Written by Fenway Foundation for Friesian Horses.
The equine heart is an impressive organ. The heart is composed of four chambers that provide the pumping function needed to supply blood to the body. The upper chambers are called the left and right atria. The lower chambers are called the left and right ventricle. There are also various in and out flow vessels in the heart to support blood flow. In simple terms, the function of the right side of the heart (right atrium, right ventricle) is to pump blood to the lungs, where oxygen is added and carbon dioxide is removed from the blood. The function of the left side of the heart (left atrium, left ventricle) is to pump blood to the rest of the body, where oxygen and nutrients are delivered to tissues, and waste products (such as carbon dioxide) are removed. Together, the four chambers of the heart perform an impressive and complicated symphony, pumping and supplying blood to the body, every moment of every day throughout the horse’s life.
A recent study conducted by Ghent University in Belgium compared the cardiac measurements of 100 Friesians and 100 Warmbloods and found that in general, the left and right ventricle internal diameter measurements of Friesian horses were significantly smaller than those of Warmbloods. Additionally, the measurement of the heart’s muscular output contractions (Fractional Shortening and Ejection Fraction) in Friesian horses was higher, indicating the heart of a Friesian horse contracts harder than a Warmblood’s heart to achieve sufficient blood output.
Research has demonstrated an “athletic heart” is linked to positive endurance related performance in horses. One study in particular found cardiac measurements, specifically left ventricle measurements, correlated with performance and showed strong heritability levels. Another study, which specifically studied fitness levels of Friesian horses, confirmed for the first time that genetics do indeed influence fitness in horses- something proven long ago in humans.
Research has confirmed Friesians have a different response to training and reach their anaerobic threshold at a lower workload than other breeds. It is possible there is a correlation between the smaller cardiac measurements of Friesian’s and aerobic endurance. Riders and trainers of Friesian horses should understand continuous exercise, particularly at the canter, might initially exceed the aerobic threshold of some Friesian horses early in their athletic conditioning. Some Friesian horses may require shorter periods of more intense aerobic exercise until they can work up to longer periods.
The Gent University study led to the publication of echocardiographic (heart ultrasound) reference intervals specific to Friesian horses. These reference intervals may be shared with your veterinarian for use when conducting clinical diagnostics of the heart.
Link to study, including echocardiographic reference intervals: https://onlinelibrary.wiley.com/doi/10.1111/jvim.15938
Vernemmen, Ingrid & Lisse, Vera & van steenkiste, Glenn & van Loon, Gunther & Decloedt, Annelies. (2020). Reference values for 2‐dimensional and M‐mode echocardiography in Friesian and Warmblood horses. Journal of Veterinary Internal Medicine. 34.
M., Younes & Robert, Celine & Barrey, Eric. (2014). Genetic Component of Endurance Ability. Equine Veterinary Journal. 46.
Munsters CCBM, van den Broek J, van Weeren R, Sloet van Oldruitenborgh-Oosterbaan MM. Young Friesian horses show familial aggregation in fitness response to a 7-week performance test. Vet J. 2013;198(1):193-199.