No one questions the roles oxygen has in wound healing. In almost every aspect of wound healing, from the inflammatory process to remodeling and from tissue survival to infection control, adequate tissue oxygen tensions are essential. The roles of hyperbaric oxygen (HBO), however, are not as well defined. When wound healing is not progressing in a normal fashion and ischemia/hypoxia is a contributing factor, it makes sense to use HBO as an adjunct to manage this aspect of the problem. Even more controversial is the role of free radicals in wound healing and whether the production of free radicals by HBO is detrimental to healing and survival. This article addresses these three aspects of the oxygen molecule spectrum and dispels misconceptions about the harmful nature of free radicals.
Things to Know about Oxygen
The chemical element oxygen has many unique features. It has the ability to oxidize almost any organic substance to modify it. It does this by “gob- bling up” electrons from the compound, which defines oxidation. It also actively forms oxides with inorganic elements such as iron, which results in the rusting process. No other element is so active in the degrading/oxidizing process. As “destructive” as oxygen is, life as we know it would not exist on earth without oxygen. The question is how is this paradox resolved? The answer is that defenses against oxygen were developed as evolution of life on Earth progressed. As Nick Lane said in Oxygen: The Molecule that Made the World, oxygen is the "elixir of life —and death."1 Much is known about the role of oxygen in wound healing. A gradient exists from inspired oxygen tensions of 160 mmHg while breathing air to 0.5 mmHg in the mitochondria (Table 1). This is a 99.7 percent decrease in oxygen tensions from the start to the end of the gradient. Without the 0.5 mmHg oxygen tensions in the mitochondria, energy for cell metabolism will not occur. Consequently, any interference in the oxygen tension at each step of the gradient can be detrimental to mitochondrial function. Clinical conditions that ultimately interfere with oxygen delivery to the mitochondria occur at each level of the gradient (Table 2).
Table 1: Oxygen Fall-off from Inspired Air to the Mitochondria
From the wound healing consideration, the tissue fluid oxygen tension is the one that is so critical for healing. Hunt (1969) confirmed that in order for fibroblasts to function and for wounds to heal, the tissue oxygen tension needs to be in the 30- 40 mmHg range (Figure 1).2 Healing is unlikely to occur below 30 mmH; above 40 mmHg healing is likely. Failure for wounds to heal with oxygen tensions 40 mmHg or greater indicate that other potential causes of non-healing, such as bioburden, deformity, inadequate protection/stabilization, malnutrition, cicatrix/bursa barriers to angiogenesis, matrix metalloproteins, and/or inadequate protection/stabilization, are also present.
Table 2: Conditions that Interfere with Oxygen Availability to the Mitochondria
||Ventilation-perfusion inequalities, obstructive lung disease
||Atherosclerosis, vasoconstriction, shunting, anemia
|Capillaries/Red Blood Cells
||Thickening of the basement membrane, sludging, hemoglobinopathies
||Relative barriers; e.g., edema, cicatrix, bursa, exudates
Oxygen, the “elixir of life,” is not only required for wound healing—its intracellular tensions and radicals generated by biochemical reactions are an integral part of many other cellular processes. These range from maintaining consciousness to initiating angiogenesis and from intermediary metabolism (Krebs cycle) to white blood cell oxidative killing. For example, during the oxidative burst in the neutrophil phagocytic vesicle, oxygen consumption increases 100-fold or more to kill bacteria through the generation of superoxides and peroxides.3,4 Oxygen is carried to every cell in the body by the bloodstream through a “loose” attachment to the iron in the hemoglobin molecule. With high oxygen tensions, as in the lungs, oxygen attaches to the hemoglobin molecule. In the tissues, oxygen diffuses through the capillary to the lower oxygen tension tissue fluids and into the cell. The physiology is explained by the Henderson-Hasselbalch equation and the biochemistry by oxygen attachment to iron to convert it to the ferric state in the oxygen-rich lung environment and release of oxygen and conversion to the ferrous state because of the lower oxygen tension tissue fluids.
As the “elixir…of death,” oxygen, when not physiologically regulated, is harmful and can kill every cell in the body. Obviously, too low oxygen tensions result in interference with cell function, which has important ramifications for wound healing. With low oxygen tensions, the cell goes into a state of suspended animation (hibernation). While not dead, it is not functioning at a level where it can continue its physiological functions. For the fibroblast, this means the arrest of wound healing; however, with restoration of adequate oxygen tensions it can recover these functions. This, of course, is the justification for revascularization and hyperbaric oxygen therapy (discussed in the next section). Oxygen tensions below a certain point for a sustained period of time result in cell death and no chance of return of function. A big question is whether oxygen utilization for metabolism, analogous to combustion, is ultimately responsible for the cell’s death just as after the consumption of all combustible material, the fire ceases to burn. Cells are programmed to die, i.e., apoptosis, at specific times. Is this because oxidation eventually “burns out” every cell in the body and results in the death of the organism?
