Cardiac Arrhythmia: What the Research Says
Evidence from 21 peer-reviewed studies
What Professionals Should Know
- •Surface ECG measurement of atrial fibrillatory rate may help predict which horses with AF are likely to recur after cardioversion, allowing better treatment planning
- •AFR could be used as a non-invasive screening tool to identify horses with electrical remodelling before attempting cardioversion
- •Knowing AFR status may help practitioners counsel owners on prognosis and likelihood of successful long-term conversion to normal sinus rhythm
- •Horses diagnosed with AF postrace, particularly persistent AF, carry substantial risk of recurrence and should receive appropriate monitoring and management protocols before return to racing
- •The wide variation in time between episodes (34-1065 days) means clinical decision-making about return-to-racing timelines must be individualized and evidence-based rather than following fixed protocols
- •Racing regulatory bodies should consider AF recurrence risk data when establishing rules around horse eligibility for return to competition following cardiac events
- •HRV monitoring via portable heart rate monitors could help identify horses at risk for atrial fibrillation recurrence without requiring ECG equipment
- •This non-invasive monitoring approach may enable early detection of arrhythmia recurrence in horses recovering from AF treatment
- •Incorporating HRV assessment into post-cardioversion monitoring protocols could improve outcomes for horses prone to AF recurrence
- •Diltiazem shows promise for controlling heart rate in equine atrial fibrillation through AV nodal inhibition, but dosing must be carefully individualized due to variable plasma concentrations
- •Hypotension from vasodilation and direct sinus node suppression are the primary dose-limiting factors; monitor blood pressure and cardiac rhythm closely during administration
- •Baroreceptor reflex-mediated sympathetic activation may partially offset diltiazem's negative effects on the heart, suggesting careful monitoring is needed to distinguish true cardiac depression from compensatory mechanisms
- •Persistent equine AF involves metabolic and structural remodeling rather than primary electrical changes, suggesting that targeting metabolic pathways and fibrosis may improve treatment outcomes beyond current electrical management approaches.
- •Understanding that AF-induced myocardial changes accumulate over 2-12 weeks reinforces the importance of early diagnosis and intervention to prevent irreversible transcriptional changes.
- •Future AF therapies in horses should consider metabolic modulation (fatty acid and glycolytic pathways) and anti-fibrotic strategies alongside conventional antiarrhythmic treatments.
- •For horses with frequent PACs refractory to medical therapy, 3D electro-anatomical mapping can identify the precise ablation target, but success rates remain variable and procedure carries risks of complications such as phrenic nerve injury
- •Radiofrequency ablation shows promise as a potential permanent treatment for PACs in horses, though this remains an advanced referral procedure best performed at specialized cardiac centres
- •Standard 12-lead ECG and vectorcardiography may not accurately localize PAC origin—3D mapping can reveal the true focus differs from non-invasive predictions
- •Equine clinicians should be aware that accessory pathways are a differential diagnosis for unexplained cardiac arrhythmias in horses, particularly when preexcitation patterns are noted on ECG
- •OAVRT should be considered in horses presenting with recurrent tachycardia or bradycardia episodes of cardiac origin that do not respond to conventional antiarrhythmic therapy
- •Recognition of this condition in horses may allow for targeted diagnostic and therapeutic approaches previously unavailable in equine medicine
- •Radiofrequency catheter ablation guided by lesion size index and three-dimensional mapping can effectively treat sustained atrial tachycardia in horses with long-term success
- •This advanced electrophysiological technique may be considered for horses with drug-refractory arrhythmias when referral to centres with appropriate mapping technology is available
- •Careful identification and isolation of the arrhythmogenic substrate (such as caudal vena cava myocardial sleeves) appears crucial for preventing recurrence
- •Recurrent syncope in donkeys warrants electrocardiographic investigation to rule out paroxysmal atrioventricular block, a potentially life-threatening arrhythmia.
- •Cardiac pacemaker implantation is a viable treatment option for equine species with PAVB when medical management is ineffective or unavailable.
