Long-term Use of an Insertable Cardiac Monitor (Loop Recorder) in Horses

Background: Short-term use of insertable cardiac monitors (loop recorders) has been reported in horses; however, information about long-term use in this species is lacking. 
Objectives: The aim of the study was to investigate the outcomes of long-term use of Reveal XT cardiac monitors in adult horses. 
Study design: Longitudinal. 
Methods: The cardiac monitors were implanted in 12 horses in standing position, under sedation and local anaesthesia. The site for implantation was the ventral body wall, approximately 5 cm from the midline on the left side, just behind the girth area. Stored data were downloaded from the devices every 8 weeks. A Televet 100 electrocardiograph was used to obtain standard electrocardiograms from each horse. Horses were regularly exercised. Arrhythmias were recorded by the cardiac monitors using the built-in human algorithm: number of total episodes, ventricular tachycardias, asystoles, bradycardias, atrial tachycardias, atrial fibrillations, and the percentage of time spent in atrial tachycardia or atrial fibrillation. 
Results: The mean duration of data recording was 420.4 days (minimum: 138 days, maximum: 671 days, standard error: 53.1 days). The number of total episodes was 425,437 (mean ± SD: 35,453±11,666 per horse). The number of asystolic and bradycardic episodes was 413,363; 97.2% of the total episodes. Based on manual evaluation of the data, these were falsely detected. Ventricular tachycardias (818 episodes) were also falsely recorded during exercise, as most T waves were recognised as QRS complexes. One horse had a naturally developed, persistent atrial fibrillation which was detected correctly 8,159 times, although when the heart rate was low, false bradycardia or asystole were recorded. 
Main limitations: Only one horse had pathologic arrhythmia in the population. 
Conclusions: The Reveal XT loop recorders worked reliably without complications. The human algorithm could not be used for automatic detection of arrhythmic episodes in the study population. 
Ethical animal research: Approved by the Institutional Animal Care and Use Ethical Committee of the University of Veterinary Medicine Budapest, Hungary (reference: PE/EA/1442-7/2019). 
Informed consent: All horses are owned by the University of Veterinary Medicine Budapest, Hungary. 
Competing interests: None declared. 
Sources of funding: Supported by Medtronic Hungary Ltd.

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