Figure 1: Oxygen Tensions Necessary for Wound Healing
Legend: Oxygen is required for fibroblasts to elaborate their functions of secretion and collagen formation. If deficient, the fibroblast may remain viable but not function for angiogenesis, a matrix needs to be generated by the fibroblast so capillary budding can grown into it and advance the blood supply.
Things to Know about Hyperbaric Oxygen
Too much oxygen is likewise harmful, the other aspect of the “elixir…of death.” Sustained periods of breathing 100 percent oxygen can cause pulmonary edema. Hyper-physiological doses of oxygen, as achieved with hyperbaric oxygen, are toxic to every cell in the body. The target tissue for this side effect is the brain, and the consequence of this toxic insult is the oxygen-induced seizure. Intermittent exposures and air breaks while breathing HBO mitigate this side effect. Why seizures occur with increased oxygen tensions of the brain is not clear. Some attribute it to hyper-metabolism just as increased oxygen percentages cause a fire to burn more vigorously. Another idea (which will be further elaborated later) is that the scavengers of the reactive oxygen species are overwhelmed by the free radicals generated by the hyperbaric oxygen exposure.
The mechanisms of hyperbaric oxygen are support- ed by physics and physiology.5.6 Hyperbaric oxygen at 2 ATA (33 FSW) increases the inhaled oxygen partial pressure tenfold from 160 mmHg partial pressure of oxygen to over 1600 mmHg. This increases the oxygen diffused from the alveolus to the plasma of the alveolar capillary tenfold but does not change the hemoglobin-carried oxygen of the red blood cell since in the absence of lung disease and/or red blood cell diseases, it would already be approaching 100 percent.
In room air, 97.5 percent of the oxygen in the blood is carried by the hemoglobin in the red blood cell and 2.5 percent is physically dissolved in plasma. With hyperbaric oxygen, the tenfold increase in plasma oxygen adds 25 percent to the blood’s oxygen carrying capacity. All oxygen delivery (as well as nutrients) to cells, be it from hemoglobin or physically dissolved in the plasma, must first diffuse through the capillary, then through tissue fluids, to the cell. The oxygen diffusion is in response to gradients, which are high in the blood, lower in the tissue fluids, and lowest in the cell. With HBO the tenfold increase in plasma oxygen content supplements the hemoglobin-carried oxygen and mitigates conditions where blood flow, hemoglobin-carried oxygen, or diffusion distance problems interfere with oxygen delivery to the cell (Figure 2). Under HBO conditions enough oxygen is physically dissolved in the tissue fluids to meet cellular oxygen requirements in the absence of hemoglobin-carried oxygen.
Oxygen diffuses through relative barriers poorly, especially as compared to carbon dioxide. Carbon dioxide’s ability to diffuse through tissue fluid is 20-times greater than that of oxygen. Commonly encountered relative barriers include atherosclerotic vessels, which interfere with perfusion; thickened capillary membranes, which slow diffusion through the endothelium; edema fluid, which increases the diffusion distance from the capillary to the cell; and cicatrix, which acts as an obstruction (Table 2). The tenfold increase in tissue fluids achieved with HBO promotes oxygen tissue diffusion through these barriers, which are considered relative because they can vary in extent from inconsequential to totally obstructing oxygen availability to the cell.
Figure 2: Blood Oxygen Content, Hyperbaric Oxygen and "Life without Blood"
Legend: The physically dissolved oxygen from the hyperbaric oxygen exposure adds to the hemoglobin-carried oxygen. Once the hemoglobin-carried oxygen becomes fully saturated it cannot carry additional oxygen. At about a 2000 mmHg oxygen partial pressure there is enough oxygen content in the plasma to meet oxygenation requirements without hemoglobin-carried oxygen. This was demonstrated by Boerema's "Life without Blood" experiment.
Key: A-V O2 = arterial-venous oxygen extraction, HBO = hyperbaric oxygen, Rx = treatment, w/o = without
In our experience, there are three fundamental reasons why wounds, especially diabetic foot ulcers, do not heal in the usual and customary fashion.8 These are failure to address adequately the underlying deformity; persistence of deep infection of bone, cicatrix, and/or bursa; and ischemia/hypoxia. Of the three, the confirmation of ischemia/ hypoxia is perhaps the easiest. This can be ascertained with the clinical exam (e.g., palpable pulses, Doppler pulses, skin coloration and temperature, and capillary refill time), imaging studies, and juxta-wound transcutaneous oxygen measurements (TCOMs). As mentioned before, tissue fluid oxygen tensions in the 30-40 mmHg range are needed for wounds to heal.2 Juxta-wound TCOMs measure and reflect the tissue fluid oxygen tensions and can be used as a guide to determine whether the ischemic/hypoxic wound will heal or if measures to increase perfusion/oxygenation are needed.