- •Long-term outcomes following pacemaker implantation in equines appear favorable, with sustained symptom resolution beyond 12 months possible.
- •Magnesium sulfate should be considered as a treatment option for sustained ventricular tachycardia in horses when lidocaine alone is insufficient, particularly in colic cases
- •Multiple boluses of magnesium sulfate administered over several hours may be necessary to achieve conversion to normal sinus rhythm
- •Sustained ventricular tachycardia can occur secondary to gastrointestinal disease and should prompt both cardiac and abdominal investigation
- •Horses presenting with intermittent pre-excitation on ECG (short PQ interval with abnormal QRS morphology) may be candidates for 3D electro-anatomical mapping and radiofrequency ablation rather than medical management alone
- •Advanced catheter ablation techniques previously used in human and small animal cardiology can be successfully adapted for equine cardiac arrhythmia treatment
- •Post-procedure monitoring should include 24-hour Holter ECG and exercise ECG testing to confirm complete elimination of pre-excitation
- •While second-degree AV blocks are common and usually benign in horses at rest, a high frequency of blocks (~300/hour) with clinical signs of syncope or collapse warrants advanced diagnostic investigation including echocardiography and rhythm monitoring
- •Structural abnormalities such as cartilaginous aortic root intrusion into the AV nodal region may underlie refractory AV conduction problems and should be considered in the differential diagnosis of horses with frequent blocks and collapse
- •Implantable loop recorders can provide valuable long-term arrhythmia documentation in horses with recurrent collapse, though obtaining recordings during clinical events remains challenging
- •Sudden cardiac death accounts for a notable proportion of unexplained exercise-associated deaths in racing trotters, but genetic predisposition linked to inbreeding cannot be confirmed as a risk factor in this population
- •Veterinarians investigating sudden death in racehorses should pursue thorough necropsy examination to differentiate between cardiac causes, vessel rupture, and other fatal conditions, as diagnosis impacts breeding and training decisions
- •Pedigree screening based on inbreeding coefficients alone is not justified as a preventive strategy for SCD in these horse populations based on current evidence
- •24-hour ECG at 5 days post-cardioversion can help identify horses at higher risk of AF recurrence; those with ≥25 APDs/24h warrant closer monitoring and consideration of additional preventive measures
- •Structural cardiac changes (mitral regurgitation and atrial dysfunction) are stronger predictors of recurrence than APD burden alone, so echocardiographic assessment remains essential in post-cardioversion management
- •Smaller horses and those with valvular disease have elevated recurrence risk and may benefit from more aggressive management strategies or closer follow-up intervals
- •Understanding pulmonary vein anatomy and conduction pathways is essential for developing effective ablation strategies to treat performance-limiting atrial fibrillation in horses
- •Current knowledge gaps regarding the histological and immunohistochemical features of equine pulmonary vein myocardial sleeves limit optimization of interventional treatments
- •Unexplained performance loss or reduced fitness in horses warrants cardiac evaluation including auscultation and ECG; subtle cases may require long-term monitoring to confirm pAF diagnosis
- •Early detection and treatment of paroxysmal AF is important because untreated episodes can lead to permanent atrial fibrillation and chronic performance issues
- •Consider adopting newer long-term ECG monitoring technologies adapted from human medicine to improve diagnostic capability and capture intermittent arrhythmias
- •Atrial fibrillation is treatable with good prognosis, but recurrence is common—work with your veterinarian on a long-term monitoring strategy if your horse has been diagnosed.
- •Multiple factors influence AF development (training intensity, age, underlying health issues), so a comprehensive approach to managing comorbidities may help prevent recurrence.
- •Modern diagnostic tools like cardiac mapping and implantable ECG devices can now better characterize individual cases, potentially allowing more targeted treatment decisions for your horse.
