What You Should Know About Using HBO In Diabetic Wounds
When the patient breathes air at sea level, the hemoglobin is already fully saturated so increasing the amount of respired oxygen can affect only the plasma-dissolved oxygen. Breathing oxygen at an elevated atmospheric pressure produces an increase in the plasma-dissolved oxygen fraction, which is proportional to the atmospheric pressure of the respired gas. Monoplace hyperbaric chambers are usually compressed with oxygen whereas multiplace chambers are compressed with air while the patient breathes 100 percent oxygen using a hood or aviator’s face mask. Typical treatments involve 90 minutes of oxygen breathing at 2.0 to 2.5 atmospheres absolute (ATA) with air breaks administered at 20- to 30-minute intervals in order to reduce the risk of central nervous system oxygen toxicity. (While HBO treatment is remarkably safe, be aware that otologic and pulmonary barotrauma and central nervous system, pulmonary and ocular oxygen toxicity can occur. Central nervous system oxygen toxicity is rare, but it can manifest as seizures. Seizures are less likely to occur if there are brief periods of air breathing.) Arterial PO2 elevations of 1500 mmHg or greater are achieved when the body is exposed to pressures of 2 to 2.5 ATA. Soft tissue and muscle PO2 levels can be elevated to about 300 mmHg. Oxygen diffusion varies in a direct linear relationship to the increased partial pressure of oxygen present in the circulating plasma caused by HBO. At pressures of 3 ATA, the diffusion radius of oxygen into the extravascular compartment is estimated to increase from 64 micons to about 247 micons at the pre-capillary arteriole. This significant level of hyperoxygenation allows for the reversal of localized tissue hypoxia, which may be secondary to ischemia or to other local factors within the compromised tissue. Hypoxia is a biochemical barrier to normal wound healing. In the hypoxic wound, HBO treatment allows an acute correction of the pathophysiology related to oxygen deficiency and impaired wound healing. Using HBO increases oxygen levels within the marginally vascularized periwound compartment, enhancing leukocyte bacteriocidal function. It may also potentiate some antibiotic effects. There are direct toxic effects on anaerobic bacteria and suppression of exotoxin production. It also enhances collagen synthesis and cross-linking, and other matrix deposition. What The Clinical Evidence Reveals Additionally, recent evidence suggests that HBO may induce specific growth factor receptors (PDGF) and stimulate growth factor (VEGF) release. There is also evidence that employing HBO may ameliorate or prevent leukocyte-mediated ischemia reperfusion injury.10-13 There have been 13 published peer-reviewed studies (including seven randomized, controlled trials) of HBO in diabetic foot wounds. A total of 606 diabetic patients received HBO with a 71 percent bipedal limb salvage rate, compared to 463 control patients who had a 53 percent bipedal limb salvage rate. All diabetic wounds were Wagner III-IV. It is interesting to compare this to the becaplermin clinical trials that involved Wager II ulcers. Control patients had healing rates of 25 percent while those receiving becaplermin had healing rates of 43 percent. A large retrospective series of 1,144 diabetic foot ulcer patients demonstrated the effectiveness of using adjunctive HBO in modified Wagner III, IV and V (equivalent to Wagner grade II, III and V) ulcers, based on ulcer/wound improvement, healing and salvage of bipedal ambulation (see “The Impact Of HBO: What One Study Shows” above).14 Currently, CMS policy reimburses only for treatment of Wagner III and greater ulcers. Final Thoughts On Appropriate Patient Selection The justification for using HBO adjunctively for a specific problem wound rests upon the clinical presentation of the wound and whether there is evidence of malperfusion or persistent or progressive local infection. Measuring transcutaneous PO2 (TcPO2) in tissue adjacent to the wound can be useful in discriminating those patients without significant hypoxia who do not require HBO from those who do. In general, TcPO2 values below 40 mmHg indicate microcirculatory impairment. Impairment in healing increases as the value decreases below this threshold. Obtaining TcPO2 values during HBO treatment may offer the best prediction of who will respond favorably to adjunctive HBO.14 The best way to use TcPO2 is in a step-wise manner.