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RMSSD and other HRV measurements

Table of Contents

Heart Rate Variability is used by researchers and practitioners across many fields like medicine, the health sciences, psychology, exercise, and sports sciences to understand how one’s body adjusts to stressors. Variation in time intervals between heartbeats (scientifically named R-R intervals ) is measured in milliseconds. NN intervals are interbeat intervals from which electrocardiographic artifacts (defined as electrocardiographic alterations, not related to cardiac electrical activity) have been removed. 

HRV consists of changes in the time intervals between consecutive heartbeats called interbeat intervals (IBIs). These tiny changes between heartbeats are measured in milliseconds. However, to understand the HRV phenomenon we need physiological knowledge and analytic methods

HRV measurements have the following limitations:

  • for correct results, a natural state of rest and a correct posture are necessary: ideally, you need to sit still with a straight back leaning on the back of the seat.
  • it is necessary to breathe naturally (do not try to “calm” your breath, take longer breaths or exhalations, or hold your breath)
  • you should avoid talking and should not be exposed to loud sounds and events that can put your body under positive or negative stress
  • if you were physically active before the measurement, you must sit down quietly for about 5 minutes 

Sneezing, yawning, coughing, and laughing during measurement can also distort the results. We will try to filter out the unwanted segment, but there is no 100% guarantee. 

For Welltory users, we advise starting building your personal baseline by taking measurements in the morning right after waking up, before you experience stressors like caffeine, sports, breakfast, and so on.

There are many methods to calculate and analyze HRV that can be divided into 3 subgroups: time-domain, frequency-domain, geometric, and nonlinear. We are going to cover the most popular ones that are widely used among the general population in daily life. 

Comparison of four different HRV measurements:

The two most popular formulas that are used to calculate HRV are RMSSD and SDNN:

RMSSD - the root mean square of successive differences

RMSSD is a standard statistical measure of HRV. RMSSD score meaning represents the root mean square of successive differences between normal heartbeats for a given set of heartbeat data that is used to investigate topics like the impact of training loads and recovery processes. RMSSD HRV measures the time difference between each successive heartbeat in real-time, meaning you can see the relevant result at the exact moment. Statistical analysis can reveal patterns in large amounts of data and turn a list of numbers into valuable information about one’s health

Researchers have proposed the following HRV RMSSD normal ranges in different studies:

  • 19 – 48 ms — healthy adults in the age group of 38 – 42 years
  • 35 – 107 ms — elite athletes

What is a good RMSSD score?

These standard ranges vary considerably, and if we tried to unify them, the reference range for RMSSD would be 19 – 107 ms, which seems like a pretty broad generalization. Even though it’s possible to get an RMSSD score that falls out of our range, it would most likely mean that your body is working overtime and is under a lot of pressure. 

The same generalization issue happens with other not-so-common HRV scores like SDANN, pNN50, or Amo50. They are too broad to be useful for everyday measurements because heart rate variability is an extremely sensitive metric. Differences in the timing and duration of the measurement also affect the results. 

To understand what to rely on in our calculations, we have thoroughly analyzed normal ranges and metrics to measure and evaluate heart rate variability (HRV) from various scientific sources.

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What is a bad RMSSD score?

RMSSD figures below 16 or above 107 ms are something you want to pay attention to. A low or high RMSSD score does not necessarily mean it is something to worry about, however, it might be a good idea to monitor it and consult with your doctor if some figures are out of line. The 2020 study on reduced HRV in individuals with chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) reveals a relationship between two HRV indices with fatigue symptoms. Low RMSSD values are specifically associated with high fatigue symptoms in CFS/ME patients and mostly during sleep, but not in healthy individuals. 

When it comes to high HRV, many reasons can cause a spike in your heart rate and HRV. A 2019 research demonstrates an interesting correlation between negative news and an increase in RMSSD measures. It suggests that the average human is more physiologically activated by negative than by positive news stories. Participants exhibited higher variability in heart rate during negative news stories than during positive news stories. 

HRV measurements significantly correlate with perceived psychological stress and can be used as a stress assessment tool. For example, Welltory’s developers designed a proprietary algorithm that assesses psychological stress levels based on heart rate variability analysis, which may or may not correlate with psychological stress depending on your personal stress tolerance.

Why do we use RMSSD?

An important feature of RMSSD is that it characterizes short-term rapid changes in heart rate. Such rapid changes can only occur under the influence of the parasympathetic. This makes RMSSD the main parameter that we can use to evaluate the work of one’s parasympathetic system. Researchers agree that RMSDD is a measure of vagus-mediated control of the heart. 

Assessing parasympathetic regulation is critical because only the vagus nerve provides a ‘protective’ effect for the heart. The activity of the vagus nerve decreases heart rate, increases coronary artery perfusion, and stabilizes the myocardium to prevent ventricular fibrillation.

In any condition accompanied by hypoxia (epilepsy, respiratory failure, pneumonia), the protective function of the vagus nerve becomes even more relevant as it allows the heart to provide a sufficient blood supply. 

In general, insufficient vagal activity usually goes under the radar. A person may not feel sick or tired, however, low RMSSD values ​​can have negative consequences in an “emergency” situation. That is why it is important to assess the performance of the parasympathetic system and analyze HRV, taking into account rapidly changing metrics.

SDNN - the standard deviation of NN intervals

SDNN is a 24-hour measure of HRV that calculates the average value of HRV in milliseconds and shows how far your HRV is from that average at any point in the day. It can contain some false positives because it is not a real-time measure. 

Some confuse SDNN and SDANN measures but SDANN is the standard deviation of the average NN intervals calculated over short periods – usually, 5 minutes and it measures changes in heart rate due to cycles longer than 5 minutes. SDNN, on the other hand, reflects all the cyclic components responsible for variability for 24 hours and, therefore, represents total variability.

The SDNN is considered the “gold standard” for medical stratification of cardiac risk when recorded over a 24 h period. SDNN values predict both morbidity and mortality.

There are optimal healthy HRV range thresholds for SDNN HRV measure: 

SDNN HRV chart

Worth-knowing fact is that acute myocardial infarction patients with SDNN meaning values over 100 ms have been reported to have a 5.3 lower mortality risk than those with under 50 ms. In general, a higher number usually indicates that your body is coping better with stress.

pNN50 - another widely used measure of HRV

This statistic is derived from the 1984 study, which introduced the NN50 count. It is defined as the mean number of times an hour in which the change in successive NN intervals exceeds 50 ms. It is believed that this measure helps assess parasympathetic activity from 24-hour ECG recordings and presents supporting data from healthy subjects compared with those with diabetes mellitus and patients after cardiac transplantation.

pNN50 HRV chart

pNN50 shows how active your parasympathetic system is relative to the sympathetic nervous system. The higher the value, the more relaxed the body is. If the pNN50 is low, you’re either tired or over-stressed.

What we use at Welltory

Overall, you can’t say that one method is better or more reliable than the others. It all depends on the purpose of the analysis. At Welltory, we are trying to evaluate the work of your regulation systems, we are trying to understand what contribution the nervous and humoral systems make to the regulation of the body’s work, therefore, we try to choose metrics that we can use to separately evaluate different “regulation branches”. We take into account the metrics showing the short-term NN interval variability and the metrics of the long-term NN interval variability, some correlation rhythmography parameters, and the main statistical metrics.

Welltory Team, 21 Dec. 2022

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