Oxigênio e cicatrização

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    O x y g e n i n W o u nd H e a l i n gNutrient, Antibiotic, Signaling Molecule,and Therapeutic Agent

    David E. Eisenbud, MD

    INTRODUCTION

    Common observations made many decades agoby mountain climbers who noted the inability toclear skin infections at high altitude, and JacquesCousteaus deep sea divers who noted that theirwork wounds healed fastest when they werediving, brought general appreciation of the impor-tance of oxygen in healing.1 Recent years have

    brought an increased and more detailed scientificappreciation of the diverse roles that oxygen playsin normal physiology and disease states.2,3 As theindividual steps of the wound healing cascadehave become elucidated in greater detail, theinvolvement of oxygen at nearly every stage hasbecome evident. More oxygen is not always better;nature seems to have adapted us to respond

    constructively to the relative hypoxia that charac-terizes the healing edge of many wounds.

    There remain many gaps in understanding of thebiochemical events of healing. Some of the currentknowledge regarding oxygen, growth factors, andother mediators is seemingly contradictory, andclassification of molecules as promoters or inhibi-tors of healing (eg, oxygen is good, tumor necrosisfactor a [TNF-a] is bad) is simplistic. However, it

    seems possible to reach a unified understandingof healing that reconciles most of the thousandsof basic science investigations into individual stepsin the chain, and oxygen is central to this. Thisarticle summarizes oxygen physiology in woundbiology, and discusses the supporting literature.

    Given the central role of oxygen in healing,there is the potential to manipulate the wound

    The author has no financial conflicts of interest to declare in relation to the writing and publication of thisarticle.Millburn Surgical Associates, 225 Millburn Avenue, Suite 104-B, Millburn, NJ 07041, USAE-mail address: [email protected]

    KEYWORDS

    Oxygen Hypoxia Hyperbaric Wound Healing Diabetes Epithelium Fibroblast

    KEY POINTS

    With deeper scientific understanding of oxygen physiology, and with support from randomized,prospective clinical investigations, the judicious, individualized use of oxygen therapy in woundmanagement may now be considered mainstream.

    Each of the most common categories of chronic wounds (arterial, venous, diabetic, pressure)become established or are perpetuated because of factors that limit oxygen delivery to the woundbed.

    At the low physiologic concentration of H2O2 (0.15%), topical angiogenesis is favorably influenced,distinguished from the 3% v/v strength available commercially; at this high concentration, severeoxidative damage to wounds is noted, and is thus contraindicated in modern wound management.

    Given that correction of wound hypoxia is beneficial to many aspects of healing, it does not neces-sarily follow that more is better, and that hyperoxygenation of normally nourished wounds confersa benefit to justify the risks.

    Clin Plastic Surg 39 (2012) 293310doi:10.1016/j.cps.2012.05.0010094-1298/12/$ see front matter 2012 Elsevier Inc. All rights reserved. p

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    mailto:[email protected]://dx.doi.org/10.1016/j.cps.2012.05.001http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://plasticsurgery.theclinics.com/http://dx.doi.org/10.1016/j.cps.2012.05.001mailto:[email protected]
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    environment by treatment with supplementaloxygen. Oxygen therapy in various forms hasbeen used to ameliorate many medical conditionsfor centuries. However, clinical results have beenvaried, and frequently disappointing. There hasbeen an indiscriminate use of oxygen treatments

    in the past, and there is still an aura of quackeryassociated with this area of medicine. However,in the face of deeper scientific understandingof oxygen physiology, and with support fromrandomized, prospective clinical investigations,the judicious, individualized use of oxygen therapyin wound management may now be consideredmainstream. This article reviews the current ratio-nale, regimens, and preclinical and patient dataregarding various oxygen treatments that havebeen used to improve the outcomes of dermalwounds. See Box 1 for a summary of roles ofoxygen in wound healing.

    Oxygen Delivery

    In normal conditions, oxygen delivery to peripheraltissues is the net result of:

    Cardiac output Peripheral vascular resistance Oxygen saturation of hemoglobin (usually

    90% or greater).

    Oxygen in serumMinimal amounts of oxygen are dissolved in theserum. Release of oxygen is governed by thehemoglobin dissociation curve. Serum PO2 is typi-cally about 100 mm Hg. Once released at the

    capillary level into normal tissue, oxygen candiffuse up to 64 mm.4 Given normal capillarydensity, this diffusion ability is sufficient to nourishand support the viability of the skin. Hunt1 empha-sized the often-neglected but relevant point thatoxygen delivery can sometimes be increased sig-

    nificantly by reversing the local vasoconstrictionthat may result from pain, cold, or other noxiousstimuli.

    Intact skin as barrier to oxygenThe keratin layer of intact epithelium is a barrier tooxygen diffusion; probes designed to exclude airdetect only 0 to 10 mm Hg on the skin surface.Warming of the skin makes this layer more perme-able and enables PO2 to increase substantially,although it does not reach the ambient level. Strip-ping the statum corneum with tape enables free

    diffusion of oxygen into the upper layers of dermis,and the PO2there closely matches the oxygentension in the environment. However, within abouta day, exudation of serum and accumulationof inflammatory cells lead to formation of a softeschar that again prevents oxygen diffusion intothe skin. Therefore, the oxygen tension in subcuta-neous tissue and dermis of intact skin depends ondelivery through the underlying circulatory system.

    Inadequate Oxygen Delivery is a Causal Factorin Many Chronic Wounds

    Each of the most common categories of chronicwounds (arterial, venous, diabetic, pressure)become established or are perpetuated becauseof factors that limit oxygen delivery to the woundbed. Scheffield5 noted that chronic wounds havea PO2in the range of 5 to 20 mm Hg, comparedwith 35 to 50 mm Hg measured in normal tissue.

    In the case of venous leg ulceration, the essen-tial disturbance is abnormal venous hypertension,

    which is propagated back to the capillary level.The capillary-tissue pressure gradient is in-creased, causing water to diffuse out of the intra-vascular space and into the interstitium; largemolecules such as fibrinogen, albumin, and a2-macroglobulin are also forced out of the vascularsystem, and pericapillary cuffs are formed thatcan be noted histologically.6 These cuffs and thelocal edema impair oxygen diffusion and renderthe cells furthest from the capillary hypoxic.

