2.1 HRV in Cardiology: Historical Context

• So the title of this talk is a meta perspective on heart rate variability, antiquated or indispensable. This is part two. So as mentioned my first talk, this is the way I interpret how Heart Rate Variability has evolved through the past 50 years and is divided to five stages, understanding heart rate variability, heart rate variability as a marker of autonomic nervous system, peripheral ability as a marker of body wide function, or rate variability within the construct of Mind Body interaction, and heart rate variability itself as a desirable target. Now, why these categories have been divided quarterly will become clearer when this Talk series is over. But I really must apologize in advance that this series is goes a lot in depth. And what I thought was going to be a two to three part series is turning out to be a five part. So I apologize for that. But you know, I think my intent as well as physio cues intent is to really empower you to be able to do research well. And so in order to do well, is to really get a deep understanding of heart rate variability to the point where you become the resident expert. So just a quick review.
• Last talk for part one, we talked about understanding heart rate variability, this largely focused in the 1970s and 80s, where physiology was the emphasis or the focus, I stated that 1970s 80s was the golden age of physiology. And that is the time when they started to really understand or develop certain methods for measuring heart rate variability. And just as a reminder, this graph shows heart rate for more specifically, our heart interval over time.
• And our intervals are heart rate is much more about the tone or the sea level, while the heart rate variability is the fluctuations that operate around this sea level or the tone.
• If you remember, I gave more details about the physiology underlying this. But a lot of it happens at the sign of aging, you'll note the SA node. And at the SA node, we talked about how the vagal nerve for the parasympathetic nervous system operates much quicker, it has a rapid onset, with quick execution and shortened recovery time, you can see that the vagal nerve being stimulated here, that the heart rate quickly drops down. And then the moment you stop, it quickly recovers to its baseline rates. And then for the sympathetic system, it has a delayed onset with slow execution and a longer recovery time. So when you stimulate the sympathetic nerve directly, it has a lag, there's a slow rise in the heart rate. And then as you turn off the stimulation, you have slow recovery. And these dynamics explain a little bit of why Heart Rate Variability shows a little different things at different frequencies. I talked about the various measures of heart rate variability, there's frequency and time domain. The frequency domain has these measures high frequency, low frequency, very low frequency, ultra low frequency and then the ratio between low frequency and high frequency. And the high frequency correlates with the respiratory sinus arrhythmia, which is largely parasympathetic nervous system because the parasympathetic nervous system operates at a fast dynamics. The low frequency has a combination of parasympathetic and sympathetic and is accounted for by the bare reflex and some vascular mechanisms. Very low frequency probably correlates with temperature regulation, hormones, some intrinsic nervous system. And then the ultra low frequency deals with thermal regulation, hormonal changes in the circadian rhythm.
• Then we talked about the time domain, the standard deviation and intervals, the RMS of success and intervals. And then p and n 50 are the ones that I talked about. And again, I'm going through this quickly because there's just all a review of part one. And just as a reminder, once you remember that when we're talking about in it notable over time, when we a lot of these measures include the successive differences of and intervals. And what that does is it eliminates the long term variability, and we're just focusing on the short term variability. So the measures that incorporate this successive difference are in SSD and pn and 50 deal with the high frequency heart rate variability, so it deals with the parasympathetic nervous system. Okay. So the summary of part one is the 1970s 80s saw major advances in heart rate variability physiology measures, at rest, the parasympathetic nervous tone is normally greater than the sympathetic tone, and then the heart rate variability frequency domain, each frequency ranges and associated with a specific physiologic process. And then high frequency Heart Rate Variability correlates with the vagal parasympathetic nervous system. And then basically, the faster es sympathetic activity filtered out at the SA node
• In the time domain, the MSSD, pn and 50 correlates with high frequency heart rate variability. And then the standard deviation and an integral depends on the duration of time series analyzed.
• So that was part one. Now we're focusing on part two. And part two is heart rate variability as a marker of autonomic nervous system. And what I'm talking about is it this time is the 1980s and 1990s, when cardiovascular medicine was pretty much at its golden age, and where we were research is what I say is focused on reductionistic approaches, but he's basically breaking down a lot of the components focusing on specific factors that are involved in disease processes. And so as a result of this approach, the heart rate variability became a way to sort of focus in and hone in on the autonomic nervous system. In my perspective, this was sort of the golden age of cardiology. The reason I say this, I mean, this is from my personal experience. This is when I was in medical school and residency during this time, there was a buzz in cardiology, there was a lot of interest, basically, a lot of the top residents wanted to go into cardiology fellowship. And I remember our chair in cardiology, I went to University Michigan for residency. His name was Bertram Pitts, who was head of large, a lot of these large RCTs walked around with a lot of sort of composure and respect. And I think a lot of people are afraid of him. So it you know, cardiology to me in the 1980s and 90s, with the golden age. And the reason for this was that there was a convergence of a lot of things that developed during that time. So that was when coronary catheterization pretty much reached its its forefront in the 1980s was when you had the first treatment for our minds. 
• Then in 1990s, you saw rapid growth of that. You also saw so the incorporation of devices, such as pacemakers, defibrillators, and then you had availability of medical treatments in the 1986 was the the lowest time which was the first statin drug available, which was approved by the FDA. And in addition, you had a lot of the infrastructure things that you were needed in order to deliver good cardiovascular care. That included cardiac care units, the first one established and Nederburg, and 1964. And then you had a rapid adoption of CCU all across the country in the 1970s, and 80s. And you have emergency medical services, which started to begin in the 1969. And then really sort of expanded in the 1970s. This is a picture, I don't know if this may sort of reveal my age. But this is a TV show in the 1970s called emergency 51. And this was sort of the the first time where EMS sort of played into everyone's, you know, home TV, people got to recognize the importance of EMS.
• In the 1970s, and later in the 1980s, you started to have ACLs guidelines, basically how to deal with a significant cardiovascular compromised or failure, such as ventricular arrhythmias are critical of the population.
• The reason also that cardiology received a lot of attention was because cardiovascular death was the leading cause of death through much of the 20th century. This was true the 1980s and 1990s. This also included stroke, which was number three as a leading cause. So cardiovascular is sometimes incorporate stroke as well. This little blip happens to be the 1918 influenza. But otherwise, you could clearly see that heart disease, cardiovascular disease was an important cause of death in the United States and all across the world.
• This is a slide that I used in part one, and this breaks down the heart rate variability publications done by each discipline. And you could see that in the 1980s and 1990s and the 2000s. In the 2000 10s. Cardiovascular Research played prominently, and heart rate variability research, this sort of was number two behind OBGYN because OBGYN at that time was very interested in fetal heart rate monitoring. But then due to the rise of cardiovascular research you saw that really gained prominence in the 1990s and 2000s. And many of the heart rate variability studies were embedded in a lot of these large randomized controlled clinical trials. You know, the cardiology field has these very cute acronyms for large trials such as hope and salt and dynamite. A lot of that did incorporate that heart rate variability in their studies and so heart rate variability played prominently in along with cardiovascular research.
• So the goals of this talk for part two is is to appreciate how signs of heart rate variability has evolved over the past 50 years to understand how different perspectives of Heart Rate Variability have emerged, to review the clinical uses for heart rate variability and to recognize the various ways to assess autonomic nervous system and to appreciate the factors that influence heart rate variability.

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