I sweat a lot! In spin class the sweat actually flows to the lowest spot on the floor and forms a puddle. Not a few little drops here and there but puddles. My clothes are completely soaked during a spin class or an early morning run, I can actually wring out my clothes.
One of the first signs of dehydration that I can monitor for is cardiac drift. Cardiac drift is when your heart rate rises due to dehydration. Since you have lower blood volume when you are dehydrated your heart beats faster trying to keep the blood pressure up. My first experience with this was at Buffalo Springs Half Ironman. A friend of mine, who happens to be a physician and a very good IM athlete, told me that you can only absorb about 28 ounces of fluid per hour and that I shouldn’t drink more than that because it will just sit in my gut. Well, It turns out I can and should drink more than the standard water bottle per hour. By the time I realized the folly of my ways it was way to late to re-hydrate. I ended up cramping on the bike and walking a good part of the run. At the medical (thanks to IronMaiden for walking me to the tent) the doc was shocked that I could still stand. My heart rate was around 140 (normally in the 50’s) and my BP was something like 80 over 40. So, after taking three bags of fluid in the medical tent, my HR was much lower and my BP was high enough so I could actually stand up without getting dizzy. I’m not sure where those three bags of fluid went because they sure didn’t come out over the course of a few days. Oh, and when you get that dehydrated you feel horrible.
At IMLou I consumed about 14 bottles of fluid on the bike – in SIX HOURS. I must have been a little closer to the mark because I only needed one bag of fluid at the medical tent!