Cardiac Arrhythmias: What the Research Says
Evidence from 33 peer-reviewed studies
2 RCT
13 Cohort Study
10 Case Report
8 Expert Opinion
What Professionals Should Know
- •Levothyroxine supplementation at performance doses does not improve athletic performance and carries significant cardiac risk; reconsider use in racing programs
- •Horses receiving thyroid supplements should be monitored with baseline and post-supplementation ECGs due to arrhythmia risk, particularly atrial fibrillation
- •Most racehorses maintain adequate thyroid function naturally; supplementation is rarely indicated and may be contraindicated in athletic horses
- •Sotalol may provide a viable oral anti-arrhythmic treatment option for horses requiring long-term arrhythmia management, addressing a current gap in equine anti-arrhythmic therapy
- •Doses of 2-4 mg/kg twice daily appear safe and effective with minimal side effects, though dosing above 2 mg/kg may not provide additional electrophysiological benefit
- •Clinicians should monitor QT interval changes via electrocardiography when initiating sotalol therapy, but echocardiographic and other cardiac function measures appear unaffected
- •Cardiac arrhythmias are extremely common in apparently healthy horses at rest; their presence alone does not indicate clinical disease requiring treatment
- •Monitor body condition and heart girth in sedentary horses, as increased heart girth may indicate elevated arrhythmia risk
- •Consider 24-h ECG screening in horses with increased heart girth or when arrhythmias are suspected, but understand that frequent arrhythmias in resting horses are often benign
- •Cardiac screening protocols for racehorses should include assessment of QT interval and restitution ratio changes during exercise, particularly at maximal effort, as these may identify high-risk individuals before sudden death occurs
- •Training programs should incorporate baseline cardiac restitution assessments to stratify arrhythmia risk and potentially prevent fatal cardiac events during racing or intense work
- •Veterinarians managing racehorses should monitor for QT interval abnormalities and restitution ratio complexity as non-invasive markers of sudden cardiac death risk
- •High prevalence of cardiac arrhythmias (56%) in event horses during cross-country competition suggests baseline ECG screening and cardiac monitoring may be warranted for competition horses
- •Upper-level event horses face substantially increased arrhythmia risk; veterinarians should consider pre-competition cardiac assessment for advanced competitors
- •Horses exhibiting arrhythmias at rest require careful monitoring during recovery periods post-competition, as resting arrhythmias predict continued arrhythmias during recovery
- •Investigate and manage lower airway inflammation in underperforming trotters, as it has both immediate and career-long impacts on racing success.
- •Upper airway problems and gastric ulcers may temporarily limit racing but don't necessarily predict permanent career damage if managed appropriately.
- •The role of exercise-induced pulmonary hemorrhage and cardiac arrhythmias in poor performance remains uncertain; assess these findings alongside other clinical signs rather than assuming causation.
- •Poor performance in Standardbreds is multifactorial; respiratory disease (asthma, EIPH, airway obstruction) and gastric ulcers account for the majority of subclinical causes in nonlame horses—exercise testing with endoscopy and BAL is valuable for diagnosis.
- •Multiple concurrent disorders are common; horses with elevated inflammatory markers on BAL or muscle enzymes often have additional pathology affecting fitness that may not be apparent on clinical exam alone.
- •Fitness variables measured on treadmill testing (lactate threshold, velocity at HR 200) are reliable indicators of performance impairment and can guide treatment priorities and return-to-racing decisions.