- •Detect AF early by noting irregularly irregular rhythms at rest and disproportionate heart rate responses during exercise; confirm with ECG before treatment planning
- •Severe underlying cardiac disease is a contraindication for cardioversion due to high recurrence risk; reserve cardioversion for performance horses or those with abnormal exercise responses (>220 bpm maximal heart rate)
- •Understand that TVEC offers superior success (>95%) compared to pharmacological options (~80%), but expect up to 39% recurrence regardless of method; plan long-term monitoring and consider preventive sotalol therapy post-conversion
- •Consider paroxysmal atrial fibrillation as a differential diagnosis in racehorses presenting with intermittent poor performance that cannot be explained by other causes
- •Implantable loop recorders offer a practical diagnostic tool for detecting PAF in horses when traditional ECG monitoring fails to capture episodic arrhythmias
- •Early detection of PAF may enable targeted management strategies to improve or maintain performance in affected racehorses
- •Lone AF may present with absent clinical signs at rest but performance horses can show exercise intolerance, respiratory distress, weakness, or collapse during work—be alert to these signs even without cardiac murmurs
- •Diagnosis may require advanced imaging beyond routine techniques as many AF cases lack obvious structural heart disease, necessitating specialist referral
- •Recovery after successful cardioversion is prolonged (weeks to months); allow adequate rehabilitation time before returning to work
- •Horses appear genetically susceptible to QT prolongation and sudden cardiac death similar to humans, warranting cardiac screening in at-risk populations
- •Certain medications (like terfenadine) may pose cardiac safety risks in horses by blocking KV11.1 channels; veterinarians should review drug safety profiles for cardiac effects
- •Understanding equine cardiac electrophysiology may help explain some cases of sudden, unexplained deaths in horses and guide preventive management strategies
Key Research Findings
Atrial fibrillatory rate (AFR) derived from surface ECG is a biomarker for electrical remodelling in equine AF
AFR has potential utility for predicting successful cardioversion and AF recurrence in horses
AFR may help identify horses at higher risk of AF recurrence after treatment with medical therapy or electrical cardioversion
Atrial fibrillation was identified in 4.9% of Thoroughbred racehorses with an overall incidence of 2.7 episodes per 1000 starts
Horses with previous AF episodes had 5.3 times higher risk of recurrence (12.8 per 1000 starts) compared to those with no prior history (2.4 per 1000 starts)
Persistent AF showed higher recurrence rate (64%) compared to paroxysmal AF (23%; OR 5.9)
Median duration between AF episodes was 343 days with wide variation (34-1065 days)
Heart rate variability parameters can distinguish between atrial fibrillation and sinus rhythm in horses
HRV parameters remain effective at differentiating rhythms before and after successful electrical cardioversion
Heart rate monitors with HRV calculations offer a practical monitoring tool for detecting arrhythmias in horses
Diltiazem achieved plasma concentrations of 390-910 ng/mL with considerable inter-individual variation among horses
Diltiazem caused intermittent depression of sinus and AV nodes with mild impairment of systolic and diastolic LV function
Diltiazem produced arterial vasodilatation, increased limb blood flow, and decreased systemic vascular resistance
Two horses developed transient high-grade sinus arrest with severe systemic hypotension, limiting safe dosage
Horse cardiac transcriptome shows distinct molecular identities across four chambers with ion channel expression largely similar to humans except for ventricular repolarising potassium channels.
Evidence Base
Atrial fibrillatory rate as predictor of recurrence of atrial fibrillation in horses treated medically or with electrical cardioversion.
Buhl Rikke, Hesselkilde Eva M, Carstensen Helena et al. (2022) — Equine veterinary journal
Incidence, recurrence, and outcome of postrace atrial fibrillation in Thoroughbred horses.
Nath Laura C, Elliott Adrian D, Weir Joe et al. (2021) — Journal of veterinary internal medicine
Heart rate variability parameters in horses distinguish atrial fibrillation from sinus rhythm before and after successful electrical cardioversion.
Broux B, De Clercq D, Decloedt A et al. (2017) — Equine veterinary journal
Effects of diltiazem on hemodynamic variables and ventricular function in healthy horses.
