Atrial Fibrillation: What the Research Says

Evidence from 37 peer-reviewed studies

16 Cohort Study

9 Case Report

12 Expert Opinion

What Professionals Should Know

•Quinidine dosing should be individualized based on population pharmacokinetic parameters rather than using one-size-fits-all regimens to improve treatment outcomes and safety
•Understanding inter-individual variability in drug metabolism helps predict which horses may require dose adjustments to avoid therapeutic failure or toxicity
•This pharmacokinetic approach provides a scientific foundation for optimizing quinidine therapy in racehorses with atrial fibrillation
•Postrace endoscopic evaluation showing high-grade EIPH should raise suspicion for underlying AF, as the association is 7.9 times stronger in AF horses
•AF presents suddenly in previously earning racehorses; declining race performance with poor finishing position may warrant cardiac evaluation regardless of prior earnings history
•Airway disease management may be relevant to AF prevention or management, given the demonstrated association between EIPH grades and AF occurrence
•TVEC offers high success rates for AF in horses, but consider echocardiographic findings (especially mitral regurgitation) when predicting outcomes and counseling owners on prognosis
•Stallions with AF require closer monitoring post-cardioversion due to significantly elevated recurrence risk
•Procedural factors like catheter type and body weight influence energy requirements—heavier horses and those requiring certain catheter types may need more energy delivery
•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
•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
•Breeders of Standardbreds should be aware that certain sire lines carry significantly elevated risk for atrial fibrillation and should consider genetic screening and pedigree analysis in breeding decisions
•Veterinarians managing Standardbred racehorses should counsel owners on familial risk factors; horses from high-risk pedigrees warrant proactive cardiac monitoring
•While heritability is modest, the identification of specific ancestral contributors suggests that selective breeding away from affected lineages could reduce AF incidence in the population over time
•Heart rate monitors with automated RMSSD calculation could be used as practical field tools to monitor for recurrence of atrial fibrillation in treated horses
•Ensure your HRM software includes appropriate artifact correction algorithms, as different filters significantly affect the reliability of RMSSD values for AF detection
•RMSSD can be reliably assessed across multiple gaits (rest, walk, trot), giving flexibility in when and how you monitor at-risk horses
•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
•Atrial fibrillation appears to have a heritable genetic component in Standardbred racehorses, warranting consideration in breeding decisions
•Breeders should be aware that certain sire lines may carry increased risk for AF and should monitor pedigree patterns
•Veterinarians treating Standardbred racehorses should maintain awareness of rising AF prevalence and potential genetic factors in client communication
•Detomidine sedation facilitates noninvasive measurement of atrial fibrillation cycle length via tissue Doppler imaging, making it a useful clinical tool for equine AF assessment without requiring invasive electrogram recordings
•The +4 ms increase in AFCL with sedation is clinically negligible and should not affect interpretation of results when monitoring AF progression or drug effects
•Noninvasive AFCL measurement can be used in practice to track atrial electrical remodelling and evaluate responses to anti-arrhythmic therapy in horses with AF
•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
•Monitor older male horses and heavy-bodied horses more closely for cardiac disease development, particularly atrial fibrillation in heavier individuals
•Racehorses of middle age warrant targeted cardiac screening for tricuspid regurgitation; mitral regurgitation patients should be assessed for secondary complications including arrhythmias and heart failure
•Establish breed and age-specific cardiac monitoring protocols in sport and performance horses to identify predisposing factors early
•TVEC is a highly effective treatment option for lone atrial fibrillation in horses, with nearly 99% success rate in this referral population
•Sex and age of the horse influence the energy levels needed for successful cardioversion, which should inform pre-treatment planning and anesthetic management
•Chronicity of atrial fibrillation does not predict treatment success, so delayed presentation should not discourage treatment attempts
•Diltiazem can be used effectively to control ventricular response rate during rapid atrial pacing in horses receiving quinidine, offering a practical approach to managing atrial arrhythmias
•Effective diltiazem dosing in horses requires titration (0.125-1.125 mg/kg IV) and monitoring for blood pressure effects, but side effects were well-tolerated in this study
•Electrophysiologic testing is feasible in standing horses and can guide selection and optimization of antiarrhythmic therapy for atrial fibrillation
•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
•For horses with suspected paroxysmal atrial fibrillation affecting performance but negative surface ECG findings, implantable loop recorders provide a diagnostic solution to detect intermittent arrhythmias
•ILR technology allows definitive diagnosis without requiring the arrhythmia to be present during examination, improving case resolution in athletic horses with unexplained performance issues
•Consider ILR placement when paroxysmal AF is suspected but conventional ECG cannot capture the arrhythmia during clinical evaluation
