The immune system has a lot to do with the tissue damage caused by vasculitis. The immune system becomes "hyperactive" in Lupus and in vasculitis and this leads to the inflammation within you body’s tissues. In Vasculitis, the inflammation in the blood vessel walls leads to narrowing of those vessels. It would seem to follow that heat might, indeed, aggravate the inflammation that occurs and could result in even more tissue damage in those areas.
If heat is relieving some of your symptoms, maybe you should try alternating between heat and cold in order to hopefully minimize the damage.
Maybe someone will have more information that I do on this subject. Otherwise, I hope I've helped you at least a little
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