    Lower extremity arterial and diabetic wounds,are prone to suffer macrovascular and/or micro-

    vascular occlusive disease, limiting blood flowand therefore oxygen delivery to the lesion. Pres-sure wounds that are not properly off-loadedbecome ischemic (and therefore also hypoxic)when capillary closing pressure is exceeded by

    Box 1Roles of oxygen in healing

    Energy source to fuel biochemical reactionsand cellular function

    Nutrient essential to the synthesis and cross-linking of collagen

    Cofactor that is manufactured into signalingmolecules such as nitric oxide and hydrogenperoxide

    Substrate for generation of reactive oxygenspecies (ROS) that combat wound coloniza-tion and infection

    Essential component of the redox switch thatturns on and off genes that encode proteinscritical to the healing cascade

    Deliberate hyperoxygenation recruits endo-thelial progenitor cells to the wound, in-creases vascular endothelial growth factor(VEGF), and promotes angiogenesis

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    the weight of the body part pressing against asupport surface.

    ReperfusionIn addition to ischemia/hypoxia, another mecha-nism of injury has been shown to play a major

    role in a variety of chronic wounds: reperfusion.6,7

    Patients with impaired arterial inflow or venousreturn have repeated episodes of ischemia and re-perfusion related to leg elevation or dependency.Restoring circulation induces endothelial sticki-ness, which draws white cells into the lesion; thealready established proinflammatory environmentestablished by ischemia intensifies as ROS floodthe wound and cause further tissue destruction.Repeated episodes of ischemia and reperfusionare more detrimental to wound healing than areprolonged phases of uninterrupted ischemia.8,9

    Inflammatory cycleIt is a popular current concept that many chronicwounds are stuck in a self-perpetuating inflam-matory cycle. Hypoxia may contribute to thispathophysiology in many cases. Under significanthypoxic conditions, mitochondrial adenosine-triphosphate (ATP) production ceases and ATP-dependent transmembrane transport systemssuch as sodium/potassium ATPase or calciumATPase fail. Intracellular accumulation of calciumpromotes release of proinflammatory cytokinessuch as TNF-a and interleukin (IL)-1, which attractneutrophils and macrophages. Endothelial adhe-sion molecules are overexpressed in hypoxia,and enable white blood cells to localize to thewound. The net effect is a self-perpetuating in-flammatory vicious cycle in which tissue destruc-tion leads to increased white cell recruitment andrelease of proinflammatory mediators and ROS,which leads to even more tissue destruction.7

    Bacterial colonization, a nearly universal featureof chronic wounds, adds to the inflammatory

    burden by attracting and activating leukocytes.Although inflammatory cells are capable of pro-ducing ROS at low oxygen tensions, the antidotesto ROS (the most potent of which is nitric oxide)require higher oxygen tension for their synthesis.

    Measurement of Wound Oxygen

    Accurate, repeatable measurements of woundoxygen are central to many in vivo investigationsinto the role of oxygen. Although numerous inves-tigators have refined research-grade systems,

    measurement of oxygen at the tissue/cellular levelin routine clinical practice is difficult and imprecise.There is a vast literature on oxygen measurementand a detailed review is beyond the scope of thisarticle. Most methods are indirect and measure

    oxygenation of periwound skin rather than thewound bed.

    Transcutaneous oximetryPerhaps the most popular of these techniques,transcutaneous oximetry (TcPO2) is subject to

    high variability related to fluctuations in vasomotortone at the site of measurement, light penetrationof skin, and hemoglobin level.7 Even perfectly per-formed TcPO2 typically overestimates wound PO2because the skin is warmed to the point ofmaximal local vasodilatation, which is not repre-sentative of the ordinary state of the local vascula-ture. In addition, there can be significant oxygenconsumption along the path from periwound intactskin to the healing tissue edge in the center of thewound.10 Thus, arterial blood PO2 is ordinarilyabout 100 mm Hg; the PO2 of dermal wounds

    ranges from 60 mm Hg at the periphery to 0 to10 mm Hg centrally. There are many reports sup-porting the usefulness of TcPO2 in determininglevels of amputation healing, but many practi-tioners have found that the method is cum-bersome and yields results that have poorrepeatability.3,11 Measurements at 1 point in timeand only a limited number of skin sites may notaccurately portray the wound microenvironment,because wounds are not uniform and vasomotortone may change from moment to moment.12

    Luminescence imagingLuminescence lifetime imaging has recently shownsignificant advantages compared with earliermethods of wound oxygen estimation.7 A phos-phorescent indicator is held in place on a plasticmatrix; quenching of the phosphorescence is pro-portional to the amount of oxygen present. Thetechnology provides a noninvasive, painless, reli-able, sensitive estimate of wound PO2.

    13 Althoughoffering potential for widespread clinical adoption,the technology is not yet available commercially

    in a format and at a cost that is compatible withordinary clinic operations and economics.

    Role of Oxygen in the Essential Stepsof Dermal Wound Healing

    Collagen synthesisExtracellular matrix deposition is inadequate inmany chronic wounds because of poor fibroblastproduction and inadequate remodeling of col-lagen, which are both oxygen dependent, andbecause of excessive degradation of extracorpo-

    real membrane oxygenation (ECM) by matrixmetalloproteinases (MMPs). Molecular oxygen isrequired for the hydroxylation of proline and lysineduring collagen synthesis and for the maturation ofprotocollagen into stable triple-helical collagen. In

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    the absence of sufficient oxygen, only protocolla-gen, which does not have the functional abilitiesof collagen, can be made.2 Collagen synthesisproceeds in direct relation to PO2 over the rangeof 25 to 250 mm Hg.10,12 Prolyl hydroxylase under20 mm Hg O2 functions at 20% of maximal speed;

    the enzyme requires more than 150 mm Hg toreach 90% of maximal speed.4,14

    AngiogenesisMany authorities have noted an apparent incon-sistency in well-known observations of woundhealing, whereby hypoxia is noted to increaseVEGF production from fibroblasts and macro-phages, but angiogenesis seems to proceed moresuccessfully under normoxic or even hyperoxicconditions.12,15,16 In one set of instructive experi-ments, mice underwent subcutaneous injection

    of a gel alone, gel with VEGF, or with anti-VEGFantibodies. The animals were then maintained invarious environments of 13% to 100% oxygenat 1 absolute atmosphere (ATA) to 2.8 ATA tosimulate hypoxia, normoxia, and hyperoxia. Theexplanted gel plugs were then sectioned andgraded for the degree of angiogenesis. Angiogen-esis was significantly decreased in the hypoxicanimals (P 5 .001) and increased in those whowere rendered hyperoxic (P

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    patients more than 60 years of age. Aging fibro-blasts show: 6,7,20

    Reduced proliferative ability Diminished capacity to respond to growth

    factor stimulation

    Increased production of destructive en-zymes such as MMPs.

