What is Heart Rate Variability?

When you are diagnosed with “heart rate variability,” your doctor or cardiologist may explain it to you as a variation (variable) in the regular amount of time between your heartbeats. Typically these variations or fluctuations are very small, measured in fractions of a second between heart beats. Below we will describe the some of the testing used to determine heart rate variability.

The clinical importance of HRV cannot be overestimated.

A high heart rate variable demonstrates the resiliency of your body to cope with “fight-flight”  or danger, stress and anxiety and have the ability to calm down afterwards. A high heart rate variable indicates an ability to manage stressful situations.

In general, low heart rate variability is considered a sign of current or future health problems because it shows your body is less resilient and struggles to handle changing situations. It’s also more common in people who have higher resting heart rates. That’s because when your heart is beating faster, there’s less time between beats, reducing the opportunity for variability. This is often the case with conditions like diabetes, high blood pressure, heart arrhythmia, asthma, anxiety and depression.

A low heart rate variability has been confirmed in numerous studies to be a strong, independent predictor of future health problems and as a correlate of all-cause mortality. A low heart rate variable demonstrates the lack of resiliency of your body to cope with “fight-flight”  or danger, stress and anxiety. Reduced HRV has been shown to occur in about every illness in man, woman, child, neonate and even fetuses. Low HRV is a predictor of both physical and emotional diseases. It also is an indicator of psychological resiliency and behavioral flexibility, reflecting an individual’s capacity to adapt effectively to changing social and environmental demands and stressors. Every person should know what their HRV is and what improves it and what diminishes it. Your long happy life depends on it.

The science of heart rate variable

The heart beats approximately 100,000 times per day or 2.5 billion times during an average lifetime. Heart rate variability (HRV) refers to the variability between successive heart beats, specifically the R-R intervals on an EKG recording. If a person has a heart rate of 60 beats per minute, the average R-R interval would be 1 second but some R-R intervals may be 0.8 seconds and others 0.12 seconds.

The variability between successful beats determines the HRV level, as well as its subcomponents. The vagus nerve has the greatest impact on heart function, as the cardiovascular afferents make up the greatest extent (compared to other organs) of the 85-90% of sensory fibers which make up the vagus nerve. These afferents go through the nodose ganglion which sits in front of the atlas (C1), thus giving a structural reason why upper cervical instability can affect HRV.

When a person is relaxed, the heart rate is low due to increased vagal tone. In this relaxed, calm, and restful state, breathing is slow and deep (belly or yoga breathing). Normally, during inspiration the R-R intervals are shortened as the heart rate speeds up so more blood is oxygenated, and they are lengthened during exhalation, so the maximum amount of blood is pumped out of the heart to the various parts of the body. Any breathing, therefore, is going to cause some variability in the time frame between successive heart beats. When vagal tone is high, the influence of breathing on this variability is maximized. The combination of a low heart rate and the large amount of time between heart beats; along with the large amount of time taken to breathe slowly and deeply, enhances the influence on the beat to beat variability. HRV is highest when vagal PSNS activity is highest; thus high HRV is associated with health and wellness. The higher the HRV the better and the more resilient a person will be to life’s many stressors!

Emotional, financial, structural (cervical instability) or physical stress (real or imagined) causes the heart rate to increase. The fast heart rate results in the time between heart beats to be very small. This coupled with the fact that breathing is very shallow (chest breathing) results in a low HRV.

Fluctuations in parasympathetic (vagus) nerve activity are a major source of HRV, particularly under resting conditions. Parasympathetic nerves can exert their effects more rapidly (<1s) than sympathetic nerves (>5 s). The magnitude of heart rate changes called oscillations, is increased at lower breathing rates (respiratory frequencies) and deeper breaths (higher tidal volumes). So slow deep breathing increases parasympathetic activity and increases HRV. This change in HRV with increased vagal tone is known as respiratory sinus arrhythmia and results from changes in the R-R interval during breathing.

Once baseline measurements are taken, they can be compared with those taken when with the patient under various stressors. As health improves, the changes in HRV from baseline lessen. The stronger the PSNS, the better the patient is able handle any stressor!