Schwarzwald Colin C, Bonagura John D, Luis-Fuentes Virginia (2005) — Journal of veterinary internal medicine
The horse cardiac transcriptome: Moving towards a molecular understanding of atrial fibrillation.
Haugaard Simon Libak, Nissen Sarah Dalgas, Schneider Mélodie Jil et al. (2025) — Equine veterinary journal
Three-dimensional electro-anatomical mapping of premature atrial complexes originating from the right atrial free wall and treatment by radiofrequency ablation in three horses.
Buschmann Eva, Van Steenkiste Glenn, Bulckens Hannes et al. (2025) — Equine veterinary journal
Orthodromic Atrioventricular Reentry Bradycardia and Tachycardia Caused by an Accessory Pathway in Horses.
Buschmann Eva, Easton-Jones Charlotte, Van Steenkiste Glenn et al. (2025) — Journal of veterinary internal medicine
Lesion size index-guided radiofrequency catheter ablation using an impedance-based three-dimensional mapping system to treat sustained atrial tachycardia in a horse.
Buschmann Eva, Van Steenkiste Glenn, Vernemmen Ingrid et al. (2025) — Equine veterinary journal
Treatment of a Paroxysmal Atrioventricular Block by Implantation of a Bipolar, Single-Chamber Cardiac Pacemaker in a Donkey.
Heun Frederik, Niebuhr Tobias, Gutierrez Bautista Alvaro et al. (2023) — Animals : an open access journal from MDPI
Resolution of sustained ventricular tachycardia in a horse presenting with colic with magnesium sulfate
Pratt Stefanie L., Bowen Mark, Redpath Adam (2023) — Equine Veterinary Education
Three-dimensional electro-anatomical mapping and radiofrequency ablation as a novel treatment for atrioventricular accessory pathway in a horse: A case report.
Buschmann Eva, Van Steenkiste Glenn, Boussy Tim et al. (2023) — Journal of veterinary internal medicine
Cartilaginous Intrusion of the Atrioventricular Node in a Quarter Horse with a High Burden of Second-Degree AV Block and Collapse: A Case Report.
Nissen Sarah Dalgas, Saljic Arnela, Kjeldsen Sofie Troest et al. (2022) — Animals : an open access journal from MDPI
Exercise-Associated Sudden Death in Finnish Standardbred and Coldblooded Trotters - A Case Series With Pedigree Analysis.
Trachsel Dagmar S, Calloe Kirstine, Mykkänen Anna K et al. (2021) — Journal of equine veterinary science
Atrial premature depolarisations five days post electrical cardioversion are related to atrial fibrillation recurrence risk in horses.
Vernemmen I, De Clercq D, Decloedt A et al. (2020) — Equine veterinary journal
Morphological and histological investigation of the conduction system in the equine atrial muscle sleeve of pulmonary veins.
Kovacs Szilvia, Racz Bence, Sotonyi Peter et al. (2024) — Equine veterinary journal
Paroxysmal Atrial Fibrillation in Horses: Pathophysiology, Diagnostics and Clinical Aspects.
Kjeldsen Sofie Troest, Nissen Sarah Dalgas, Buhl Rikke et al. (2022) — Animals : an open access journal from MDPI
Fundamentals of arrhythmogenic mechanisms and treatment strategies for equine atrial fibrillation.
Premont Antoine, Balthes Samantha, Marr Celia M et al. (2022) — Equine veterinary journal
Atrial fibrillation in horses Part 2: Diagnosis, treatment and prognosis.
Decloedt Annelies, Van Steenkiste Glenn, Vera Lisse et al. (2021) — Veterinary journal (London, England : 1997)
Implantable loop recorders can detect paroxysmal atrial fibrillation in Standardbred racehorses with intermittent poor performance.
Buhl Rikke, Nissen Sarah D, Winther Marie L K et al. (2021) — Equine veterinary journal
Atrial fibrillation in horses part 1: Pathophysiology.
Decloedt Annelies, Van Steenkiste Glenn, Vera Lisse et al. (2020) — Veterinary journal (London, England : 1997)