    Advanced age induces greater sensitivity to thenegative effects of hypoxia. The migratory responseof fibroblasts to TGF-b1 stimulation is blunted inolder patients, compared with younger ones.Mustoe and colleagues6 compared the effects ofhypoxia on young (age 2433 years) human dermalfibroblasts in tissue culture with fibroblasts fromolder (age 6173 years) donors. Under 1% oxygen,there was greater decrease in TGF-b1 receptor

    expression in the aged cells (decrease of 12% inyoung fibroblasts vs 43% in old fibroblasts).

    Responsiveness to TGF-b1 stimulation wasreduced by advanced age:

    Activity of p42/p44 mitogen-activated ki-nase increased 50% in young cells versusdecreasing 24% in the aged cells

    Unstimulated fibroblast migration in-creased 30% in young cells exposed tohypoxia, whereas the aged cells showedno change

    When TGF-b1 stimulation was added tothe migration assay, young cells increasedactivity by 109% versus only 37% in theaged cells.2

    Mendez and colleagues20 studied fibroblastsharvested from the venous ulcer wound bedsand used, as controls, cells taken from normalskin of the thigh.

    The 7 patients who were evaluated (meanage 51; range 3667 years) had suffered

    wounds for a mean of 12.7 months (range1117 months)

    Chronic wound fibroblast growth rates wereonly one-third of those observed in thecontrol cells (P5 .006).

    b-Galactosidase (b-GAL) activity was used asa sign of cellular senescence, a state of irreversiblearrest of proliferation despite maintenance ofmetabolism.

    Six of 7 controls had no senescence asso-

    ciated (SA)-b-GAL activity All samples from the chronic wounds had

    measurable activity (mean 6.3%, median 2%) Level of SA-b-Gal correlated inversely with

    cellular growth rate (R2 5 0.77).

    The issue of cellular senescence may be morecomplex than mere chronologic age of thewounded patient. It may be the number of cell divi-sions that fibroblasts have undergone, rather thanthe age of the host, that defines the age of a cell.Many cell lines, including fibroblasts, are capable

    of only a finite number of cell divisions. Chronicwounds in young individuals may contain fibro-blasts that have already reached the limit of theirproliferative ability and are prematurely senes-cent.7 Thus, although the lessons learned aboutthe roles of oxygen in wound physiology aregenerally applicable, the degree to which patientsmay react to hypoxia and hyperoxia with the pre-dicted responses may vary according to the phys-iologic age of the host cells.

    Nitric oxide generation

    Nitric oxide synthetase metabolizes the aminoacid L-arginine into nitric oxide using oxygen asa substrate. Although nitric oxide is well knownfor its diverse, generally beneficial, effects oninflammation, angiogenesis, and cell proliferation,a full discussion of the putative benefits ofenhanced nitric oxide is beyond the scope of thisarticle.21

    Role of Oxygen in Infection Control

    During the initial phase of wound healing, acti-vated leukocytes enter the wound and engulfbacteria. In the presence of adequate oxygen, anoxidative burst ensues, in which oxygen consump-tion increases as much as 50-fold comparedwith baseline conditions, and persists for hours,creating ROS that destroy the invaders.22 ROSinclude:

    Peroxide anion (HO2)

    Hydroxyl ion (OH) Superoxide anion (O2

    )

    Hydrogen peroxide (H2O2).About 98% of oxygen consumption by leuko-

    cytes is related to this respiratory burst, which isfacilitated by phagocyte (neutrophil, eosinophil,monocyte, and macrophage) cell membranebound nicotinamide adenine dinucleotide phos-phate (NADPH) oxidase23:

    NADPH12O2/NADP12O

    21H1

    Glucose provides the energy to drive the reac-tion, generating substantial amounts of lactate in

    the process.1ROS, and especially superoxide, are toxic and

    kill bacteria, then are rapidly degraded to H2O2and other by-products.10,24 The kinetics are suchthat the killing process works at 50% of maximal

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    speed in the presence of oxygen at 40 to 80 mmHg, and as much as 400 mm Hg are required toincrease the velocity to 90%.4 Neutrophils there-fore lose most of their ability to kill bacteria atlessthan40mmHg.7 Patients afflicted with chronicgranulomatous disease, which is characterized by

    defects in the genes that encode NADPH oxidase,have increased susceptibility to infection, and alsoshow impaired wound healing.10

    The results of many investigations suggest that,in a reduced PO2 wound environment, the abilityof leukocytes to generate ROS is substantiallyimpaired, and therefore the ability to ward off colo-nization/infection is lowered. In vitro studies ofleukocytes obtained from venous blood showthat neutrophil oxygen consumption increases asambient oxygen concentration increases, and sug-gest that increasing ambient oxygen to more thanphysiologic levels may induce even greater ROSsynthesis. However, there is the theoretic potentialthat excess ROS may be destructive and counter-productive to wound healing. Thebody hasa robustsystem for removal of ROS by superoxide dismu-tase, catalase, and reduced glutathione, and thewound concentration of ROS is the net result ofsynthesis and destruction. ROS clearance requiresan adequate circulatory supply.25

    The potential to increase local wound oxygena-tion to supraphysiologic levels and cure or prevent

    infection was evaluated in 30 patients with chronicdiabetic wounds. Patients were randomized toreceive standard care (wound dressings; antibi-otics guided by culture results) alone or in combi-nation with 4 hyperbaric oxygen therapy (HBOT)treatments given during a 2-week period. In thecontrol group, cultures grew significant colonycounts of 16 different isolates at baseline, and 12at the end of the observation period; in the patientsreceiving HBOT, isolates were reduced from 19before treatment to only 3 afterward (P

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    defects in healing that are associated with hy-poxia. Absolute hypoxia is usually defined as anoxygen level less than 30 mm Hg.2 However,hypoxia is more typically a relative term, indicatinginsufficient oxygen for the tissue and physiologicsituation under consideration.12 For example,

    with infection or an open wound, oxygen demandincreases and levels of delivery that might beadequate for intact, uninfected dermis may bedeficient. Cells challenged with hypoxia mustreduce activity and rely on anaerobic metabolism,or die. Acute and mild/moderate hypoxia usuallyleads to cellular adaptation and survival; pro-longed and more extreme hypoxia may result incellular death.

    Hypoxia is a more frequent issue in wound repairthan is commonly appreciated. Wound bedoxygenation depends on:

    Pulmonary uptake Hemoglobin level Cardiac output Vascular patency Capillary density Factors that deplete oxygen such as

    parenchymal consumption and inflamma-tory cell activity.

