VOL: 97, ISSUE: 09, PAGE NO: 7
Keith Cutting, MN, RMN, RGN, DipN, Cert Ed (FE), is principal lecturer, Faculty of Health Studies, Buckingham Chilterns University College, Chalfont St GilesHyperbaric oxygen therapy (HBO) delivers oxygen at up to three times atmospheric pressure and will raise arterial levels of oxygen 10-15 times higher than that achieved by normal atmospheric pressure (Steenblock, 2000).
Hyperbaric oxygen therapy (HBO) delivers oxygen at up to three times atmospheric pressure and will raise arterial levels of oxygen 10-15 times higher than that achieved by normal atmospheric pressure (Steenblock, 2000).
HBO as therapy
HBO is often associated with treatment for deep-sea divers who have suffered from decompression sickness or 'the bends'. This results in the release of nitrogen bubbles in the blood stream when a diver ascends too quickly from the ocean depths.
During HBO therapy an individual will breathe 100% oxygen. Oxygen in these instances should be regarded as a drug and will be delivered to the recipient inside a chamber at two to three times atmospheric pressure. The amount of oxygen delivered depends on the pressure generated. It should be noted that a low dose may be ineffective (Hammarlund, 1995) and, more important, an elevated dose of oxygen can be toxic (Clarke, 1982). Therefore, it can only be administered intermittently.
Boyle's Law states that a gas will reduce in volume when subjected to pressure (Frost and Farine, 2000). Thus any free gas in the circulatory system, for example nitrogen in a case of 'the bends', will decrease in volume and hopefully pass through the vascular system or at worst into a smaller vessel, consequently limiting any resulting infarction. Raising oxygen levels also assists in the removal of toxic gasses, for example carbon monoxide. HBO produces a rapid dissociation of CO from haemoglobin (Wright-Patterson Medical Centre, 2000).
Apart from decompression sickness, HBO has been found to be of use in the treatment of a number of different pathologies (Box 1) It can also be used to heal deep wounds (Allen, 1991).
HBO and wound-healing
The importance of oxygen in the repair of tissue is widely recorded and the reparative process will expend large quantities (Silver, 1966). Its direct unhumidified application at atmospheric pressure is of dubious benefit, however, as this will desiccate the wound surface.
HBO therapy has been shown to be of benefit in non-healing chronic wounds by countering the effects of hypoxia (Sheffield, 1988). Hypoxia leads to damage to the micro-vascular endothelium, which in turn causes oedema. Oedema compresses the capillary bed, diminishing the perfusion of tissues and enhancing ischaemia (Allen, 1991). HBO causes vasoconstriction by having an adrenergic effect on blood vessels. This reduces oedema but does not impede oxygenation of tissues. The extra oxygen ensures enough is delivered to the tissues.
The presence of oxygen-free radicals in wounds is the result of ischaemia-reperfusion injury and may lead to cell death. Zamboni et al (1989) have demonstrated that HBO can be used in such cases.
The risk of infection increases as tissue oxygen tension decreases. HBO is therefore able to assist in the management of wound infection. Hypoxia prevents phagocytosis of bacteria by leucocytes. When the partial pressure of oxygen assisted by HBO returns to normal tissue concentrations, leucocytes are able to carry out their antibacterial role (Park et al, 1995).
Poor vascularity impedes wound-healing. Rohr et al (1992) showed how vital oxygen is in the healing process and how increased angiogenesis may be achieved under pressurised oxygen.
The ability to apply topical oxygen to a wound therapeutically has been achieved in recent years. With this method, humidified oxygen is applied to the wound via a local chamber. Although this strictly cannot be called HBO, as it does not comply with international guidelines (Donlin and Bryson, 1995), it is on occasions referred to as such. Alternatively, the term topical hyperbaric oxygenation (THO) has been used (Upson, 1986).