# Introduction

There are several parameters for Heart Rate Variability (HRV). We introduce the most important ones with a short explanation. A distinction is made between time-based parameters and frequency-based parameters.

What an evaluation of the parameters can look like in reality is shown in the screenshot. This is the evaluation of an HRV measurement from our Mindfield eSense app. A look at the various parameters provides an insight into what the heart rate variability of a user of our eSense Pulse looks like.

Of particular interest is not only the current state of the measured values at the time of a measurement, but the change in the measured values over a certain time and several measurements. Biofeedback training can improve heart rate variability. This should be reflected in the individual HRV parameters over time.

The different HRV parameters after a measurement from the eSense app

## Note: RR and NN interval

As an introduction to all measured parameters, we would like to point out that the terms RR and NN mean the same thing, but in science and literature they are often used alternately. The RR or NN interval is the timely distance between two heartbeats, in milliseconds.

In simple terms, you train to achieve the greatest possible variability between these distances. The analysis of this data is possible in different parameters, which allow different conclusions about the condition of your nervous system.

## Time-based parameters

## SDNN

("Standard Deviation of the NN Interval") is the standard deviation of the RR intervals. In simple terms, how much do the intervals differ on average from the average of all intervals. Let's say you have a value of 68ms. This means, on average, one heartbeat deviates from the next heartbeat by 68ms, ie 68ms earlier or later than the previous one. The larger this value, the higher your overall heart rate variability, which means the better your body can adapt and adapt to change. You can also see how well sympathetic and parasympathetic work together.

## RMSSD

("Root Mean Square of Successive Differences") is one of the most important parameters that provides information about the activity of the parasympathetic nervous system. It describes the short-term variability of the heartbeat, how much the heart rate changes from one heartbeat to the next. The RMSSD value is calculated as follows: The time difference of successive RR intervals is multiplied by itself to obtain only positive values. These intermediate values are summed and divided by the number of RR interval differences to obtain the mean. From the mean of these squared differences, the square root is drawn. The RMSSD is also often referred to as a value for the recovery of the body or as a "brake" of the nervous system. The larger this number is, the better.

## Stress index (SI)

The exact formula: Stress index (SI) = AMo / 2Mo x MxDMn. Where AMo is the amplitude of the modal value and represents the percentage in comparison to all other RR intervals. Mo (in the formula 2Mo) is the modal value for the duration of a RR interval that has been measured the most often. MxDMn is the variability width, or in other words the difference between the maximum and minimum measured RR interval. Simply put, this formula calculates how adaptive the heart is or the value of the heart rate variability (HRV). A good relationship between the sympathetic and the parasympathetic reaches scores of 30 to 150 points. At 500 - 1000 points, limitations of adaptability are already assumed. Over 1000 points is an indication of a high level of exertion or stressful circumstances. Of course, it must also be noted that the SI depends on the respective situation. It makes a difference whether you are sitting quietly or exercising physically. However, at least in general terms, the lower the stress index (SI), the better. The more rigid and less variable the RR intervals are, the higher the stress index scores.

## NN50

The number of pairs of RR intervals that are more than 50ms apart. The larger this number is, the better. However, comparison of this parameter requires that only sessions of the same length and under the same conditions are compared (for example, sitting position, before / after exercise, morning / evening, ...).

## pNN50

The percentage of pairs of RR intervals that are more than 50ms apart. The larger this number is, the better. However, comparison of this parameter requires that only sessions of the same length and under the same conditions are compared (for example, sitting position, before / after exercise, morning / evening, ...)

## Frequency-based parameters

## LF („low frequency“)

This is the HRV in the range of 0.04 to 0.15 Hz. This captures vibrations in the range of about 10 seconds. Here the activity of the sympathetic nerve develops well, with very slow breathing also in the upper range at 0,15 Hz the parasympathetic nervous system. The greater this value is, the more pronounced is the focus of the sympathetic active.

## HF („high frequency“)

This is the HRV in the range of 0.15 to 0.40 Hz. This includes oscillations between about 2-7 seconds, in which frequently affects the heartbeat by breathing (RSA = Respiratory Sinusarrhytmie). Here also clearly shows the activity of the parasympathetic. The larger this value, with even, quiet breathing, the more pronounced the parasympathetic activity.

## Average RR

The average RR (also sometimes called RR interval or NN interval) is the average interval between two heartbeats. He is thus the reciprocal of the heart rate.

To get a feeling for this value, just think of the following 3 examples: If you have a rest pulse of 60 beats per minute, the average RR is exactly 1,000ms or one second (because 1 beat per second). If you're doing light to medium physical effort and you're getting 120 beats per minute, then the RR is 500ms or half a second (because 2 beats per second). If you make a lot of effort and get your pulse to 180 beats per second, the RR is 333ms or one-third of a second (that's 3 beats per second). There is no better or worse for this value, the bigger or smaller. At most, the statement can be made, the greater the value at rest, the slower your resting pulse, which is e.g. In endurance athletes is a hallmark of fitness and a powerful heart muscle.

Image illustration of a frequency spectrum from the eSense app

##### Read more > Explanation Heart rate variability (HRV)