•Consider rate control and cardioversion with IV antiarrhythmic drugs (metoprolol and/or quinidine) as treatment options for persistent lone atrial fibrillation in neonatal foals, even without structural heart disease
•Monitor neonatal foals presenting with irregular tachycardia and nursing difficulties, as AF may be the underlying cause and is treatable
•Successful cardioversion can result in sustained normal sinus rhythm and marked clinical improvement in demeanor and nursing ability
•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
•Oral torsemide may offer a viable alternative to intravenous furosemide for managing equine CHF, with apparent clinical benefit demonstrated in this case
•High-dose torsemide should be reserved for cases unresponsive to lower doses and requires close monitoring of electrolyte panels and renal function throughout treatment
•Expect electrolyte disturbances (hypokalemia particularly) with high-dose loop diuretic therapy and plan supplementation or dietary adjustments accordingly
•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
•Consider adding digoxin to quinidine sulfate treatment for atrial fibrillation, as combination therapy significantly improved conversion rates to normal rhythm
•Expect approximately 80% success rate with quinidine-based treatment in equine AF cases, even when exact duration of arrhythmia is unknown
•Breed does not appear to influence treatment response to digoxin and quinidine combination therapy for atrial fibrillation
•Veterinarians can now noninvasively assess atrial fibrillation severity and electrical remodelling using tissue Doppler ultrasound, eliminating need for invasive cardiac catheterization in many cases
•Atrial fibrillation cycle length measurement via ultrasound allows better monitoring of drug efficacy and disease progression in horses undergoing treatment for AF
•The wide limits of agreement (-18 to +9 ms) between methods suggest tissue Doppler can be useful for monitoring but should be used cautiously for precise absolute measurements in clinical decision-making
•Transvenous electrical cardioversion for atrial fibrillation in horses causes a measurable but clinically insignificant rise in cardiac troponin I
•The procedure does not appear to cause substantial cardiomyocyte damage regardless of shock parameters used
•Longer convalescent periods may be warranted post-cardioversion due to potential persistent atrial dysfunction despite troponin elevation
•When performing electrical cardioversion for atrial fibrillation in horses, have temporary pacing capability available as atrioventricular conduction disturbances may occur post-shock
•Be prepared for transient but significant bradycardia following defibrillation attempts; temporary pacing support may be necessary while waiting for spontaneous recovery
•This complication highlights the risks of transvenous electrical cardioversion in horses and suggests careful case selection and preparation are essential
•Atrial fibrillation may increase risk of exercise-induced pulmonary haemorrhage; affected horses show reduced exercise tolerance and visible tracheal bleeding after exertion
•AF-positive horses demonstrate impaired aerobic performance with lower maximum velocities and disproportionately elevated heart rates, suggesting compromised cardiac output during exercise
•Consider cardiac screening before intensive exercise programmes, particularly in untrained or performance horses, as AF may predispose to EIPH
•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
•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.
•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
•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
•Fifth rib resection provides reliable access to the entire circumference of the heart in horses and can be completed within 45 minutes, making this approach suitable for cardiac emergencies and experimental procedures
•Expect a significant drop in PaO2 during thoracotomy even with 100% oxygen and positive pressure ventilation; this is a normal physiologic response and does not indicate inadequate anesthesia management
•This surgical approach enables complete visualization of the heart for procedures addressing pericarditis, thoracic trauma, and other cardiac pathologies in cases where medical management has failed
•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
•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
•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
•Early conversion of atrial fibrillation in horses is critical, as electrical and contractile remodeling begins within hours and can progress to persistent AF within 7 days
•The rapid reversibility of AF-induced remodeling within 48 hours of successful conversion suggests early intervention can prevent permanent structural damage and preserve athletic function
•Early return to training may be feasible after AF conversion given the demonstrated complete recovery of atrial and ventricular function within 2 days
•Horses recovering from atrial fibrillation conversion may experience temporary left atrial dysfunction for 24-72 hours; echocardiographic monitoring can assess recovery trajectory
•Both pharmacological (quinidine) and electrical cardioversion can be effective in horses, with similar post-conversion mechanical outcomes
•Most horses show spontaneous improvement in atrial function within 72 hours of successful cardioversion, suggesting close monitoring rather than intervention may be appropriate during early recovery
•TVEC is a viable alternative treatment for equine atrial fibrillation in cases where horses cannot tolerate or do not respond to quinidine therapy
•The procedure requires right jugular vein catheterization under general anesthesia with imaging-guided electrode placement—collaborate closely with your referral center's cardiology team
•No additional antiarrhythmic medications are needed during the procedure, reducing medication-related adverse effects seen with conventional treatment

