Supraventricular Tachycardia - SVT

Supraventricular Tachycardia is dysrhythmias 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. People with coronary blood vessel disease and SVT 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. What is worrisome is that this disorder 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, in adults, it is often sequela of chronic hypertension, lung disease, or coronary blood vessel disease. Initial evaluation of the child with idiopathic or unexplained SVT includes an echocardiogram. It 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 SVT. Some ventricular SVT may disappear 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 SVT. 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: 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 SVT is that of an accessory pathway participating in orthodromic SVT. The term orthodromic implies that the activation wave front 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 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 SVT, 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.

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