- •SCD occurs more frequently during training than racing, suggesting that exercise intensity alone may not be the primary trigger; training protocols should incorporate monitoring for early warning signs in young horses
- •SCD predominantly affects horses early in their racing careers (median 3.6 years), indicating screening and cardiac evaluation should be prioritized before race entry
- •Entire males appear to have some protective effect against SCD; sex-specific risk stratification may be warranted in pre-race cardiac assessments
- •Cardiac arrhythmias are highly prevalent in poorly performing Standardbreds; baseline electrocardiographic screening during exercise testing should be considered part of performance evaluation protocols
- •The relationship between arrhythmia occurrence and poor athletic performance remains unclear—further investigation is needed before attributing performance issues solely to detected arrhythmias
- •Age and fitness level (indicated by lactate levels) may influence arrhythmia presentation, suggesting individualized assessment strategies based on horse demographics and conditioning status
- •Cardiac arrhythmias are very common in poorly performing trotters (77-78%) but do not appear to be causally related to upper airway obstruction, suggesting other etiologies should be investigated
- •Recovery period arrhythmias (particularly in the first 2 minutes) may be more clinically significant than exercise-induced arrhythmias; monitor peak exercise heart rate as a potential predictor
- •Standard categorization of arrhythmias as supraventricular versus ventricular may be unreliable in horses; descriptive reporting of arrhythmia characteristics is more useful clinically
- •Smartphone ECG can reliably screen for cardiac arrhythmias in horses at the stall, improving point-of-care diagnostics without requiring expensive equipment
- •Always obtain tracings from both sides of the thorax to maximize diagnostic quality—right side for P wave analysis, left side for QRS/QT measurement
- •This tool is most valuable for identifying horses with arrhythmias that warrant further veterinary investigation, though it cannot replace full diagnostic ECG for measurement accuracy
- •2D speckle tracking offers a diagnostic tool to monitor horses with aortic regurgitation and detect early signs of cardiac dysfunction before clinical decompensation occurs
- •Regular echocardiographic assessment using this technique can help determine when affected horses should be retired or have activity restricted
- •Early detection of myocardial dysfunction changes may improve management decisions and outcomes for horses with AR
- •Use the 4-electrode system when performing exercise ECG tests to minimize motion artifacts and obtain cleaner diagnostic tracings during work
- •Use the 7-electrode system for extended resting ECG recordings when monitoring for intermittent arrhythmias over 24 hours
- •Method selection should depend on clinical context: exercise-induced arrhythmia assessment versus detection of resting or paroxysmal arrhythmias
- •ECG interpretation in horses now has evidence-based threshold criteria for distinguishing normal rhythm variation from clinically significant arrhythmias, improving diagnostic accuracy
- •The two-step algorithm provides a standardized, reproducible method for analysing equine ECG recordings, reducing subjective interpretation errors
- •Understanding normal cardiac beat-to-beat variation in horses helps clinicians better identify premature beats and other arrhythmias that warrant intervention
- •Track veterinarians should consider implementing pre-race and post-race cardiac screening protocols, particularly for older horses, as a component of fitness and safety assessment for racehorses
- •Poor racing performance warrants cardiac evaluation, especially given the 1.3-2.0% frequency of atrial fibrillation in underperforming horses, which may indicate underlying cardiac pathology affecting performance
- •Baseline prevalence and frequency data provided can serve as a reference standard for monitoring trends in cardiac arrhythmias and evaluating the effectiveness of track safety and conditioning programs
- •Horses with atrial arrhythmias may have identifiable anatomical substrate in the vena cava that could guide catheter-based intervention strategies
- •Recognition of the caudal vena cava as an arrhythmogenic predilection site could improve diagnostic and therapeutic approaches to equine cardiac arrhythmias
- •Advanced electro-anatomical mapping techniques may help identify which arrhythmia cases are amenable to interventional procedures versus medical management
- •3D EAM-guided RFCA offers a novel interventional approach for treating equine atrial arrhythmias that may be refractory to medical management
- •This technique requires specialized equipment and expertise; referral to tertiary centres with catheterization capabilities is necessary for appropriate candidate selection
- •Successful isolation of myocardial sleeves in pulmonary and caval veins may improve outcomes in horses with atrial fibrillation or tachycardia
