After much deliberation about buying a heart rate monitor capable of recording Heart Rate Variability (HRV), I have decided to be profligate, and have bought a Polar RX800cx. I had been vacillating between a Polar RS800cx and a Suunto t6r. For the purpose of measuring HRV, the Polar RS800cx and the Suunto t6r have very similar capability. Both devices record R-R interval (the interval between the ventricular contraction of consecutive heart beats), and either can be used to provide the raw data required by the Firstbeat software which I have discussed on several recent postings. This software computes a stress/ recovery index which promises to provide a sensitive indicator of impending over-training. This stress/recovery index is based on estimate of the balance between activity of the parasympathetic nervous system (which promotes recovery) and the sympathetic nervous system (which promotes fight or flight).
When it came to the final decision about which device to buy, I was swayed by the fact that RS800cx comes with a stride sensor that appears to provide a reliable measure of cadence. I am very interested in assessing my cadence when running, but a discussion of the reasons for my interest will have to wait for later posting as my first objective was to see how useful R-R data might be for measuring stress levels.
The Polar Own Optimizer
Although I have focused mainly on First beat software in recent weeks, the software that comes with the Polar RS800cx does include a utility called Own Optimizer, which assess stress levels on wakening in the morning. Unfortunately, the information provided in the manual provides little scientific justification for the Own Optimizer. As far as I can gather, Own Optimizer is based largely on the changes in both heart rate and HRV in response to rising from sitting to standing. In principle such measurements might provide a sensitive measure of the degree of withdrawal of parasympathetic nervous activity and increase in sympathetic activity associated with the mild challenge of standing-up. Therefore, I will be curious to experiment with Own Optimizer, but the output of Own Optimizer is difficult to interpret unless one has baseline data recorded while in a relaxed state. There is little point in me trying out Own Optimizer until I am fully recovered from my recent episode of illness. Nonetheless, I hope eventually to compare Own Optimizer with the Firstbeat stress/recovery index.
Back to the Poincare scatter plot
Meanwhile, I exported the R-R data from my new RS800cx and subjected it to the same Poincare analysis which I had presented in my post on 26th June. The Poincare analysis requires the production of a scatter plot which can be produced easliy using software such as Excel. Interpreting the scatter plot taxes the grey cells, but the effort is probably worthwhile.
The Poincare analysis is based on a scatter plot in which each heart beat is plotted on the x-y plane with the x-coordinate equal to the interval between the preceding beat and the beat of interest, while y coordinate is equal to the interval between the beat of interest and the following beat. A heart beat for which the preceding inter-beat interval is equal to the subsequent inter-beat interval must lie on a line that represents the equation x=y. This line slopes upwards and to the right at 45 degrees, as shown in the figure below. Strong parasympathetic activity is associated with large beat by beat variability in heart rate, so consecutive interbeat intervals will differ substantially in duration and the points representing the heart beats will lie at a substantial distance for the 45 degree line.
Conversely, if inter-beat interval varies slowly (over a time scale long compared with the average inter-beat interval) then each heart beat will be represented by a point near the 45 degree line, though the slow variation will cause the location of the points representing the heart beats to wander along the 45 degree line. Slow fluctuations are largely due to sympathetic nervous activity, though there is evidence that parasympathetic activity can also contribute to slow variations – however for the present purpose, let us assume that sympathetic nervous activity is mainly responsible for slow variation, resulting in a spread of points along the 45 degree line, while parasympathetic activity is responsible for rapid variation that causes a spread of points away from the 45 degree line.
As I described on 26th June, if there is a good balance between sympathetic and parasympathetic activity, the scatter plot should produce a cluster of points that looks like a swarm of bees the spreads out both along the 45 degree line and away from the 45 degree line. If we draw an ellipse that captures most of the points, this ellipse will be almost circular. Conversely, if the person is stressed, so that there is an excess of sympathetic activity, the scatter plot will produce a cigar shaped cluster extending along the 45 degree line. In the data which I had recorded using my ‘home-made’ ECG device before my recent illness, the scatter plot had exhibited a large spread away from the 45 degree line in addition to a large spread along the line. The ellipse that embraced most of the points was pleasingly round.
Here is a comparison of the scatter-plot for data recorded today (using my new RS800cx) with data recorded before my recent illness. Both sets of data were recorded while sitting in a relaxed state, breathing slowly and deeply. The pre-illness recording was in fact made at around 6pm at the end of a day at work, while today’s recording was made in the early afternoon after a relaxing Sunday morning. Thus, if all other circumstances were the same, today’s plot might have been expected to show an even more favorable balance between parasympathetic activity and sympathetic activity. However, all other circumstances were not the same. Today, I am in the fourth day of convalescence after a peculiar and debilitating illness that lasted almost 4 weeks.
There are two striking differences. First, the cluster of pink dots presenting today’s data has moved down and to the left, indicating that the average inter-beat interval was shorter. A shorter inter-beat interval corresponds to a faster heart rate. Before my illness, my average heart rate was around 46 beats per minute. Today’s value was 59 beats per minute. Secondly, the ellipse that embraces the majority of the points is thinner – somewhat more like a cigar. Today’s data reveals substantially reduced heart rate variability, especially a reduction of the parasympathetic activity that produces rapid changes in heart rate and scatters the points far from the 45 degree line.
Occasional parasympathetic surges
There are a few points that are outside the ellipse that embraces the general trend. Several of these points represent heart beats for which a preceding short inter-beat interval was followed by a longer than usual inter-beat interval (causing a large displacement above the 45 degree line). In fact these points represent a sudden drop in heart rate from around 58-62 beats per minute to 52-55 beats per minute. It appears that I was experiencing occasional surges of parasympathetic activity. These surges were also apparent when I examined an even longer recording, so I suspect that they are not just random chance events but in fact represent some fairly consistent pattern of autonomic nervous activity. I do not have an explanation for those occasional sudden surges of what appears to be parasympathetic activity.
The main conclusion
However, the main conclusion is very clear. My recent illness has left me in a quite stressed state – at least compared to my relaxed pre-illness condition. In fact, although today’s recording reveals a marked deterioration, it is not too bad for a 63 year old, so there is no reason for me to be too alarmed. However, it would probably be unwise for me to resume vigorous training until the scatter plot of data recorded in a resting state returns to something more like the widely dispersed shape exhibited in my pre-illness data.
The apparent sensitivity of the Poincare scatter-plot to stress level makes me wonder how useful if might prove to be as a measure of training stress. Once I have recovered fully, it will be interesting to compare the scatter plot following a hard training session (‘over-reaching’) compared with that following easier training sessions. Maybe the Poincare scatter plot of data record during relaxed deep breathing will be as informative as either Polar’s Own Optimizer or the stress/recovery index computed by Firstbeat software. Whatever the relative merits of the different ways of assessing autonomic imbalance from HRV data, it appears that HRV is a sensitive indicator of one’s internal milieu.