Ventricular Tachycardia

Introduction

Ventricular tachycardia (VT) is a rhythm disturbance classified as four or more ventricular premature complexes (VPC's) in sequence, producing a rapid heart rate. It is a serious condition which has the capability of causing ventricular fibrillation and sudden death. It most commonly occurs secondary to an existing systemic or cardiac abnormality. It is classed as a grade 4 rhythm disturbance because it is electrically unstable and causes haemodynamic compromise.

Small Animals

It is particularly common in Dobermanns secondary to Dilated CardioMyopathy and in the Boxer secondary to Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC). The rhythm disturbance is much less common in cats and normally the result of end-stage cardiomyopathy.

Causes

• Significant structural damage to the heart, heart failure or a breed specific myopathy
• Systemic disease such as hypoxia, anaemia, electrolyte and acid-base disturbances, hyperthyroidism (cats) etc.
• Drug reactions

Clinical Signs

On cardiac auscultation, heart rate is abnormally high (>160-180 in dogs). There may be signs of haemodynamic compromise and output failure such as collapse, hypotension, syncope and congestive heart failure. Sudden death may be the only presenting sign, especially in predisposed breeds.

Diagnosis

The condition can be diagnosed by ECG. The ECG shows wide and bizarre QRS complexes and P waves that are not consistency related to the QRS complexes. Heart rate should be high (>160-180 in dogs). QRS complexes can be monomorphic (similar in appearance with a regular R-R interval) or polymorphic (variable appearance and irregular R-R interval). A polymorphic appearance is suggestive of a more unstable myocardium. The P-P interval should be regular.

Sustained Ventricular Tachycardia lasts for more than 30 seconds, unsustained VT lasts less than thirty seconds.

Treatment

The underlying cause should be identified and treated where possible.

Treatment of the arrhythmia itself is always warranted because V-tach is classed as a grade 4 rhythm disturbance. The aim of treatment is to restore a normal sinus rhythm and reduce the ventricular rate thus improving cardiac output and preventing sinus arrest.

Dog: lidocaine (1st line of treatment), quinidine or procainamide (2nd line of treatment). Side effects of lidocaine include hypotension and gastrointestinal signs.

Cat: propranolol (1st line of treatment), small dose of lidocaine (2nd line of treatment). A high dose of lidocaine can cause neurotoxicity in the cat.

Following 1st line treatment, serum potassium levels should be assessed and corrected if necessary.

Prognosis

Prognosis depends on the underlying cause of the arrhythmia and the success of its treatment.

Large Animals

Ventricular tacycardia is most likely to be diagnosed and treated in the horse.

Causes

VT normally occurs secondary to myocardial disease, endotoxaemia or electrolyte and acid-base abnormalities. It can also occur idiopathically.

Clinical Signs

Cardiac auscultation should reveal an abnormally high heart rate. There may also be a history of exercise intolerance and dyspnoea.

Diagnosis

The condition can be diagnosed by ECG. The ECG shows a fast heart rate with wide and bizarre QRS complexes and P waves that are not consistency related to the QRS complexes. QRS complexes can be monomorphic (similar in appearance with a regular R-R interval) or polymorphic (variable appearance and irregular R-R interval). A polymorphic appearance is suggestive of a more unstable myocardium. The P-P interval should be regular.

Sustained Ventricular Tachycardia lasts for more than 30 seconds, unsustained VT lasts less than thirty seconds.

Treatment

Exercise should not be continued whist the horse has VT.

If the cause is thought to be idiopathic then box-rest and steroids are the treatment of choice. Otherwise the treatment options are:

• Procainamide (in the conscious horse) or lidocaine (in the unconscious horse)

• Quinidine Gluconate

• Magnesium Sulfate supplementation

Prognosis

VT can progress to a fatal arrhythmia (ventricular fibrillation). Prognosis depends on the underlying cause of the arrhythmia and the success of its treatment.

References

Dennis, S (2011) Ventricular arrhythmias RVC Cardiology Elective Course, Royal Veterinary College

Menzies-Gow, N (2001) ECG interpretation in the horse In Practice 2001 23: 454-45

RVC staff (2009) Cardiovascular System RVC Intergrated BVetMed Course, Royal Veterinary College