- •Echocardiography-guided catheterization may enable more equine veterinarians to perform diagnostic electrophysiological studies without expensive 3D mapping equipment
- •This feasibility work could improve diagnosis and treatment planning for horses with problematic arrhythmias
- •Minimally invasive catheterization guidance represents a practical advancement for equine cardiac specialist practice
- •Vectorcardiography may provide equine practitioners with a more reliable diagnostic tool than standard 12-lead ECG for identifying the specific site of origin of ventricular arrhythmias, potentially enabling more targeted therapeutic interventions
- •This technique could improve the diagnostic workup and prognosis assessment of horses presenting with clinically significant ventricular ectopy by pinpointing anatomical origin
- •Results suggest the equine heart's conduction system requires species-specific diagnostic approaches; VCG characteristics cannot be directly extrapolated from human cardiology protocols
- •ILRs offer a solution for diagnosing horses with suspected arrhythmias that are difficult to detect during routine clinic visits or standard ECG examinations
- •Continuous monitoring with ILRs can help establish definitive diagnoses in performance horses with exercise intolerance or unexplained poor performance related to cardiac issues
- •This technology may improve clinical decision-making regarding treatment options and prognosis for horses with suspected atrial fibrillation
- •Catheter-based electroanatomical mapping is now feasible in standing horses, offering a diagnostic tool to identify arrhythmia mechanisms without general anesthesia
- •This technique could guide ablation strategies and characterize electrical substrates in horses with cardiac arrhythmias, potentially improving treatment outcomes
- •Further development is needed for left atrial mapping and application in larger horse breeds before widespread clinical adoption
- •This advanced mapping technology is now feasible for clinical and research applications in horses with complex arrhythmias, offering detailed electrophysiological characterization beyond conventional methods
- •Veterinary cardiologists working with horses experiencing unexplained arrhythmias may have access to a sophisticated diagnostic tool that was previously unavailable in equine medicine
- •The technique requires specialized equipment and general anaesthesia, limiting its application to referral centers and research institutions
- •Sotalol may be a viable oral option for managing chronic equine arrhythmias, with intermediate bioavailability and a convenient 15-hour half-life supporting once or twice daily dosing protocols
- •The significant QT prolongation effect indicates sotalol works as expected in horses, but baseline and periodic ECG monitoring would be prudent to assess individual response and safety
- •Single-dose tolerance appears good in healthy horses, but this study was limited to acute administration in a small sample—clinical efficacy and long-term safety data for chronic use in arrhythmic horses are still needed
- •Oesophageal ECG is a practical, feasible technique for equine practitioners to better visualize P waves and diagnose supraventricular arrhythmias that may be missed on standard surface ECG
- •When performing oesophageal ECG, position the electrode at withers height plus 10 cm with 10 cm spacing between electrodes for optimal P wave visualization
- •Consider this technique for horses with suspected arrhythmias where surface ECG recordings show unclear or small P waves, and for potential future cardiac pacing or electrophysiological interventions
- •Cardiac dysfunction should be considered a potential complication in horses presenting with piroplasmosis; monitor for arrhythmias and elevation of cardiac biomarkers
- •Horses with babesiosis showing clinical signs of fever, icterus, depression, and brown urine warrant cardiac evaluation including ECG and troponin I assessment
- •Early recognition and treatment of piroplasmosis can prevent or resolve myocardial involvement and associated arrhythmias
- •When selecting ECG recording equipment for your practice, ensure the device has been validated for compatibility with non-linear analysis software if you plan to use AI-based diagnostic tools
- •Results from ECG complexity analysis may not be directly comparable between different recording devices until standardization protocols are established
- •Stay informed about emerging ECG technology validation studies as the field develops AI-assisted diagnostic capabilities for equine cardiac conditions
- •Adoption of the Delta configuration electrode positioning could standardize ECG recordings across equine practices, improving consistency and comparability of arrhythmia diagnoses
- •The new configuration may enhance early detection and characterization of cardiac arrhythmias by providing larger signal amplitudes and better spatial information about electrical activity
- •Implementing this standardized electrode placement in your practice prepares for future multicentre studies and advanced diagnostic techniques like vectorcardiography
