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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • An irregularly irregular cardiac rhythm, with variable intensity heart sounds and pulses and inconsistent diastolic intervals
  • Can be paroxysmal (resolving spontaneously within 48 h of onset), persistent, or permanent

Pathophysiology!!navigator!!

  • A critical atrial mass must be present for AF to occur
  • Predisposing factors—large atrial mass, atrial remodeling and fibrosis, high vagal tone, shortened and nonhomogeneous effective refractory period, potassium depletion, atrial premature complexes, bradycardia, predisposing arrhythmias (atrial tachycardia or atrial flutter), and rapid atrial pacing
  • Produces no change in cardiac output at rest in the absence of significant underlying cardiac disease
  • During high-intensity exercise, produces a significant increase in exercising HR (often 40–60 bpm higher than when in NSR) and subsequent fall in cardiac output and exercise capacity
  • Present in many horses with CHF but is not the cause of CHF

Systems Affected!!navigator!!

Cardiovascular

Signalment!!navigator!!

Higher incidence in Standardbred, draft, and Warmblood horses.

Signs!!navigator!!

General Comments

Exercise intolerance in high performance animals; often an incidental finding in horses performing only light work.

Historical Findings

  • Exercise intolerance
  • Exercise-induced pulmonary hemorrhage—often profuse
  • Weakness or collapse

Physical Examination Findings

  • Irregularly irregular heart rhythm
  • Variable intensity heart sounds and arterial pulses
  • Absent fourth heart sound
  • Cardiac murmurs with predisposing cardiac disease

Causes!!navigator!!

  • Normal horses have sufficient atrial mass and high vagal tone to develop AF without evident underlying heart disease—“lone” AF
  • Diseases causing atrial enlargement further predispose horses to AF

Risk Factors!!navigator!!

  • AV valve insufficiency
  • CHF
  • Electrolyte disturbances

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Second-degree AV block—regular rhythm is interrupted by pauses containing fourth heart sound
  • Atrial tachycardia with second-degree AV block—rhythm usually is regularly irregular; fourth heart sounds are present
  • Atrial flutter—rhythm usually irregularly irregular; need ECG to differentiate
  • NSR with multifocal ventricular premature complexes—need ECG to differentiate

CBC/Biochemistry/Urinalysis!!navigator!!

Low plasma potassium or urinary fractional excretion of potassium may be present.

Other Laboratory Tests!!navigator!!

Elevated cardiac troponin I or cardiac troponin T possible but usually within the normal range.

Imaging!!navigator!!

ECG

  • No P waves, replaced by baseline “f” waves (Figure 1)
  • The “f” waves may be coarse or fine and may occur 300–500 times per minute
  • Irregular R–R interval
  • Some variation in the amplitude of QRS and T complexes usually is present, but these complexes are otherwise normal in appearance

Echocardiography

  • Many have little or no discernible underlying cardiac disease; therefore, the echocardiogram is normal
  • Some have low shortening fraction (24–32%). This should return to normal within several days of conversion to normal NSR
  • Mild left atrial enlargement occasionally occurs with permanent AF
  • Atrial enlargement due to AV valve insufficiency, underlying myocardial disease, or congenital defects may be present

Other Diagnostic Procedures!!navigator!!

Continuous 24 h Holter Monitoring

  • Use with suspected paroxysmal AF to identify underlying arrhythmias
  • Use with persistent AF to determine if other concurrent arrhythmias are present

Exercise ECG

Use to detect exercise-induced arrhythmias and conduction abnormalities and to determine horse and rider safety and exercise limitations if the AF is not or cannot be converted.

Pathologic Findings!!navigator!!

  • Grossly and histopathologically normal heart in horses with no underlying cardiac disease
  • Focal or diffuse atrial fibrosis may be present in horses with longstanding AF
  • Myocarditis, myocardial necrosis, and fatty infiltration have been documented in affected horses
  • Both atrial and ventricular enlargement in horses with significant AV valvular disease

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

  • Restoration of NSR and athletic performance in horses with no, minimal, or mild underlying heart disease
  • Assessment of rider/driver/handler safety if cardioversion is not successful or not desired; rate control if indicated
  • Palliative care for horses with AF in conjunction with CHF

Appropriate Health Care!!navigator!!

  • Monitor horses for 24–48 h to determine if the condition will spontaneously resolve (i.e. paroxysmal)
  • In horses with AF and CHF, institute treatment for CHF—using digoxin (0.0022 mg/kg IV every 12 h or 0.011 mg/kg PO every 12 h), furosemide (1–2 mg/kg IV every 8 h, not PO), or torsemide (0.5–1 mg/kg PO every 12 h) and ACE inhibitor (benazepril at 1 mg/kg PO every 12 h), if indicated
  • If AF is persistent, CHF is not present, and exercise intolerance is present, pharmacologic cardioversion or TVEC should be considered

Nursing Care!!navigator!!

  • Perform continuous ECG throughout attempted conversion to NSR
  • Keep horses quiet during pharmacologic cardioversion

Activity!!navigator!!

