What is HRV? — The Breath Space

Since a number of people have been asking me about HRV, I thought I would provide a brief explanation about what exactly is HRV. HRV is something I use with patients and clients continually as a way to monitor the state and function of the nervous system. All the mind-body approaches I use have an impact on the nervous system and we can assess this through monitoring HRV. Personally, I monitor my HRV with a variety of devices pretty much daily. If you would like to learn more about HRV, let me know. I hope this provide some insights into the world of HRV.

Below is the transcript of the video.

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If you haven’t heard by now, there is this thing people are monitoring and measuring called heart rate variability or HRV. Athletes, “biohackers", biofeedback practitioners, cardiologist and other health care providers are all looking at this noninvasive measurement as a way to gain some insight into the function and workings of the nervous system.

A number of people have been asking me about HRV so I thought I would offer a brief explanation of HRV, as this is something I use with patients and clients continually, and I myself monitor my HRV with a variety of devices pretty much on a daily basis. If you are remotely interested in health, as an athlete or someone trying to minimize the impacts of stress on your life, you most likely have come across these three letter - H R V, or heart rate variability. In fact, so many wearable, devices and apps are on the market right now all touting the benefit of monitoring and tracking this measurement.

But what it is?

DEFINITION

Simply put: Heart rate variability is defined as the time difference between subsequent or consecutive heart beats.

That is it.

You might be under the impression that the heart is like a metronome, consistently beating at a steady pace and rhythm, but it’s not like that at all. Our heart rate changes continually, adjusting constantly to the changes in our lives, and this variability provides some valuable insight into the function of our nervous system as well as our body’s ability to adapt to stressor.

Our cardiovascular system is regulated by the autonomic nervous system, which sends signal from the brain to the heart, and from the heart back to the brain. The two main branches of the autonomic nervous system - the sympathetic and the parasympathetic - both have specific influences on the heart and its rate. The sympathetic branch sends signals down to the pacemaker of the heart, known as the sinoatrial node or SA node, and throughout the heart muscle itself, while the parasympathetic sends signals down to SA node, as well as the atrioventirical or AV node, and the muscles of the atria of our heart.

But in broad strokes, our heart rate speeds up due to either an increase in the activity of the sympathetic nervous system or due to an inhibition or reduction in the activity of the parasympathetic system.

The slowing down of our heart rate seems to be primarily a result of parasympathetic activity, and the nerve of the parasympathetic system that is most responsible for this is the 10th cranial nerve, known as the vagus nerve.

A host of complex interactions effect nervous system's influence on the heart. Physiological processes like:

The levels of oxygen and carbon dioxide in our blood stream
The sensitivity and activity of pressure receptors, called baroreceptors, in our aorta and neck, which monitor blood pressure
The rate and volume of our breathing
The regulation of our body temperature
and even the function of our kidneys

MEASUREMENT OF HRV

So how do we measure HRV. There are basically two ways to calculate HRV - the most commonly known one being through ECG or electrocardiograph, and the other called through photoplethysmograph or PPG which measures the changes in the volume of our blood in our blood vessels - something called blood volume pulse.

Let’s first look at ECG.

An ECG is tool to detect and measure the electrical impulses through the heart. This is called a passive electrical reading and requires sensors to be attached to the body. When you see a cardiologist for a stress test, you typically have 12 leads or sensors attached to your torso. However, most home HRV systems use only two sensors. In biofeedback training it is common to place each sensor on an opposite wrist, and then have an extra sensor, known as the ground. Sports enthusiasts and athletes are probably familiar with heart straps worn around the chest to monitor your heart rate. This is another version of the ECG.

Let’s look at an ECG reading.

This little bump here is called the p wave, and it reflects the depolarization of the atria, the chambers of the heart which receives blood returning to it from either the body or the lungs. This big spike here is called the QRS complex, and represents the depolarization and contraction of the ventricles, which are responsible for sending the blood out of the heart and into the body. The peak of the complex is the represented by the letter R. Each of these R peaks represents another heart beat, so the heart rate is calculate by measuring the time between each R to R interval. When a cardiologist orders a 24-hour Holter monitor, which I would like to say here is still the gold standard for HRV measurements, they are using an ECG recording.