Hyperbaric oxygen is an intervention to increase wound oxygenation (in addition to revascularization, edema reduction, improved cardiac function, and pharmacological agents).9 The value of using TCOMs to predict healing with use of HBO as an adjunct to manage wound hypoxia and achieve healing is established. We reported that hypoxic wounds (that is, wounds with juxta-wound TCOM levels in room air of less than 40 mmHg) heal in 87 percent of cases if the TCOMs increase to 200 mmHg or greater with HBO exposure and HBO treatments are subsequently used in wound management.12 This information using TCOMs objectifies the indications for HBO in problem wounds.
Hyperbaric oxygen has applications to many other medical conditions in addition to hypoxic wounds. The indications for using HBO in other conditions are based on its mechanisms. We divide the mechanisms into primary and secondary.5,6 The primary mechanisms hyperoxygenation and pressurization (to reduce bubble size) are immediate and act in a drug dose-duration fashion. Applications in addition to decompression sickness and arterial gas embolism include threatened flaps, acute blood loss anemia, acute peripheral ischemia, burns, crush injuries and compartment syndromes, and central retinal artery occlusions. Secondary mechanisms occur as a result of hyperoxygenation and pressurization acting on body tissues and microorganisms. In contrast to the dose-duration effects of the primary mechanisms, the effects of the secondary mechanisms tend to be additive and require repetitive HBO treatments. They include edema reduction, stimulation of host healing responses (including fibroblast function and angiogenesis), gas washout (for carbon monoxide poisoning and decompression sickness), reperfusion injury, delayed radiation damage of soft tissue and bone, cerebral abscess, refractory osteomyelitis, gas gangrene, and necrotizing soft tissue infections. Awareness of the mechanisms of HBO helps justify its use. In addition, mechanisms of HBO may have applications to current off-label uses of HBO such as acute brain and spinal cord events, sports injuries, osteonecrosis, and fracture healing.
Things to Know about Free Radicals and Hyperbaric Oxygen
For decades researchers have known that highly reactive molecules called free radicals cause ag- ing by damaging the DNA (deoxyribonucleic acid) of cells and thus disturbing the carefully regulated functions of tissues and organs. Free radical formation is an integral part of intermediary metabolism (Krebs cycle) and, with glucose plus oxygen, provides the energy for cell metabolism, cell/tissue function, and generation of cell products. In addition, free radicals generated by the phagosomes in the neutrophil kill bacteria. How- ever, reactive oxygen species in the wrong place or in super-physiological numbers damage tissues. This is observed with radiation injury and ischemia-reperfusion injury. There is increasing recognition that cardiovascular diseases, neurodegenerative diseases such as Alzheimer’s and chronic inflammation, apoptosis, and necrosis have oxidative stress components.
With evolution, cells have generated antioxidants (oxygen radical scavengers) to mitigate oxidative stresses. This phenomenon is so important that Lane asserts that life would not have evolved to its present form without the generation of antioxidants to “tame” the highly reactive oxygen radicals it generates.1 Defenses to mitigate the oxidative stresses include vesicles such as the mitochondrion shell, which contains the reactive oxygen species as glucose is metabolized to generate energy, and allows the train of reactions to proceed in a regulated fashion. The other mechanism to handle reactive oxygen species is the generation of antioxidants such as glutathione peroxidase and superoxide dismutase.
While HBO is believed to generate reactive oxygen species, oxygen is required for the generation of oxygen radical scavengers.14 Consequently, in the hypoxic environment insufficient oxygen to generate the oxygen radical scavengers may cause damage such as cell death and tissue necrosis, which is always of concern in problem wounds. Much is known about oxygen radical scavengers/ antioxidants, oxygen transporters, and reactive oxygen species.15 For example, 84 genes in the human genome are related to oxygen stresses.
Reactive oxygen species may have salubrious effects with respect to longevity and disease prevention. There is increasing appreciation of noncoding DNA sequences (epigenes, or “junk” genes) as being able to influence genetic information through the expression of DNA. The physiological reactive oxygen species generated by HBO could be one of the “missing links” in our understanding of how they are beneficial to the organism. Reactive oxygen species may remove damaged DNA segments that lead to aging, diseases, neoplasms, or prevent wound healing.
