Heart rate variability (HRV) is an important value when looking at equine cardiac health. But how does it differ from heart rate? What does it tell us? And why is it so important? Let’s take a look.
Heart rate vs. Heart rate variability
Heart rate (HR) is measured in beats per minute (BPM)—simply the average number of beats in 60 seconds. The heart rate of an adult horse sits between 28 and 44 beats per minute. And generally speaking, a low heart rate indicates rest, while a high heart rate corresponds with exertion, be it from exercise, excitement, or stress.
Heart rate variability
Heart rate variability (HRV) on the other hand measures the specific changes (or variability) in the time between beats. This variability between beats is referred to as either an R-R interval or inter-beat interval (IBI).
Less variability in the heart beats (a low HRV) indicates that the horse is under stress, while greater variability (a higher HRV) typically means that the horse has a strong ability to tolerate stress or is recovering well from prior accumulated stress. So when a horse is at rest, a high HRV is favorable over a low HRV. But in an active state, lower relative HRV is generally favorable over a high HRV.
HRV is linked to the autonomic nervous system (ANS) and reflects the balance between the parasympathetic nervous system (PNS) and the sympathetic nervous system (SNS)—the “rest and digest” and “fight or flight” branches, respectively. By balancing the two forces, the ANS helps horses respond to stress in their environment and regulate things like heart rate, respiration and digestion. Essentially, HRV reflects the heart’s ability to respond to different situations.
Understanding Heart rate variability
Heart rate variability is a noninvasive measurement that has been used to assess autonomic nervous system regulation of cardiovascular function. Evidence suggests that several factors play a role in the variations of one horse’s HRV to another, including genotype, behavior, environment, temperament, and nutritional status, among others.
Understanding the variations in a horse’s heart rhythm help veterinarians confirm important diagnoses, such as arrhythmias. But heart rate variation has also been used in numerous studies to better understand the effects of stress, pregnancy, exercise, disease, massage, human-horse interaction, behavior, equipment use, transport and more.
History of HRV
Techniques to measure HRV were developed with advances in electronics and digital signal processing in the 1960s, which generated an explosion of interest in HRV in health and disease. In equine medicine today, the study of HRV is primarily used in cardiology and research settings. But veterinarians see increasing applications for HRV analysis as new technologies emerge. The ability to gather cardiac data without disturbing the horse opens up research and discovery in areas where human presence has historically skewed the findings.
Influences on HRV
The primary inputs that influence the Sino-atrial (SA) node—which controls heart rate—are respiration, blood pressure and thermoregulation. But stress, hormones, electrolytes, acid-base balance, activity, eating, sleep, arousal and disease are also modifiers. Those primary inputs originate from the activity of the sympathetic and parasympathetic nervous systems. Basically, increased SNS activity and/or decreased PNS activity leads to decreased HRV.
Because there are many environmental and physiological influences on HRV, it’s important that vets are able to measure HRV over both short and long periods, and also perform recurring measurements across days or even weeks. This helps vets to identify outlier data and gain confidence in their baseline values.
Whether you use manual methods or rely on sensor-based devices, heart rate detection in horses is quite simple today. But obtaining an accurate ECG with visible HRV—while vital for veterinarians, cardiologists, and researchers—is not always so straightforward.
When looking at a heart rhythm that has been measured with a stethoscope or heart rate monitor, it may appear to have very little variation over time. But ECG recordings are able to detect the R wave in the QRS complex and calculate the time between R waves—the R-R interval. This reveals the often subtle pattern of variation over time in an otherwise “normal” heart rhythm. HRV is generally measured either in short (e.g., 5–10 minutes) or long (12–24 hour) periods, and the time between successive R-wave peaks is measured in milliseconds. The most commonly used parameters to study HRV in horses are the HR, the standard deviation of all the R-R wave intervals (SDNN), and the root mean square of the successive differences of the R-R intervals (RMSSD).
Measuring HRV is commonly done at rest—a value that has been well described in horses. However, publications documenting the normal R‐R variation during exercise are limited. It’s challenging to separate truly premature beats from normal beat‐to‐beat variation during an exercise ECG. When the degree of normal variation is unclear, P‐QRS‐T morphology is difficult to differentiate and motion artifacts impair ECG quality.
In veterinary medicine today, physiological monitoring methods that use plastic conductive electrodes or standard Ag/AgCl electrodes are still widely used. Some are invasive, some are not. But either way, these are limited to a restrictive set of applications.
Newer technologies such as wearable devices are finally making waves in the equine world, but there are more products geared toward equestrians than veterinarians. One reason may be that few of the wearable technologies on the market are medical grade, so translating their use to clinical applications isn’t feasible. In other words, many are just heart rate monitors and do not produce a detailed ECG from which HRV can be derived. Another likely roadblock is the relatively slow adoption of new technologies in the veterinary field compared to other industries. But with the growing prevalence of new technologies at veterinary conferences and an emphasis by industry associations on the importance of incorporating technology into daily practice, veterinarians are becoming more and more interested.
Advances in sensor technology, embedded computing, digital signal processing, artificial intelligence and machine learning are steadily influencing new, built-for-purpose technologies that offer immediate value in veterinary medicine. “Smart” measuring and monitoring solutions are the future that will move vets beyond the standard telehealth practices seen today.
New technologies powered by real-time data and artificial intelligence will not only reduce the reliance on manual, subjective monitoring, but also aid veterinarians in diagnostic decision making, increase their service offering, and open new areas for research.
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