- •VCG analysis of P wave characteristics may enable practitioners to identify the anatomical origin of atrial arrhythmias in horses without invasive mapping procedures
- •This exploratory work suggests a non-invasive diagnostic tool could help guide targeted management decisions for horses with atrial premature depolarizations
- •Further validation studies are needed before clinical implementation, but this technique adapts established human cardiology methods to equine practice
- •Current standard ECG protocols may produce misleading results due to inappropriate lead placement; consider alternative lead positioning strategies based on electrophysiological principles
- •High variability in ECG configurations from traditional guidelines means abnormalities in individual horses may be difficult to interpret without breed or individual baseline comparisons
- •Adoption of improved lead placement techniques could standardize equine ECGs and make them more clinically useful for detecting genuine cardiac pathology
- •This anatomical reference data is essential for veterinarians developing catheter-based electrophysiological interventions and ablation procedures in horses with arrhythmias
- •Expect anatomical variation based on horse size and breed; these measurements apply specifically to Warmblood horses and may not translate directly to other breeds
- •Advanced imaging techniques will be needed to visualize these structures in living horses before attempting minimally invasive intracardiac procedures clinically
- •HRV analysis offers a non-invasive diagnostic tool to help identify cardiac arrhythmias in horses, potentially improving early detection of conditions like atrial fibrillation
- •Proper ECG data collection protocols and understanding of HRV measurement variables are essential for reliable results in clinical practice
- •Advanced HRV analysis using machine learning may enhance diagnostic accuracy for equine arrhythmias, particularly during exercise when some arrhythmias are more apparent
- •Refer horses with refractory atrial fibrillation to specialists equipped for transvenous electrical cardioversion, as this catheter-based approach offers superior outcomes compared to medical management alone
- •Echocardiography-guided catheter procedures are feasible in equine practice and represent the most practical imaging solution for interventional cardiology in horses
- •Advanced electrophysiological mapping and ablation techniques are now available for equine arrhythmias and should be considered for animals with atrial tachycardia or other complex arrhythmias
- •This minimally invasive endomyocardial biopsy technique offers a safe diagnostic option for investigating suspected myocardial disease in standing horses, which may be underdiagnosed due to lack of specific diagnostic tools
- •Expect transient arrhythmias during and immediately after the procedure; monitor with continuous ECG and allow time for resolution before assuming complications
- •The technique is currently validated for research and diagnostic purposes in healthy horses, but safety in clinical patients with active myocardial disease requires further investigation
Key Research Findings
Supra-physiologic levothyroxine at 0.25 mg/kg resulted in significantly higher heart rates during standardized exercise testing compared to placebo (239±9 vs 219±6 bpm at 10 m/s)
Three of six horses (50%) receiving high-dose levothyroxine developed cardiac arrhythmias including atrial fibrillation
Levothyroxine supplementation decreased V200 (velocity at 200 bpm heart rate) during exercise testing, contrary to intended performance benefits
Survey of 50 racehorses showed median T4 of 2.00 µg/dL within normal range with only 3 horses (6%) exceeding laboratory reference limits
Oral sotalol at 2, 3, and 4 mg/kg twice daily achieved steady-state plasma concentrations of 287, 409, and 543 ng/mL respectively in unfasted horses
Sotalol significantly increased QT interval and effective refractory period (ERP) in right atrial and ventricular tissue
Higher dosages did not produce progressive increases in QT interval or ERP despite higher plasma concentrations, suggesting a plateau effect
No significant side effects were observed except transient local sweating, making sotalol a potential long-term anti-arrhythmic option for equine use
92.6% of clinically healthy sedentary horses experienced at least one arrhythmia during 24-h ambulatory ECG recording
Supraventricular premature complexes were detected in 86.2% of horses and ventricular complexes in 24.5%
38.3% of horses met criteria for being affected by arrhythmias (>1 SVPC/h or any VPC)
Increased heart girth score was significantly associated with arrhythmia detection (OR 1.06, p=0.05)
QT interval and cardiac restitution ratio (QT/TQ) show complex changes from rest to maximal effort in Standardbred racehorses
Cardiac restitution ratio complexity may serve as a novel indicator of arrhythmia risk during maximal intensity exercise
QT interval dynamics during exercise have not been previously characterized in horses and may explain predisposition to sudden cardiac death in racehorses
Evidence Base
A randomised, controlled trial to determine the effect of levothyroxine on Standardbred racehorses.
