Things have been busy at the Equine Science Center. Our students have returned to campus, and they have filled it with such positive energy! Our labs have also been busy as research projects get under way or continue. Sometimes I have to take a quick cat nap just thinking about all the fun things going on. It can be exhausting. One of the ongoing research projects is our big project with veterans and horses. I know I’ve given you some hints in the past about this wonderful project. Today, though, we’re going to set off on an investigative journey to take a deep dive into one particular aspect of this project. Journey along with me as I learn about heart rate and heart rate variability.

Most of us are probably at least somewhat familiar with heart rate. Measuring heart rate involves counting how many times the heart beats in a given period of time. Heart rate is generally reported in beats per minute, so we’re counting the number of times the heart beats in a one minute period. Some of you might have taken this measurement on yourself or your horse by using a stethoscope to the listen to the heart. In a research setting, heart rate is typically measured using a heart rate monitor or electrocardiogram (ECG) machine. In my reading of the literature, I discovered that this equipment detects and records the electrical activity causing contractions in the heart muscles. This electrical activity can be detected by looking at an ECG trace. You can check out the picture below to see for yourself.

An ECG showing 6 successive heart beats. Each set of peaks represents a change in the electrical charge across the heart muscles and thus, a contraction of the heart.

Heart rate can provide some information about what is going on with a horse or any other animal or human. When a horse is exercising or scared, its heart rate will go up. This faster beating of the heart pumps blood through the body at a faster rate providing more fuel and oxygen to tissues in the body. In contrast, heart rate decreases when a horse is relaxed or standing still.

An ECG trace can provide more information than just heart rate, however. We can also use it to calculate heart rate variability. If you look at the ECG trace above, you’ll notice the beats are not evenly spaced across the x-axis which represents time. Many software programs will calculate the R-R interval or the interval between successive heart beats. This information can then be used to provide more information about the horse or human. High heart rate variability (the time between successive heart beats varies) is associated with activation of the parasympathetic and sympathetic branches of the autonomic nervous system. I had to go do a little more research about the nervous system. It turns out the autonomic nervous system is responsible for many of our reactions to stressful and frightening situations. Under these conditions, there is greater activation of the sympathetic system which causes things like an increased heart rate. In contrast, when we are relaxing or eating the parasympathetic system is more activated leading to a decrease in heart rate and routing of blood flow to the digestive organs. Low heart rate variability (a fairly constant amount of time between successive heart beats), in contrast, represents activation of the sympathetic nervous system. Thus, heart rate variability can provide information on the balance between sympathetic and parasympathetic activation in the autonomous nervous system. We can use this information to determine how stressed a horse is. In a stressful situation we would expect increased activation of the sympathetic nervous system leading to an increase in heart rate and a decrease in heart rate variability.

I’m curious to see what my ECG trace looks like. Hopefully, it shows I’m relaxed and content! I think I’m going to go talk to the students about hooking me up to the ECG unit. If you ever get the chance, you should do the same thing! After reading today’s blog, you’ll know what you’re looking at.

Until Next Time.

Your Friend,

Lord Nelson  

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