

In today's larger "multiplace" chambers, both patients and medical staff inside the chamber breathe from "oxygen helmets", flexible, transparent soft plastic helmets with a seal around the neck similar to a space suit helmet. They may also breathe from tightly fitting aviators type oxygen masks, which supply pure oxygen and remove the exhaled gas from the chamber. During treatment patients breathe 100% 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. Administration of 100% breathing oxygen maximizes the patients treatment. 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. Smaller "monoplace" chambers can only accommodate the patient. No medical staff can enter. The chamber is flooded with pure oxygen or compressed air. The cost of using pure oxygen in a monoplace chamber is much higher than using compressed air. If pure oxygen is used no oxygen breathing mask or helmet is needed. If compressed air is used then an oxygen mask or helmet is needed as in a multiplace chamber. In monoplace chambers that are compressed with pure oxygen a mask is available to provide the patient with "air breaks," periods of breathing normal air, in order to reduce the risk of hyperoxic seizures.

Effects of Pressure

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 increases further, mist may form in the air inside the chamber and the air may become warm. When the patient speaks, the pitch 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 fall.

Home treatment

There are portable HBOT chambers, which are used for home treatment. These are usually referred to as "mild chambers", which is a reference to the lower pressure of soft-sided chambers. Those commercially available in the USA go up to 4 PSI (1.27 ATA 8.92 FSW). International portable chambers can go to 7.35 psi (1.5 ATA 16.38 FSW) or higher. These chambers are operated with oxygen concentrators (typically 95% oxygen) or with 100% oxygen as the breathing gas.

The soft chambers are FDA approved only for the treatment of Altitude Sickness but are commonly used off label primarily for the treatment of autism and other neural conditions. The FDA has a specific warning that supplemental oxygen is not to be used. Terrell Owens of the Dallas Cowboys has one in his house to aide his recovery from injuries as well as teammate Kevin Burnett. Similarly, Jimmy Rollins of the Philadelphia Phillies reported he has one he is using to speed recovery from a sprained ankle. J.D. Drew of the Boston Red Sox has one as does Zach Thomas of the Miami Dolphins.

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 then re-absorbed into the body. After elimination of bubbles, the pressure is gradually reduced back to atmospheric levels.

Protocol

The slang term 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.

Emergency HBOT for diving disorders typically follows one of 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.

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

Possible 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 trapped underneath dental fillings. Breathing high-pressure oxygen for long periods can 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).

Contraindications

The only absolute contraindication to hyperbaric oxygen therapy is untreated pneumothorax.

Also, patients should not undergo HBO therapy if they are taking or have recently taken the following drugs:

Doxorubicin (Adriamycin) - A chemotherapeutic drug.
Disulfiram (Antabuse) - Used in the treatment of alcoholism.
Cis-Platinum - A cancer drug.
Mafenide Acetate (Sulfamylon) - Suppresses bacterial infections in burn wounds

The following are relative contraindications:

Upper respiratory infections - These conditions can make it difficult for the patient to clear their ears, which can result in what is termed 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: Since cancers both thrive in blood rich environments and may be suppressed in high oxygen environments, cancer and HBO poses a dilemma since HBO both increases blood flow via angiogenesis and also raises oxygen levels. Taking an anti-angiogenic supplement may provide a solution to this problem.

Neuro-rehabilitation

The Collet (Quebec) trial that was published in the Lancet in 2001 was the largest randomized trial of Hyperbaric Oxygen Therapy (HBOT) for children with cerebral palsy (CP); it followed the McGill pilot study on the same subject.

The evidence showed both groups of children treated with two very different hyperbaric treatment dosages improved significantly. The motor improvements that were seen and measured with the gross motor function measure were greater, more generalized, and were obtained in a shorter period of time than most of the changes found in any other studies of recognized conventional therapies in the treatment of children with cerebral palsy. The children in both groups improved an average of ten times more during the two months of HBOT (whilst all other therapies and medication were stopped) than during the three months follow-up (when medication and all the ancillary treatments were restarted). This impressive change in the rate of improvements clearly indicates the probable effectiveness of hyperbaric treatment. Both the Lancet commentary and the tech report by the Agency for Healthcare Research and Quality (AHRQ) concluded that the hypothesis of both treatments being equally effective should be retained.

Since the Quebec study of HBOT for children with CP, many reports have been made on the possible efficacy of a low pressure hyperbaric treatment and all the trials conducted with HBOT in CP have demonstrated positive results.

An editorial on CP published by the Undersea and Hyperbaric Medical Society in 2007 called for further research that will include "basic science research to determine a reasonable mechanism of action" for hyperbaric oxygenation as well as "clinical studies of the highest possible methodological rigor".

Middle ear barotrauma (MEBT) is always a consideration in treating both children and adults in a hyperbaric environment, but most children currently being treated with HBOT are being pressurized to 1.3 ATA which greatly reduces the risks of potential side effects of any kind.

Some medical practitioners recommend the use of HBOT for the treatment of acute tinnitus but this treatment has not been verified by independent evidence and the treatment was withdrawn from support by the German health insurance. There is evidence that the therapeutic effects could be greatly due to psychological mechanisms triggered by the patients attitude towards therapy prior to the treatment.

It has been postulated that HBOT might relieve some of the core symptoms of autism..

Fischer et al. in New York University performed the first randomized, placebo-controlled, double-blind trial on MS patients treated with HBOT. Improvements in balance and bladder function were found in 12 of 17 patients (p<0.0001). Those patients with a less severe form of the disease had a more favorable and long lasting response. After a year with no further treatment, the treated group showed a positive change (p<0.0008). Barnes et al. found overall benefit in their treated group (p<0.03) and a year later there was less deterioration in cerebellar function (p<0.03). Two other controlled studies have reported sustained benefit with follow-up.

In the 2004 Cochrane review, Bennett and Heard "found no consistent evidence to confirm a beneficial effect of hyperbaric oxygen therapy for the treatment of multiple sclerosis and do not believe routine use is justified".