# What is HRV?

HRV, or heart rate variability, is a measurement of how much a person’s R-R time interval varies. It is useful as a non-invasive method to measure a person’s autonomic nervous system activity, especially the vagal nerve, but it is also useful as a tacker for a patient’s health for certain conditions. HRV is usually measured using an ECG [insert link later]. There are many variables used to describe a patient’s HRV and they are mainly split between the time-domain and the frequency-domain.

## Time-domain measurements of HRV

This is the simplest way to analyse HRV and are based on the NN time intervals of the patient. The most important variables are:

Mean NN (Mean of NN intervals in the sample)

SDNN (Standard deviation of NN intervals) – Describes overall HRV

RMSSD (Root mean square of successive differences) – Describes parasympathetic activity

## Frequency-domain measurements of HRV

Frequency-domain methods of measuring HRV uses bands of frequency and count the number of NN intervals that fall within each band. The bands are usually denoted as:

LF (low frequency) – 0.04-0.15Hz – Reflects sympathetic and parasympathetic activity.

HR (high frequency) – 0.15-0.40Hz – Reflects only parasympathetic activity.

Some studies will also use VLF (very low frequency) bands in their analysis. LF/HF is another measurement that is sometimes used and describes the low and high frequency power ratio. This estimates the balance between sympathetic and parasympathetic activity. Frequency-domain variables are usually found by applying Fourier transform to the original ECG signal, which is in the time-domain, to create a PSD (power spectral density).

## What does normal HRV look like?

Healthy adults usually have short-term HRV values of:

Mean NN: 926±90ms

SDNN: 50±16ms

RMSSD: 42±15ms

(Work in progress, paper reference doesn't want to work (https://onlinelibrary.wiley.com/doi/full/10.1111/j.1540-8159.2010.02841.x))