If the time between each R-R interval was 1000 milliseconds, that equates to a heart rate of 60. If, however, the time between heart beats is less than 1000 ms, then the heart rate is speeding up and getting faster. If the time between heart beats is increasing and getting longer, then the heart rate is slowing down.

The numbers you see at the top of this graph is the calculated heart rate based on the time interval between the R-R interval of adjacent beats. As you can see here, the heart rate is slowing down, and then begins to speed up again.

This is a recording of an ECG on one of my various biofeedback systems, called Alive. Keep your eye on the middle graph. You can see the blue line indicating the ECG recording. Here you can see the each successive R-R interval, also known as the interbeat interval, or IBI. The software calculates the heart beat, moment to moment, based on the amount of time, in milliseconds, between adjacent heart beats or R-R peaks.

Now the second way of measuring heart rate and heart rate variability that is common to most wearables and apps out there on the market today uses something called a photoplethysmograph or PPG. This involves a light source which shines through your tissues to measures the amount or volume of blood in the vessels, hence the name of this measurement - blood volume pulse. Each time your heart beats, a volume of blood gets pushed out of the heart creating a pulse wave throughout the blood vessels. The amount of blood present affects the amount of light that gets reflected back to the sensor and that is how each beat is determined. Finger sensors, and devices like the Oura Ring, the Whoop, and the Fitbit all use this method. Even your cameraphone can function as a very effective and accurate PPG.

Here is an example of this type of reading. It looks very similar to an ECG recording but it is not.

Blood Volume Pulse Reading on Alive Software

The time between each of the peaks of the pulse wave is used to calculate the HRV and this is called an interbeat interval.

IMPORTANCE OF HRV

So why is any of this important? For what reason would we want to monitor our HRV?

As I mentioned already, the rate of our heart beat reflects the amount of sympathetic or parasympathetic influence on our heart.

When we are in a state of chronic stress, whether physically, mentally and emotionally, our nervous system activity tends to shift towards the sympathetic over parasympathetic. In such a state, resting heart rate tends to increase which in turn decreases the amount of variability in our heart rate (lowering HRV). By monitoring and tracking our HRV daily we can determine the impact of our lifestyle on our nervous system. For example, overtraining in exercise and sports, tends to shift the nervous system to a predominately sympathetic state , thus decrease our HRV.

However, other factors contribute to lowering our HRV.

Physical stressors like:

lack of sleep, inefficient sleep or staying up late
eating highly processed food or even eating foods to which you are allergic
consuming large amounts of alcohol or begin dehydrated

Mental and emotional stressors also can have an impact on HRV.

worry, anxiety, and rumination whether about finances, mortgage, presentations or exams, or work all impact HRV

Therefore, tracking HRV is one way to get a picture about the state of our nervous system and its resiliency and adaptability - its ability to maintain balance in the presence of stressors. The science of HRV is quite complex, but what the research does tend to suggest is this:

Decreased HRV values are correlated to poor health outcomes, such as cardiovascular disease and diabetes.

Correlations have been made between low HRV and host of physical and psychological conditions. By tracking daily HRV, you can better determine what types of activities you wish to do that day - for instance if HRV is low, you might take extra precautions, if possible, to ensure getting more sleep, drinking more water, adjusting your diet or exercise routine and engaging in resiliency practices to strengthen the parasympathetic system and increase vagal tone. Furthermore, you can also begin to assess how lifestyle factors, like diet and exercise, are affecting your health.

Finally, in addition to tracking HRV, you can also use some HRV programs to practice resiliency exercises to strengthen PNS and vagal tone, and this is the crux of my work with people - teaching them mind-body medicine practice, whether breathwork, muscle relaxation and postural changes, mindfulness training or gratitude and lovingkindness practice - to help them decrease sympathetic arousal and to help them tonify and strengthen their vagus nerve.

If you are interested in learning more about HRV and how to use, feel free to contact me.

I hope this provided some insight as to what is HRV