Bit & Bridle

Equine Ventricular Tachycardia

Other Names

  • VT in horses
  • Ventricular arrhythmia
  • Rapid ventricular rhythm

What is Equine Ventricular Tachycardia?

Equine ventricular tachycardia (VT) is a serious cardiac arrhythmia characterised by an abnormally fast heart rate originating from the ventricles — the lower chambers of the heart — rather than from the normal pacemaker site in the sinoatrial node. In a healthy horse, electrical impulses travel in an organised sequence from the atria down to the ventricles. In VT, aberrant electrical activity in the ventricular myocardium generates a rapid, often chaotic rhythm that impairs the heart’s ability to fill and eject blood effectively.

Ventricular tachycardia can occur as a brief, self-limiting episode (non-sustained VT) or as a sustained rhythm lasting more than 30 seconds, which represents a cardiac emergency. At very high rates, VT can deteriorate into ventricular fibrillation — a fatal arrhythmia in which the heart ceases to pump effectively. VT is therefore one of the most clinically significant arrhythmias encountered in equine practice.

Causes

VT in horses may arise from a variety of underlying conditions:

  • Myocardial disease: Equine myocarditis (viral or otherwise) is a significant predisposing cause, as inflamed or scarred myocardium generates abnormal electrical foci.
  • Electrolyte imbalances: Hypokalaemia, hypomagnesaemia, and hypocalcaemia all affect myocardial excitability and can trigger ventricular arrhythmias.
  • Gastrointestinal disease: Colic, particularly large colon displacement or strangulation, frequently causes electrolyte disturbances that precipitate VT.
  • Toxic causes: Ionophore toxicity (e.g., monensin contamination in feed) is highly cardiotoxic in horses and a well-documented cause of ventricular arrhythmias.
  • Cardiac neoplasia: Rare but reported; tumours of the myocardium can create arrhythmogenic foci.
  • Anaesthesia: Halothane anaesthesia in particular is associated with sensitisation of the myocardium to catecholamine-induced arrhythmias.
  • Idiopathic: Some cases occur without an identifiable underlying cause.

Symptoms and Signs

Clinical presentation varies with the rate and duration of the arrhythmia:

  • Rapid, irregular heartbeat detectable on auscultation
  • Weakness or sudden collapse
  • Exercise intolerance or sudden deterioration in performance
  • Pale, grey, or cyanotic mucous membranes
  • Distended jugular veins
  • Profuse sweating
  • Muscle trembling or fasciculations
  • Syncope (fainting) or seizure-like episodes
  • Sudden death in severe or sustained cases

Horses in colic, post-exercise, or those with known myocardial disease should be assessed for arrhythmias even when cardiac signs are not the presenting complaint.

Diagnosis

A definitive diagnosis of VT requires electrocardiographic confirmation:

  • Electrocardiography (ECG): The definitive diagnostic tool. VT appears as a series of three or more wide, aberrant QRS complexes at a rate typically exceeding 60 beats per minute above the normal resting rate. A base-apex lead ECG can be performed in the field.
  • Holter monitoring: Ambulatory ECG monitoring over 24 hours can detect intermittent or exercise-induced VT not apparent at rest.
  • Echocardiography: Assesses underlying structural myocardial disease, chamber dilation, or valvular abnormalities contributing to the arrhythmia.
  • Serum electrolytes: Measurement of potassium, magnesium, calcium, and sodium to identify correctable causes.
  • Cardiac troponin I: Elevated levels suggest active myocardial damage.
  • Toxicology: Feed analysis or toxin screening where ionophore contamination is suspected.

Treatment

Treatment depends on the underlying cause, the horse’s clinical status, and whether the arrhythmia is sustained:

  • Emergency management: Sustained VT with clinical deterioration is a veterinary emergency. Intravenous lidocaine is the first-line antiarrhythmic in acute equine VT — administered as a bolus followed by a continuous rate infusion (CRI) under close monitoring.
  • Electrolyte correction: Intravenous supplementation of potassium, magnesium, or calcium where deficiencies are identified.
  • Treat underlying cause: Management of colic, myocarditis, or withdrawal from toxic feed sources as appropriate.
  • Quinidine: May be used for conversion of certain arrhythmias, though its use in VT specifically requires expert guidance due to proarrhythmic potential.
  • Procainamide: An alternative antiarrhythmic occasionally used in refractory cases.
  • Rest and monitoring: Non-sustained, haemodynamically stable VT may be managed conservatively with stall rest and serial ECG monitoring.

The British Equine Veterinary Association (BEVA) and specialist equine cardiologists provide updated guidance on antiarrhythmic protocols.

Prevention

  • Ensure feeds are free from ionophore contamination — read all feed labels and source from reputable suppliers.
  • Maintain appropriate electrolyte balance in horses undergoing intensive exercise, particularly in hot weather.
  • Allow adequate recovery time following respiratory viral illness before returning to work.
  • Perform pre-purchase cardiac auscultation on all competition horses, with referral for ECG or echo if any abnormality is detected.
  • Regular veterinary monitoring for horses with known myocardial disease.

Prognosis

Prognosis varies considerably depending on the cause and response to treatment. Horses with VT secondary to a correctable electrolyte imbalance or transient colic often recover well once the underlying issue is resolved. Cases associated with myocarditis or structural cardiac disease carry a more guarded prognosis. Ionophore toxicity has a poor prognosis due to the severity of myocardial damage typically involved. Horses that have experienced VT require thorough cardiac re-evaluation before returning to any ridden or competitive work, and some may be permanently retired from performance.

When to Call the Vet

Seek immediate veterinary attention if your horse:

  • Has a very rapid or irregular heartbeat that is not settling
  • Shows signs of weakness, distress, or collapse
  • Has mucous membranes that are pale, grey, or bluish
  • Is known to have eaten potentially contaminated feed
  • Develops cardiac signs during or after a colic episode

Sustained ventricular tachycardia is a life-threatening emergency and should be treated as such.

Frequently Asked Questions

Is ventricular tachycardia always an emergency in horses?

Not always — brief, non-sustained episodes in otherwise stable horses may be managed with monitoring and treatment of the underlying cause. However, sustained VT with clinical deterioration is a genuine emergency requiring immediate intravenous antiarrhythmic therapy.

Can a horse return to competition after ventricular tachycardia?

This depends entirely on the underlying cause and whether it has been fully resolved. A horse that experienced VT due to a correctable electrolyte imbalance may return to work after full cardiac reassessment. Horses with VT secondary to myocardial disease face a more uncertain outlook regarding athletic performance.

How is VT different from atrial fibrillation in horses?

Atrial fibrillation (AF) originates in the upper chambers (atria) and, while it causes an irregular rhythm, it is generally less immediately life-threatening than ventricular tachycardia, which involves the lower chambers and carries a higher risk of cardiac output failure and sudden death.

What feed contaminant commonly causes VT in horses?

Ionophores — particularly monensin and lasalocid — are antimicrobial feed additives used in poultry and cattle nutrition. They are highly toxic to horses and, if ingested through contaminated feed, cause severe myocardial damage that can precipitate fatal ventricular arrhythmias.