Horse Trust-funded research discovers cardiac arrhythmias in horses

A research project funded by The Horse Trust has found that the majority of horses that undergo surgery have an abnormal heart rhythm afterwards. The project was led by Ruth Morgan, who is currently working as The Horse Trust's Senior Clinical Scholar in Equine Internal Medicine at the University of Liverpool.

Her research is the first to report the prevalence of arrhythmias in horses following general anaesthesia and surgery. 67 horses undergoing colic surgery and 37 horses undergoing orthopaedic surgery were included in the study.

A telemetric electrocardiogram (ECG) was fitted to each horse after they had recovered from anaesthesia and was left in place for 24 hours. Various breeds of horses were included in the study including Cobs, Thoroughbreds, Warmbloods, Welsh ponies and Irish drafts.

Morgan found that arrhythmias occurred in over 80% of horses and that there was no significant difference in the prevalence of any type of arrhythmias between the horses with or without gastrointestinal disease.

"We had thought that horses undergoing emergency colic surgery would be more at risk of developing arrhythmias as they are very unwell before the surgery," said Morgan. "However, we found that almost all the horses had arrhythmias after surgery, so maybe it is the anaesthesia or surgery itself causing the arrhythmia."

The most common types of arrhythmias found in the horses after surgery were Supra-ventricular premature depolarisations (SVPD), Sinus Arrhythmia (SA), and Ventricular premature depolarisations (VPD), which were found in 84%, 81% and 58% of the horses respectively.

Other arrhythmias found were 2nd degree AV block, Sino-atrial Block and Ventricular Tachycardia. Morgan also looked at some risk factors to see if they were associated with the development of arrhythmias following surgery, however very few factors came out as significant. It appears that the main reason for the development of arrhythmias is that the autonomic nervous system, which controls the horse's heart rate, is altered by anaesthesia and surgery.

"This research has given us a better understanding of what is going on with the horse's heart following surgery - their heart is undergoing stress during surgery and more attention needs to be paid to it," said Morgan.

"However, this research should also be reassuring to horse owners as it shows how common arrhythmias are following surgery, and that they are not normally caused by heart disease."

Morgan said that further research is needed to investigate the risk factors that impact the development of arrhythmias. She would also like to monitor the horses for five days following surgery to see when the heart rhythm normalises - many horses do not have an arrhythmia when they leave hospital, which suggests that they only occur for a brief period of time after surgery. Morgan's research has been accepted for publication with Acta Veterinaria Scandanavica.