As hyperbaric-oxygen therapy (HBOT) – the use of oxygen under pressure – gains wider acceptance in human medicine, its application in equine and small-animal veterinary medicine has been growing, too.
They are used in human medicine to help treat some diseases (with Medicare approval) and conditions such as acute carbon-monoxide poisoning, decompression illness, gas embolism, gas gangrene, progressive necrotizing infections and preservation of skin grafts. The list has become extensive.
For horses, HBOT is used to assist healing and to speed recovery of poor-healing wounds, certain aerobic and anaerobic infections, bone (osteomyelitis) and necrotizing soft-tissue infections. It is also reportedly showing some benefit in patients with hypoxic-ischemic encephalopathy, tendinous and ligamentous injuries, respiratory conditions, as well as intestinal ischemic injury like colon torsion and small-intestinal strangulation.
In a hyperbaric oxygen chamber, horses breathe air at 2 to 3 atmospheres absolute, or about two to three times the normal environmental pressure at sea level.
Normal oxygen delivery to the tissues is dependent upon the proportion of O2 in the air, lung function, red-cell hemoglobin (Hb) and blood pressure.
Under normal circumstances, Hb is 97 percent saturated with oxygen, with a very small amount dissolved in the blood plasma.Via the mitochondrial oxygen-transport system, the partial pressure of oxygen is reduced in a cascade of succeeding decreases, ultimately delivering oxygen to tissues at a significant decrease in partial pressure as blood flow exits the lungs and progresses to the cells.
HBOT and wound healing
The failure of wound healing often is due to persistent infection, malperfusion and hypoxia, cellular failure, unrelieved pressure or recurrent trauma.
Hyperbaric oxygen therapy produces a directly proportional increase in the plasma volume fraction of transported oxygen and delivery to the injured tissues. With increased cellular and muscle pO2, the tissues become saturated with oxygen, assisting the healing process.
Hyperoxygenation allows for reversal of localized tissue hypoxia, which may be secondary to ischemia or other factors of the compromised, non-healing wound site. For the hypoxic wound, HBOT corrects the pathophysiology related to oxygen deficiency and impaired wound healing.
According to the Hyperbaric Oxygen Committee report in 2003, "a key factor in HBOT's enhancement of the hypoxic wound environment is its ability to establish adequate oxygen availability within the vascularized connective tissue compartment that surrounds the wound."
Neutrophils, fibroblasts, macrophages and osteoclasts all depend on an oxygen-sufficient environment to adequately promote healing. Proper wound healing is promoted with "improved leukocyte function of bacterial killing, antibiotic potentiation and enhanced collagen synthesis," which occurs during periods of increased tissue pO2.
According to the 2003 report, the result of continuous HBOT treatments is "improved local host immune response, clearance of infection, enhanced tissue growth and angiogenesis with progressive improvement of local tissue oxygenation and epithelialization of hypoxic wounds."
Because HBOT drives the arterial oxygen content exceedingly high, it reduces swelling. As it increases oxygen to the mitochondria, it stimulates healing and works synergistically with several antibiotics.
"Bacteria like pseudomonas normally develop a slime layer that is difficult to penetrate with antibiotics, but hyperbaric oxygen therapy breaks it down, so you can then penetrate the cells and kill a lot of pseudomonas," says Mindy Johnson, DVM, BARHC, at Bluegrass Animal Rehabilitation and Hyperbaric Center in Versailles, Ky.
"There are a lot of 'bugs' HBOT can kill even without any antibiotics."
"HBOT is very beneficial for infections," says Natanya Nieman, DVM, at Winstar Farm, also in Versailles, Ky.
Applications of HBOT
"The high oxygen level is actually bactericidal for certain bacteria," says Roger Gfeller, DVM, Dipl. ACVECC, at the Central California Veterinary Specialty Center in Fresno.
"You have to be conscious that you can't get to your patient in the chamber, so you need to watch for signs of discomfort, or how they're breathing," Gfeller cautions. "If they're becoming anxious you may want to stop your compression and discontinue the procedure. If they've got indwelling catheters, you must monitor to make sure they won't dislodge them. You need to monitor what you're treating for, be cautious of pneumothorax. Most importantly, you must make sure there is nothing going into the chamber that can spark, and create a combustible situation.
"Treating the multiples of diseases with high-pressure pure oxygen has so many healing benefits due to the fact that it drives oxygen to the cells, it is hard to put into words how beneficial it is," Gfeller says.
For small animals, he says "it's really effective for pneumonia, intestinal ileus, post-surgery, gas gangrene, pancreatitis, neurological injuries, including spine injuries, ivermectin poisoning, post-seizural cortical blindness."
"Hyberbaric oxygen therapy started in equine medicine about six years ago," says Fairfield Bain, DVM, vice president of Equine Oxygen Therapy in Lexington, Ky. "We try to follow the general indications for human HBOT treatment therapies, those of the American College of Hyperbaric Medicine and the Undersea and Hyperbaric Medical Society, organizations that oversee and provide guidelines to the human medical community.
"In equine medicine, the major categories for use of HBOT are those situations where you are salvaging tissues, i.e., difficult wounds, bone infections and sloughing-tissue injuries. ...We use it in an attempt to salvage tissue that we might ordinarily lose due to poor blood supply," Bain explains.