  • Horses with AF should not perform high-intensity exercise
  • Horses with AF can usually perform successfully in lower level athletic work, as broodmares, and as breeding stallions
  • An exercising ECG is indicated to ensure safety of horse and rider for intended use if cardioversion is not successful or not elected

Diet!!navigator!!

  • Oral potassium supplementation may be indicated with low plasma potassium, or low urinary fractional excretion of potassium or with excessive sweating
  • Potassium chloride salt can be added to the feed (1 tablespoon every 12 h, gradually increasing to 28 g (1 oz) every 12 h)

Client Education!!navigator!!

  • Discuss treatment-associated risks with owners—see Possible Complications
  • Discuss predisposing factors to minimize the likelihood of recurrence

Surgical Considerations!!navigator!!

  • TVEC under general anesthesia is usually successful (success rate similar to quinidine)
  • This utilizes a biphasic current delivered between electrodes placed in the right atrium and left pulmonary artery using pressure waveforms, echocardiography, and radiography or robotic fluoroscopy to guide and confirm electrode placement

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

The drug of choice for conversion is quinidine sulfate or gluconate.

Quinidine Gluconate

  • Indicated with AF of duration ≤2 weeks and no underlying cardiac disease
  • Administered in boluses of 0.5–1 mg/kg every 5–10 min to a total dose of 12 mg/kg

Quinidine Sulfate

  • Indicated in horses with persistent AF
  • Administered via nasogastric intubation at 22 mg/kg every 2 h to a total of 4–6 treatments, then every 6 h until the horse shows signs of toxicity or has converted to NSR

Contraindications!!navigator!!

  • Do not administer quinidine sulfate or gluconate to horses with AF and CHF
  • Horses with a resting HR of >60 bpm and/or grade 3/6 or louder systolic murmurs are likely to have CHF
  • TVEC may be preferable in horses with wide QRS morphology with increased ventricular response rate

Precautions!!navigator!!

Quinidine is associated with the following complications.

Cardiovascular

  • Prolonged QRS duration—indicates quinidine toxicity
  • Prolonged QT interval—increases risk of ventricular arrhythmias
  • Rapid supraventricular tachycardia—treat aggressively with digoxin to slow HR
    • Digoxin is recommended in conjunction with quinidine in horses with myocardial dysfunction or rapid HR during quinidine treatment
    • If HR exceeds 100 bpm, consider digoxin—0.011 mg/kg PO or 0.0022 mg/kg IV
    • If HR exceeds 150 bpm, consider digoxin (0.0022 mg/kg IV) and sodium bicarbonate (1 mEq/kg IV)
    • Detomidine or diltiazem may also be used to control rate with close monitoring of blood pressure
    • If a horse receiving quinidine only on day 1 does not convert, consider adding digoxin orally on day 2
    • Base subsequent digoxin administration during quinidine treatment on serum digoxin concentration and need to control HR or to improve myocardial contractility
  • Ventricular arrhythmias do not require treatment if ventricular rhythm is slow (<120 bpm), uniform, and no R-on-T is detected
    • If treatment indicated use magnesium sulfate—2–5 mg/kg bolus IV every 5 min to 50 mg/kg total
  • Hypotension—monitor and treat, if severe, with IV fluids to effect and, if necessary, phenylephrine (0.1–0.2 μg/kg/min IV to effect)
  • Sudden death—try to prevent with continuous ECG monitoring and treatment of any concerning arrhythmias that occur

Gastrointestinal

  • Flatulence—resolves on return of quinidine plasma concentrations to negligible levels
  • Oral ulcerations—prevent by administering quinidine via nasogastric tube
  • Diarrhea—resolves on return of quinidine plasma concentrations to negligible levels
  • Colic—associated with increasing number of doses; treat with analgesics as needed

Respiratory

Upper respiratory tract obstruction—indicates quinidine toxicity; treat with passage of a nasotracheal tube to relieve the upper airway obstruction; administer corticosteroids and antihistamines; emergency tracheotomy, if necessary.

Dermatologic

Urticaria—treat with corticosteroids and antihistamines.

Reproductive

Paraphimosis—resolves on return of plasma quinidine concentration to negligible levels.

Musculoskeletal

Laminitis—if the horse is uncomfortable, administer analgesics.

Neurologic

  • Indicates quinidine toxicity
  • Ataxia—resolves on return of plasma quinidine concentration to negligible levels
  • Convulsions—administer anticonvulsants
  • Bizarre behavior—resolves on return of plasma quinidine concentration to negligible levels

Patient Monitoring During Treatment With Quinidine!!navigator!!

  • Perform continuous ECG during treatment, because antiarrhythmic drugs are also arrhythmogenic
  • Measure QRS and QT duration before each dose; discontinue treatment if QRS or QT duration ≥25% of the pretreatment value
  • Discontinue treatment if rapid supraventricular tachycardia, ventricular arrhythmias, diarrhea, colic, ataxia, convulsions, bizarre behavior, urticaria, upper respiratory tract obstruction, or laminitis occurs

Possible Interactions!!navigator!!