Key Research Findings

Population pharmacokinetic analysis identified substantial inter-individual variability in quinidine plasma concentrations in Thoroughbred racehorses with atrial fibrillation

Kuroda Taisuke, 2024

Rational dosage regimen based on population pharmacokinetics developed to optimize quinidine efficacy while minimizing adverse effects

Kuroda Taisuke, 2024

Study addresses the wide variability in treatment response to quinidine sulfate in equine atrial fibrillation cases

Kuroda Taisuke, 2024

Horses with postrace AF finished substantially further behind winners (OR 1.41 per length) compared to horses performing to expectation

Nath Laura C, 2023

AF horses had significantly higher grades of exercise-induced pulmonary hemorrhage (21.1% with grade ≥3) versus poor performers without AF (3.3%), suggesting a mechanistic relationship

Nath Laura C, 2023

Career earnings before the AF event were not inferior in AF horses despite acute performance impairment, indicating AF develops in established performers

Nath Laura C, 2023

Exercise volume did not promote AF development, contradicting the high-dose exercise hypothesis observed in human populations

Nath Laura C, 2023

Transvenous electrical cardioversion achieved 94.4% success rate in 231 procedures across 199 horses with atrial fibrillation

Vernemmen Ingrid, 2022

Mitral regurgitation and longer AF cycle length were independent determinants of cardioversion success

Vernemmen Ingrid, 2022

31.9% of horses experienced recurrence after successful first cardioversion (51/160 cases)

Vernemmen Ingrid, 2022

Stallions had 3-fold higher recurrence risk compared to other sexes, with mitral regurgitation and longer AF duration also increasing recurrence risk

Vernemmen Ingrid, 2022

Atrial fibrillatory rate (AFR) derived from surface ECG is a biomarker for electrical remodelling in equine AF

Buhl Rikke, 2022

AFR has potential utility for predicting successful cardioversion and AF recurrence in horses

Buhl Rikke, 2022

AFR may help identify horses at higher risk of AF recurrence after treatment with medical therapy or electrical cardioversion

Buhl Rikke, 2022

Atrial fibrillation was identified in 4.9% of Thoroughbred racehorses with an overall incidence of 2.7 episodes per 1000 starts

Nath Laura C, 2021

Evidence Base

Rational quinidine dosage regimen for atrial fibrillation in Thoroughbred racehorses based on population pharmacokinetics.