    Decreased wound oxygen tension occursnot only from macrocirculatory issues but also

    because oxygen is consumed by metabolicallyactive and proliferative cells located along thepath from capillaries to the healing edge of tissueBox 2. As much as a 150-mm distance from thenearest capillary to the healing edge may need tobe nourished by oxygen diffusion.4 This effect ismagnified in the presence of heavy bacterial colo-nization or infection, further consuming oxygenand decreasing the available oxygen to the healingwound. A vicious cycle may ensue in whichbacterial oxygen consumption reduces the ability

    of leukocytes to synthesize ROS, enabling furtherbacterial proliferation, and so on. The centers ofeven seemingly well-oxygenated wounds may behypoxic because of high oxygen extraction alongthe path of diffusion from capillaries.7 Althoughcirculating blood may contain a PO2 of 100 mmHg, the periphery of a dermal wound may be

    60 mm Hg and, at the center of the wound, read-ings can be as low as 0 to 10 mm Hg.1,35 Manychronic wounds suffer local hypoxia; in one study,normal, nonwounded tissue showed oxygentensions of 30 to 50 mm Hg, whereas measure-ments of PO2 in nonhealing chronic wounds in the

    same patients were in the range of 5 to 20 mm Hg.4At present, it is not easy to reconcile all the

    scientific knowledge about effects of hypoxiaand hyperoxia on wound tissues and to synthesizeall the evidence into a coherent story.19 Hypoxiahas been traditionally regarded as an importantstimulus to fibroblast growth and angiogenesis.Hypoxia encourages angiogenesis by increasinglevels of hypoxia-inducible factor 1 (HIF-1), whichin turn binds to the promoter segment of theVEGF gene and activates transcription, leadingto higher synthesis of VEGF, the principal angioge-netic growth factor in human physiology.2,7 Para-doxically, multiple studies have shown thathyperoxic conditions induce greater angiogenesis,perhaps by increasing local ROS.32,36

    Acute hypoxia induces temporary increases incellular replication (3-fold increase under 5 mmHg compared with 150 mm Hg). This has beenassociated with a 6.3-fold increase in the ex-pression of TGF-b1 and enhanced procollagensynthesis.7 However, these increases in prolifera-tion and metabolic activity are short lived and,

    when these conditions are maintained for morethan a week, cellular growth and synthetic activitydecrease to significantly less than baseline physio-logic levels.16 The situation is reversible; restorationof normal oxygen levels restores typical prolifera-tion rates, and a second bout of hypoxia inducesa second temporary burst of fibroblast activity. Inchronic hypoxia, the production and secretion ofthe most important cytokines and chemokines cen-tral to healing (including TNF-a, TGF-a, TGF-b1,KGF, EGF, PDFG, and insulin growth factor) require

    oxygen and are reduced or absent.There seem to be at least 2 ways to reconcile theolder concept that hypoxia is beneficial withmodern understanding that healing proceedsmore quickly with increased oxygen delivery.

    1. The true primary stimulus to VEGF secretion,angiogenesis and collagen deposition may belactate, more than hypoxia.4,5 Even in well-perfused and properly oxygenated wounds,lactate may accumulate because leukocytes,fibroblasts, and endothelial cells lack mito-

    chondria and rely on anaerobic glycolysis forenergy production; a principal by-product ofthis glycolysis is lactate.1 Thus, lactate levelsmay still be increased in hyperoxia, albeit lessso compared with hypoxia.

    Box 2Clinical pearl: wound hypoxia

    Clinical pearl: wound hypoxia is under-recog-nized and may occur in even normally perfusedlesions because of high oxygen extraction alongthe diffusion path from the feeding capillary tothe edge of healing tissue.

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    2. It is possible that the PO2 gradient between thecapillary and the most distant cell may driveangiogenesis more than the absolute level ofPO2. The immediate effect of hyperoxygenationof the blood may be to increase this gradient,and increased oxygen diffusion to areas of

    low PO2 may follow.

    Therapeutic Oxygen Supplementation

    Because of the high incidence of wound hypoxiaand the knowledge of the deleterious effects ofinadequate oxygen on healing, it is natural toconsider the potential of oxygen supplementationto improve wound repair Box 3. In some instances,the most effective way to improve wound oxygen-ation may involve measures to enhance blood flowrather than changing the oxygen content of the

    blood. Such maneuvers may include hydrationand relief of vasoconstriction using local warmthand analgesics. Because hemoglobin is nearlysaturated in most individuals, enhancing inspiredoxygen may only modestly increase peripheraldelivery. Other techniques for hyperoxygenationof wounds include breathing oxygen under supra-normal pressures (hyperbaric oxygen [HBO]) orbathing the wound topically with an enhancedoxygen environment (topical oxygen therapy[TOT). The remainder of this article focuses onvarious attempts to supplement oxygen for the

    benefit of healing acute and chronic wounds.

    Enriched inhaled oxygen to prevent surgicalsite infectionsUnder normal circumstances, hemoglobin isalmost completely saturated while breathing roomair under normobaric conditions, so the opportunityfor increasedoxygen delivery with enhanced forcedinspiratory oxygen (FiO2) is limited. However, byincreasing the serum PO2, the tissue diffusiongradient is increased. For example, in the rabbitischemic ear model, breathing 100% O

    2at 1 ATA

    increases blood PO2 from 90 to 450 mm Hg. Atless than 20% O2, most of the oxygen is consumedwithin 70 mm of the nourishing capillary, but, after

    breathing 100% O2 for 45 minutes, there is meas-ureable increase in PO2 even 150 mm away.

    5

    Knighton and colleagues37 in 1984 showed thathigher inspired concentrations of oxygen loweredthe extent of infection induced by intradermalinjection of bacteria. Guinea pigs were given intra-

    dermal injections containing 108 E coli, then weretreated with either 12%, 21%, or 45% inhaled nor-mobaric oxygen. The end point, the size of theresulting lesion, was substantially reduced withincreased oxygen (P

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    A similar prospective, randomized trial of 30%versus 80% inspired oxygen was conducted byBelda and colleagues.40

    Three hundred patients were randomized toyield 291 who were evaluable

    The rate of SSI was unusually high (57patients [39.3%]) and there was no clearexplanation for this finding

    The incidence of SSI was reduced signifi-cantly in response to higher inspiredoxygen: 24% versus 15% infected forpatients treated with 30% and 80%oxygen, respectively (P5 .04).

