1.2 Fundamental HRV Physiology & Frequency Domain

• To sort of illustrate the way I interpret how Heart Rate Variability has evolved over the years, I have put together so divided into five stages. And this is based on the prevailing scientific paradigms that existed in each of these stages. And unfortunately, I won't be able to go through all of these five stages, we'll probably take three talks overall, to go over all of them. But you know, I think as of today, we'll just largely focus on part one, understanding heart rate variability. 
• And the stages are first understanding heart rate variability, and then current rate variability as a marker of autonomic nervous system, then heart rate variability as a marker of body weight function, heart rate variability within the construct of Mind Body interaction, and then finally, the heart rate variability itself as a desirable target. 
• So the first stage understanding heart rate variability happens in the 1970s and 1980s. Predominantly, there was a focus on physiology and development of methods. 
• In my view, 1970s and 1980s was the golden age and physiology. That you know, this occurred about 20 years after Hodgkin and Huxley. Finally there's their axon signal transduction have gone along the squid axons. This was also 20 years after Watson Crick figured out the DNA double helix structure. And there was enough time for development of imaging states such as Eliza 1971 electrophysiology techniques, such as voltage plants and machines that are were important for obtaining electrophysiological measures, neural transmitter blockades, such as atrophy and propranolol, and he also had the infrastructure for animal studies. 
• So, it is only natural that you would see a significant development or advancement of our physiological understanding of our autonomic nervous system around this time. And by this time, by the 1970s, they really had a pretty good understanding of the autonomic nervous system. 
• And they have divided into two divisions actually, there are probably three because there's the parasympathetic nervous system, the sympathetic nervous system. And the third one is the enteric nervous system, which we don't talk about too much here. And during the 1970s, and 80s, they've really much outlines the various physiological roles that each of these nervous systems play, and also importantly, understood the anatomical pathways that were responsible for the execution and functions of both of these nervous systems. 
• In regards to the hearts, we knew that the parasympathetic nerves slows heart rates, and the sympathetic nerves increases heart rate. Now, the contracting functions of the heart are coordinated basically through the electrical conduction system as depicted here in blue and purple. 
• The part that sets the pace for the heartbeats is the sino atrial node or the SA node located in the right atrium, and it helps sets the rate of the heartbeats the conduction, the electrical conduction, as you have, essentially pacemaker cells, and the SA node sends electrical signals down the atrium, to the AV node. And then down the Purkinje is of his his Purkinje cells, and then down the bundles that lead to the contractions of various chambers. 
• And both the parasympathetic and the nervous, sympathetic nervous system x app, the essay notes and helps set the rate of the heartbeat. And they both have sort of opposing effects. 
• The sympathetic nervous system increases heart rate, which is shown here going up to 200 beats per minute. The vagal system, on the other hand, slow is heart rate. And if you have exact cancellation of each other, and if you'd let the heart beat intrinsically alone on its own, the intrinsic heart rate is about that 100 210 beats per minute. And sets are resting heart rate is around 70 beats per minute, that indicates that the vagal tone is greater than the sympathetic tone at rest. And we knew that actually in the 1970s, and even as early as the 1950s, actually was because they had done some experimentations with transplanted hearts. 
• And you can see here, the heart rate over time in a healthy heart. And you can see that the average heart rate is about 71 beats per minute. And then when they transplanted the heart, either in a dog and a human, you can see that the heart rate was much higher than 99 beats per minute or even higher 210 beats per minute. And the other thing that you note is that the variability that you see in the heart rate disappears in a transplanted heart rate.
• And the reason is that case is because of transplanted heart. In order to transfer that part from a donor to the recipient, you need to disconnect all the autonomic nervous connections to that heart. So that includes the parasympathetic nervous system sympathetic nervous system. And so you lose this variability and you have a significant rise in her heart rate. 
• And this was known as or at least in 1960s, because the first human cardiac transplantation was done by Christiane Barner and South Africa in December of 1967. So we knew this way early on. And this is an example of sort of the complexity that you get with heart rate variability. 
• This is our interval, which is the inverse of heart rate. And you can see the changes in the are mutable with time or with the number of beats. And the average of this graph gives you the resting b2b interval, or in the analogy that I like to say is sort of like the sea level. The fluctuations that you see around this quote unquote, sea levels is the tides in the ways. 
• And this is the heart rate variability that we're talking about. And as early as the 1970s, clearly, physiologists were able to identify the important factors that were involved in heart rate variability, and even develop a schematic representation. And this is a pretty good representations. And this was a publication in 1973. 
• They've identified sort of the central neural controls, which included the brainstem control centers, and peripheral mechanical factors. And they even developed a schematic to help understand how these various factors interacted with each other. And, as noted, they identify the important factors which included the control centers of the autonomic nervous system in the brainstem. 
