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MBW:Chaotic Heart Rate

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Introduction

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According to the the Centers for Disease Control and Prevention (CDC), heart disease is the leading cause of death for both men and women[5], accounting for approximately 600,000 deaths per year in the United States. Because the number of deaths caused by heart disease is so high, considerable effort is spent trying to understand the causes and implications while also creating better ways to predict disease onset. One physiological aspect that is studied is the heart rate, or the rate at which the heart contracts, pumping blood through the arteries. Heart rate is usually studied via electrocardiography a branch of medicine which uses sensors to detect the electrical activity in the heart. These sensors, also known as EKGs, generate time series data of electrical signals. The time between contractions is commonly referred to simply as "heart rate."

In this work I further investigate heart rate dynamics by analyzing time series data. I use various nonlinear dynamics techniques such as delay coordinate embedding, Lyapunov exponents, and capacity dimension calculations in the hope of determining whether heart rate time series data is chaotic. However, there are two novel differences I introduce. First, I conduct nonlinear time series analysis of heart rate data collected from a triathlete, training and racing at an Ironman distance. Secondly, this data is not collected by a clinical EKG machine, but by wearable, consumer-grade electronics such as a Garmin Heart Rate Monitor. The main source of data for previous research comes from clinical or hospital settings. The data is usually gathered from different, broad categories of people by age, such as "normal," "male, ages 30-34," or congestive heart failure patients. Also this data is collected while the patients are asleep or at rest. Results from the work presented here can be used in determining differences between health people and congestive heart failure patients, for example. Also, it may be possible to help create better training programs for endurance multi-sport athletes.

Three different regimes are investigated. The hypothesis is that each of the following regimes will provide measureably different dynamics: training vs. racing, early cycle vs. late cycle, and sequential vs. isolated events. I believe that racing data will exhibit more evidence for chaos than training data. This is based on the personal experience that there are significantly more things out of your control during a race, and these added variables will cause the system to be chaotic. Also, early cycle training data will demonstrate more chaotic evidence than late cycle data due to the fact that early in a cycle the body is struggling in more ways. Lastly, isolated events will show more indications of chaos than sequential events. This is a mostly a guess from experience: the longer the race goes on the less fluctuations you experience.

Background

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Due to the large number of people who suffer from heart disease, research into cardiac functions, such as heart rate, is prevalent. Previous research has analyzed the heart rate (as time series data), especially from the perspective of nonlinear dynamics, in an effort to further understand the details of the cardiovascular system. In fact, in 2008, the journal Chaos , introduced a new feature: "Controversial Topics in Nonlinear Dynamics," and the topic of the first edition was entitled "Is the Normal Heart Rate Chaotic?" The edition included eight, relatively introductory, papers on this broad topic. One paper by Baille, Cecen, and Erkal makes the claim that heart rate is not chaotic[1]. This results corroborates early work by Kanters et al. [4]. However, both of theses papers directly conflict with earlier work by Ivanov et. al. [3]. The work by Ivanov et al. is related to other multifractality investigations by Sassi et al. [6], which suggest that heart rate variability may be multi-fractal for certain conditions, but not all. There is also work that suggests the original question, "Is the Normal Heart Rate Chaotic?" is too general, and the authors, Wessel et al. suggest that "chaotic-ness" of the heart rate is related to respiration [9]. Clearly, the subject is not well understood, and I intend to further the discussion by taking a somewhat different approach.

Methods

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Delay Coordinate Embedding

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An implied assumption I have made is that the heart rate is a single state-variable in a much larger biological system. For example, from experience I know that both hydration-level and temperature affect the heart rate variable. Also, pace, terrain, energy stores, rest, and injuries all play a role in the heart rate. However, using a heart rate time series, there is no access to any of these other variables. Therefore, I use delay coordinate embedding to "generate" these other variables. Now, these generated values are not used directly; however, Takens theorem says that certain quantities, such as Lyapunov exponents and capacity dimension, are topologically invariant from the variables in the original system [8]. To say this another way, the reconstructed state space is diffeomorphic to the orignal state space.