Time-domain indices of HRV are calculated just from measurements of the inter beat interval or R-R interval. The SDNN is the standard deviation of the normal-to-normal sinus node-initiated R-R intervals measured in milliseconds (ms). “Normal” means that abnormal beats, like ectopic beats (heartbeats that originate outside the right atrium’s sinoatrial node), have been removed. pNN50 is the percentage of adjacent NN intervals that differ from each other by more than 50 ms; whereas NN50 is the number of pairs of successive NN (R-R) intervals that differ by more than 50ms. Again, the higher the variability the better; thus, the larger the pNN50 and NN50 the greater the vagus activity and likelihood of health.

SDRR is the standard deviation of all the R-R intervals. RMSSD is the root mean square of successive R-R interval differences. Each successive time difference between heart beats in ms is obtained and then averaged before the square root of the total is obtained. Both SNS and PNS contribute to SDNN and RMSSD. SDNN, especially over a 24-hour period, predicts morbidity and mortality, especially in heart patients. Patients with a SDNN value below 50ms are classified as unhealthy, 50-100 ms have compromised health and above 100 ms are healthy.

Power spectral analysis is used to separate HRV into its component rhythms that operate within different frequency ranges. The power is the area under the height of a peak over a certain time period. The frequency reflects the period over which the rhythm occurs. For example, a 0.1 HZ frequency has a period of 10 seconds. The absolute power is calculated as ms squared divided by cycles per second (ms2/Hz). Relative power is an estimate of the percentage of the total HRV power. It divides the absolute power for a specific frequency band by the summed absolute power of the LF and HF bands. HRV can be broken down into four frequency domains: ultralow (<0.003 Hz), very low (0.0033-0.04 Hz), low (0.04-0.15 Hz) and high (0.15-0.40). So HRV can be broken down into ultralow (ULF), very low (VLF), low (LF) and high frequency (HF) rhythms that operate within different frequency ranges.

ULF band is not utilized much because it requires at least a 24-hour recording period. Circadian rhythms appear to drive this rhythm. Likewise, the VLF band is a more useful reflector of HRV when recorded over a long period of time (at least 5 minutes, but 24 hours is preferred). The very low frequency (VLF) band is below 0.04 hertz which involves the heart rate fluctuations between 25 and 300 seconds.  VLF power is more strongly associated with all-cause mortality than LF or HF power. Low VLF is associated with low testosterone, high inflammatory state and appears to be fundamental to health. It appears to be generated by the PSNS. Because of the long-time frame needed to get an accurate measure of VLF, it is not used as prevalently in day to day monitoring of HRV.

The most commonly measured and applicable frequency ranges of HRV measured for most patients are the LF and HF. LF and HF band measurements can be extremely useful even with a 2-minute recording for LF and as low as 1-minute recording for HF. These are often reported in terms of power, the signal energy found within a frequency band.  The absolute power is the calculated milliseconds squared divided by cycles per second (ms2/Hz) The HRV frequency-domain measurements can be expressed as percentages for instance LF/HF, the ratio of LF-to-HF power. This LF/HF power ration estimates the ratio or balance between the SNS and PSNS.


The HF or respiratory band (0.15-0.40 Hz) reflects parasympathetic vagus activity during the respiratory cycle. The HF oscillations coincide with the typical respiration frequency, which is 15 breaths per minute or 0.25 Hz.  During inhalation, the cardiovascular center inhibits vagal outflow resulting in an increase in heart rate, so the heart can empty, and the blood can be pumped throughout the body. Conversely, during exhalation, it restores vagal outflow, resulting in a slowing of the HR (so more blood can be oxygenated). Lower HF power is correlated with stress, anxiety, worry and ultimately to increased illness and death. Thus, a person literally must vagofyTM or die!

HF power is an indicator of vagus activity but not entirely.  At a breathing rate from 9 to 24 breaths per minute, HF power increases and heart rate slows, indicating an increase in vagus tone. But when the breathing slows to 8 or less breaths per minute, the heart rate does not correspondingly slow more, but what does change is LF power. So, at normal breathing rates of 9-14/minute, the LF represents SNS activity (tone) and the HF represents PSNS activity (vagal tone). At lower breathing rates, vagus nerve activity influences the LF, causing it to lower.  The LF/HF ratio is commonly used as a measure of sympatho-vagal balance, but again is primarily valid used as a baseline.

