I just looked at my power file from New Orleans. I noticed that my heart rate decreased over the course of the ride. It spiked at the very beginning to 169, then steadied about 155 for the first half of the ride, then slowly decreased so my average heart rate on the last half of the ride was 150, and the last 1/4 was 145. Normally at the end of rides I see it go up.
I’m just wondering if this was due to lack of nutrition or hydration? I suffered from both as I (stupidly) opted to skip the last water stop and ran out of water at mile 50 and as a result I (again stupidly) decided not to eat anything since I couldn’t wash it down with liquid. But I didn’t really suffer until the run, but maybe I started to suffer earlier and just didn’t realize it?
I thought my nutrition was right on and actually peed on the run, but I also got the chills quite a few times on the run…
On your heart rate, it would go up and/or down for a variety of reasons. First off, if you’ve got to mirror HR to your workload. Did your power fall off as well?
My power remained pretty steady. My PE went up but that may have been the wind.
The rule of thumb is that hr too low is a lack of nutrition and hr too high is heat stress or dehydration or both. Like every rule of thumb there are exceptions.
As Jason pointed out if your power dropped with hr then you can get a better idea of what was going on. It could have been lack of attention, maybe after being out on the bike that long your focus kinda wavered. Although even in this case, wavering focus can also be a sign of a lack of nutrition. If your power stayed the same and hr dropped, maybe you just finally calmed down toward the end 
As for the chills, my personal experience is that the only times I have had them was when I was exercising and felt badly depleted in terms of blood sugar. Granted I didn’t have a blood sugar test done, just that everything else pointed to low blood sugar. But all I have in that regard is personal experience for the chills.
If your HR is way too low, you could be dead
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Your post brings up the important point of not being a slave to your heart rate monitor. If you looik at the hrm and it says 0 bpm, it is pretty important in race situations not to die. You may feel compelled to do so since the hrm says you are dead, but don’t do it; it will not give good race results.
I’m no expert on this, but I know that your gearing can play a role in heart rate. When I was a newbie cyclist and my cycling mentor was leading me around at the local practice race, when I was about to blow up, he’d have me switch to a harder gear and slow my cadence to help get my heart rate back down without losing much speed. Now, years later, I know when I’m time trialing that I can find my rhythm and start rolling a bigger gear and maintain a high speed with a relatively low heart rate. Maybe you’re finding a better rhythm later in the race.
Watch pro cycling and you’ll often hear commentators at the end of a race say that the riders who are maintaining a higher cadence appear “fresher” and they favor them for the finish. When you tire, you naturally tend to lower your cadence and increase your gear. You maintain speed, but not necessarily for all that long. So maybe it’s just a natural function of tiring as you get farther along in the race.
Just some possibilities.
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