#193 - AMA #31: Heart rate variability (HRV), alcohol, sleep, and more

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Hey everyone. Welcome to Ask Me Anything Episode number 31. I'm joined once again by Bob Kaplan. Although as you'll learn at the end of this episode, this will be the last time we're joined by Bob Kaplan. But let's not get ahead of ourselves. In today's episode, we talk about really two things in detail. One is HRV heart rate variability. We've had so many questions on this subject for the past couple of years, and we've just been slowly kind of collecting questions until we thought we had enough to do kind of one of our thematic shows on it. And the second thing we talk about is ethanol. All things related to ethanol. So how does it affect HRV is really our segue. But then also what are the effects of alcohol on sleep? And we really go after mortality, probably where we spend most of our time, and we talk about it through the lens of all of the epidemiology. As most of you or may some of you know, there's a confusing body of literature out there that suggests that maybe some alcohol is good for you and no, alcohol is not as good for you. And so we go really deep on this subject matter. So again, if you have any questions about alcohol or if you drink at all, and if you're, or if you don't drink and you're wondering if you should be drinking, I think this episode will be very helpful for you. So one thing I want to point out for this episode, this episode was just due to some limitations of timing when we recorded it, this was not recorded in video. So we will only be doing this in audio. That might make the show notes a little more important, although we didn't really have a lot of content here that necessitated being able to see. So I think you're going to be fine listening to this in audio. But obviously the show notes will have any of the graphs and other images that we talk about. Now, if you're not a subscriber, of course you can get a sneak peek of this as usual, and then you'll have to obviously subscribe if you want to Enjoy the full content. So without further delay, I hope you enjoy AMA number 31. Hey, Bob, how's it going today?

Going well, Peter. How are you?

It's good. It's a little unusual today we're, we're doing this a little bit on the old school tip. No video today. We're going to have to be a little bit more descriptive in our terms since people can't watch our facial expressions, although that somehow provided any value, we won't do any screen sharing.

I think we can manage.

What do you have in store for us today?

So I aggregated a whole bunch of questions around hrv, heart rate variability and alcohol. Many questions on both those topics.

I know this has been something you've been asking me about for a year. When are we going to do one on hrv? When are we going to do one on hrv? So I think the answer is now.

All right, we'll dive in. First question seems relevant. What is hrv?

And it's funny, this is a tougher question to answer without an image, but I'll do my best. So, as you pointed out a second ago, HRV stands for heart rate variability. And that's a pretty descriptive term because what it measures is the variation in time between heartbeats. And that's measured in milliseconds. So 1,000 milliseconds is one second. So if a person's heart is beating 60 times per minute, you might say, well, there's 1,000 milliseconds between every beat. But it turns out that it's not really that way. You see, even if your heart is beating 60 times per minute or once per second, between the first beat and the second beat, it might be 10, 10 milliseconds. And between that beat and the next beat, it might be 960 milliseconds. And between that beat and the beat thereafter, it might be 10, 27 milliseconds. When you start looking at this thing at the level of thousandths of a second, you realize that there is actually some variability. If anybody's ever seen an ekg, which I'm sure everybody has seen, but maybe you haven't thought so much about what all the little squiggly lines mean. Without going into the details of what the P wave and the QRS and T waves, meaning, I think most people will recognize that there is a very big spike for each of those beats, and that's actually the R spike. If you now measure the distance between the Rs, we call that the RR interval. And you take the root Mean square of the successive differences between the heartbeats. So that means you calculate the time difference between each RR interval, Each of those values is squared. So multiplied by itself, then the result is averaged before the square root of the total is obtained. So it's just kind of a mathematical method for trying to approximate differences. You get what's called the rmssd. Again, if you just think about that in units, you took something that was in milliseconds, you squared it, so it became milliseconds squared, you added them all up, it was still milliseconds squared, you took the square root of it, it's back to milliseconds. So RMSSD is reported in milliseconds. If anybody's been paying attention to wearables, a number of these things will calculate your heart rate variability, typically while you're sleeping, and you'll notice it gives you a number in milliseconds. I think prior to the advent of wearables, this was typically just done with an EKG or another sort of chest strap like device that was very accurately measuring the electrical activity of the heart.

