Just looking for a little help this year as I'm going to start doing a few multi-day events this year. I'm generally good for one huge day (like 7-8 hours at avg heart rate 155-160).

Twice last year I had big rides the day right after. Both times I was ok for the first 3 hours or so, but even with drinking and eating generally well, my heart rate started going down and I was unable to hold anything above 145 or 150 (I can hold 170+ for an hour w/ max 194, 35 yo).

Is it the legs not being strong enough to push the heart any higher or is it a lack of power out of the cardio system? How do I address this weakness? I come from more sprint/recovery sports (tennis, basketball) and am adept at going into the red for long periods and recovering quite well from it, but I think I probably overuse this a bit.

Thanks!

Low heart rate is a common sign of over extending or dehydration. Its just a fact of the body saying its tried.

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Heart rate suppression is a very complex issues, a whole bunch of things can effect what the heart does and does not do during exercise. I did try to speak to a cardiologist about this once and... well I don't think I really understood most of the concepts being explained.

However in relation to excerise there are a couple of things to keep an eye on being, HR, power (if applicable) and RPE. If some of those things don't match up they need to be looked at. If your HR is low but power and RPE is where it should be then this may be of no issue (even a good thing). If HR and RPE is way up and power down then could be fatigue/overtraining. It's very important to listen to your body. If doing your usual climb and things just don't feel right sometimes throwing in the towel for the day is the best training you can do.

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"Physiology is all just propaganda and lies... all waiting to be disproven by the next study."
"I'm not a real doctor; But I am a real worm; I am an actual worm." - TMBG

Yeah absolutely listen to my body quite a bit and maybe even too much sometimes. RPE is generally not very high at that point, it's just that the motor is petering out. I think it basically has to do with overextending myself on the first day to the point that recovery is just not possible the next day at this point. I've had a full ekg, stress test etc etc there's no issues whatsoever there as far as I know.

I think the main issue is that I come from sports that aren't purely aerobic and I haven't had time to build up my engine to the level of my riding quite yet if that makes any sense?

Heart rate is a very good indicator of how tired your body is. I believe that a typical progression of fatigue will be that HR will go down, then RPE will start to get pretty bad and power will follow the downward trend. Start to take that low HR as an indicator that your body is beginning to tire (even if you don't quite feel it), and just be careful to not push it too hard because you'll likely fade faster than normal at that point. Although, if I remember correctly, when I was in a similar situation I was able to hold my power pretty well when I couldn't hit my max HR. I just don't think I was a efficient as I possibly could have been.

As a kind of related point, I've always noticed that when I do training rides in the morning, my HR is always perfect and super reactive, whereas if I do my workouts in the later afternoon there's a bit more lag to my HR. It's a simple example of how the body gets tired even throughout the day. I think you're right that you haven't built up your engine yet, but that'll come with time. Just listen to your body--it's good that you're recognizing these signs!

Except that it's not. HR in isolation tells us next to nothing about a person's fatigue, fitness or state of arousal etc etc. Just because HR is higher or lower in a given circumstance needs to be compared to a range of other factors before and determination about what that HR means.

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"Physiology is all just propaganda and lies... all waiting to be disproven by the next study."
"I'm not a real doctor; But I am a real worm; I am an actual worm." - TMBG

I wasn't saying to simply look at HR in isolation. I meant in relation to all those other things, like you said. Thanks for clearing that up.