Heart rate variability (HRV) appears to be on everyone’s mind these days, at least those interested in tracking their own or their customers’ sleep, recovery, performance, or overall health. The phenomenon is not new in and of itself, but its application in everyday language and on widely available gadgets is a relatively recent development. HRV is one of the markers measured by many professional and consumer wearables, and sports, health, and performance coaches and practitioners should understand the fundamentals of this exciting phenomena.
Without delving too far into the science of HRV, this blog will address it from a practical standpoint:
- What it is.
- What it can tell you about the physiology of your body.
- What you should be mindful of when interpreting it.
Future blogs in this series will expand on the issue to include practical applications and the rather complicated relationship of HRV to things like stress and recovery.
“Higher HRV has been linked to lower morbidity and mortality, as well as increased psychological well-being and quality of life.”
The physiological phenomenon of fluctuation in the time interval between successive heartbeats in milliseconds is known as heart rate variability (HRV). A typical, healthy heart does not tick evenly like a metronome; instead, there is continual variance in the milliseconds between heartbeats. We are generally unaware of this volatility; it is not the same as our heart rate (beats per minute) increasing and dropping as we go about our everyday activities.
Feeling your pulse on your wrist while taking a few deep breathes in and out might give you an idea of your HRV: the gap between beats gets longer (heart rate slows) when you exhale and shorter (heart rate increases) when you inhale, a phenomenon known as respiratory sinus arrhythmia. HRV is acutely influenced by various factors, including exercise, hormone reactions, metabolic processes, cognitive activities, stress, and recuperation.
How Do You Calculate HRV?
Accurate monitoring of each heartbeat and the period between beats is required for reliable HRV analysis. There are other technologies for estimating HRV, but it is beyond the scope of this blog to go into detail about them. If you want to learn more about the basics of measuring HRV and the many HRV factors, I recommend the Task Force article on heart rate variability.
In summary, ECG-based algorithms identify the R wave in the QRS complex and calculate the time interval between R waves (R-R interval; Fig. 1). This is what the Firstbeat Bodyguard does, for example: it can detect the heartbeat at 1 ms accuracy (1000HZ) for very accurate HRV analysis in most persons of all body kinds and ages. Most commonly accessible wearable devices employ PPG or photoplethysmography to detect the heartbeat optically by detecting the wave of blood flow, such as from the wrist or ear and then calculating the inter-beat interval, or IBI. Comparison of multiple methodologies is usually difficult, and this is especially true with HRV – which is beyond the scope of this blog. However, if used correctly and consistently, several methods and gadgets can create interesting and relevant information for the user.
Figure 1: An electrocardiogram (ECG) graph displaying a sequence of QRS complexes. The period between heartbeats (R-R interval) changes naturally from beat to beat. A more in-depth investigation of this variation (HRV) reveals a wealth of information about the body’s physiological state.
The Autonomic Nervous System and Heart Rate Variability
HRV is a non-invasive index of autonomic nervous system activity that is governed by the autonomic nervous system (ANS) and its sympathetic and parasympathetic branches. The sympathetic branch of the ANS is the stress or fight or flight system, preparing us to act, react, and perform — to meet the various demands of life.
The parasympathetic nervous system is known as the rest and digest system because it helps the body to power down and recover “once the struggle is ended.” The sympathetic branch stimulates the synthesis of stress hormones, increases the pace and power of heart contraction (cardiac output), and decreases HRV, which is required during exercise and psychologically or physically stressful situations. After the tension has passed, the parasympathetic nervous system decreases the heart rate and boosts HRV to restore balance. This natural interaction between the two systems enables the heart to respond swiftly to a variety of events and needs.
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Why Is Heart Rate Variability Important?
In a normal, healthy condition, HRV should rise during relaxing activities such as meditation or sleep, when the parasympathetic nervous system takes over (see Fig 2 for an example). HRV, on the other hand, naturally diminishes during stress, when increased sympathetic activity assists the body in meeting the demand. As a result, HRV is often higher while the heart is beating slowly and lower when the heart begins to beat quicker, such as during stress or exercise.
The HRV level naturally changes from day to day, depending on activity level and amount of, say, work-related stress, but if a person is chronically stressed or overloaded – physically or mentally – the natural interplay between the two systems can be disrupted, and the body can get stuck in a sympathetically dominant fight state, with low HRV and high stress hormone levels, even when the person is resting. This is extremely taxing on the body and can lead to a variety of mental and physical health issues.
In the HR Firstbeat Lifestyle Assessment graph, RMSSD reflects the beat-to-beat variation.
Figure 2: The graph of a person’s HRV (RMSSD in ms) over 24 hours demonstrates how HRV reduces to virtually nil while exercise (parasympathetic activity is withdrawn) and considerably increases during meditation and sleep. This is reflected in the Firstbeat Lifestyle Assessment graph as a green recovery stage and is considered a meaningful, healthy response.
Although genetic variables account for approximately 30% of overall HRV levels, individuals can enhance their individual HRV by improving their health, exercise, stress management, and recovery skills. High HRV is usually seen as a sign of a healthy heart, and higher HRV has been linked to lower morbidity and mortality, as well as increased psychological well-being and quality of life in numerous studies. We must live with what the genetic lottery has given us, and even if some general reference values are available, comparing our HRV values to those of other people is meaningless. The good news is that lifestyle has a significant impact on HRV. We may actively improve our lifestyle by being physically active and striving for a better balance in our lives, and we will likely experience improvements in our HRV as a result.