The first step in creating an embedding is to estimate the delay value, \tau . To do this, I use the TISEAN program =mutual= . The mutual program uses adjusted mutual information to approximate this value. A typical method is to use the first minimum in the mutual information calculation for the delay value, \tau . Once we have a value for the delay, the next step is to estimate the number of dimensions. The TISEAN program false_nearest is what I used to do this. The false_nearest program reports the percentage of "false" neighbors found when projecting all points onto a single dimension. A common technique is to choose the dimension where the false neighbor count drops below a threshold of 10 percent.


Lyapunov Exponents

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Now that a reconstructed state-space, or embedding, has been created the calculation of the maximal Lyapunov exponent can proceed. TISEAN's program lyap_k is used to do this. In doing so, I supply both the estimated delay, \tau , and the approximate dimension of the data, both of which were calculated previously in Section 4. Without explaining in too much detail, the output of the lyap_k program is basically an estimated "stretching factor." The lyap_k calculates this stretching factor for various points and different values of a paramemter called \epsilon , which controls how big the local neighborhood for each calculation is. For more information please see [2]. When analyzing the lyap_k output, I am looking for scaling regions with constant slope, where the value of that slope is the maximal Lyapunov exponent.

Capacity Dimension

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When we think of dimension, either in a physical sense or a higher, multi-dimensional sense, we usually think of integer-valued dimensions. For example, a line is one dimension, while physical objects exist in three dimensions. However, some objects, such as the Koch curve, exhibit different properties. Any two points points on the Koch curve are infinitely far apart. However, the curve encloses a finite area (or volume). There is as calculation called the capacity dimension which captures this relationship. The Koch curve, as well as many other fractals, have a non-integer valued capacity dimensions. Chaotic systems often have fractal structure. Therefore, computing the capacity dimension of the embedded time series data can provide more evidence of a chaotic system. To compute the capacity dimension of the heart rate data I used the TISEAN program d2. Again, without going into the details of the d2 algorithm, the output gives a correlation sum for each value of the parameter \epsilon , as discussed in Section 5. Instead of looking for a constant slope scaling region, we are looking for a region which is asymptotically invaraiant . This means region of the curve where the correlation sum does not change for many values of \epsilon .

Process

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The first question I investigated was this: do events completed in isolation (a single run, for example) show different properties than events completed in succession (a run preceded by a bike ride)? In a triathlon, each event, swimming, biking and running, is performed in quick succession. That is, there is no resting between events. In fact, the time between events (called transition ) is typically a very stressful period for athletes. However, most athletes train events singularly: go to the pool and swim, or just leave your house for a run. To study this, I looked at two similar length bike rides. The first was a 3 hour, 30 minute bike ride in isolation. The second was a 2 hour 45 minute bike ride which was preceded by a 10 kilometer (approximately 56 minute) run. Even though a race is swim before bike, sometimes getting to a pool and biking right afterwards can be difficult, so we simulate that action by running first instead of swimming. This figure shows the TISEAN output for mutual and false_nearest , used to approximate the delay and the dimension, respectively.

3-30-bike-mut.png 3-30-bike-fnn.png 2-45-bike-after-run-mut.png 2-45-bike-after-run-fnn.png

From these graphs I chose the following parameters. For the isolated bike event I determined that \tau =19 and the dimension, m=9. For the sequential bike I found \tau =39 and the dimension, m=10. Now that we have estimated these parameters we can calculate the Lyapunov exponents. The next figure shows the TISEAN output: the y-axis is what is called the "stretching-factor" and the x-axis is the iteration. Each of the different colors represent a different value for the algorithms parameters \epsilon , (the local size), and m, the dimension. Looking at the left side of each graph in this figure, a positive and constant slope region is visibile before the graph becomes much more scattered. This is exactly the scaling region that indicates the Lyapunov exponent.

3-30-bike-lyap.png 2-45-bike-after-run-lyap.png

In the left-hand figure the scaling region is clear, but in the right-hand figure, while the region is still present it is significantly less pronounced. I found the slope of both these regions: isolated = 0.31881 and sequential = 0.27798. To do these calculations, I took the average slope of the scaling regions for all the different parameters. The next figures shows lines fitted to the scaling regions for both the isolated and sequential bike events.

3-30-bike-lyap-line.png 2-45-bike-after-run-lyap-line.png

Next we look at the capacity dimension results, which can be seen here.

