Ventricular tachycardia

Ventricular tachycardia (V-tach or VT) is a tachycardia, or fast heart rhythm that originates in one of the ventricles of the heart. This is a potentially life-threatening arrhythmia because it may lead to ventricular fibrillation and sudden death.

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Classification

  Ventricular tachycardia can be classified based on its morphology: Monomorphic ventricular tachycardia means that the appearance of all the beats match each other in each lead of a surface electrocardiogram (ECG). Polymorphic ventricular tachycardia, on the other hand, has beat-to-beat variations in morphology. This most commonly appears as a cyclical progressive change in cardiac axis referred to by its French eponym Torsades de Pointes (literally twisting of the points).

Another way to classify ventricular tachycardias is the duration of the episodes: Technically, three or more beats in a row on an ECG that originate from the ventricle at a rate of more than 100 beats per minute constitute a ventricular tachycardia. If the fast rhythm self-terminates within 30 seconds, it is considered a non-sustained ventricular tachycardia. If the rhythm lasts more than 30 seconds it is known as a sustained ventricular tachycardia(even if it terminates on its own after 30 seconds).

A third way to classify ventricular tachycardia is on the basis of its symptoms: Pulseless VT is associated with no effective cardiac output, hence, no effective pulse, and is a cause of cardiac arrest. In this circumstance it is best treated the same way as ventricular fibrillation (VF)and is recognized as one of the shockable rhythms on the cardiac arrest protocol. Some VT is associated with reasonable cardiac output and may even be asymptomatic. The heart usually tolerates this rhythm poorly in the medium to long term, and patients may certainly deteriorate to pulseless VT or to VF.

Pathophysiology

The morphology of the tachycardia depends on its cause.

In monomorphic ventricular tachycardia, the reason all the beats look the same is because the impulse is being generated from either increased automaticity of a single point in either the left or right ventricle, or due to a reentry circuit within the ventricle. The most common cause of monomorphic ventricular tachycardia is damaged or dead (scar) tissue from a previous myocardial infarction (heart attack). This scar cannot conduct electrical activity, so there is a potential circuit around the scar that results in the tachycardia. This is similar to the re-entrant circuits that are the cause of atrial flutter and the re-entrant forms of supraventricular tachycardia. Other rarer congenital causes of monomorphic VT include right ventricular dysplasia, and right and left ventricular outflow tract VT.

Polymorphic ventricular tachycardia, on the other hand, is most commonly caused by abnormalities of ventricular muscle repolarisation. The predisposition to this problem usually manifests on the ECG as a prolongation of the QT interval. QT prolongation may be congenital or acquired. Congenital problems include Long QT syndrome and Catecholaminergic polymorphic ventricular tachycardia. Acquired problems are usually related to drug toxicity or electrolyte abnormalities, but can occur as a result of myocardial ischaemia. Class III anti-arrhythmic drugs such as sotalol and amiodarone prolong the QT interval and may in some circumstances be pro-arrhythmic. Other relatively common drugs including some antibiotics and antihistamines may also be a danger, particularly in combination with one another. Problems with blood levels of potassium, magnesium and calcium may also contribute. High dose magnesium is often used as an antidote in cardiac arrest protocols.

Diagnosis

The diagnosis of ventricular tachycardia is made based on the rhythm seen on either a 12 lead EKG or a telemetry rhythm strip. It may be very difficult to differentiate between ventricular tachycardia and a wide-complex supraventricular tachycardia in some cases. In particular, supraventricular tachycardias with aberrant conduction from pre-existing bundle branch block are commonly misdiagnosed as ventricular tachycardia. Other rarer phenomena include ashman beats and antedromic atrioventricular re-entry tachcyardias.

Various diagnostic criteria have been developed to determine if a wide complex tachycardia is ventricular tachycardia or a more benign rhythm.^[1][2] In addition to these diagnostic criteria, if the individual has a past history of a myocardial infarction, congestive heart failure, or recent angina, the wide complex tachycardia is much more likely to be ventricular tachycardia.^[3]

The proper diagnosis is important, as the misdiagnosis of supraventricular tachycardia when ventricular tachycardia is present is associated with worse prognosis. This is particularly true if calcium channel blockers, such as verapamil are used to attempt to terminate a presumed supraventricular tachycardia.^[4] It is therefore wisest to assume that all wide complex tachycardia is VT until proven otherwise.

Treatment

Therapy may be directed at either terminating an episode of the arrhythmia or for suppressing a future episode from occurring. The treatment is tailored to the specific patient, with regard to how well the individual tolerates episodes of ventricular tachycardia, how frequently episodes occur, their comorbidities, and their wishes.

Electrical Cardioversion / Defibrillation

It is usually possible to terminate a VT episode with a direct current shock across the heart. This is ideally synchronised to the patient's heartbeat. As it is quite uncomfortable, shocks should be delivered only to an unconscious or sedated patient. A patient with pulseless VT will be unconscious and treated as an emergency on a cardiac arrest protocol. Elective cardioversion is usually performed in controlled circumstances with anaesthetic and airway support.

The shock may be delivered to the outside of the chest using an external defibrillator, or internally to the heart by an implantable cardioverter-defibrillator (ICD) if one has previously been inserted.

An ICD may also be set to attempt to overdrive pace the ventricle. Pacing the ventricle at a rate faster than the underlying tachycardia can sometimes be effective in terminating the rhythm. If this fails after a short trial, the ICD will usually stop pacing, charge up and deliver a defibrillation grade shock.

Antiarrhythmic drug therapy

Drugs such as amiodarone, epinephrine and vasopressin may be used in addition to [defibrillation] to terminate VT while the underlying cause of the VT can be determined. Possible causes or contributing factors to VT can be remembered as the six H's and five T's: Hypovolemia, Hypoxia, Hydrogen ion (acidosis), Hypo- or Hyperkalemia, Hypoglycemia, Hypothermia; and Toxins, Tamponade (cardiac), Tension pneumothorax, Thrombosis, Trauma.

Long term anti-arrhythmic therapy may be indicated to prevent recurrence of VT. Beta-blockers and a number of class III anti-arrhythmics are commonly used.

For some of the rare congenital syndromes of VT, other drugs, and sometimes even catheter ablation therapy may be useful.

The implantation of an ICD is more effective than drug therapy for prevention of sudden cardiac death due to VT and VF, but may be constrained by cost issues, and well as patient co-morbidities and patient preference.

Popular culture

VT is frequently referenced in the 1970s television series Emergency!

In the 2007 film, Casino Royale, the protagonist, James Bond, suffers ventricular tachycardia from intoxication of digitalis, and goes into cardic arrest.^[5]

"V-Tach" is a commonly used term in the television shows Scrubs, ER and HOUSE.

"V-Tach" is what the "The Satin Slayer" on the American soap opera All My Children used to kill his victims.^[6]

References