Paroxysmal Supraventricular Tachycardia

Introduction
Background

Supraventricular tachycardia (SVT), a common clinical condition, is any tachyarrhythmia that requires atrial and/or atrioventricular (AV) nodal tissue for its initiation and maintenance. It is usually a narrow-complex tachycardia that has a regular, rapid rhythm; exceptions include atrial fibrillation (AF) and multifocal atrial tachycardia (MAT). Aberrant conduction during SVT results in a wide-complex tachycardia. SVT occurs in persons of all age groups, and treatment can be challenging.

Paroxysmal supraventricular tachycardia (PSVT) is episodic, with an abrupt onset and termination. Manifestations of SVT are quite variable; patients may be asymptomatic or they may present with minor palpitations or more severe symptoms. Results from electrophysiology studies have helped determine that the pathophysiology of SVT involves abnormalities in impulse formation and conduction pathways. The most common mechanism identified is reentry.15,41,3,50 This article focuses on SVT, including the pathophysiology, clinical presentation, diagnosis, management, and treatment options of this condition.

Pathophysiology

The development of intracardiac electrophysiology studies has dramatically changed the classification of SVT. Intracardiac recordings have identified the various mechanisms of SVT. Depending on the site of origin of the dysrhythmia, SVTs may be classified as an atrial or AV tachyarrhythmia.27,7

Atrial tachyarrhythmias include (1) sinus tachycardia, (2) inappropriate sinus tachycardia (IST), (3) sinus nodal reentrant tachycardia (SNRT), (4) atrial tachycardia, (5) multifocal atrial tachycardia, (6) atrial flutter, and (7) atrial fibrillation.

AV tachyarrhythmias include (1) AV nodal reentrant tachycardia (AVNRT), (2) AV reentrant tachycardia (AVRT), (3), junctional ectopic tachycardia (JET), and (4) nonparoxysmal junctional tachycardia (NPJT).
Atrial Tachyarrhythmias

Sinus tachycardia

Sinus tachycardia is an accelerated sinus rate that is a physiologic response to a stressor. It is characterized by a heart rate faster than 100 beats per minute (bpm) and generally involves a regular rhythm (see Media file 1). Underlying physiological stresses such as hypoxia, hypovolemia, fever, anxiety, pain, hyperthyroidism, and exercise usually induce sinus tachycardia.47,19 Treatment involves addressing the basic underlying stressor. Certain drugs, such as stimulants (eg, nicotine, caffeine), medications (eg, atropine, salbutamol), recreational drugs (eg, cocaine, amphetamines, ecstasy), and hydralazine, can also induce sinus tachycardia.

Inappropriate sinus tachycardia

IST is an accelerated baseline sinus rate in the absence of a physiological stressor. In this situation, healthy adults may have an elevated resting heart rate and an exaggerated heart rate response to even minimal exercise. This tachyarrhythmia is observed most commonly in young women without structural heart disease.8,29,55 The underlying mechanism of IST may be hypersensitivity of the sinus node to autonomic input or an abnormality within the sinus node, its autonomic input, or both.8,29,55

Sinus nodal reentrant tachycardia

SNRT is frequently confused with IST. SNRT is due to a reentry circuit, either in or near the sinus node. Therefore, it has an abrupt onset and offset. The heart rate is usually 100-150 bpm, and ECG tracings usually demonstrate normal sinus P-wave morphology.8,29,55

Atrial tachycardia

Atrial tachycardia is an arrhythmia originating in the atrial myocardium. Enhanced automaticity, triggered activity, or reentry may result in this rare tachycardia.51,17,10,30,55 The heart rate is regular and is usually 120-250 bpm. The P-wave morphology is different from the sinus P waves and is dependent on the site of origin of the tachycardia (see Media file 2). Because the arrhythmia does not involve the AV node, nodal blocking agents such as adenosine and verapamil are usually unsuccessful in terminating this arrhythmia. Atrial tachycardia has also been associated with digoxin toxicity via the triggered mechanism.51,17,10,30,55

Multifocal atrial tachycardia


Multifocal atrial tachycardia is a tachyarrhythmia that arises within the atrial tissue; it is composed of 3 or more P-wave morphologies and heart rates. This arrhythmia is fairly uncommon and is typically observed in elderly patients with pulmonary disease. The heart rate is greater than 100 bpm, and ECG findings typically include an irregular rhythm, which may be misinterpreted as atrial fibrillation (see Media file 3). Treatment involves correcting the underlying disease process.38,22,43 Magnesium and verapamil may sometimes be effective.

Atrial flutter

Atrial flutter is a tachyarrhythmia arising above the AV node with an atrial rate of 250-350 bpm. The mechanism behind atrial flutter is generally reentrant in nature. Typically, counterclockwise atrial flutter is due to a macroreentrant right atrial circuit. It is commonly observed in patients with ischemic heart disease, myocardial infarction, cardiomyopathy, myocarditis, pulmonary embolus, toxic ingestion (eg, alcohol), or chest trauma. It may be a transitional rhythm and can progress to atrial fibrillation. ECG findings of typical atrial flutter include negative sawtooth flutter waves in leads II, III, and aVF. AV conduction is most commonly 2:1, which yields a ventricular rate of approximately 150 bpm (see Media file 4).2,47,24

Atrial fibrillation


Atrial fibrillation is an extremely common arrhythmia arising from chaotic atrial depolarization. The atrial rate is usually 300-600 bpm, while the ventricular rate may be 170 bpm or more. ECG findings characteristically include an irregular rhythm with fibrillatory atrial activity (see Media file 5). This arrhythmia is associated with rheumatic heart disease, hypertension, ischemic heart disease, pericarditis, thyrotoxicosis, alcohol intoxication, mitral valve prolapse and other disorders of the mitral valve, and digitalis toxicity.2,47,24 When atrial fibrillation occurs in young or middle-aged patients in the absence of structural heart disease or any apparent cause, it is called lone or idiopathic atrial fibrillation.