• The bearer receptors, which were important for maintaining a stable blood pressure, and also the effect of breathing by changing the intro pleural pressures. And so they've clearly developed a very good understanding of this in the 1970s. And one of the important parts, sort of physiological contributions to this heart rate variability is respiratory sinus arrhythmia. This is the change in your heart rate as you take a breath in and out. And you may notice as you breathe in your heart rate, which is illustrated here by the gray increases with inspiration or inhalation, and it goes down with exhalation.
• And there's certain sort of time dynamical time parameters or information about the typical timescales is about every three to seven seconds that you take a breath, or about nine to 24 cycles per minute. So this is the respiratory sinus arrhythmia. And then the other important physiological factor in heart rate variability is the barrel reflex. Their reflex is a way to control or stabilize blood pressure. And you have essentially these mechanical receptors located in the aorta and the carotid sinus. And then through the vagus nerve and the glossopharyngeal nerve, you have signals sent to the brainstem.
• And then this brainstem signals then get sent to the court the autonomic nervous system nuclei, including the parasympathetic, and the sympathetic systems, and then they feed back that response. So that affects the SA node and other regions of heart. 
• The timescale for the bear reflex is a little longer, it's about three to 20 seconds, and it happens to be greatest problems approximately about six cycles per minute or every 10 seconds. And around this time in the 1970s, in order to capture these variability, dynamics, physiologists began to create multiple measures. And it's very hard to see sort of this in sort of granular detail here. 
• But what's important to know is already by the 1970s, they've established important time to name measures and frequency domain measures. And this is a summation of the multiple Heart Rate Variability measures that are out there in the literature right now. This was in shaper in 2017. These are sort of highlighting the most important ones, there are at least 60 to 70 different heart rate variability measures now. And they're categorized into three big buckets. 
• One is Heart Rate Variability by the time-domain measures. The second is Heart Rate Variability by frequency domain measures. And the third one is nonlinear measures. And I'm going to talk about each of these in a little more detail.
• The first one that I want to talk about is the frequency domain. And the way I want to help you understand how this works is to sort of imagine light. 
• So if you take prism, and then you shine, what light box awaits, then the prism actually breaks down the white light into its different components. So spectral lights, you have different speed of light, secondary to the refraction within the prism. And as a result, device like into its different frequencies. And you could imagine that if you had a photo sensor in this wall, and you let that photo sensor accumulate, the amount of light that goes onto that sensor, over time, you will develop an idea of how strong each of these individual components of light are. 
• You could do the same thing with heart rate time series. And we use different techniques. And sort of analogous to the prism, you have something called for a transport and autoregressive spectrum analysis, there are some others as well. And what these techniques do is that it permits us to identify the spectral power, each frequency range. 
• So you can imagine this heart rate variability is composed of multiple signals with different frequencies. And these techniques enables us to sort of divide them into identify which component is the strongest. And what you see here is you can essentially produce a spectral power. And you can see that the spectral power is divided according to the frequency and frequency, the higher it is, that means the faster the cycle. So the higher frequency is on the right here, and the lower frequency is on the left. And over time, we've got a pretty good understanding of the various categories of the cycles of the frequencies that are involved in heart rate variability, and we've divided them into several categories. 
• There's high frequency, low frequency, very low frequency and ultra low frequency. And then we have a ratio of lower frequency versus higher and higher frequencies. And the frequencies that are associated with each of these frequency ranges are listed here. 
• The ones that I want to have you pay attention to is the high frequency and low frequency. So high frequency is point one five 2.4 hertz, which is about nine to 24 cycles per minute. Low frequency is three to nine cycles per minutes, very low frequency is 25 seconds to five minutes per cycle, five minutes to 24 hours for the ultra low frequencies per cycle. And in order to evaluate the cycles, you would need to acquire the heart rate over time. And because these cycles are shorter, high frequency low frequency, oftentimes, you just need five minutes in order to get information about these cycles. Whereas for longer cycles, you would definitely need to require as as long as 24 hours. And each of these cycles have physiological significance. 
• So high frequency is associated with a respiratory sinus arrhythmia. The low frequency is associated with bear reflex in vascular sympathetic nerves. And there's something called a mayor waves which is the oscillations the intrinsic oscillations that you see in the vessels. 
• And when I'm talking about the artery system, very low frequencies have slower physiological systems involved, such as temperature regulation, hormones, such as reading engine tension, thyroxin, reproductive hormones and steroids. And then the heart has an intrinsic nervous system as well, which is believed to have a very low frequency range. And then at the ultra low frequency range involves thermal regulation, other hormones such as cortisol and growth hormones, and the circadian rhythm. And they knew a lot of this as early as the 1970s. And this is a publication in ergonomics. And again, they have created this frequency spectral analysis of heart rate variability, and I'm gonna read this to you because it may be small on your screen, but they have already noticed that the respiratory activity is largely predominantly near point three five hertz, which falls into our high frequency range. 
• The vasomotor activate lies in the point one hertz, which lies in the low frequency range and thermal activity at point O two five hertz, which is in the very low frequency range. So already they knew this in the 1970s

‍