The Unusual Suspects, Part 2 - When Oxygen Turns Lethal
Last week we looked at a condition in pigs called water toxicity. This week, let’s look at the sinister side of another life-sustaining compound: oxygen.
Most school kids can recite the percentage of oxygen in the air we breathe: 21%. It’s sort of odd to think about this element as not even making up the majority of the air we breathe (the majority of our breathable air is made up of nitrogen), but this is how life on earth evolved. What’s interesting is that oxygen concentrations higher than 21% can cause problems, and exposure to high levels of oxygen under high pressure can be toxic, even lethal.
Hyperbaric chambers are used for divers recovering from the bends — a very painful condition resulting from excessive nitrogen build-up in the joints from improper decompression — and are some of the main causes of oxygen toxicity because the subjects inside are breathing 100% oxygen. It turns out that oxygen at high concentrations creates high levels of free radicals in the body. These chemical troublemakers are detrimental to cell membranes and other important cellular structures. Oxygen toxicity can begin with fluid accumulation in the lungs and graduate to neurological symptoms as the brain becomes affected.
Veterinarians recently began borrowing this modality from human physicians and are using high levels of oxygen to treat equine patients. Enter the use of equine hyperbaric chambers.
In the past few decades, physicians have started using hyperbaric chambers to help stimulate healing in otherwise non-healing wounds — wounds with infection so deep they were non-responsive to antibiotics. The theory is that the increased levels of oxygen under higher atmospheric pressure — like what you get in a hyperbaric chamber — forces higher concentrations of the element into the blood, which would, for brief periods of time, increase wound healing.
Hyperbaric oxygen therapy (HBOT) in horses is still in its infancy and only a few large equine veterinary clinics offer it at the moment. Although there is some scientific research supporting the use of HBOT in human patients, similar evidence in horses is lacking. The Veterinary Hyperbaric Medicine Society (VHMS) has been created at the University of Tennessee’s College of Veterinary Medicine and despite a comparatively underwhelming amount of scientific evidence for this technology, the society at least offers some guidance for veterinarians interested in offering the fledgling therapy to equine patients.
In addition to oxygen toxicity from exposure to a hyperbaric chamber, such therapy also carries a much more dramatic threat: explosion. Pure oxygen is extremely flammable and when combined with an excellent fuel source such as horsehair, can be catastrophic in the presence of a spark. This is what occurred on February 10, 2012 at a Florida equine rehabilitation center. A horse and a technician were killed when a spark from the horse’s shoe ignited the chamber.
The VHMS maintains that there are protocols in place for these chambers to safeguard against such events and individuals offering such therapy need to be properly trained. I’m not sure of the number of equine hyperbaric chambers in the U.S. currently, but I can’t help but wonder if their numbers have plateaued as a result of this 2012 accident. Are the safety threats (meaning both risk of explosion and risk of oxygen toxicity) worth the as-of-yet mostly unsubstantiated benefits of HBOT? My guess is that we just need more data.
Dr. Anna O'Brien
