My heart.

I had a catheterization done yesterday and all went well. No complications with the procedure. But, the doctors still don't know what's causing the increase in tachycardia. I may just have to live (or die) with it.
That's life.
I was just leafing through Strunk and White's "The Elements of Style" (a book I find to be invaluable and maddening), and I came across an example of a quotation. It said, "Never worry about your heart till it stops beating."