Originally Posted by Carbonfiberboy

Sure.

On a long ride, if your HR goes down below where it normally is for your RPE or power, the probable cause is low blood sugar. The fix is to eat something, preferably something fast acting, like a squirt of Hammer Gel or a Shot Blok. I don't use packaged gels because of the packaging.

If your HR goes up beyond where is should be for your RPE, especially if it's up quite a bit, the probable cause is dehydration. When you get dehydrated, your blood volume drops which causes your HR to go up to compensate. The fix is to drink. My usual fix is to drink 3-5 swallows every 5 minutes. If things have gotten so far that your forearms are dry, you need to get in the shade while you drink ASAP, you're on the verge of a medical emergency.