## Time-domain, non-linear und frequency domain parameters

### Time-Domain Parameter

**RR: i**nterval between two heartbeats (R spikes in the QRS complex / ECG).

**NN: i**nterval between two heartbeats (emphasis on "normal" heartbeats)

**SDNN: t**he standard deviation of all NN intervals. The SDNN describes a median of the variabilty. It conists of parts from sympathetic and parasympathetic nervous system. The SDNN can be described as an overall variability or total power. You can describe it to the patient as the total power of the regulation system.

**SDANN: t**he standard deviation of the average of the NN interval for all 5-minute periods of the entire recording (higher values indicate increased parasympathetic activity)

**SDANN-i: **the standard deviation of the average normal NN interval for all five-minute periods of a 24-hour recording (higher values indicate increased parasympathetic activity)

**RMSSD: **the square root of the root mean square of the sum of all differences between successive NN intervals (higher values indicate increased parasympathetic activity)

**SI: **stress index, reflects sympathetic activity

**pNN50: **the percentage of successive intervals that differ by more than 50 ms (higher values indicate increased parasympathetic activity)

**SDSD:** the standard deviation of the differences between successive NN intervals

**NN50: **the number of pairs of successive NN intervals that differ by more than 50 ms in the entire recording (higher values indicate increased parasympathetic activity)

There are many more HRV parameter. However, many of them are redundant. For ANS Analysis, the most important time-domain parameters, RMSSD (parasympathetic nervous system), SI (sympathetic nervous system) and SDNN (standard deviation) are calculated and illustrated. Additionally, the important non-linear parameter Alpha 1 /DFA 1 value will be calculated. It reflects the quality of the regulation as well as the interplay between the individual regulation systems.

### Non-linear HRV Parameter

**Alpha 1 oder DFA 1: **(detrendet fluctuation analysis)

The Alpha 1 does not just measure the timely changes of the heart rate variabilty but rather the quality of the regulation. By checking the HRV signal for random and recurring intervals, it can be determined how the individual control systems work together.

The ideal value for the Alpha 1 is 1.0. This states a heart rate variability consisting of 50% random signals, indicating a fast reactivity and 50% recurring signals, indicating a fundamental stability of the control systems.

Values above 1.0 mean more stability and indicate compensations processes within the individual control systems.

Values below 1.0 mean a lot of randomness. Values below 0.8 indicate a hindered cooperation of the control systems. That condition can be described as a chaos within the system.

### Frequency-Domain HRV Parameter

The pure RR intervals can be decomposed into their frequency constituents using a fast Fourier transformation (FFT). By this method, frequency components in the VLF, LF and HF spectra are obtained from the signal. The total power is indicated as TP.

**VLF - Very Low Frequency**

Frequenzbereich: 0,00 - 0,04Hz

**LF - Low Frequency**

Frequenzbereich: 0,04 - 0,15Hz

**HF - High Frequnecy**

Frequenzbereich: 0,15 - 0,4Hz

**LF/HF Ratio**