    Chura and colleagues41 reviewed the literatureand conducted a meta-analysis on the valueof supplemental oxygen in preventing SSI incolorectal surgery. Among thousands of articlesthat emerged from a keyword search on surgicalsite infection and perioperative oxygen only4 studies met criteria for sufficient scientific rigorto warrant analysis. Although there was signifi-cant heterogeneity among these studies, theinvestigators concluded that there was suf-ficient evidence to support the assertion thatperioperative oxygen supplementation reducesSSI. The aggregate number of patients in thesestudies was 943 (477 who received supplemental

    O2 and 466 who were controls). The pooled rela-tive risk for SSI with perioperative O2 supplemen-tation was 0.68 (95% confidence interval, 0.49to 0.94).

    Only 1 major study failed to show reduction ofSSI with supplemental oxygen. Meyhoff andcolleagues42 studied 1400 Danish patients under-going laparotomy at 14 hospitals. Patients andobservers were blinded to the random assignmentto 30% or 80% O2. The study end point wassuperficial or deep wound infection, or intra-

    abdominal infection, using the Centers for DiseaseControl and Prevention definitions. The low-oxygen group had an SSI rate of 20.1% versus19.1% for 80% O2 (not significant [NS]). Mostpatients received perioperative antibiotic prophy-laxis with cefuroxime and metronidazole or benzyl-penicillin and gentamycin; perhaps this accountsfor the apparently discrepancy between this studyand the others cited earlier.

    On balance, it seems that increasing theconcentration of inhaled oxygen is likely of benefit.In 2008, the UK National Institute for Health and

    Clinical Excellence concluded that, The mecha-nism for improved blood oxygen carriage dueto increased FiO2 is physiologically not clear.However, this simple, cheap intervention deservesfurther investigation.43

    Systemic HBOTHBOT is conducted in single-person or multiple-person chambers that completely enveloppatients in an environment of 2 to 2.5 atmospheres(atm) and 100% oxygen.44 Sessions are typicallydelivered for 90 to 120 minutes daily (sometimes

    twice daily), and therapeutic courses lasting 2to 8 weeks are typical. Because most patientsunder room air conditions show hemoglobin ox-ygen saturations of 90% or greater, the incre-mental binding of oxygen to hemoglobin inducedby HBOT is marginal. However, independently ofthe normal mechanism of oxygen transport totissues via hemoglobin binding, HBOT dissolvessubstantial amounts of oxygen directly into serum,much as carbon dioxide is dissolved under pres-sure into carbonated beverages.

    Serum PO2 can reach 1200 to 2000 mm Hgduring treatment sessions.35

    Total blood oxygen content, which is ordi-narily in the range of 20 volume %, in-creases to about 27 volume % during100% oxygen breathing at 3 ATA pressure.

    Because oxygen delivery from capillariesinto tissue is driven by diffusion alonga gradient, large increases of capillary PO2result in substantial increases in oxygena-tion at the cellular level. When the PO2 is

    increased to 2000 mm Hg, oxygen maydiffuse as far as 246 mm.

    As shown by the classic report of Boeremaand colleagues45 in 1960, sufficient oxygencan be dissolved in the bloodstream tomaintain life and vital organ function evenin the absence of hemoglobin.

    Following a session of HBOT, skin oxygentension remains increased for a period of30 minutes to 4 hours.

    The details of nearly 400 years of hyperbaricmedicine have been described by Kindwall.46

    The history of HBOT is checkered. On a navelevel, hyperoxygenation of tissue seems to bepotentially beneficial in a wide variety of pathologicconditions. Given the sensitivity of peripheral andcentral nervous tissue to hypoxia, for example,HBOT has been attempted in a range of neurologicdiseases and conditions. In past decades, hyper-baric operating rooms were used to conductprocedures such as carotid endarterectomy, inwhich temporary brain ischemia was anticipated.

    In general, results of HBOT for these and manyother conditions were disappointing.

    The combination of overly optimistic ex-pectations and disappointing outcomes led tothe perception that HBOT was not useful in any

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    medical condition, and the therapy was relegatedto the realm of quackery in the opinion of manypractitioners since the 1970s. Nevertheless, anincrease in interest and effort to understand thephysiology of wound healing in the past 20 yearshas generated a large body of evidence both in

    favor of the results of HBOT and to explain themechanisms by which HBOT is beneficial. Thus,opinion has returned to a middle ground in whichHBOT is understood to be a valuable adjunctivewound healing therapy but not a panacea. Never-theless there are still skeptics of HBOT who thinkthat, for most patients, the therapy is unnecessaryand that oxygenation of most wounds can beachieved in other ways, such as by improving localperfusion.1

    HBOT: growing scientific basisMedical societies, government agencies, andhealth insurers now agree that there is a legitimaterole for HBOT in a variety of conditions, includingmany aspects of chronic open wounds. The Amer-ican Diabetes Association endorsed HBOT fortreating recalcitrant diabetic foot ulcers in 1999;3 years later, the Centers for Medicare and Med-icaid Services announced its concurrence andits policy to reimburse for such treatments inpatients who had failed to heal with a month ofstandard care.14 Professional societies such as

    the Undersea and Hyperbaric Medical Societyand the Wound Healing Society have includedadjunctive HBOT in suggested algorithms of carefor diabetic foot ulceration.

    There is a large and growing body of scientificinformation to indicate the potential physiologicbenefits of HBOT in wound healing. Typical HBOTincreases PO2 to 1200 mm Hg and increases O2diffusion distance from 60 to 250 mm.47 Fibroblastproliferation is improved by this treatment.Hehenberger and colleagues22 described a series

    of experiments in which fibroblasts harvested fromnormal patients undergoing reduction mammo-plasty were compared with cells derived fromspecimens of diabetic foot ulcer beds. Cells wereplated in tissue culture medium and placed insidea monoplace hyperbaric oxygen (HBO) chamber,then subjected to air (as control, at 795 and 1875mm Hg) and 100% oxygen at various pressuresfrom 795 to 2250 mm Hg. Total cell count wasdetermined by measuring DNA content in thespecimens immediately before treatment and after24 hours. Although pressurized air did not induce

    fibroblast growth, fibroblasts proliferated more asPO2 was increased, and maximal proliferationrate was observed at 1875 mm Hg (2.4 ATA).22

    HBOT was able to restore fibroblast growth in dia-betic ulcer cells to the level seen in the normal

    control cells. Other investigators have noted thatHBOT induces fibroblasts to differentiate into my-ofibroblasts, which are responsible for woundcontraction.48

    See Box 4 for a summary of HBOT wound heal-ing indications.