The next question is probably, why do we measure hrv? So you talked about heart rate. I think people would understand measuring heart rate and maybe why that matters, why you might want a lower heart rate in general, why do we measure hrv? What does it tell us? Or what can it tell us?

Possibly, as I alluded to above the heart, even when your heart rate is not changing and if you're at rest, which is when we measure hrv. So we don't really measure HRV when you're out and about moving around, because just the movement itself that you're undergoing is going to change your heart rate. So if I stand up from my desk and go and walk over to the kitchen and do something, well, that's going to increase my heart rate just because I stood up. And if I'm exercising, obviously my heart rate is changing quite a bit. So this is really something that we care about when you're at rest. But as I said, there is variability between those beats, and it turns out that that variability is heavily influenced by which of the autonomic nervous systems is most dominant. So this probably warrants a slight detour. So we have, broadly, broadly speaking, two nervous systems. We have one that is under our control and one that is not under our control. And that's a very good thing if you want to reach forward and grab a pencil or walk. Obviously that requires voluntary control. You want to be in control of those nerves firing and making Muscles do their thing, but you certainly don't want to have to be thinking and consciously making things happen that need to happen constantly. You certainly wouldn't want to have to think about breathing. You certainly wouldn't want to have to think about your heart beating. You certainly wouldn't want to have to think about digesting food. You certainly wouldn't want to have to think about a lot of things that take place beneath the surface. So all of those things are regulated by this thing called the autonomic nervous system. We further divide this autonomic nervous system into two branches. One is called the sympathetic system and the other is called the parasympathetic system. The parasympathetic system. The way we would always learn to remember this in medical school was this was the rest and digest system. So this is the down regulating system. It conserves energy, it aids in digestion, it slows heart rate. Our favorite little fact was it was responsible for erections, but not ejaculation. Ejaculation somehow came from the sympathetic system. So the sympathetic system, of course, is the fight or flight system. So it's making energy more available. It's dilating the pupils, right? It's getting you ready to see as much information as possible. It's slowing digestion and peristalsis, meaning it's slowing down anything that's not essential and it's increasing heart rate. Let's just think about what this looks like from a practical standpoint. If you're laying in bed and you hear a loud bang in your house, you have no idea if it's an intruder or if a picture fell off the wall. But you don't even have to worry about your brain isn't going to even force you to make that decision. It's going to make the decision for you, which is this is a threat. And so your heart rate's going to shoot up, your pupils will dilate. Any amount of digestive energy going on right now will cease, and your liver's gonna start cranking out glucose and making you available for fight or flight. Okay, so what does this have to do with hrv? It turns out when the sympathetic system is revved up, HRV goes down. And when the parasympathetic system is in control, HRV goes up. And I think it's easy to think about this, right? As the heart rate speeds up, which is what's happening under sympathetic tone, there's less variability between the beats. When the heart rate slows down, when the body is relaxing, there's more variability between beats.

I don't know if that's intuitive or not. I was thinking about that. If you have over the course of a minute and your beats per minute is, say you're, I don't know, Lance Armstrong when he's at the height of his powers. I don't even know what that is.

Yeah, probably 30 beats per minute. Yeah, 30.

And then you've got somebody else who has 100 beats per minute. Just the RR intervals, whatever you call it, there's just less time between them. Intuitively, I would think there would be less room for variation between each beat, if that makes sense.

Yeah, I mean, I don't know how strong the correlation is between resting heart rate and hrv. It's almost undoubtedly positive, but I don't know what the R squared is on that. Is that a fair explanation, Bob? Does that make sense on why we would care about the hrv?

Yes, I think so. As a knock on to that, when you get your HRV output, if it's high or low, what is that telling us? Or how does that inform us? What do we do with that information?

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