Sudden heart rate drop from 65 to low 30's.Dehydration ?

In my limited (non-expert) experience, dehydration is more likely to elicit tachycardia...elevated HR, not bradycardia (slow heart rate). I also had problems with beta blocker when taking an anti-arrhythmic drug, or when I was in atrial fibrillation and taking too much metoprolol, the beta blocker prescribed for me. My HR dropped to 30, which is about the rate that one's AV node sustains when the SA node is blocked for some reason. The AV node takes over to maintain a rock-bottom rate for the heart when it isn't relaying normal signaling from the SA node. Beta blockers that are present in too high a concentration might block the SA node and cause the AV node to take over, but it's rate is bare minimum for survival.
Please contact a competent health professional and explain your circumstances.

I think you should consult an electrophysiologist or a cardiologist and explain what is happening. I can only guess that you're taking too much of the beta blocker, or that some other development has rendered it somewhat dangerous for you.

I've heard of medications interfering with each other, for example if they are both processed by the same enzyme, having both meds in your system may raise the level of one or the other above what is ideal. You could ask your doctor and pharmacist if anything else you are taking could have such an effect.

Out of curiosity, I'm wondering if you would have noticed anything if the Apple watch hadn't alerted you to the episode? Did it feel different?

Scolio , I be been told by an allergist some years ago that I tend to accumulate certain meds. Maybe I experienced a buildup of beta blocker/ lorsartan.

I first felt lightheaded.
A sense of heat flash and malaise. I only thought of my watch later. There was no alert/ notification but the screen clearly informed me that my HR had fallen below 40 …several times in 1/2 hour. I’ll check my settings.Then I returned to a normal HR and had no problem sleeping well
Through the night.

I used my health provider portal to let my cardiologist know…
Loe BP low heart rate.
Answer” Watch both for a week or so I then report in”.

My husband, age 73, had been feeling light headed and went to the doctor. His pressure was 102/60 and pulse was 40. The doctor told him to stop his blood pressure meds, he did and is so far doing alright. Also during th winter his pulse was always in the 40's, one time even in the 30's. However, the doctor wasn't concerned said it was due to the cold and when he rubbed his hands the pulse rate did go up. Don't know if any of this helps...good luck !

Thanks, Casey, that does help a lot. All things point to beta blockers on an empty stomach plus dehydration. If it happens again, I’ll need to reevaluate the situation.
ThisMayo Connect site really helps.