Kuroda Taisuke, Minamijima Yohei, Kinman Christopher Ken et al. (2024) — Frontiers in veterinary science

Atrial Fibrillation: What the Research Says

What Professionals Should Know

Key Research Findings

Evidence Base

Rational quinidine dosage regimen for atrial fibrillation in Thoroughbred racehorses based on population pharmacokinetics.

Associations between postrace atrial fibrillation and measures of performance, racing history and airway disease in horses.

Transvenous electrical cardioversion of atrial fibrillation in horses: Horse and procedural factors correlated with success and recurrence.

Atrial fibrillatory rate as predictor of recurrence of atrial fibrillation in horses treated medically or with electrical cardioversion.

Incidence, recurrence, and outcome of postrace atrial fibrillation in Thoroughbred horses.

Stall-side screening potential of a smartphone electrocardiogram recorded over both sides of the thorax in horses.

Marginal ancestral contributions to atrial fibrillation in the Standardbred racehorse: Comparison of cases and controls.

Can heart rate variability parameters derived by a heart rate monitor differentiate between atrial fibrillation and sinus rhythm?

Heart rate variability parameters in horses distinguish atrial fibrillation from sinus rhythm before and after successful electrical cardioversion.

Estimates of heritability of atrial fibrillation in the Standardbred racehorse.

Influence of detomidine on atrial fibrillation cycle length measured by intracardiac electrogram recording and by colour tissue Doppler imaging in horses.

Occurrence of cardiac arrhythmias in Standardbred racehorses.

Prevalence and risk factors for cardiac diseases in a hospital-based population of 3,434 horses (1994-2011).

Transvenous electrical cardioversion of equine atrial fibrillation: patient factors and clinical results in 72 treatment episodes.

Atrial, SA nodal, and AV nodal electrophysiology in standing horses: normal findings and electrophysiologic effects of quinidine and diltiazem.

Effects of diltiazem on hemodynamic variables and ventricular function in healthy horses.

Validation and clinical application of implantable loop recorders for diagnosis of atrial fibrillation in horses.

Intravenous administration of quinidine and metoprolol for treatment of atrial fibrillation in 2 neonatal foals.

Successful caudal vena cava and pulmonary vein isolation in healthy horses using 3D electro-anatomical mapping and a contact force-guided ablation system.

Clinical effect of torsemide in a horse with congestive heart failure and atrial fibrillation.

Detection of atrial fibrillation with implantable loop recorders in horses.

Clinical Research Abstracts of the British Equine Veterinary Association Congress 2015.

Noninvasive determination of atrial fibrillation cycle length by atrial colour tissue Doppler imaging in horses.

Effect of transvenous electrical cardioversion on plasma cardiac troponin I concentrations in horses with atrial fibrillation.

Transient complete atrioventricular block following transvenous electrical cardioversion of atrial fibrillation in a horse.

Atrial fibrillation as a risk factor for exercise-induced pulmonary haemorrhage following a standardised exercise test.

Morphological and histological investigation of the conduction system in the equine atrial muscle sleeve of pulmonary veins.

Fundamentals of arrhythmogenic mechanisms and treatment strategies for equine atrial fibrillation.

Paroxysmal Atrial Fibrillation in Horses: Pathophysiology, Diagnostics and Clinical Aspects.

Heart rate variability analysis in horses for the diagnosis of arrhythmias.

Thoracotomy and Pericardiotomy for Access to the Heart in Horses: Surgical Procedure and Effects on Anesthetic Variables.

Atrial fibrillation in horses Part 2: Diagnosis, treatment and prognosis.

Catheter-based electrical interventions to study, diagnose and treat arrhythmias in horses: From refractory period to electro-anatomical mapping.

Atrial fibrillation in horses part 1: Pathophysiology.

Atrial and ventricular electrical and contractile remodeling and reverse remodeling owing to short-term pacing-induced atrial fibrillation in horses.

Echocardiographic evidence of left atrial mechanical dysfunction after conversion of atrial fibrillation to sinus rhythm in 5 horses.

Transvenous electrical cardioversion of equine atrial fibrillation: technical considerations.