Supraventricular Tachycardia - SVT

Supraventricular Tachycardia is dysrhythmia in which the heart rate is dangerously high. Diastole is shortened and the heart does not have sufficient time to fill. Heart output drops dangerously low and heart failure can occur, especially in clients with heart disease or damage. Clients with coronary blood vessel disease and Supraventricular Tachycardia can develop chest pain because coronary blood flow cannot meet the increased need of the myocardium imposed by the fast rate. Besides tachycardia and angina, hypotension, syncope, and reduced renal output are signs and symptoms of low cardiac output and impending heart failure. Digitalis, adrenergic blockers, and calcium channel blockers are used to slow heart rate.

Disorders of rate and rhythm in the pediatric population are rare. Worrisome Supraventricular Tachycardia, such as ventricular tachycardia, can be asymptomatic in children and have a benign natural history. Rhythm disturbances, such as sinus bradycardia, can be life threatening in the neonate.

Supraventricular Tachycardia in children is usually the result of cardiac lesions. In contrast, Supraventricular Tachycardia in the adult is often sequela of chronic hypertension, lung disease, or coronary blood vessel disease. Initial evaluation of the child with idiopathic or unexplained Supraventricular Tachycardia includes an echocardiogram. Supraventricular Tachycardia associated with structural or congenital heart disease has a poorer prognosis than that with structurally normal heart.

Age is an important consideration in the child with Supraventricular Tachycardia. Some ventricular Supraventricular Tachycardia disappears with age. Other conditions associated with an escape pacemaker, worsen with age. The ventricular rate in third degree heart block may be adequate for the two month old child but will not provide an adequate cardiac output for the child at age twelve. Age is also a factor in the clinical presentation of the Supraventricular Tachycardia. The infant, unable to express, may present with poor nourishment, irritation. The older child presents with specific symptoms, such as syncope from decreased cerebral blood flow, chest pain from decreased coronary blood flow, or palpitations. Adolescents involved in competitive athletics with syncope, palpitations or worrisome chest pain should be investigated promptly.

An essential requirement, for effective management of any Supraventricular Tachycardia, is identification of the precise mechanism. There are two types of Supraventricular Tachycardia exist: re-entry and ectopic.

Generally, the more common re-entry tachycardias are characterized by:

• Paroxysmal onset and termination, with fairly fixed rates.
• Reproducible termination with cardio version and rapid overdrive pacing.
• Predictable response to agents such as adenosine.

The less common ectopic tachycardias demonstrate gradual warm-up and cool-down in rates, largely proportional to autonomic tone, unresponsiveness to electrical cardio version and attempts at overdrive pacing, and minimal response to conventional antiarrhythmic drugs.

The most common mechanism underlying pediatric Supraventricular Tachycardia is that of an accessory pathway participating in orthodromic Supraventricular Tachycardia. The term orthodromic implies that the activation wave front during Supraventricular Tachycardia proceeds in an antegrade fashion down the atrioventricular node to the ventricles and then retrogrades up the accessory pathway lack to the atrium. This is in contract to the rare antidromic form of Supraventricular Tachycardia using an accessory connection, wherein the impulse travels antegrade down the accessory pathway to the ventricle and retrograde up the atrioventricular node, resulting in a wide orthodromic tachycardia difficult to distinguish clinically from ventricular tachycardia.

Atrioventricular node re-entry, the next most common form of Supraventricular Tachycardia, uses two functionally and physiologically distinct atrioventricular node components, the slow and fast pathways. Typical atrioventricular node re-entry uses the slow pathway in the antegrade direction and fast pathway in the retrograde direction. This is the opposite case in atypical atrioventricular node re-entry Supraventricular Tachycardia.

Following recent advances of clinical electrophysiology, almost all Supraventricular Tachycardias can be abolished by radiofrequency catheter ablation. Therefore, the pharmacological treatment of such tachycardias has become less frequent.