Hyperbaric oxygen therapy

British Medical Journal, Oct 24, 1998 by R.M. Leach, P.J. Rees, P. Wilmshurst

Over the past 40 years hyperbaric oxygen therapy has been recommended and used in a wide variety of medical conditions, often without adequate scientific validation of efficacy or safety. Consequently a high degree of medical scepticism has developed regarding its use. The Undersea and Hyperbaric Medical Society approves use of hyperbaric oxygen for a few conditions for which there is thought to be reasonable scientific evidence or well validated clinical experience. In these conditions early referral is essential.

Hyperbaric oxygen has been shown ineffective in diseases such as multiple sclerosis and dementia, but it continues to be used despite the risks of the treatment. For conditions where its use remains unproved--for example, rheumatoid arthritis, cirrhosis, and gastroduodenal ulcer--hyperbaric oxygen should be used only in the context of well controlled clinical trials.

Biochemical and physiological effects

At sea level the plasma oxygen concentration is 3 ml/l. Tissues at rest require about 60 ml of oxygen per litre of blood flow (assuming normal perfusion) to maintain normal cellular metabolism, although requirements vary between tissues. At a pressure of 3 atmospheres (304 kPa) dissolved oxygen approaches 60 ml/l of plasma, which is almost sufficient to supply the resting total oxygen requirement of many tissues without a contribution from oxygen bound to haemoglobin. This has advantages in situations such as carbon monoxide poisoning or in severe anaemia where difficult crossmatching or religious belief prevents blood transfusion.

Oxygen at 300 kPa increases oxygen tension in arterial to nearly 270 kPa and in tissue to about 53 kPa. This improves the cellular oxygen supply by raising the tissue-cellular diffusion gradient. The hyperoxia has potential benefits including improved angiogenesis. The formation of collagen matrix is essential for angiogenesis and is inhibited by hypoxia. In irradiated tissue hyperbaric oxygen is more effective than normobaric oxygen at raising tissue partial pressure of oxygen and promoting angiogenesis and wound healing. The healing process may also be helped in non-irradiated tissues with compromised perfusion, but this requires further validation.

The value of hyperbaric oxygen therapy in decompression illness and arterial gas embolism depends on the physical properties of gases. The volume of a gas in an enclosed space is inversely proportional to the pressure exerted on it (Boyle's law). At 300 kPa bubble volume is reduced by about two thirds, Any intravascular bubbles causing obstruction move to smaller vessels, which reduces extravascular tissue damage. Dissolution of the gas bubble is enhanced by replacing the inert gas in the bubble with oxygen, which is then rapidly metabolised by the tissues.

Availability and administration

Multiplace chambers are available in a few NHS hospitals (Aberdeen, Craigavon, Newcastle upon Tyne), Royal Navy centres, private units, police diving units, professional diver training schools, and sites associated with the North Sea oil industry. The United States has over 250 facilities.

Often early treatment is essential for maximum benefit. This poses appreciable practical problems as severely ill patients may have to be transported long distances and may require intensive medical support, including mechanical ventilation, between treatment sessions. It is important to discuss the potential benefits and risks for each patient with the regional hyperbaric oxygen facilities.

Multioccupancy chambers are required for critically ill patients who require an attendant within the chamber and are usually used for acute problems. Monoplace chambers can be used to treat patients with chronic medical conditions. Hyperbaric oxygen is inhaled through masks, fight fitting hoods, or endotracheal tubes.

Inside the chambers pressure is usually increased to about 250-280 kPa, equivalent to a depth of 15-18 m of water. The duration of treatment varies from 45 to 300 min and patients may receive up to 40 sessions. Appropriate monitoring is essential during treatment, and facilities for resuscitation and immediate mechanical ventilation should be available.

Dangers of hyperbaric oxygen

The potential risks and risk-benefit ratio of hyperbaric oxygen have often been underemphasised in therapeutic trials. The side effects are often mild and reversible but can be severe and life threatening. In general, if pressures do not exceed 300 kPa and the length of treatment is less than 120 minutes, hyperbaric oxygen therapy is safe. Overall, severe central nervous system symptoms occur in 1-2% of treated patients, symptomatic reversible barotrauma in 15-20%, pulmonary symptoms in 15-20%, and reversible optic symptoms in up to 20% of patients.

Reversible myopia, due to oxygen toxicity on the lens, is the commonest side effect and can last for weeks or months. Epileptic fits are rare and usually cause no permanent damage. A suggested carcinogenic effect of hyperbaric oxygen has not been substantiate in extensive studies.