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HRV Metrics & Terms

Time & Frequency Domain Measures:


Depending on the way you get your HRV readings; which app or device you use, you will have different data available. Here are some of the most common.


Time Domain Measures:


Time domains essentially average the beat to beat interval time distance between each heart beat over a given period of time.  There are many variations of this including; SDNN, RMSSD, pNN50, HR Max -HR Min, non-linear measures and others not included here. For the most part, devices and apps tend to use RMSSD, some use SDNN too, we will discuss those two here. Time domain measures can be from as little as 30 seconds to over 24 hours depending on the purpose for the measurement. Most people measuring outside of a hospital will be doing short term measures, that is 5 minutes or less. 




This stands for; Standard Deviation of Normal to Normal. SDNN is referring to is the standard deviation of the measurements taken between two normal R-R intervals on a ECG. “Normal” refers to artifacted data, so data that has imperfections such as arrhythmias removed. SDNN is more common for long term or 24 hour measures of HRV. The Apple Watch only measures data in this way. Most other apps use rMSSD, and some will give you both. 




This stands for; Root Mean Square of Successive Differences. rMSSD is the more commonly used form of HRV calculation for short term measurements as this is what research supports best for measures of this length. Although it seems as more research comes out, SDNN may be just as good. rMSSD is most tied to vagal tone or the activity level of the vagus nerve which indicates largely how parasympathetics are functioning. 


Frequency Domain Measures:


Similar to a ray of light going through a prism and being separated into its various color frequencies; Frequency domain measures separate the power sources of the heart beats. This can be viewed as a measure of the power of the autonomic nervous system and what is supplying the power. There are 4 frequency bands:


     - Ultra Low Frequency (ULF): Generally only seen in 24hr measures. The sources of power for this frequency band include biological rhythms, things such as circadian rhythm, body temperature, metabolism and more. 

     - Very Low Frequency (VLF):  Sources of power in this frequency band are the heart’s intrinsic nervous system and there are sympathetic influencers here too. High VLF power may indicate low vagal tone or SNS dominance (Gevirtz, 2017)

     -Low Frequency (LF):  This is the baroreflex range, the largest influx here is fluctuations from blood pressure and the parasympathetic nervous system. High LF during normal breathing may indicate vagal brake dysfunction (Khazan, 2020). 

     -High Frequency (HF):  This is the respiratory range and is most influenced by breathing at normal rates (12-20 breaths per minute). The sources of power here are respiratory sinus arrhythmia (the speeding and slowing of heart rate with each breath cycle) and the parasympathetic nervous system. 


There is a ton more to learn about how to apply these metrics and what they mean. We encourage you to take the extended course at to learn all about it. 

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