3-30-bike-d2-line.png 2-45-bike-after-run-d2-line.png

In both plots we clearly see an asymptotically invariant region where the points are clustered around a line before dropping off. In fact, both plots are very similar. To find the capacity dimension, I fitted a line to these points (only the asymptotically invariant ones) and found where that line interesects that y-axis. This happens at approximately y=1, for both plots, therefore I estimated the capacity dimension to be 1 in both cases. This does not indicate a fractal nature to the time series. And while it does not support the claim of a chaotic system, it does not necessarily detract from it either.

The above illustrates the process I used for the entirety of this work. I do not detail the other investigations in this way (isolated run vs. sequential run, training vs. racing, etc), but present the results in Section 22.

Results

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Isolated vs. Sequential

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I have shown the isolated vs. sequential results in Section 7, for the biking case. The next images present the results for the running case. Here the scaling region in the Lyapunov calculation is significantly less pronounced. In fact, I believe it is not actually present, indicating the maximal Lyapunov exponent is not positive. Also the capacity dimension calculations are similar to what was calculated above: d_{{cap}}=1. Again, this result does not support the evidence of a chaotic system. However, for these running examples, there has been no strong evidence of a chaotic system.

1-30-run-lyap.png 1-30-run-d2-line.png

Here we have the sequential results:

Bp-run-lyap.png Bp-run-d2-line.png


Training vs. Racing

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For this case I am displaying results from biking events. This is for two reasons: first, the biking events are longer and provide more data, and second, the batteries on my heart rate device sometimes do not last the whole race, which means they truncate the running data. We compare the 3 hour 30 minute bike presented previously against data collected during the bike event at the Boulder Half Ironman. This case provides significant information, because these events were completed on the same bike course, removing some variables. However, the race data is, by definition, sequential. All race data is sequential, therefore, I am unfortunately unable to prevent the introduction of this new variable. Race results are shown in the next figure. Figure~33.

Boulder-ironman-bike-lyap.png Boulder-ironman-bike-d2-line.png


From this figure, we clearly see a scaling region similar to the isolated bike event. This is strong evidence that there is a positive Lyapunov exponent. The capacity dimension is also very similar to the previous results, and indicates a capacity dimension, d_{{cap}}=1.

Early vs Late in the Training Cycle

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One strategy for endurance training is the concept of periodicty. This a loose organization of workouts that gradually build in duration and intensity for a period of several weeks. At the end of this period, both the duration and intensity are signficantly reduced allowing the athelte some recovery before starting the next cycle. To investigate the heart rate dynamics that occur in different places in this cycle I compared the same 3 hour 30 minute bike ride, which occured just a few weeks before a half-Ironman race, against the very first bike ride of this year's season, a 2 hour ride. The calculations of the 2 hour ride are depicted here.

Early-bike-lyap.png Early-bike-d2-line.png

These results are, again, very similar to the isolated bike investigation. We see a pretty clearly defined scaling region indicating a positive maximial Lyapunov exponent, and the capacity dimension still appears to be around one.

Dicussion

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Dealing with Real Data

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One of the main challenges of this work was working with real, actual data. For example, for all the above algorithms and ideas to work, the assumption is made that the data are sampled at evenly spaced points. Unfortunately, this is not the case with the device I used. Therefore I attempted to use inter-spike interval embedding [7]. Inter-spike interval embedding basically states that if your data events are the result of some integral-like action, firing of a neuron and a heart beat are good examples, than you can simply use the difference in time of the firing of the events. My device does not report when the heart actually beats, it reports (as far as I can tell) a running average of the last several seconds. This value is not the result of some integral like process and therefore inter-spike interval embedding is not applicable. The only other thing I could do was to interpolate the points at regular intervals. This is usually a pretty bad idea, but because I am at the mercy of the data and the sensor, I had no other choice. I would also like to point out that because I did use interpolation, the reported results are cast under a shadow of uncertainty.

Another issue I ran into was the manual intervention required at many of the steps. For example, all of the algorithms I used make the assumption that the trajectory is tending towards infinity. To say that another way, the algorithms do not operate on the transient part of the trajectory. But, my data definitely has a transient portion, while the heart rate climbs to an endurance level. Therefore, I had to manually remove the transient from each data file. This was a time-consuming and tedious process.