The LF to HF power (LF/HF ratio) has been used historically as a guide to estimate the ratio between SNS and PSNS activity.  A high ratio meant SNS dominance and a lower ratio indicated high vagal tone.  As discussed above, this is only valid during “normal” breathing paces, as the slower and deeper the breathing, the greater the variability of the LF band, up to 50% is due to the vagus nerve.

HRV amplitude is at its highest when the inspiration/expiration ratio is 1 to 1 (expiration should take as long as inspiration, each taking 5 seconds) during slow breathing at 0.1 Hz or 6 breaths per minute. While each person has a breathing rate that maximizes HRV, in most people the resonance frequency of the cardiovascular system is around 0.1 hertz (10 seconds), which is when the HRV is at its maximum, or PSNS activity is highest. Specifically, it is believed that at this resonance frequency breathing, the heart rate, blood pressure, HRV and baroreflex (which senses arterial pressures in heart and carotid arteries and aortic arch) are all coordinated and at their highest efficiency. In other words, at a certain breathing rate, the heart rate and blood pressure are the lowest to adequately perfuse the bodies tissues. It is presumed at this rate of breathing, the cardiopulmonary system runs most productively, as the pulmonary gas exchange efficiency is maximized, and cardiac ejection fraction its most powerful. It takes several months of practice to breathe at this rate, speed and depth but cardiovascular and cardiopulmonary health and overall body health are maximized. So, at maximum HRV, the heart pumps well oxygenated blood to the most amount of places in the least amount of time with the least amount of energy!

Learning about HRV can have many positive effects on health including increased energy and vitality. Perhaps instead of a balanced nervous system what one needs for health is an extremely efficient one!  Above 0.1 hertz, the SNS has very little influence, whereas the PSNS can affect heart rhythms down to 0.05 hertz (20-second rhythm). What this means is that when a person breathes once every 10 seconds or 0.1 Hz, which is 6 breaths per minute, the rhythmic oscillations peak in the LF band, PSNS activity and heart rate variability is maximum and thus HRV is at its highest level.  It should be noted that HRV amplitude is at its highest when the inspiration/expiration ratio is ½ during slow breathing at 0.1 Hz.

HRV is often reported in a 0 to 100 scale, which is based on the RMSSD calculations. The 0 to 100 scale allows people to easily track their HRV levels. When monitoring HRV levels, you will see it jump around moment to moment. One second it is 52, the next 68, then 55 and an average of a certain period, such as 2 minutes in then obtained.  RMSSD is the root mean squared of successive differences in the R-R intervals, as discussed above. The natural log is then applied to the RMSSD (lnRMSSD) to help show the magnitude of changes in the R-R variability. The lnRMSSD typically ranges from 0 to 6.5, but to better show the change it is expanded to a 0 to 100 score. A consistent baseline score of 70 or higher is associated with health; whereas levels between 50 and 70 are compromised health and diseases; whereas a regular HRV below 50 puts the person at risk for catastrophic illness and even death.


Most analysis for autonomic nervous system function relies on changes in blood pressure with various position changes. Blood pressure, unfortunately, is a latter finding in dysautonomia and vagopathy; whereas changes in HRV occur much sooner.  Baseline HRV and HRV parameter changes to stressors is a much more sensitive test and shows up much sooner when dysautonomia and vagopathy are present. Also, since HRV testing can be done at home it can be utilized to assess the progress of treatment.


Many Caring Medical patients, especially those with systemic illnesses and/or cervical instability find monitoring and improving their HRV important to regaining their health. Typically, a finger probe is used which can measure the EKG and this is synced to a cell phone app.  Each morning it is checked, and trends noted.  A person then tests HRV doing various activities and to determine which ones lower and which ones raise their HRV. Just because you like a certain type of music, for instance, does not mean that your nervous system does. On a day when the HRV is low, it is helpful then to do something to raise it, like take a cold shower, meditate, pray or better yet, pray before you take that freezing cold shower! We put more tips on our page about how to improve vagal tone. Obviously slowing down the breathing rate and increasing the depth of breaths always has a great effect on HRV. Mostly obtaining a high HRV involves getting adequate sleep and having an attitude of gratefulness.


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