    HBOT treatment and wound vascularityA common observation among hyperbaric physi-cians is the large increase in wound vascularityafter patients are treated for several weeks.HBOT has been shown to increase VEGF mRNAlevels in endothelial cells and macrophages, andincreased VEGF is noted in wound fluid of patientsreceiving this treatment.7 Much of this effect maybe mediated by increased nitric oxide.49,50 HBOTinduces endothelial progenitor cells (EPCs) tomigrate out of bone marrow, circulate, and settlein the peripheral wound, forming vascularbuds.51,52 Circulating EPCs in the peripheral bloodare diminished in diabetes mellitus; HBOT re-verses this. The HBOT-induced steep oxygengradient between capillary and hypoxic woundbed prompts macrophage migration and releaseof angiogenetic growth factors. There is a distinc-tion between angiogenesis (proliferation andmigration of resident endothelial cells supportedby fibroblasts) and vasculogenesis (a de novoprocess whereby EPCs enter a wound, differen-

    tiate into endothelial cells, and create a newvascular network). HBOT facilitates both theseregenerative processes by increasing woundVEGF, basic fibroblast growth factor, and TGF-b1.Patients undergoing 20 HBO treatments in pre-paration for dental procedures related to osteora-dionecrosis of the mandible were found to havea 5-fold increase in EPCs. Under inhibition of nitricoxide synthetase, this effect is blocked. Maximalstimulation of peripheral nitric oxide synthetaserequires up to 2.8 ATA.12

    HBOT and hypoxia/hyperoxiaWe have noted that both hypoxia and hyperoxiahave certain salutary effects on healing. Perhaps

    Box 4Generally accepted wound-related indicationsfor HBOT

    Necrotizing soft tissue infections

    Radiation damage to soft tissue and/or bone

    Moderate and deep diabetic foot and legwounds

    Crush injury

    Gas gangrene

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    by fortune more than design, the current regimenof HBOT makes use of both stimuli: during HBOtreatment, and for about 4 hours afterward,the supplemental oxygen facilitates fibroblastgrowth and collagen deposition and maturation,whereas during the remaining 20 hours before

    the next treatment, relative hypoxia stimulatesangiogenesis.

    HBOT and antibioticsHBOT also increases the susceptibility of variousbacteria to antibiotics, but is this strain specificand the effect cannot be generalized to all patho-gens, whether aerobic or anaerobic.5,7

    HBOT and MMPSSander and colleagues36 used a model of impairedhealing created by depleting mice of macrophages

    to study the effects of HBOT. Healing was restoredto normal pace in HBOT-treated animals, eventhough the wounds were not ischemic or hypoxic.The author proposed that the benefit of HBOTwas mediated through contradictory effectson MMPs. The activity of tissue inhibitor ofmetalloproteinase-1 (TIMP-1) was increased bythe treatment. However, HBOT increased TNF-ain the wound bed, which typically increases thesupply of MMPs available for selective tissuebreakdown necessary for neovascularization andepithelial migration.36

    HBOT and granulation tissueIn the rabbit ischemic ear model, HBOT increasedgranulation tissue and epithelial regrowth; theresponse was accentuated when growth factorswere added to the HBOT.53 Two of the 3 arteriessupplying the external ear were ligated to createtissue ischemia, then 6-mm, full-thickness dermalwounds were created (n 5 42). Wounds weretreated with PDGF-BB, TGF-b1, or buffered saline;animals underwent HBO for 90 min/d at 1, 2, or 3

    ATA 100% O2 for up to a week. Treatment at 1ATA did not alter epithelial advance or granula-tion tissue. Hyperoxygenation at 2 and 3 ATAincreased granulation and increased wound PO2to as high as 300 mm Hg, increasing tissue volumeby 100% (P 5 .03) but not influencing epithelialadvance.54 In combination with topical growthfactors, granulation tissue increased by 200%(P 5 .0001) and epithelial advance increasedsignificantly.

    Obstacles to defining benefits of HBOT

    A major obstacle to precise definition of the bene-fits of HBOT is the paucity of scientifically rigorousclinical studies. In 2005, Roeckl-Wiedmann andcolleagues55 performed an extensive searchthrough some 24 electronic databases as well as

    a manual search through texts to gather 78 articlesthat purportedly offered clinical evidence on therole of HBOT in wound management. Of these,only 21 were deemed to be suitable human clinicaltrials, and 15 were excluded because they werenot randomized, not clearly focused on 1 wound

    cause, used TOT instead of HBOT, or reporteddata that had already been published elsewhere.Six publications of at least moderate scientificmerit were left: 5 on diabetic foot ulcers and 1 onvenous leg ulcers.

    Kessler and colleagues study on diabetic footulcersKessler and colleagues56 noted that HBOTdoubled the rate of wound healing (P 5 .037)compared with controls during 2 weeks of treat-ment, but, once the twice-daily HBOT treatmentswere discontinued, the rate of healing in thecontrol and HBOT groups equalized.56 Twenty-eight patients were randomized, 15 to HBOT and13 to control therapy, including off-loading.Patients were excluded if they suffered significantarterial occlusive disease or serious contraindica-tions to HBOT (emphysema, claustrophobia,proliferating retinopathy). The groups were similarin terms of demographic factors and indicators ofdiabetic complications (renal, ocular, vascular),and all suffered with neuropathy. Lesions ranged

    from Wagner grade I to III. HBOT was deliveredas 100% oxygen at 2.5 ATA pressure, giving two5-minute air breaks during each treatmentsession. Only 1 patient had barotrauma to theear and had to discontinue participation. Peri-wound TcPO2 increased from a mean of 22 to454 mm Hg with first treatment, and 26 to 550mm Hg after 20 treatments (P

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    underwent HBOT and 33 did not. There were nodifferences in the treatment groups in terms ofdemographic features, depth of foot ulceration,other diabetic complications, circulatory status,or duration of hospitalization. In the treatmentgroups, Wagner grade III and IV lesions predomi-

    nated (25.7% and 62.8% in the HBOT group, and24.2% and 60.6% in the control groups, respec-tively). All patients underwent an initial aggressiveexcisional debridement shortly after admission tothe hospital, then twice-daily dressing changesand antibiotics guided by frequent wound cultures.Patients receiving HBOT received an average of 38treatments; daily in the beginning and then 5 daysper week later on (2.5 ATA 100% O2, 90 minutes).The end point of the study was the incidence ofmajor amputation. Three patients receiving HBOTrequired below-knee or above-knee amputations,compared with 11 controls (8.6% vs 33.3%, re-spectively; P5 .016). TcPO2 measurements wereincluded in the study: at baseline the 2 patientgroups had similar readings (23.210.7 mm Hgvs 21.310.7 mm Hg, respectively; NS). However,at hospital discharge, the values had increased to37.3 (16.1) mm Hg and 26.3 (13.5) mm Hg,respectively. The greater improvement in theHBOT group was highly statistically significant(P5 .0002). The investigators concluded that theenhanced increase in TcPO2 was related to

    improved angiogenesis in and around the wound,and they proposed that HBOT is effective inlowering the incidence of major amputation in dia-betic wounds.

