2.5 years ago

FitEyes Team 260

@fiteyes_team

From a recent FitEyes email discussion:

I truly do not understand HRV

..... and why it should be at least 50??

2.5 years ago

david 4.3k

@david_fe

I truly do not understand HRV

That's probably because the articles you have read so far either oversimplify HRV or assume you already have that background and jump into complex details. What you need is a good introductory explanation about HRV that doesn't oversimplify, but does explain it fully. If I find such an article, I'll share it here. However, for now, let's revisit the first paragraph from the article I recommended:

An Overview of Heart Rate Variability Metrics and Norms - PMC

Healthy biological systems exhibit complex patterns of variability that can be described by mathematical chaos. Heart rate variability (HRV) consists of changes in the time intervals between consecutive heartbeats called interbeat intervals (IBIs). A healthy heart is not a metronome. The oscillations of a healthy heart are complex and constantly changing, which allow the cardiovascular system to rapidly adjust to sudden physical and psychological challenges to homeostasis.

I'll give an example to make that paragraph more concrete. If your heart rate is 60 beats per minute you might think that means that your heart is beating steadily at a rate of once per second. If your heart was beating steadily at this rate, it would be a gap of 1.0 seconds between each pair of successive heart beats. We used to think a steady heartbeat like that was ideal and healthy. That's wrong.

As the article linked above says, "A healthy heart is not a metronome." You should have small variations in the amount of time between successive heart beats. That's the variability in the name HRV. It's good.

For example (using made up numbers) you could have a gap of 0.8 seconds between two beats and 1.20 seconds between two other beats. The average of 0.8 + 1.2 is 1 second, but that average heart rate (HR) metric doesn't tell you about the variability -- HRV.

With gaps between heart beats of 0.8 and 1.2 (example numbers only), you have variability in the intervals between beats (a good thing). If the gaps between all heart beats is exactly 1.0 second, you have no variability and that's bad.

HRV measures the variations (fluctuations) in time between your heartbeats. HRV is a measure of your nervous system activity and hence it can reflect stress (among other things). See this related article:

Daily worry is related to low heart rate variability during waking and the subsequent nocturnal sleep period - PubMed

..... and why it should be at least 50??

This is referring to a statement from the article I linked above:

An Overview of Heart Rate Variability Metrics and Norms - PMC

The SDNN is the "gold standard" for medical stratification of cardiac risk when recorded over a 24 h period (source). SDNN values predict both morbidity and mortality. Based on 24 h monitoring, patients with SDNN values below 50 ms are classified as unhealthy, 50–100 ms have compromised health, and above 100 ms are healthy (source).

However, you can't directly compare a 24-hour SDNN value (which was referenced above) to a short term generic "HRV" metric.

From the same article:

The authors survey published normative values for clinical, healthy, and optimal performance populations. They stress the importance of measurement context, including recording period length, subject age, and sex, on baseline HRV values. They caution that 24 h, short-term, and ultra-short-term normative values are not interchangeable.

Some of the ways to measure HRV include PNN50, rMSSD (or SD1), SDNN, HF/LF, and many more. When someone says, "My HRV is X," we need more information to know exactly what they are referring to. For some measures of HRV and for some individuals at a given moment, a value of 50 may be OK. It's not OK on a 24-hour SDDN conducted with reliable equipment. But don't compare that value to your fitness tracker, which is measuring some other aspect of HRV.