Quinidine results in increased steady-state digoxin concentration, causing potential digoxin toxicity.

Alternative Drugs!!navigator!!

  • IV amiodarone has been successful but has a lower cardioversion rate
  • Flecainide has been associated with fatal ventricular arrhythmias and is not recommended

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring Following Conversion!!navigator!!

  • Following conversion, perform 24 h Holter ECG. If atrial ectopy is found, rest and corticosteroid therapy may be indicated
  • Evaluate left atrial contractile function; if absent or decreased, rest for 4 weeks and reevaluate
  • Regularly monitor cardiac rhythm; any irregularities or poor performance should prompt reexamination
  • If in permanent AF, informed adult rider should monitor exercising HR with HR monitor
  • Retire horse if exercising HR >220 bpm or ventricular arrhythmias or conduction abnormalities are induced by exercise or sympathetic nervous system stimulation

Prevention/Avoidance!!navigator!!

  • Discontinue administration of furosemide and bicarbonate milkshakes
  • Administer potassium or other electrolyte supplementation, if indicated
  • Consider administration of ACE inhibitor to minimize atrial remodeling and fibrosis
  • Consider administration of vitamin C
  • Avoid thyroid hormone supplementation
  • See chapter Supraventricular arrhythmias

Possible Complications!!navigator!!

  • If AF is not or cannot be treated, clinical signs will persist
  • Some horses with AF also have exercise-induced aberrant conduction, ventricular arrhythmias or R-on-T complexes; if AF is not or cannot be converted and the horse is to continue to be used for ridden exercise, exercising ECG is indicated to ensure horse and rider safety—see chapter Ventricular arrhythmias

Expected Course and Prognosis!!navigator!!

  • Most horses with little or no underlying cardiac disease convert to NSR with quinidine or TVEC cardioversion
  • Recurrences occur in ≅15% of horses with a suspected duration of ≤4 months
  • Recurrences occur in ≅45% of horses with a duration of AF of >4 months
  • Recurrence is most likely during the first year after conversion but can occur at any time
  • Recurrence likely in horses with left atrial enlargement or mitral regurgitation
  • Prognosis for return to the previous level of athletic performance is excellent in converted horses without significant underlying cardiovascular disease
  • Horses with permanent AF that do not convert to NSR with treatment or that are not candidates for conversion usually have a normal life expectancy and can be safely used for lower level athletic performance, as long as their exercising HR is <220 bpm and there is no aberrant conduction, ventricular arrhythmias, or R-on-T complexes
  • With significant valvular insufficiency, severity of the valvular heart disease and its progression determine the horse's useful performance life and life expectancy
  • Horses with CHF usually have severe underlying valvular heart or myocardial disease and have a guarded to grave prognosis for life
  • Most affected horses treated for CHF respond to the supportive therapy and improve for a short time but are euthanized within 2–6 months of initiating treatment

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

Any cardiac disease resulting in atrial enlargement predisposes to AF.

Age-Related Factors!!navigator!!

Older horses are more likely to have significant underlying cardiac disease, with valvular insufficiency and atrial enlargement and are not usually candidates for conversion.

Pregnancy/Fertility/Breeding!!navigator!!

  • Affected pregnant mares without underlying cardiac disease and CHF should not experience any problems
  • Affected pregnant mares with CHF—treat for the underlying cardiac disease with digoxin and furosemide. Although not studied in the horse, ACE inhibitors are contraindicated in pregnant mares due to the risk of birth defects documented in other species

Synonyms!!navigator!!

A fib, AF

Abbreviations!!navigator!!

  • ACE = angiotensin-converting enzyme
  • AF = atrial fibrillation
  • AV = atrioventricular
  • CHF = congestive heart failure
  • HR = heart rate
  • NSR = normal sinus rhythm
  • TVEC = transvenous electrical cardioversion

Suggested Reading ⬆ ⬇

De Clercq D, van Loon G, Baert K, et al. Effects of an adapted intravenous amiodarone treatment protocol in horses with atrial fibrillation. Equine Vet J 2007;39:344–349.

Decloedt A, Schwarzwald CC, De Clercq D, et al. Risk factors for recurrence of atrial fibrillation in horses after cardioversion to sinus rhythm. J Vet Intern Med 2015;29:946–953.

McGurrin MK, Physick-Sheard PW, Kenney DG, et al. Transvenous electrical cardioversion of equine atrial fibrillation: technical considerations. J Vet Intern Med 2005;19:695–702.

Reef VB, Levitan CW, Spencer PA. Factors affecting prognosis and conversion in equine atrial fibrillation. J Vet Intern Med 1988;2:1–6.

Reef VB, Reimer JM, Spencer PA. Treatment of equine atrial fibrillation: new perspectives. J Vet Intern Med 1995;9:57–67.

van Loon G, Blissitt KJ, Keen JA, et al. Use of intravenous flecainide in horses with naturally-occurring atrial fibrillation. Equine Vet J 2004;36:609–614.

Author(s) ⬆

Author: Virginia B. Reef

Consulting Editor: Celia M. Marr and Virginia B. Reef