    Baroni and colleagues study on diabetic footulcersBaroni and colleagues58 conducted a nonrandom-ized comparison of the outcomes of 18 hospital-ized patients with diabetic foot ulcers treatedwith standard of care plus HBOT with another

    group of 10 patients treated identically but withoutHBOT. All patients had advanced diabetes,and most suffered retinopathy, neuropathy, andvascular occlusive disease. In the HBOT group,16 patients healed versus 1 in the comparatorgroup (P 5 .001). Two patients receiving HBOTand 4 comparator patients underwent amputation.The investigators concluded that there is majorbenefit in HBOT for amputation avoidance.

    Abidia and colleagues study on ischemic lowerextremity ulcers

    Abidia and colleagues59 conducted a smalldouble-blind study in which patients with ischemiclower extremity ulcers were randomized to hyper-baric air or hyperbaric oxygen treatment. By 6weeks, 5 of 8 wounds healed in the oxygen group

    versus 1 of 8 of the controls (NS because of smallnumbers). At 1 year, all the HBOT wounds re-mained healed but the wound that had healed inthe control group reopened, and the differencein 1-year closure was significantly better in thetreated group (P 5 .026). The investigators

    analyzed overall direct patient treatment costs(fees for dressings, clinic visits, and hospitaliza-tions) and found that HBOT was also ableto show cost-effectiveness (based on 100 UKpounds per HBO treatment session). At 6 weeks,median wound surface area closure was 100% inthe HBOT group and 52% in controls (P5 .027).Although the study included few patients, therandomization was strict and the methodologywas rigorous.

    Kranke and colleagues study on diabetic foot

    ulcersKranke and colleagues60 found only 5 publicationsthat met the standards for inclusion in a CochraneReview of the evidence in favor of HBOT forchronic wounds. The investigators concludedthat HBOT significantly reduces the chance ofamputation and may increase the chance of heal-ing at 1 year for diabetic foot ulcers. However, theyconcluded that there is a need for further substan-tiation because the number of patients on whichthese conclusions were based is small, and the

    trial methodologies imperfect. They could notjustify the use of HBOT in the management of otherwound causes based on current evidence. Thereport estimated that 4 diabetic ulcers needed tobe treated with HBOT to prevent 1 amputation.

    Lin and colleagues study on diabetic footlesionsLin and colleagues61 randomized 29 patients withWagner 0, 1, and 2 foot lesions to standard carewith and without HBOT. Although patients startedthe study with similar values, after 30 HBO treat-

    ments, the patients receiving HBOT had signifi-cantly reduced HgA1c (6.61.7 vs 9.34.3) TcPO2(57.520.7 vs 35.821.2), and laser Doppler perfu-sion scan flux (35.67.0 vs 25.84.1; all 3 compar-isons were statistically significant at the P

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    months, at great expense ($1000 per treatment atour center) but with minimal tangible improvement.Fife and colleagues34 found only moderate predic-tive ability of TcPO2 to distinguish patients whowould benefit from HBOT, whether measuredunder room air, breathing 100% oxygen at sea

    level, or in the chamber. Barnes47 noted that, fordiabetic lesions, the extent of the wound accord-ing to the Wagner scale correlated well withresponse to HBOT, with Wagner 3, 4, and 5 lesionsresponding well 77%, 64%, and 30% of the time,respectively. In the same report, patients whoachieve a periwound TcPO2 of 200 while breathing100% oxygen at 2.5 ATA were likely to heal.

    Complications in HBOTHBOT generally is safe and comfortable; neverthe-less, there are well-known complications that mayoccur. One set of risks relates to maintaining thepatient under supranormal external pressure.About 60% to 70% of patients experience meas-ureable, but reversible, myopia; patients with pres-byopia may experience temporary improvement invisual acuity during the course of therapy. The inci-dence of otic barotrauma has been quotedbetween about 2% and 20%.35,47,55 In my experi-ence, the incidence is even higher, and fullyone-quarter of my patients require temporarytympanostomy tubes to equalize pressures in the

    middle ear and prevent permanent damage. Thetubes are easily removed (or sometimes fall outspontaneously) and the tympanic membranes typi-cally heal within days afterward. Other barotraumais less frequent; the incidence of pneumothoraxhas been cited at 1 in 1 million treatments. HBOTimposes increased afterload on the left ventricleand patients with New York Heart Associationclass 3 and 4 congestive heart failure, with ejectionfraction less than 35%, should not be treated,because their cardiac situation may worsen.

    The other set of potential complications relatesto the effects of oxygen as a medicine. Perhapsthe most feared complication is oxygen toxicityseizure, the risk of which is about 0.01% to0.03%. These seizures, which are usually overwithin the brief time it takes to switch the breathingmixture from 100% to air and to decompress theHBO chamber, do not cause permanent braindamage and are random events that do not neces-sarily preclude further HBOT. Regimens dictatinga 90-minute treatment and limiting pressure toabout 2.5 ATA are based on known tolerance of

    patients to high levels of oxygen for given periodsof time. The risk of seizure may be lowered bygiving a 5-minute period of air breathing, ratherthan 100% oxygen, every half hour while in thechamber. There are other theoretic risks of

    excessive oxygen, such as pulmonary toxicityand of enhancing tumor growth, but these havenot been borne out in everyday practice.

    Indications for HBOTAt present, the generally approved wound-related

    indications for HBOT include moderate and deepdiabetic foot/leg lesions, necrotizing soft tissueinfections, radiation damage to soft tissue and/or bone (osteoradionecrosis), selected problemwounds (Figs. 1 and 2), compromised skin graftsand flaps, crush injury, clostridial myositis, gasgangrene, and refractory osteomyelitis. Periopera-tive treatments have been particularly useful inpatients with osteoradionecrosis of the mandiblewho are undergoing dental extractions or im-plants; typical regimens call for 10 preoperativetreatments and then postprocedure treatment until

    full healing is observed. Similarly, in anticipation ofskin grafts or flaps that may be compromisedbecause of tension or because the tissue to beclosed has previously been irradiated, a briefperiod of preoperative treatment followed by re-sumption of HBOT as soon as practical aftersurgery optimizes graft/flap take.