Lastly, for the algorithms to work correctly they must operate on the same regime of data. But many of my longer training sessions have breaks, or in several workouts during the summer when it is very hot outside, I am forced to walk during a run, causing my heart rate to drop and leave the regime I am trying to investigate. Stopping or resting also introduces another transient period, which I would need to remove. I did not use data files where multiple regimes were present. This limited my choices, especially for longer workouts.

Selecting Data

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Before calculating embeddings, Lyapunov exponents, and capacity dimensions, I first selected specific workouts to analyze. Selecting these workouts was a somewhat arbitrary task. I have over two years of data, hundreds of hours and thousands of miles. However, as mentioned in Section 39, many of the data files contained problems, such as transient trajectories and regime changes. Therefore, I tried to choose the data files that best fit the requirements of the algorithms. I picked longer time series that did not contain rest. I also removed the initial transient trajectories. This is not to say that I chose files that would support my claim of chaos more than others. Longer files with no-regime changes were given preference.

Data Chunks

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To further validate the results, I also ran the same calculations on portions of the sequential bike data. The idea is that if we analyze, say, the middle two-thirds of the data, we should get similar results. The next figure shows the results of doing just that. The average slope in this is calculated to be 0.12842, and while this is a different number it is still positive and similar. Also the plot does not change much. This further strengthens our confidence in the positve maximal Lyapunov exponent.

3-30-bike-chunk-lyap.png

Conclusion

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I have shown that for biking data there is evidence of a positive maximal Lyapunov exponent in all test scenarios: isolated vs. sequential, racing vs. training, and early vs. late season. In that way, my intial hypothesis was incorrect in that the dynmacics were chaotically similar. All of the running data, across all regimes, showed little evidence for chaos. The capacity dimensions for all cases provided little support because the calculations were all approximately one. I also looked at operating on different chunks of data, and this further strengthened the case for a positive Lyapunov exponent. There are many issues in this work, not least of which is the interpolated data and the amount of data. Future work should focus on a better data collection mechanism while also focusing on a method to collect more data.

References

[1] Richard T Baillie, Aydin A Cecen, and Cahit Erkal. Normal heartbeat series are nonchaotic, nonlinear, and multifractal: New evidence from semiparametric and parametric tests. Chaos: An Interdisciplinary Journal of Nonlinear Science, 19(2):028503–028503, 2009.

[2] Rainer Hegger, Holger Kantz, and Thomas Schreiber. Practical implementation of nonlinear time series methods: The tisean package. Chaos: An Interdisciplinary Journal of Nonlinear Science, 9(2):413–435, 1999.

[3] Plamen Ch Ivanov, Lu ́ıs A Nunes Amaral, Ary L Goldberger, Shlomo Havlin, Michael G Rosenblum, Zbigniew R Struzik, and H Eugene Stanley. Multifractality in human heartbeat dynamics. Nature, 399(6735):461–465, 1999.

[4] Jørgen K Kanters, Niels-Henrik Holstein-Rathlou, and Erik Agner. Lack of evidence for low-dimensional chaos in heart rate variability. Journal of Cardiovascular Electrophysiology, 5(7):591–601, 1994.

[5] Kenneth D Kochanek, Jiaquan Xu, Sherry L Murphy, Arialdi M Minin ̃o, and Hsiang-Ching Kung. National vital statistics reports. National Vital Statistics Reports, 59(4):1, 2011.

[6] Roberto Sassi, Maria Gabriella Signorini, and Sergio Cerutti. Multifractality and heart rate variability. Chaos: An Interdisciplinary Journal of Nonlinear Science, 19(2):028507–028507, 2009.

[7] Tim Sauer. Interspike interval embedding of chaotic signals. Chaos: An Interdisciplinary Journal of Nonlinear Science, 5(1):127–132, 1995.

[8] Floris Takens. Detecting strange attractors in turbulence. In Dynamical systems and turbulence, Warwick 1980, pages 366–381. Springer, 1981.

[9] Niels Wessel, Maik Riedl, and Ju ̈rgen Kurths. Is the normal heart rate chaotic due to respiration? Chaos: An Interdisciplinary Journal of Nonlinear Science, 19(2):028508–028508, 2009.