Friday, June 18, 2010
Notes from The Doctor's Desk - June 18, 2010
Hyperbaric Oxygen Therapy for Horses
by Fairfield T. Bain, DVM, MBA
Diplomate, ACVIM, ACVP, ACVECC
Internist/Pathologist
Hyperbaric oxygen therapy is certainly not new, but it is relatively recent on the scene in equine medical care. It has a long history in human medicine – most widely known for its use in treatment of decompression sickness (aka “the bends”) for divers – more recently for a variety of medical conditions. Hyperbaric oxygen therapy is an FDA-approved medical therapy for certain conditions in human medicine.
Figure 1. Indications for hyperbaric oxygen therapy for humans (courtesy Duke University – http://hyperbaric.mc.duke.edu/indications.htm)
Air or gas embolism
Decompression illness
Carbon monoxide poisoning and smoke inhalation
Clostridial myonecrosis (gas gangrene)
Crush injury, compartment syndrome and other acute traumatic ischemias
Enhance healing in selected problem wounds
Exceptional blood loss or severe anemia
Necrotizing soft tissue infections (subcutaneous tissue, muscle, fascia)
Refractory osteomyelitis
Radiation tissue damage (osteoradionecrosis and soft tissue radionecrosis)
Compromised skin grafts and flaps
Our horse patients have many conditions similar to those on this list – especially wounds with poor blood supply, those with severe trauma to the skin and underlying tissue, or those with large skip flaps, soft tissue infections (ex. tendon sheath infections), and bone infections (osteomyelitis). We have treated a variety of other conditions including birth asphyxia, peripheral nerve injury (brachial plexus injury), intestinal injuries that compromise the blood supply (colon torsion and small intestinal strangulation), and spinal trauma.
So, how does hyperbaric oxygen therapy work? It works using the principles of gas under pressure. It is a mode of therapy in which the patient breathes 100% oxygen at pressures greater than normal atmospheric pressure. In the hyperbaric chamber, the animal is exposed to almost 100% oxygen under increasing pressure. Some physics lessons are needed to understand what is happening during HBOT treatments. At sea level, breathing air we are exposed to a pressure of 14.7 pounds per square inch (psi) or 760 millimeters of mercury (mmHg). The air we breathe is comprised of 79% nitrogen and 21% oxygen, meaning that it contains a pressure of approximately 160 mmHg of oxygen (21% x 760 mmHg). In hyperbaric medicine, pressure is measured in Atmospheres Absolute (ATA). At sea level, we are exposed to 1 ATA (“normal atmospheric pressure”). In diving, with each 33 feet (or 10 meters) of sea water, the pressure increases one atmosphere. Thus, 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 since these are the relative pressures that a patient is exposed to for 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 mmHg x 2 = 1520 mmHg. At 3 ATA, the animal is breathing in 3 atmospheres worth of 100% oxygen – 760 mmHg x 3 = 2280 mmHg – 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 a patient would be treated (due to the increased risk of oxygen toxicity seizures at that concentration of oxygen) with most treatments being between 2 to 2.5 ATA. This high concentration of inspired oxygen has several valuable effects in the body: it causes constriction of blood vessels – reducing swelling, it stimulates tissue healing – increasing cell division in the endothelial cells that line blood capillaries, increasing division in fibroblasts – the cells that make collagen in a variety of tissues. The importance of the effect on blood vessels is that new capillary blood vessel growth is critical in tissue healing in a variety of 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. It also has been shown to enhance the effects of certain antibiotics – especially those that do not work well in low oxygen environments (especially the aminoglycoside family of antibiotics such as amikacin and gentocin) known to occur in severely infected tissues. Much of the basic research showing these effects has actually been performed in animal models of human disease, and now, we are able to use this information for our horse patients.
What happens when the horse enters the hyperbaric chamber? For the first one or two treatments, the animal may need sedation for the new experience. It is not really any different than riding in a horse trailer or van. The most commonly available equine chamber is an open room design that allows the horse the comfort factor of being able to move around, thus being more relaxed. The technician constantly monitors the horse using a series of video monitors from different perspectives. The pressure is gradually increased – delivering oxygen into the floor of the chamber and gradually removing the air through the roof over several cycles. After about 15 minutes, the treatment pressure (usually 2.0 – 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 treatment protocols – time of treatment, maximal pressure, and number of treatments depend upon the condition being treated. We have tried to follow the experience in the animal models and in human medicine for our horse patients. In general, a horse might receive up to 10-12 treatments for a bone infection.
Another issue worth considering is the response of the horse patient to pressure. Horses have guttural pouches – a distensible air pouch within the auditory tube. The openings readily open on swallowing and apparently allow ease in equalization of pressure. While disorders of the sinuses or guttural pouches could create an air-trapping scenario, this rarely seems to be the case in the horse patient.
As you might expect, these chambers are large structures and require an environmentally-controlled building to house them. Most chambers operate using liquid oxygen as it is the most cost-effective supply.
One important thing to understand is that hyperbaric oxygen therapy is a supportive or adjunctive treatment. The condition is still being managed with other medical treatments – such as appropriate antibiotics for a bone or tendon sheath infection. Hyperbaric oxygen therapy should be viewed as something to enhance tissue healing. Our goal is to shorten the recovery time in injuries and illnesses – the result being improved survival rates for devastating illnesses like colon torsions, and less time in the hospital overall.
For those interested in more information and training in hyperbaric medicine, the following resources are available:
Veterinary Hyperbaric Medicine Society - http://www.vet.utk.edu/vhms/
The Veterinary Hyperbaric Medicine Society is a recently formed organization to foster communication between facilities and to collate information specific to veterinary applications of hyperbaric oxygen therapy.
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