Kritchevsky Janice, Olave Carla, Tinkler Stacy et al. (2022) — Equine veterinary journal
RCT
Pharmacokinetics and electrophysiological effects of sotalol hydrochloride in horses.
Broux B, De Clercq D, Decloedt A et al. (2018) — Equine veterinary journal
RCT
Cardiac arrhythmia prevalence and risk factors in 24-h electrocardiograms of sedentary horses.
Maas Lauren T, Louie Elizabeth Williams, Finno Carrie J et al. (2025) — Equine veterinary journal
Cohort Study
QT Interval and Cardiac Restitution Ratio Complexity in Standardbred Racehorses From Rest to Maximal Effort: Insights Into Arrhythmia Risk.
Avison Amanda, Goderre Beverley G, Pyle W Glen et al. (2025) — Journal of veterinary internal medicine
Cohort Study
Premature depolarisations in horses competing in United States Eventing Association and Fédération Equestre Internationale-sanctioned 3-day events.
Durando Mary M, Slack Joann, Birks Eric et al. (2024) — Equine veterinary journal
Cohort Study
Associations between Medical Disorders and Racing Outcomes in Poorly Performing Standardbred Trotter Racehorses: A Retrospective Study.
Lo Feudo Chiara Maria, Stucchi Luca, Stancari Giovanni et al. (2023) — Animals : an open access journal from MDPI
Cohort Study
Medical causes of poor performance and their associations with fitness in Standardbred racehorses.
Lo Feudo Chiara M, Stucchi Luca, Conturba Bianca et al. (2023) — Journal of veterinary internal medicine
Cohort Study
Risk Factors for Exercise-Associated Sudden Cardiac Death in Thoroughbred Racehorses.
Nath Laura, Stent Andrew, Elliott Adrian et al. (2022) — Animals : an open access journal from MDPI
Cohort Study
Evaluation of Cardiac Arrhythmias before, during, and after Treadmill Exercise Testing in Poorly Performing Standardbred Racehorses.
Alberti Elena, Stucchi Luca, Lo Feudo Chiara Maria et al. (2021) — Animals : an open access journal from MDPI
Cohort Study
Cardiac arrhythmias in poorly performing Standardbred and Norwegian-Swedish Coldblooded trotters undergoing high-speed treadmill testing.
Slack J, Stefanovski D, Madsen T F et al. (2021) — Veterinary journal (London, England : 1997)
Cohort Study
Stall-side screening potential of a smartphone electrocardiogram recorded over both sides of the thorax in horses.
Corradini Ignacio, Fernández-Ruiz Alicia, Barba Marta et al. (2020) — Journal of veterinary internal medicine
Cohort Study
Assessment of left ventricular function in horses with aortic regurgitation by 2D speckle tracking.
Decloedt A, Ven S, De Clercq D et al. (2020) — BMC veterinary research
Cohort Study
Two Methods for 24-hour Holter Monitoring in Horses: Evaluation of Recording Performance at Rest and During Exercise.
Vezzosi Tommaso, Vitale Valentina, Sgorbini Micaela et al. (2019) — Journal of equine veterinary science
Cohort Study
Appropriate threshold levels of cardiac beat-to-beat variation in semi-automatic analysis of equine ECG recordings.
Flethøj Mette, Kanters Jørgen K, Pedersen Philip J et al. (2016) — BMC veterinary research
Cohort Study
Occurrence of cardiac arrhythmias in Standardbred racehorses.
Slack J, Boston R C, Soma L R et al. (2015) — Equine veterinary journal
Cohort Study
Morphological evidence of a potential arrhythmogenic substrate in the caudal and cranial vena cava in horses.
Ibrahim Lara, Buschmann Eva, van Loon Gunther et al. (2025) — Equine veterinary journal
Case Report
Successful caudal vena cava and pulmonary vein isolation in healthy horses using 3D electro-anatomical mapping and a contact force-guided ablation system.