    The American Diabetes Association has since1999 endorsed the used of HBOT as an adjunctivetherapy for severe diabetic wounds that failto respond to standard therapy.14 The Centers

    for Medicare and Medicaid Services in 2002approved the therapy for treatment Wagner III toIV ulcers. Professional organizations such as theUndersea and Hyperbaric Medical Society andWound Healing Society have included HBOT intheir suggested algorithms of care. More than90% of wounds closed with HBOT remain healed4 years later. Although health economic aspectsof treatment are beyond the scope of this article,a case can be made that the expense of HBOT isjustifiable with cost-effectiveness data.47

    TOT

    TOT involves enclosing a body part (usually anextremity) in a portable chamber that circulatespure oxygen over the wound surface. Potentialadvantages to this approach, compared withHBOT, include lower cost, greater convenience,therapy not limited to 2 hours per day, andsuccess not dependent on the macrocirculationand microcirculation to the wound bed.7 Potentialcomplications of HBOT, such as oxygen toxicity totissues/organs, may be avoided by using TOT.35

    TOT Penetration of Wound Surface

    Perhaps the most controversial issue is the abilityof TOT to achieve meaningful penetration of the

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    wound surface and reach the cells/tissues thatwould benefit from hyperoxygenation. This issuehas been addressed both with direct measure-ment of tissue oxygen levels and, indirectly, by

    showing the biochemical and biologic effectsof TOT. Fries and colleagues35 showed that, at adepth of 2 mm into the wound bed, PO2 increasedfrom its baseline of 5 to 7 mm Hg to 40 mm Hg

    Fig. 1. Multiple-recurrent ventral hernia had recently been repaired with fascial flaps and mesh, but the woundbecame infected and dehisced shortly after surgery. The patient was returned to surgery and the mesh was

    removed. (A) Exposed abdominal organs thinly covered with collagen mesh. (B) After 20 HBO treatments, thereis substantial in-growth of vessels and soft tissue. (C) After nearly 40 HBO treatments, the wound has partiallycontracted and the soft tissue coverage has thickened. (D) Perioperative HBOT was used to support the take ofpartial-thickness skin grafts, achieving definitive closure of the wound.

    Fig. 2. Open transmetatarsal amputation. (A) Early postoperative period. Cut ends of metatarsals #2 to 5 areexposed. (B) After approximately 40 HBO treatments. Bone ends are covered. Healthy granulation tissue fillsthe wound.

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    after 4 minutes of TOT. Standardized full-thicknesswounds on the backs of pigs were treated witheither topical air or topical pure oxygen. By thesecond day of treatment, the surface area of theoxygen wounds was significantly (P

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    therapeutic effect came more from mandatory legelevation 6 hours per day, or from TOT. In 32 of 46TOT-treated ulcers, granulation tissue developedfirst in the center of the wound, and then extendedperipherally to cover the surface of the lesion.

    Clinical Evidence for TOT

    Although there is sufficient penetration of TOT toachieve meaningful physiologic effects within thewound bed, the current clinical evidence in favorof this modality needs to be supplemented withmore rigorous investigations. In choosing betweenHBOT and TOT, the benefits for TOT of lower costand avoidance of potential complications are out-weighed by the less potent clinical effect achievedwith this approach. Particularly in the case of dia-betic foot and leg ulceration, in which there is

    substantial risk of amputation, the preponderanceof data seems to support HBOT in preferenceto TOT.

    HOW MUCH OXYGEN IS OPTIMAL?

    Given that correction of wound hypoxia isbeneficial to many aspects of healing, it does notnecessarily follow that more is better, or that hyper-oxygenation of normally nourished wounds con-fers enough benefit to justify the risks. It ispossible that, for some individuals, imposingoxidative stress and excessive ROS may be moreharmful than helpful.2,12 The rate of production oftoxic radicals is directly proportional to localoxygen tension.4 Heng and colleagues25 makeimportant points about potential negative influ-ences of excess oxygen on wound repair. Unphy-siologically high levels of oxygen can react withNADP and be metabolized to ROS in the cytoplasmwithout the usual catalysis by the mitochondrialcytochrome system, which ordinarily also controlsthe release of ROS. In this event, the ROS created

    may be injurious to the host cell (fibroblast, endo-thelium) rather than serving a useful role in killingforeign organisms. This may be one mechanismby which oxygen exerts brain toxicity, occasionallycausing seizures in patients under HBOT.66 Inaddition, bypassing the cytochrome system sacri-fices production of ATP, so the extra oxygen iswasted. There is the potential for cell cycle arrestand genotoxicity from exposure of cells to pureoxygen.10,67 Cellular senescence is accelerated inhyperoxic conditions.12

    Cellular Adaptation to Ambient Oxygen Level

    Cells seem to accustom themselves to chronichypoxia or hyperoxia. For example, the PHD familyof proteins, which are known to suppress cellular

    proliferation, modulate the cell cycle and en-courage apoptosis, is expressed when hypoxia issensed.68 Cells cultured under 20% oxygen andthen subjected to a 5% oxygen environmentsynthesize higher levels of PHDs; over longerperiods of time, PHDs return to baseline levels.

    Similarly, cells accustomed to culture under 30%oxygen overexpress PHDs when suddenly putunder 20% oxygen.12 Thus there is a hypoxia setpoint that is tunable. In theory, the optimal oxygentreatment strategy may be to restore hypoxicareas of the wound bed to normal oxygen tensionwithout overtreating and risking oxidative stress.

    Individualized Oxygen Dosing

    In common practice, nearly all patients receiving

    HBOT are dosed similarly: 90-minute sessions,once daily at 2.0 to 2.5 ATA. It is possible thatthis one-size-fits-all approach to dosing mightnot be appropriate for all patients; some woundsare more hypoxic than others, therefore somemay be overdosed and others underdosed.Everyone has endogenous genes that encodeantioxidant molecules to protect them againstoxidative stress; however, individuals may vary inthe levels of expression of these genes and there-fore some patients may be less capable thanothers of dealing with the oxygen load. Real-timewound PO2 mapping assist understand of thedegree of hypoxia in each wound, and could bethe basis for more individualized therapy.

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