Did this happen only once, or is it reoccurring?

If you didn't feel weird, I would say it is probably equipment malfunction. I would think a rapidly plummetting hr during exercise would lead to passing out or fainting pretty damn quick.

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Long Chile was a silly place.

Most likely your monitor if you felt nothing associated with it. But.... This is exactly what I saw one day 15 years ago, but I felt light headed each time I would see this happening. Nothing big, like I was getting sick or something, but it was weird that the feeling always happened when I saw the monitor going down,

Long story short, I had 3rd degree heart bloc and the monitor was just averaging down because of my heart stopping. The fuzzy feeling was not getting blood pumped through my body, of course if it persisted I would have just died. Lucky for me it was intermittent and I put 2 and 2 together and drove myself to the ER. It took 3 more days for me to finally get a doc to recognize what was going on, 24 hours later had pacemaker put in.

But that would be the exception, most of the time is is a HR monitor issue..

Try a new battery in the monitor?

+1 on checking the battery, though you should also do a manual check (fingers / watch) to see if it's really that low or just not reading correctly. Mine (Garmin HRM) was doing this last summer (both reading artificially high and low) - I replaced the battery and it was better for a while. Ultimately I had to replace the HRM. New one works fine.

If it's not your equipment, dropping HR is an indicator of either:

1. Underfueling. Not enough blood sugar to keep the body going as hard as you were before. Assuming you have the fitness to be going that hard anyway.
2. Underhydrating for the amount of fuel you're taking on. That makes the fuel ineffective and you're back at #1 again.

On the other hand, rising HR is an indicator of becoming dehydrated, which makes your blood thicken and then your heart has to pump more often to deliver the fuel and oxygen your muscles need.

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Zen and the Art of Triathlon. Strava Workout Log
Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
http://www.zentriathlon.com

I would have agreed fully until I checked the pics posted in the OP. HR dropping that low isn't a hydration or nutrition problem. It's either an equipment malfunction or the OP died and self-resuscitated several times near the end of the run without his knowledge.

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Take a short break from ST and read my blog:
http://tri-banter.blogspot.com/

True. While HR in the 60s isn't deadly, that's one hell of a massive bonk and he certainly wouldn't have said he didn't feel different. If my HR dropped like that, I'd be laying on the ground while watching clouds and eating a Twix hoping it doesn't get any worse!

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Zen and the Art of Triathlon. Strava Workout Log
Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
http://www.zentriathlon.com

I have this issue happen to me whenever I am on a really long run. I think I'm fueling correctly, but I guess not.

Surely it's impossible for someone to run 8 min miles at 60bpm. I definitely go for equipment malfunction.

Out of interest what kind of heart rate monitor is beng used? If it's a chest strap I would replace the battery, if optical, perhaps sweat is interfering with it somehow.

Garmin HR Run (chest strap).
I have 2 I cycle thru.
Will start labeling and watching if the same occurs with each.
Will also try battery change.
I''m still here so that's a good sign!
Thanks for all the feedback.

I agree with equipment failure, but just to be sure, if it happens again, measure your pulse manually. You don't have to even be close to exact.

You should be to quickly tell the difference between 140 and 60 bpm

Oh I completely agree with you. 60bpm is a normal resting HR on the couch. If you're running 8min miles it should be at least double that. Change the battery.

"60bpm is a normal resting HR on the couch"

For some .................. mine is more like 35 - 40.
Lord knows what it is when I go to sleep.
Sometimes I wonder if I'll wake.

One other thing you may want to look at since you have the Garmin HRM is the connections (the two snaps on the chest strap that attach the transmitter). I believe that was ultimately the cause of my problem / why I needed a new one - one side was starting to rust and I think (just my speculation) it was somehow interfering with the measurement.

Good luck!