Researchers, clinicians explore more uses of new equine treatment

by Fairfield Bain, D.V.M.

BY NOW, the word has spread about the value of hyperbaric oxygen therapy as an adjunctive treatment for selected problems in horses. This innovation in equine medicine appears to be one of the most promising new technologies to cross over from human medicine.

Used successfully to treat humans since the 1970s, hyperbaric oxygen therapy has been proven to be effective in conditions where diminished circulation or decreased oxygen supply compromise healing. In horses, hyperbaric oxygen therapy has been used effectively in applications such as alleviating circulatory problems, resolving anaerobic and aerobic infections, healing difficult wounds and fractures, reducing potentially fatal complications of colic surgery, and treating foals suffering from birth asphyxia.

Many significant medical problems that exist in the horse result in prolonged disability or eventual humane destruction. While not a magic bullet or cure-all, use of hyperbaric oxygen therapy as supportive care makes sense in certain situations to heighten the success of surgical and medical treatment.

By far, wounds and bone infections remain the most common problems treated in the hyperbaric chamber. These are conditions where exposure to hyperbaric oxygen can:

•Help raise tissue oxygen levels in comprised skin and connective tissue, as well as bone;

•Enhance white-blood-cell killing of bacteria;

•Enhance antibiotic function in inflamed tissues where there are areas of low oxygen tension; and

•Stimulate the repair process (in both skin and bone).

The basic physiology of hyperbaric oxygen makes it useful for a variety of injuries and infections. As we gain experience with this technology, we are learning more about its usefulness. Areas currently being explored include treatment of equine patients after surgery for colon torsion, healing of traumatic burns and other wounds, and in resolving tendon sheath infections.

Colon torsion

In the breeding-intense region of Central Kentucky, colon torsions are a common cause of colic and subsequent loss, predominantly in broodmares.

The main issue following surgical correction is reperfusion injury to the wall of the large intestine. This occurs when the intestine is strangulated, resulting in lack of blood flow for a period of time followed by reflow, which causes an inflammatory process. This process involves increased adhesion of white blood cells (neutrophils) to the endothelial lining of cells in the blood vessels. These activated blood cells eventually release their packets of enzymes, including free radicals, into the adjacent tissues. This enzyme release results in additional damage and the formation of severe edema (swelling) from the increased leakiness of the blood vessels. This edema further contributes to injury by increasing the distance required for oxygen to diffuse outward from the blood capillary to the cells in the injured tissue.

The distance between the capillary and the cells in a particular tissue is increased in injured tissue by accumulation of fluid between the cells, which separates them from each other and the blood vessels, and cell injury results from a decrease in oxygen delivery.

All these changes often go together to result in an increasing downward spiral in the horse's condition, with extensive medical costs involved in intensive care, intravenous fluid support, and numerous medications aimed at trying to reverse the process or stabilize the intestinal wall until it can heal. Costs for such a patient often can reach $10,000 to $20,000 or more.

Colon torsions have been an area of interest for hyperbaric oxygen therapy for several reasons. One reason is its ability to deliver higher concentrations of oxygen to the tissues with an ability to diffuse farther out from the capillary into the tissues that become swollen and edematous following injury. This might help salvage some of the interior aspect of the colon wall (the mucosa) that otherwise would slough.

Another aspect of hyperbaric oxygen therapy might seem counterintuitive--providing increasing amounts of oxygen in a tissue undergoing reperfusion injury. Hyperbaric oxygen therapy actually has been shown to reduce perfusion injury by reducing the number of white blood cells sticking to the endothelial cells.

Like most things, the earlier the intervention, the better the chances are of reducing the extent of the injury or illness.

Over the past few years, veterinarians have been interested in early treatment after surgery of colon-torsion patients with hyperbaric oxygen therapy. Clinical information is still being accumulated, such as the time the horse spent in the hospital and amount of medical support required. But initial observations that hyperbaric oxygen therapy can reduce post-operative edema within the colon wall (as monitored by diagnostic ultrasound) are encouraging.

As previously mentioned, this is far from the magic bullet, but veterinarians are encouraged by the early results and are hopeful that continued experience will support the early use of hyperbaric oxygen therapy for the post-operative colon-torsion patient in an effort to salvage the colon wall and reduce the amount of post-operative medical care required.

Wound healing

Wounds are one of the most common problems for the equine patient. A variety of severe, devitalizing wounds of the limbs have been treated in the hyperbaric oxygen chamber in an effort to reduce sloughing and loss of tissue. Again, the mechanism is most likely the ability of the elevated oxygen concentration (ten to 12 times higher than normal) to diffuse farther outward from the blood capillary into the tissue, which helps to save tissue that otherwise would be compromised and lost.

Burn wounds, such as those that occur in a barn fire, are one of the most horrifying events seen by horsemen and veterinarians. The burn can leave a large area of uncovered tissue with leaky blood vessels that ooze serum. Infections can be secondary problems, and regrowth of skin over the wound to cover the raw burn can take a long time.

Hyperbaric oxygen can be beneficial in these cases by helping to reduce edema and by stimulating the growth of new skin cells and healing of the burn. As with the continuing injury following colon torsion, the same issues can exist in burned skin. Early treatment with hyperbaric oxygen is hoped to help salvage some injured skin and enhance the healing process.

Tendon sheath infections

Another area of interest is soft-tissue infections, such as tendon sheath infections. The notorious killer of horses such as 1989 Horse of the Year and leading Japanese sire Sunday Silence, these infections involved synovial sheaths that require prolonged drainage and antibiotic treatment.

The idea of using hyperbaric oxygen treatment in these cases is to enhance antibiotic function and increase the activity of white blood cells in killing bacteria. Hyperbaric oxygen therapy has been used in human medicine in a similar manner to treat necrotizing infections of soft tissue. It appears to have similar application in the horse.

In situations where hyperbaric oxygen therapy has been found to be useful, the idea is to maximize the chances of a good outcome by using the best combination of medical and surgical treatments available. Hyperbaric oxygen therapy is considered supportive therapy. Surgical drainage and administration of appropriate antibiotics based on a culture of the infected fluid remain the principal treatments for this type of infection.

Acceptance in equine medicine

Hyperbaric oxygen therapy is a well-accepted modality in human medicine. In fact, Medicare has adopted a list of indications in which use of hyperbaric medicine is an accepted and approved treatment.  Veterinary medicine is still in its very early stages of experience with hyperbaric oxygen therapy, but it is hoped that more will be learned about the particular conditions that would benefit the most from its use, as well as more about the most appropriate time to administer it in a medical or surgical episode to achieve the best results.

Fairfield Bain, D.V.M., formerly was associated with Hagyard Equine Medical Institute in Lexington and now is in practice at Woodside Equine Clinic in Ashland, Virginia. He is vice president of Equine Oxygen Therapy, a manufacturer and distributor of hyperbaric oxygen chambers.