Diagnosis and treatment of atrial fibrillation

Breed predisposition and heritability of atrial fibrillation in the Standardbred horse: a retrospective case-control study

OBJECTIVES

To assess evidence for genetic contributions to atrial fibrillation (AF) in the Standardbred horse.

ANIMALS

Equine referrals to the Ontario Veterinary College Health Sciences Centre (OVCHSC) for 1985-2009, and age and gait matched breed registry controls.

METHODS

Breeds presenting ≥ 5 times annually were tabulated (admission year and diagnosis; total 40,039; AF 396; no AF 39,643), and breed and year effects examined. Heritability and inbreeding coefficients were determined for Standardbred AF cases and racing contemporaries, and odds ratios for AF were calculated for frequently occurring sires.

RESULTS

Year and breed effects on diagnosis were highly significant (Chi-Square 212.85, p < 0.0001, and 304.25, p < 0.0001, respectively). Year effect on diagnosis by breed was significant from 1997, and due to Standardbred admissions each year. Quarterhorses were significantly less likely to present with AF (OR 0.0578-0.6048), Standardbreds were more likely (OR 4.3874-10.9006). Heritability of AF on the underlying scale (h²(u)) was estimated at 29.6 ± 3.9% and on the observed binomial scale (h²(o)), at 9.6%. For horses born in 1994 or later, h²(u) was 31.1 ± 4.3% and h²(o), 10.1%. Of 22 first generation sires appearing ≥ 10 times in the case/control file, seven pacing and one trotting sire produced affected horses more frequently than expected (OR 2.66-66.32). Inbreeding was not a factor.

CONCLUSIONS

There is genetic liability to AF in Standardbred horses, likely due to more than single genes with simple Mendelian inheritance. Genomic studies are required.

Normal electrocardiographic QT interval in race-fit Standardbred horses at rest and its rate dependence during exercise

OBJECTIVES

Cardiac repolarization, measured as QT and Tpeak to Tend (TpTe) intervals on the ECG, is important, as irregularities caused by diseases, ventricular hypertrophy, drugs and genetic defects can trigger arrhythmias which predispose human patients to syncope and sudden cardiac death. In horses, repolarization is not well described and therefore QT analysis cannot yet be used diagnostically. Therefore, we sought to describe reference values for the normal QT and TpTe intervals in Standardbreds and to determine the best method for heart rate (HR) correction.

ANIMALS

30 Standardbreds.

METHODS

QT and TpTe intervals were measured during rest and exercise and plotted against HR converted to Rpeak to Rpeak interval (RR). Data were fitted with relevant regression models. Intra- and inter-observer agreement was assessed using Bland-Altman analyses.

RESULTS

Data were best described by a piecewise linear model (r(2) > 0.97). Average prediction error of this model was smaller than for both Bazett and Fridericia corrections. Coefficient of repeatability of intra- and inter-observer variability was 8.76 ms and 5.64 ms respectively and coefficient of variation was 1.77% and 2.76% respectively. TpTe increased with RR in stallions.

CONCLUSIONS

The QT interval in Standardbred horses shortens with decreasing RR interval (increasing HR) as in humans, but in a markedly different order as it clearly follows a piecewise linear model. The equine QT interval can be measured easily and there is small intra- and inter-observer variability. This model of the equine QT interval provides clinicians with a method that could support a diagnosis of repolarization disturbances in horses.

Ventricular response during lungeing exercise in horses with lone atrial fibrillation

REASONS FOR PERFORMING THE STUDY

Atrial fibrillation (AF) is the most important dysrhythmia affecting performance in horses and has been associated with incoordination, collapse and sudden death. Limited information is available on ventricular response during exercise in horses with lone AF.

OBJECTIVES

To investigate ventricular response in horses with lone AF during a standardised lungeing exercise test.

METHODS

A modified base-apex electrocardiogram was recorded at rest and during a standardised lungeing exercise test from 43 horses diagnosed with lone AF. During the test horses walked for 7 min, trotted for 10 min, cantered for 4 min, galloped for 1 min and recovered for 7 min.

RESULTS

Individual average heart rate during walk ranged from 42 to 175 beats/min, during trot from 89 to 207 beats/min, during canter from 141 to 269 beats/min, and during gallop from 191 to 311 beats/min. Individual beat-to-beat maximal heart rate ranged from 248 to 492 beats/min. Ventricular premature depolarisations were present in 81% of the horses: at rest (16%), during exercise (69%), and during recovery (2%). In 33% of the horses, broad QRS complexes with R-on-T morphology were found.

CONCLUSIONS

Exercising horses with lone AF frequently develop disproportionate tachycardia. In addition, QRS broadening and even R-on-T morphology is frequently found. QRS broadening may originate from ventricular ectopic foci or from aberrant intraventricular conduction, for example due to bundle branch block. This might explain the high number of complexes currently classified as ventricular premature depolarisations.

POTENTIAL RELEVANCE

Prevalence of QRS broadening and especially R-on-T was very high in horses with AF and was found at low levels of exercise. These dysrhythmias are considered risk factors for the development of ventricular tachycardia and fibrillation and they might explain signs of weakness, collapse or sudden death that have been reported in horses with AF.

Use of intravenous flecainide in horses with naturally-occurring atrial fibrillation

REASONS FOR PERFORMING STUDY

It has been reported that i.v. flecainide has a high efficacy for the treatment of experimentally-induced acute atrial fibrillation (AF) in horses and that its use is associated with minimal toxic side effects.

OBJECTIVES

The objectives were to study the efficacy of i.v. flecainide as a treatment for atrial fibrillation in horses with naturally-occurring AF.

METHODS

Ten horses with naturally-occurring AF were treated with 2 mg/kg bwt flecainide i.v. at a rate of 0.2 mg/kg bwt/min. In 3 horses, the infusion was continued at 0.05-0.10 mg/kg bwt/min until a total dose of 3.0 mg/kg bwt had been administered. Heart rate, QRS duration and average interval between fibrillation waves were measured before, during and following flecainide infusion. If conversion to normal sinus rhythm was not achieved, horses were treated with quinidine sulphate per os at a dose of 22 mg/kg bwt given every 2 h.

RESULTS

None of the horses with chronic AF (n = 9) converted to sinus rhythm with flecainide i.v. The only horse treated successfully had acute AF of 12 days' duration. The QRS duration and fibrillation cycle length increased significantly (P = 0.006 and 0.002, respectively) during and following flecainide infusion. Heart rate did not increase significantly over time however, 3 horses developed heart rates in excess of 100 beats/min. Two horses developed a potentially dangerous ventricular dysrhythmia during the first 15 mins of treatment. Quinidine sulphate given per os restored sinus rhythm in 8 out of 9 horses, with minimal adverse effects.

CONCLUSIONS

Although flecainide might be efficacious in cases of acute AF, it was not possible to restore sinus rhythm in horses with naturally-occurring chronic AF at the dosages used in this study. In 2 horses, 2.0 mg/kg bwt flecainide was associated with potentially dangerous dysrhythmias.

POTENTIAL CLINICAL RELEVANCE

Intravenous administration of 2 mg/kg bwt flecainide is unlikely to convert chronic AF in horses and could induce dangerous dysrhythmias.