Buschmann Eva, Van Steenkiste Glenn, Duytschaever Mattias et al. (2024) — Equine veterinary journal
Case Report
Feasibility of transthoracic echocardiographic guidance for multicatheter electrophysiological mapping studies in horses.
Vernemmen Ingrid, Buschmann Eva, Demeyere Marie et al. (2024) — Journal of veterinary internal medicine
Case Report
An Exploratory Study on Vectorcardiographic Identification of the Site of Origin of Focally Induced Premature Depolarizations in Horses, Part II: The Ventricles.
Van Steenkiste Glenn, Delhaas Tammo, Hermans Ben et al. (2022) — Animals : an open access journal from MDPI
Case Report
Detection of atrial fibrillation with implantable loop recorders in horses.
Buhl Rikke, Hesselkilde Eva M, Carstensen Helena et al. (2021) — Equine veterinary journal
Case Report
Show 13 more references
First catheter-based high-density endocardial 3D electroanatomical mapping of the right atrium in standing horses.
Hesselkilde Eva, Linz Dominik, Saljic Arnela et al. (2021) — Equine veterinary journal
Case Report
Three dimensional ultra-high-density electro-anatomical cardiac mapping in horses: methodology.
Van Steenkiste Glenn, De Clercq Dominique, Boussy Tim et al. (2020) — Equine veterinary journal
Case Report
Pharmacokinetics of intravenously and orally administered sotalol hydrochloride in horses and effects on surface electrocardiogram and left ventricular systolic function.
Broux B, De Clercq D, Decloedt A et al. (2016) — Veterinary journal (London, England : 1997)
Case Report
Oesophageal electrocardiography in healthy horses.
Verheyen T, Decloedt A, De Clercq D et al. (2012) — Equine veterinary journal
Case Report
Cardiac arrhythmias associated with piroplasmosis in the horse: a case report.
Diana Alessia, Guglielmini Carlo, Candini Daniela et al. (2007) — Veterinary journal (London, England : 1997)
Case Report
Agreement of the performance of equine electrocardiogram recording devices for ECG complexity analysis.
Alexeenko Vadim, Anchan Dhruvpal Singh, Ter Woort Fe et al. (2025) — Equine veterinary journal
Expert Opinion
Einthoven's triangle adapted for horses: Proposal for the Delta configuration.
Paulussen Ellen, Van Steenkiste Glenn, Hermans Ben J M et al. (2024) — Journal of veterinary internal medicine
Expert Opinion
An Exploratory Study on Vectorcardiographic Identification of the Site of Origin of Focally Induced Premature Depolarizations in Horses, Part I: The Atria.
Van Steenkiste Glenn, Delhaas Tammo, Hermans Ben et al. (2022) — Animals : an open access journal from MDPI
Expert Opinion
A novel approach for obtaining 12-lead electrocardiograms in horses.
Hesselkilde Eva M, Isaksen Jonas L, Petersen Bettina V et al. (2021) — Journal of veterinary internal medicine
Expert Opinion
Right atrial-related structures in horses of interest during electrophysiological studies.
Vernemmen Ingrid, Vera Lisse, Van Steenkiste Glenn et al. (2021) — Equine veterinary journal
Expert Opinion
Heart rate variability analysis in horses for the diagnosis of arrhythmias.
Mitchell Katharyn J, Schwarzwald Colin C (2021) — Veterinary journal (London, England : 1997)
Expert Opinion
Catheter-based electrical interventions to study, diagnose and treat arrhythmias in horses: From refractory period to electro-anatomical mapping.
van Loon Gunther, Van Steenkiste Glenn, Vera Lisse et al. (2020) — Veterinary journal (London, England : 1997)
Expert Opinion
Right atrial and right ventricular ultrasound-guided biopsy technique in standing horses.
Decloedt A, de Clercq D, Ven S et al. (2016) — Equine veterinary journal
Expert Opinion