There are many different heart rate variability parameters, some of these parameters are considered indicators of parasympathetic, some of sympathetic and some of total activation. What exactly is hidden behind this, we have already learned in the first part of this series.

All HRV parameters have one thing in common, they are based on the evaluation of so-called RR-intervals.

## RR intervals

The RR interval is generally defined as the distance between two heartbeats, which provides information about the propagation of excitation in the heart chambers. As a rule, these intervals are not of equal length and are therefore subject to temporal fluctuations. These fluctuations, i.e. the duration between the individual R-peaks, can be measured accurately with an ECG. The result is a sequence of all these "durations" (RR intervals).

The figure shows two different sequences with 22 RR intervals each. In the RR sequence on the left side the RR duration varies slightly compared to the RR sequence on the right side of the figure. Accordingly, the heart rate variability is rather low on the left side and rather high on the right side.

Unfortunately, the sequence of measured RR-intervals often contains measurement errors and artifacts, which have to be removed or smoothed by appropriate filters. From the filtered RR sequence, the HRV parameters can be calculated, which in turn are divided into **time-related** and **frequency-related** parameters.

How the individual HRV parameters are calculated from the RR sequence is thankfully defined purely mathematically. However, how meaningful indicators for physical and mental stress, illness and fitness are calculated from the HRV parameters using appropriate analysis techniques is the higher art. Exactly at this point the different HRV tools on the market differ strongly from each other.

## The time-related HRV parameters

Let's start with the time-related HRV parameters. These are simple statistical measures calculated directly on the entire sequence of RR intervals of a measurement. The most important parameters are:

- Mittlere RR-Dauer
- Mittlere Herzfrequenz
- SDNN (Standard Deviation of RR-Intervals)
- RMSSD (Root Mean Sum of Squared Distance)
- pNN50

### Mean RR duration

The mean RR duration is mathematically considered to be the arithmetic mean value over all RR intervals of a measurement in [ms]. RRi is the ith RR interval and N is the number of all RR intervals of the measurement.

### Mean heart rate

The mean heart rate, also called pulse, is calculated from the mean value of the RR intervals and is given in [beats/minute].

The reciprocal parameters, mean RR duration and mean heart rate, are not HRV parameters in the strict sense, but both values have long been and are often used as indicators of exercise. Typically, they are measured during training, displayed on sports watches as pulse and interpreted as a measure of the intensity of the load. Since both parameters are average values, a high temporal measurement resolution is not necessary for their determination as in the case of the following HRV parameters. Accordingly, these values are also displayed by comparatively simple devices and can today be measured sufficiently accurately even on the wrist - a chest strap is not absolutely necessary.

For all HRV parameters described in the following, however, an exact measurement of the RR intervals and a so-called beat-by-beat recording is necessary. Some chest straps, such as the Polar H10, already offer this ECG accuracy during recording.

### SDNN (Standard Deviation of RR-Intervals)

SDNN is the standard deviation of the RR intervals in the measuring time range and is specified in [ms]. The standard deviation describes how strongly the individual RR intervals scatter around their mean value. The greater the scatter, the higher the standard deviation. Conversely, the standard deviation would be 0 if all measured values were identical.

The SDNN is a measure of the overall activation of the autonomic nervous system - i.e. the sum of sympathetic and parasympathetic activation. It indicates how well the autonomic nervous system can regulate the processes in the body. Since the SDNN value increases with the length of the measurement time, measurements should only be taken under the same measurement conditions as, for example, always lying down and with the same measurement duration. In this way, falsifications of values such as the SDNN can be excluded.

### RMSSD (Root Mean Sum of Squared Distance)

Mathematically speaking, the RMSSD is the square root of the mean over all squares of the differences of successive RR intervals and is also expressed in [ms].

The RMSSD measures the short-term changes of successive RR intervals. It is considered a sound measure of parasympathetic activation and is often interpreted as an indicator of recovery ability, fitness and health.

The RMSSD value shows how quickly the body can react to stress. A high value means that the body can cope well with the alternation between load and relief. It therefore allows conclusions to be drawn about the stress load.

Conversely, a reduced RMSSD can indicate physical strain (such as after intensive training), psychological strain (stress) or illness.

Also with RMSSD, the cross comparison between different people and their RMSSD values is not useful, but rather the longitudinal comparison of values of a single person should be used. By taking many measurements, changes in the RMSSD and, of course, other HRV parameters can be detected and personal reference values can be determined.

### pNN50

pNN50 is the fraction of all pairs of consecutive RR intervals that differ from each other by more than 50 ms. To calculate this parameter, the number of pairs of consecutive RR-intervals with a difference > 50 ms is determined and this value is divided by the number of all pairs of consecutive RR-intervals.

The pNN50 correlates strongly with the RMSSD and is also considered a well-founded measure of parasympathetic activation. Compared to the RMSSD, however, it is more sustainable and stands more for the general power reserves. First deteriorations can be read off more quickly from it.

Parameter |
Indicator |

Mean RR duration, mean heart rate |
Indicator of the burden. Is typically measured during training and interpreted as a measure of the intensity of the load. |

SDNN |
Indicates how well the autonomic nervous system can regulate the processes in the body. |

RMSSD |
Indicator of recovery ability, fitness and health. Indicates how quickly the body can react to stress. This allows conclusions to be drawn about stress levels. |

pNN50 |
Measure of parasympathetic activation. |

## The frequency-related HRV parameters

The time-related HRV parameters have already been learned, what remains are the frequency-related parameters. But we will look at them in the next article of this #HRV series.