Hyperbaric oxygen therapy certainly is not new, but it is a relative newcomer to equine medicine. It has a long history in human medicine and is most widely known for its use in the treatment of decompression sickness (i.e., "the bends") in divers as well as more recently in the treatment of various medical conditions.1 Hyperbaric oxygen therapy also is FDA-approved for certain conditions (see INDICATIONS FOR HYPERBARIC OXYGEN THERAPY IN HUMANS).
Equine patients have many conditions similar to those on the list for humans,4-9 especially wounds with poorly vascular, traumatized tissue or flaps, soft tissue infections, such as tendon sheath infections, and bone infections or osteomyelitis. Hyperbaric oxygen therapy has been used to treat various other conditions in horses, including birth asphyxia; peripheral nerve injury, such as brachial plexus injury; intestinal ischemia, such as colon torsion and small intestinal strangulation; and spinal trauma.
How Does Hyperbaric Oxygen Therapy Work?
Hyperbaric oxygen therapy works using the principles of gas under pressure. It is a mode of therapy in which the patient breathes oxygen at pressures greater than normal atmospheric pressure. In the hyperbaric chamber, the animal is exposed to 100% oxygen under increasing pressure.
Some physics lessons are needed to understand what is happening. At sea level, we are exposed to an atmospheric pressure of 14.7 pounds per square inch (psi) or 760 millimeters of mercury (mm Hg). The air we breathe is 79% nitrogen and 21% oxygen, meaning that it contains a pressure of approximately 160 mm Hg of oxygen (21% × 760 mm Hg). In hyperbaric medicine, pressure is measured in atmospheres absolute (ATA). At sea level, we are exposed to 1 ATA (normal atmospheric pressure). In diving, the pressure increases 1 atmosphere for each 33 feet (i.e., 10 meters) of sea water. Therefore, at 33 feet of sea water (fsw), the absolute pressure is 2 ATA. At 66 feet of sea water, the absolute pressure is 3 ATA.
This comparison to depths in sea water is important because these are the relative pressures that a patient is exposed to during clinical hyperbaric oxygen treatments. The critical feature of oxygen under pressure is that the amount that can be breathed into the lungs increases exponentially.
Using 100% oxygen, at 2 ATA the animal is breathing in 2 atmospheres worth of 100% oxygen, or:
760 mm Hg × 2 = 1,520 mm Hg
At 3 ATA, the animal is breathing in 3 atmospheres worth of 100% oxygen, or:
760 mm Hg × 3 = 2,280 mm Hg
That is an incredible 14 times the amount one would breathe in room air at sea level.
In medical applications of hyperbaric oxygen therapy, 3 ATA is the maximal pressure that would be used to treat a patient because of the increased risk for oxygen seizures at that concentration of oxygen. Most treatments use from 2 to 2.5 ATA. This high concentration of inspired oxygen has several valuable physiologic effects on the body:
- It stimulates tissue healing by reducing swelling.
- It increases cell division in the endothelial cells that line blood capillaries.
- It increases division within fibro-blasts, the cells that make collagen.
The importance of the effect on blood vessels is that new capillary vessel in-growth is critical in tissue healing for various injuries and increases the body's ability to deliver antibiotics into areas that previously had poor blood supply. Hyperbaric oxygen also reduces inflammation and may have immune-stimulating effects. In addition, it has been shown to enhance the effects of certain antibiotics, especially those that do not work well in low-oxygen environments, such as the aminoglycosides (e.g., amikacin and gentamicin). Much of the basic research showing these effects has been conducted in animal models of human disease, and veterinary professionals are now able to use this information for equine patients.
For the first one or two treatments, the animal may need sedation, but basically, it is no different than when the horse is in a horse trailer or van.
The most common equine chamber is an open room design that allows the horse to move around, which helps it to relax. The technician constantly monitors the horse by checking video monitors positioned to obtain different perspectives. The chamber needs to be housed in an environmentally controlled building. Most chambers use liquid oxygen, as it is the most cost-effective supply.
Pressure in the chamber is gradually increased, delivering oxygen through the floor of the chamber and gradually removing room air through the roof over several cycles. After about 30 minutes, the treatment pressure (usually 2.0 to 2.5 ATA) and maximal oxygen concentration (usually around 94%) is reached.
The total time in the chamber varies from 45 to 60 minutes. The protocol — time of treatment, maximal pressure, and number of treatments — depends on the condition being treated. Because few equine studies have been conducted to document treatment protocols, we follow the experience in animal models and human medicine. For example, a horse with a bone infection might receive 10 or 12 treatments.
The response of the equine patient to pressure is important. Horses have guttural pouches, which are distensible air pouches within the auditory tube that readily open when the horse swallows and apparently facilitates equalization of pressure. Although disorders of the sinuses or guttural pouches could create an air-trapping scenario, this rarely seems to be the case in equine patients.
One important concept to understand is that hyperbaric oxygen therapy is a supportive or adjunctive treatment. The primary condition is still managed using other medical treatments, such as appropriate antibiotics for bone or tendon sheath infection. Hyperbaric oxygen therapy should be viewed as way to enhance tissue healing. The goal is to shorten the recovery time in injuries and illnesses — the result being improved survival rates for devastating illnesses like colon torsion and less overall time in the hospital.
For More Information:
Holder TEC, Schumaker J, Donnell RL, et al. Effects of hyperbaric oxygen on full-thickness meshed sheet skin grafts applied to fresh and granulating wounds in horses. Am J Vet Res 2008; 69(1):144-147.
Krahwinkel DJ. Topical and systemic medications for wounds. Vet Clin North Am Small Anim Pract 2006;36(4):739-757.
Stewart A. Clostridial myositis and collapse in a standardbred filly. Vet Clin North Am Equine Anim Pract 2006;22(1):127-143.