1.4 Physiological Implications of HRV Dynamics
Dr. Ahn gets into the implications of HRV dynamics and the importance of the vagal brake and its relation to stress.
- 0:00- 6:00 Implications of HRV Dynamics
- 6:01 - 10:34 Vagal Brake
- So the question for you is what causes relative tachycardia during respiration then, so we know that if you breathe in, you have rapid rise in your heart rate, as you breathe out, you have a decrease in your heart rate. Because I had heard on a podcast, someone's saying that as you breathe in you inhale, this directly stimulates the sympathetic nervous system. And this is how you get a rise in your heart rate. We know that's not true based on our understanding of physiology.
- In fact, what happens is, that actually, your parasympathetic nervous system is withdrawn. And we know this is there was a recent study done by a chive eonni, in journal physiology in 2020. And they've actually done this in human life human beings, where they inserted an insulated electrode needle into the vagus nerve.
- And, you know, I'm surprised they were able to find volunteers for this because it's a relatively dangerous procedure, because the vagus nerve is right by the carotid artery and the internal jugular vein. But they were able to do it, we found three volunteers actually, I think they were the study investigators. And they're able to measure the number of activations of the vagus nerves. And you can see here by the instantaneous frequency, basically the number of stimulations that you get per second. And you can they measured it with in spoken respiration. And so as they're expiring, the vagus nerve is active. But when they're inspiring, the vagus nerve is silenced. And so that's where you get the rise in your heartbeat. And so you see this cycle as you breathe in and out. So the relative type of cardio that occurs during inspiration is due to withdrawal of the vagal nervous activity, and not to be increased or enhanced, sympathetic nervous.
- Now, does this mean that there are no respiratory components in the sympathetic nervous system? And the answer is actually no.
- In human beings, you can insert a little electrode into the deeper ulnar nerve and attain the sympathetic nervous activities. And this is an example of this. And you could see sort of the heart rates as represented by blood pressure here. And you can see the heart rhythms manifested in the are shown in the sympathetic nervous system activities. But you could also see a respiratory component in here.
- And if you decide to really directly measure electrodes, suiting the sympathetic nervous system, either with a deep peroneal nerve recording, which is invasive, or using electrical, electro dermal skin conductance, you could see here that, you know, this is a study that was published by MIT, they took five deep breaths, evaluate the electrical term skin conduction, so you can already see these five deep breaths, I forgot to mention that the sweat glands are operated by the sympathetic nervous system. So when you have these inspirations and activates the sympathetic nervous system, which enhances or increases the sweat gland activity, and that increases conductivity. And so that's what you see of the, in this in this graph here.
- So the fact is, there are respiratory components in sympathetic nervous system. However, the SA neuro no again, filters out the faster processes, the faster activities that are greater than point one five hertz. And so the only thing that you see in regards to the heart rhythm, because of this AC, no filter, is the slower activity of the sympathetic nervous system.
- The quiet next question, so what happens if you breathe slower than seven beats per minute?
- Now that leads to the RSA, the respiratory sinus arrhythmia, then moving into the low frequency range. And so, this respiratory influenced effect on to this on the SA node now has now a both a present abetik nervous system and a sympathetic nervous system component because it is slow enough to be to invoke the sympathetic nervous system.
- And when you are at the slower range, it enables this RSA effect to subsequently work with a barrel reflex which operates around the same range and the phaser motor oscillations, and through the heart rates and blood pressure interactions, it causes what's called this coherent or the resonant states and if you see in this graph here was put out by Heart Math, you see sort of normal respirations. And you can see sort of the various heart rhythms in the blood pressures.
- But the moment you set to a 10, second breathing rhythm, or point one hertz, you see this locking in the frequency locking, of all the the time series, not only of respiration, heart rate, and blood pressure as well. And some argue that this is an important way to retrain your autonomic nervous system. I don't know the details of the literature behind that quite yet. But you know, some have suggested that this resonance system is why meditation, breathing at slower rates are really beneficial to your health.
- The final thing I wanted to mention in according to this graph, was that there is something called a vagal brake. And when there is stimulation of the acetylcholine muscarinic receptor, there is a car here that inhibits the adult little cyclists that's part of the sympathetic nervous system. So you have what's called a vagal brake.
- And the reason this is important is this is related to the question, how does the heart respond acutely to stress? When I learned about this physiology of the SA node, it sort of baffled me because it goes against my instinct of what would happen. From an evolutionary standpoint, you would think that if you have a fight or flight response associated with the sympathetic nervous system, that would need to act quite rapidly. You need to, you know, so that you respond to the threat where alarm immediately. So how does this explain our ability to respond rapidly to an acute stress.
- This was assessed by studying and psychophysiology. In the 1980s, they took about 20 Norwegian undergraduate college students, and they put them to a task of doing a video game with or without an electric shock. So this electric shock was a threat when they did poorly on this video game. As you can see here, these graphs on the heart rhythm and RSA or the respiratory sympathetic arrhythmia, you can see that rest, their heart rate is relatively low. But when they have this threat, their heart rate jumps up. And actually, what they did is they randomized to either getting shot first, you know, video game with shot, either first or last. And then conversely, whether you get the elect video game without shot, or on the second task. And then for the respiratory sinus arrhythmia, what we see is that in the beginning, the respiratory sadness with me at rest is relatively high. In other words, their parasympathetic system is activated. And then with this threat, this RSA quickly goes down, and it's just sustained through their second task.
- So the heart responds to an acute stress by removing the vagal brake. And the sympathetic nervous system is simply to respond to these acutely to the stress. And I think it makes sense from a physiological standpoint why that is, it's because the sympathetic nervous system does require a lot of metabolic resources and a lot of energy to make this happen. So if they if the sympathetic nervous system responded to every little threat, then it would be highly emotionally draining probably emotionally, but also physically draining to the organism. So, the vagal system has acted as the immediate acute function or mechanism for addressing a stress stressful situation.
- This goes back to the prior slides, which shows that at rest, we have a larger parasympathetic tone when accounting for the slower heart rate than you would see for an intrinsic heart rhythm. And this gives us a little bit of a buffer. So if you have a higher pressure protect zone and if you encounter a stress, you are enable this person to pick nervous system to subsequently deactivate and enable the sympathetic nervous system to kick in, and you can understand why this tone is important.
- Now if you don't have this tone, if this parasympathetic nervous system is constantly inactivated or lacks significant activity, when you encounter a stressful situation, you don't have that significant tone reserve, to to inhibit so that you're able to address that stressful situation acutely. So I think this is one of those things that it's important to know when we're evaluating the subsequent clinical studies or some of the physiological or psychological studies and why the present back nervous system is so important.