Animal studies have shown that longevity is in- creased in animals genetically altered and missing antioxidant enzymes.16 In addition, those animals that overproduced superoxides lived 32 percent longer than the controls. The longest living rodent, the naked mole rat, is able to survive 25 to 30 years, has lower levels of antioxidants than similar sized rodents, and remains disease free eight times longer. The hypothesis to explain this observation was that the mole rat accumulates more oxidative damage to their tissues at an earlier age, so only the most healthy individuals survive. A herbicide that generates free radicals resulted in worms living 58 percent longer than untreated animals.
Oxygen is a remarkable molecule; too little is lethal and too much is lethal. Organisms have generated remarkable mechanisms to maintain oxygen in physiological settings. Hyperbaric oxygen may alter these protective responses on one hand and on the other may make them more effective. The role of reactive oxygen in wound healing has hardly been addressed, but much is known about its role in killing bacteria. We are not quite ready to recommend sleeping in the pressurized HBO chamber, but if HBO modestly generates reactive oxygen species to turn on epigenes to influence DNA messaging which, in turn, mitigate disease processes, many new roles for HBO can be expected.
- Lane N. Oxygen: The Molecule that made the World. New York: Oxford University Press; 2002. P. 1-15.
- Hunt TK, Zederfeldt B, Goldstick TK. Oxygen and healing. Am J Surg. 1969;118:521-5.
- Sabarra AJ, Karnovsky MI. The biochemical basis of phagocytosis. I. Metabolic changes during the ingestion of particles by polymorphonuclear leu- kocytes. J Biol Chem. 1959; 234(6):1355–62.
- Sabarra AJ, Karnovsky MI. The biochemical basis of phagocytosis. II. Incorporation of C14-labeled building blocks into lipid, protein, and glycogen of leukocytes during phagocytosis. J Biol Chem. 1960; 235(8):2224-9.
- Strauss MB, Hart GB, Miller SS, et al. Mechanisms of hyperbaric oxygen. Part 1 primary: hyperoxygenation and pressurization. Wound Care & Hyperbaric Medicine. 2012; 3(3):27-42.
- Strauss MB, Hart GB, Miller SS, et al. Mechanisms of Hyperbaric Oxygen. Part 2 secondary: consequences of hyperoxygenation and pressurization. Wound Care & Hyperbaric Medicine. 2012; 3(4):45-63.
- Boerma I, Meyne MG, Brummelkamp WK, et al. Life without blood. A study of the influence of high atmospheric pressure and hyperthermia on dilution of the blood. J Cardiovas Surg. 1960; 1:133-46.
- Strauss MB, Aksenov IV, SS Miller. MasterMinding Wounds. North Palm Beach, FL: Best Publishing Company; 2012. P. 27-8.
- Strauss MB, Miller SS, Aksenov IV, Manji K. Wound oxygenation and an introduction to hyperbaric oxygen therapy: interventions for the hypoxic/ischemic wound. Wound Care & Hyperbaric Medicine. 2012; 3(2):36-52.
- Strauss MB, Breedlove JW, Hart GB. Use of Transcutaneous oxygen measurements to predict healing in foot wounds. Undersea Hyperb Med. 1997; 24(Abst 40):15.
- Strauss MB, Winant DM, Breedlove JW, et al. The predictability of transcutaneous oxygen measurements for wound healing. Undersea Hyperb Med. 1998; 25(Abst 28):16.
- Strauss MB, Bryant BJ, Hart GB. Transcutaneous oxygen measurements under hyperbaric oxygen conditions as a predictor for healing of problem wounds. Foot Ankle Intl. 2002; 23:933-7.
- Fife CE, Buyukcakir C, Otto G, et al. The predictive value of transcutaneous oxygen tension measurements in diabetic lower extremity ulcers treated with hyperbaric oxygen therapy: a retrospective analysis of 1144 patients. Wound Repair Regen. 2002; 10:198-207.
- Ferrari R, Ceconi C, Curello S, et. al. Oxygen- mediated myocardial damage during ischaemia and reperfusion: role of the cellular defences against oxygen toxicity. J Mol Cell Cardiol. 1985 Oct;17(10):937-45.
- Moyer MW. The myth of antioxidants. Scientific American. 2013;308(2): 62-7.
About the Authors
Drs. Strauss and Miller are physicians at the Hyperbaric Medicine Department at Long Beach Memorial Medical Center. Additional biographical information can be found after their diving article on page 33. The authors would like to acknowledge Dr. Phi-Nga Jeannie Le for her editorial contribution to this article.