Putting on the hyperbaric pressure; undersea technology gains favor

Boosting oxygen levels can speed healing, save limbs

One of the paradoxes in wound care is that healing is expedited by oxygen in the blood, but wound tissue resists that help. To handle that quandary, some wound healers are sinking to new depths — hyperbaric chambers. Widely used to treat deep sea divers suffering from the bends, hyperbaric medicine also helps increase the amount of oxygen that goes to wound tissue to form new blood vessels as well as boost collagen synthesis and fibroblast proliferation, all of which are key to wound healing. 

In technical terms, the Undersea and Hyperbaric Medical Society defines hyperbaric oxygen (HBO2) therapy as the intermittent administration of 100% oxygen inhaled at a pressure greater than at sea level. It has been used since the 1940s to treat decompression sickness (the bends) among scuba divers who come up too quickly or stay down too long. But it also is applied to a limited number of other conditions, including certain types of chronic wounds, chronic osteomyelitis, failing flaps and skin grafts, wounds caused by soft- tissue radiation, and osteoradionecrosis. 

Pressurized oxygen sets into motion a series of physiological mechanisms, not all of which are fully understood. These include hyperoxygenation, neovascularization, antimicrobial activity, direct pressure, vasoconstriction, and attenuation of perfusion injury. (For details on the physiologic affects of HBO2 therapy, see story, p. 63.) The first three effects are extremely beneficial for wound healing, particularly neovascularization. 

"In patients with large-vessel obstruction, the mechanism of wound development is clear, and only the restoration of pulsatile blood flow will allow adequate healing," says Jeffrey A. Stone, DO, MPH, associate medical director in the Hyperbaric Medicine Department and medical director of the Aerospace Medicine Program at the Institute for Exercise and Environmental Medicine at Presbyterian Hospital of Dallas.1 

At normal atmospheric pressure, hemoglobin in the blood is saturated with oxygen. A small amount of oxygen also is dissolved in the plasma. Increasing the pressure through HBO2 therapy increases the level of dissolved oxygen in the blood significantly, resulting in about a 30% increase in oxygen-carrying capacity, Stone tells Wound Care. This effective augmentation of arterial "driving pressure" increases the perfusion distance of oxygen into tissue by up to four times, thus facilitating angiogenesis.

Diving without getting wet

HBO2 therapy is carried out in a pressurized chamber. Several designs are currently in use. Multiplace chambers accommodate two to 18 people and are filled with compressed room air. Pressurized oxygen is delivered to patients via masks, hoods, or ventilators. Staff accompany patients into multiplace chambers, and in those equipped with an air lock, they can enter and exit without interrupting treatment. 

More commonly used are monoplace chambers, transparent cylinders large enough to hold a single, supine patient. Once the patient is in the cylinder and it is sealed, the entire unit is pressurized with 100% oxygen. Tactile contact with the patient is not possible during pressurization, but patients still can be on a ventilator or receive IV medication. 

Treatment pressures typically range from 2.0 to 2.4 atmospheres (ATA), or the pressure found 33 to 45 feet underwater (feet of sea water). One atmosphere is equivalent to the air pressure at sea level and is the equivalent of 760 mmHg. At 2.4 atmospheres, pressures are equivalent to those at 45 feet underwater, or just over 1,800 mmHg. 

"With that pressure, we can get 10 to 15 or even 20 times the amount of oxygen into tissue compared to normal," Stone says. He notes that topical HBO2, in which only a portion of the body is exposed to pressurized oxygen, is an unproven technique with no data to support its efficacy. 

For emergency conditions such as carbon monoxide poisoning or arterial gas embolism, one or two HBO2 treatments might be enough. (See list of conditions treated with HBO2, p. 64.) But when angiogenesis is the primary goal, 20 to 40 sessions, and sometimes more, are needed. Sessions last from 90 minutes to two hours.

Warm and safe

During HBO2 therapy, patients have little to do but wait. In the vast majority of cases, they experience no discomfort. Despite the references to underwater pressures commonly used in hyperbaric medicine, patients never get wet. As the pressure builds, the interior of a monoplace chamber becomes warm due to the compression of air molecules. (According to Boyle’s Law, as the pressure of a gas increases while the volume remains constant, temperature rises.) Conversely, the temperature drops during decompression. 