Hagyard-Davidson-McGee found HBOT especially useful for intestinal strangulation, due to colon torsion. "The earlier it's applied, the better the outcome, especially in certain critical tissues, such as the colon, where if you apply it too late, there is already a lot of tissue sloughing and injury," says Bain.
"I view it as an adjunctive therapy. It's not a sole treatment process. We still use our traditional medical therapies, but it is just one extra tool that we use to enhance the outcome, and hopefully in some cases make the difference in survival," Bain says.
At Winstar Farm, "we use it most for infections, wounds, with newborn foals, for 'bleeders' (EIPH) and for tendon injuries," says Nieman.
Winstar is treating a foal suffering from an abdominal abscess with antibiotics and hyperbarics. The abscess is shrinking quickly, and the foal is improving more rapidly than expected.
For about three years, Winstar has been putting all of its newborn foals in the chamber about eight to 12 hours post-foaling. It has about 80 foals per year and has not had a septic joint in three years, nor a septic foal or a bad case of neonatal asphyxia.
Even foals that are a little weak and wander around the stall prior to treatment come out of the chamber nursing and a lot stronger.
Because Winstar owns the chamber and the horses, it has few limitations on what horses it can treat. This year it has used the chamber for all the mares to be bred on foal-heat, every other day, during the post-foaling period and reports a high percentage of in-foal mares off foal-heat.
In post-foaling, Winstar uses a 45-minute to one-hour treatment at 1.5 ATA. If there are problems with high white-cell count, or the foal is a potential "dummy" foal or has low IgG count, Winstar will order further treatments for three to five successive days for up to an hour and raise the pressure about 0.5 ATA a day, with a "normal" treatment at 2.5-3 ATA for one hour.
"Horses just feel better after a treatment," says Nieman. "They come out of the chamber prancing and feeling good for the first couple of hours. If they were really lame going in, they'll come out of the chamber more sound and feeling better for a short period.
"It's also amazing how horses bounce back after colic surgery," adds Nieman. "It's probably saved a number of horses' lives – those with infections that would not otherwise have been treated, such as bone infections."
At Bluegrass, the chamber is used mainly to reduce tissue swelling, for neurologic injuries (neck, back), pre-operatively to reduce swelling around the spinal chord, for wound management, bone infections and osteomyelitis. The owners would like to use it more for dogs suffering crush injuries after being hit by vehicles – wounds that otherwise would easily become necrotic – and for gastric dilitation volvulus to preserve tissue until the animal's vascular supply of oxygen improves.
Bluegrass also uses the chamber frequently after GI surgery to assist resections and anastamosis, to speed tissue growth and for fungal disease — all with good results.
"While before the patient would have a guarded or poor prognosis, we now do a lot of wound care (with the HBOT) and treat neurologic disease, and they are turning around so quickly," Johnson says.
History and potential uses
The Undersea and Hyperbaric Medical Society (UHMS) formed the Hyperbaric Oxygen Committee (HOC) in 1976 to research clinical data and offer recommendations on clinical efficacy and safety of the therapy.
The committee, made up of practitioners and researchers in several fields of medicine, meets yearly to consider new indications for HBOT, makes recommendations, after which the UHMS executive committee votes final approval.
So far in equine medicine, data from hyperbaric therapy are not from controlled studies, but mainly from anecdotal, client-driven, case-based information.
Lacking scientific data, equine practitioners tend to be skeptical about HBOT.
Groups that work with HBOT have formed the Veterinary Hyperbaric Medicine Society and created a Web site in conjunction with UHMS. They are gathering case data on HBOT with horses, and beginning to write up some of the results.
Some equine practitioners want to perform controlled research, as more university veterinary facilities obtain HBOT chambers. They expect to disseminate papers on the subject to the veterinary community, to enlist veterinarians to help promote the benefits of HBOT for horses and to interest more DVMs to obtain chambers for their practices.
There is now a cased-based registry, where facilities that have chambers for horses are collecting information into a central database, to disseminate the cased-based results. Controlled studies will take longer.
"Even if some of the practices would write some articles on a single case it would be helpful," says Dennis Geiser, DVM, Dipl. AVBP, Large Animal Clinic, University of Tennessee College of Veterinary Medicine, who serves as president of the Veterinary Hyperbaric Medicine Society.
"This is a discipline, not necessarily always just a complementary therapy," Geiser says. "There are some conditions where it would be a primary therapy, but for the most part we are going to use conventional therapy along with hyperbarics," Geiser says.
"It's a great therapy. I am convinced that, by looking at the available case literature, this is really going to help a lot of horses that might ordinarily be euthanized. It's got a lot of potential. We just need to find out how long we need to treat horses, how much oxygen and at what pressure we need to use the therapy for horses, and get that information out to veterinarians," Geiser explains.
"It is also important as the case-based data is collected to collate it, but we need to make sure the information we get is the information we want and have some follow-up. Also if there is anecdotal data as to the response of horses with various conditions to HBOT treatment, then there needs to be confirmation that the positive results were actually due to the treatment. We need to be able to compare treated and non-treated individuals because we have to be able to say to our clients that this therapy will help this (particular) condition."
Ed Kane is a Seattle author, researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background in horses, pets and livestock.
On the Web
The American College of Hyperbaric Medicine
The Undersea and Hyperbaric Medical Society
Veterinary Hyperbaric Medicine Society