How does pressure cause tissue damage?
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We all know that too much pressure between a saddle and a horse can cause pain and damage to the horse. In a previous post I explained why we don’t really know how much pressure is too much. Another commonly asked question is “How does pressure cause damage anyway?” And the answer to that is the same: We don’t really know for sure. But there are a number of things going on that could be causing, or contributing to, the damage. And the number one culprit discussed is lack of oxygen (ischemia).
Shutting off the blood flow
All tissues in our bodies need oxygen to survive and work properly, and it is blood flow which provides the oxygen. So if blood flow is totally cut off to tissues, they will be damaged due to lack of oxygen. Basic stuff. So how much pressure does it take to stop blood flow? That depends if you are cutting off blood in an artery (higher pressure) or a vein (lower pressure). This, by the way, is where we get the two numbers in our normal blood pressure measurements. It takes a lot less pressure to cut off blood flow through the tiniest blood vessels – the capillaries. Work done on humans (I know of none on horses) shows that it takes less than 1 PSI to stop blood flow through capillaries, and it is from the capillaries that tissues such as muscle actually get the oxygen they need.
Try this test
So do we need less than 1 PSI everywhere underneath our saddles at all times to prevent damage to the horse? Not at all and here is why. Try this. Take a finger and press down on your thumbnail. Look at what happens to the color under your nail. It goes from pink to white. Now take your figure off. How fast does it go back to pink? This is called your capillary refill time (CRT) and it is a quick test that can be done to see if your blood flow is normal or impaired, as it is if you are in shock. (Veterinarians do the same test on the gums of animals.) What you are doing is stopping blood flow through the capillaries so the pink color (which is from your blood) goes white. When the pressure is removed, the blood flows back and it is pink again. Normal CRT is less than 2 seconds, and often much less than that. I am sure you put much more than 1 PSI on your thumbnail when you did this, and you stopped the blood flow.
It's more than just lack of oxygen
So, did you damage your thumb when you did this? Of course not, because you didn’t keep constant pressure there for long enough for the tissue to run out of oxygen and for damage to occur. Same thing with saddles on backs. This isn’t to say that pressures never stay above the capillary occluding level for long periods of time, especially around the wither area. But if the people making Smart Underwear have found that a few seconds stimulation every 10 minutes will cause the buttock muscles to contract enough to get sufficient blood flow to prevent pressure sores in their paraplegic patients, how much more movement is in the muscles of the horse’s back under saddle?
But it is one of the causes
So am I saying that lack of oxygen isn’t a problem under saddle? Not at all. It probably is a large contributor to the muscle damage that occurs, especially in the wither area were you can get unrelenting pressure. But to say that any pressure over 1 or 1 ½ PSI is unacceptable and will cause damage is false. And damage occurs in areas that never have unrelenting pressure, so they always have oxygen available. There must be something more. And there are a few culprits. But they will be discussed in another post...