While HBO2 is extremely safe, it is not entirely risk-free. Pressures above 2.8 ATA (about 60 feet underwater) applied over many sessions can lead to oxygen toxicity, a condition that can cause a number of serious pulmonary, ocular, and central nervous system problems. "Air breaks," during which normal unpressurized room air is periodically pumped into the chamber, minimize the hazard. Stone estimates that oxygen toxicity occurs in only one out of 15,000 cases. For patients who do have a reaction, the pressure or session length is adjusted. 

Patients who are unable to equalize pressure in their ears are apt to feel discomfort similar to that which occurs when diving into water or during air travel. For the rare patient who cannot equalize ear pressure, antihistamines and nasal decongestant spray may be given a day or so before treatment, which dilates the Eustachian tubes and dries up mucus. Even a poorly filled dental cavity may cause discomfort during a pressure change.2 

Whenever pressurized 100% oxygen is used, the risk of fire also must be considered. Patients must be free of all petroleum-based products and enter the chamber wearing only cotton gowns.

Adjunctive therapy

Clinicians who use HBO2 strongly emphasize its place as an adjunctive therapy for chronic wounds. "It’s not first-line therapy, and it needs to be used in conjunction with good medical and surgical management in selected patient groups," says Barbara Steggman, MD, associate director at the Jefferson C. Davis Wound Center and Hyperbaric Medicine Center in San Antonio, TX. 

Her criteria for choosing candidates for HBO2 include a wound that has some hypoxic component and has demonstrated delayed healing. She would be hesitant to use HBO2 for patients with obvious contraindications, such as those with severe cardiac disease, those who demonstrate adequate tissue oxygenation yet still failed a course of appropriate wound care, and those who, upon initial evaluation, demonstrate no tissue oxygenation whatsoever. 

Approximately 75% of Steggman’s HBO2 patients present with diabetic foot wounds. Many face a leg amputation, and HBO2 is often used as a last resort to save the limb. She reports a salvage rate of about 70% among patients who undergo HBO2

Stone also stresses the adjunctive role of HBO2. "Many other steps should be taken before HBO2, such as revascularization, infection control, making sure patients with diabetic foot ulcers stay off the infected foot, appropriate debridements, and glucose control," he says. Smoking cessation, good nutrition, and edema control also contribute to wound healing.

Research growing

While still somewhat lean, the body of research on HBO2’s usefulness in healing diabetic foot wounds is steadily expanding. In a retrospective analysis of 501 patients with diabetic foot wounds, Stone and his colleagues found that, despite having more serious wounds, patients undergoing HBO2 had a higher limb salvage rate than those receiving standard care.

In one of the many prospective studies, investigators found patients with nondiabetic chronic wounds treated with HBO2 showed a 35.7% reduction in size at six weeks, compared with a 2.7% reduction in the control group.4 

Currently, Stone is the principal investigator of a randomized, prospective, double-blinded study, funded by the American Diabetes Association, of HBO2 for patients with diabetic foot ulcers. All patients will receive standard wound care. Patients in the treatment group will receive a series of HBO2 sessions at 2.4 ATA on 100% oxygen for a period of 100 minutes each, while control patients will be exposed to the same hyperbaric environment but will breathe a gas mixture equivalent to air. 

"In the past, many viewed hyperbaric oxygen therapy as not well-substantiated," Steggman says. "I think that’s changing. There’s a lot more physiology going on than most people are aware of. I think we have a definite role in wound healing, and it’s kind of unfortunate that people tend to overlook it because of misconceptions."
References
1. Stone JA, Cianci P. The adjunctive role of hyperbaric oxygen therapy in the treatment of lower extremity wounds in patients with diabetes. Diabetes Spectrum 1997; 10:118-123.
2. Collison L. Hyperbarics: When pressuring the patients helps. RN March 1993; 56:44-49.
3. Stone JA, Scott RG, Brill LR, Levine BD. The role of hyperbaric oxygen therapy in the treatment of the diabetic foot. Diabetes 1995; 44(Suppl 1):71A.
4. Hammarlund C, Sundberg T. Hyperbaric oxygen reduced size of chronic leg ulcers: A randomized double-blind study. Plast Reconstr Surg 1996; 93:829-823.