AV Tachyarrhythmias

AV nodal reentrant tachycardia

The most common cause of paroxysmal supraventricular tachycardia is AVNRT. AVNRT is diagnosed in 50-60% of patients who present with regular narrow QRS tachyarrhythmia.25,2,23,3 The heart rate is 120-250 bpm and is typically quite regular (see Media files 6-7). AVNRT may occur in healthy, young individuals, and it occurs most commonly in women.23 Most patients do not have structural heart disease. However, occasionally these individuals may have an underlying heart condition such as rheumatic heart disease, pericarditis, myocardial infarction, mitral valve prolapse, or preexcitation syndrome.25,3,23,3

An understanding of the electrophysiology of AV nodal tissue is very important in order to comprehend the mechanism of AVNRT. In most people, the AV node has a single conducting pathway that conducts impulses in an anterograde manner to depolarize the bundle of His. In certain cases, AV nodal tissue may have 2 conducting pathways with different electrophysiological properties (see Media file 8). One pathway (alpha) is a relatively slow conducting pathway with a short refractory period, while the second pathway (beta) is a rapid conducting pathway with a long refractory period. The coexistence of these functionally different pathways serves as the substrate for reentrant tachycardia.25,2,3,20 Electrophysiologic studies have demonstrated dual AV nodal pathways in 40% of patients.

Onset of AVNRT is triggered by a premature atrial impulse. A premature atrial impulse may reach the AV node when the fast pathway (beta) is still refractory from the previous impulse but the slow pathway (alpha) may be able to conduct. The premature impulse then conducts through the slow pathway (alpha) in an anterograde manner; the (beta) pathway continues to recover because of its longer refractory period. After the impulse conducts in an anterograde manner through the slow (alpha) pathway, it may find the fast (beta) pathway recovered; the impulse then conducts in a retrograde manner via the fast (beta) pathway. If the slow pathway (alpha) has repolarized by the time the impulse completes the retrograde conduction, the impulse can then reenter the slow (alpha) pathway and initiate AVNRT (see Media file 8).

Importantly, note that AVNRT does not involve the ventricles as part of the reentry circuit; the necessity of perinodal atrial tissue to the circuit is controversial. Because the impulse typically conducts in an anterograde manner through the slow pathway and in a retrograde manner through the fast pathway, the PR interval is longer than the RP interval. Thus, in patients with typical AVNRT, the P wave is usually located at the terminal portion of the QRS complex.25,3,2,20,24 In patients with atypical AVNRT, anterograde conduction is via the fast pathway, while retrograde conduction is via the slow pathway. For these atypical patients, the RP interval is longer than the PR interval.25,54,2,23,3,20,26,24

AV reentrant tachycardia

AVRT is the second most common form of paroxysmal supraventricular tachycardia. The incidence rate of AVRT in the general population is 0.1-0.3%. AVRT is more common in males than in females (male-to-female ratio of 2:1), and patients with AVRT commonly present at a younger age than patients with AVNRT. AVRT is associated with the Ebstein anomaly, although most patients with AVRT do not have evidence of structural heart disease. AVRT occurs in the presence of accessory pathways, or bypass tracts. Accessory pathways are errant strands of myocardium that bridge the mitral or tricuspid valves.25,33,20,55

AVRT is the result of 2 or more conducting pathways: the AV node and 1 or more bypass tracts. In a normal heart, only a single route of conduction is present. Conduction begins at the sinus node, progresses to the AV node, and then to the bundle of His and the bundle branches. However, in AVRT, 1 or more accessory pathways connect the atria and the ventricles. The accessory pathways may conduct impulses in an anterograde manner, a retrograde manner, or both.52,14,25,18,33,37,20,55 When impulses travel down the accessory pathway in an anterograde manner, ventricular preexcitation results. This produces a short PR interval and a delta wave as is observed in persons with Wolff-Parkinson-White (WPW) syndrome (see Media file 9).52

Importantly, note that not all accessory pathways conduct in an anterograde manner. Concealed accessory pathways are not evident during sinus rhythm, and they are only capable of retrograde conduction. A reentry circuit is most commonly established by impulses traveling in an anterograde manner through the AV node and in a retrograde manner through the accessory pathway; this is called orthodromic AVRT. A reentry circuit may also be established by a premature impulse traveling in an anterograde manner through a manifest accessory pathway and in a retrograde manner through the AV node; this is called antidromic AVRT (see Media file 10).6,34

While the orthodromic AVRT is typically a narrow-complex tachycardia (see Media file 11), antidromic AVRT inscribes a bizarre, wide-complex tachycardia (see Media file 12).6,5,34

Patients with Wolff-Parkinson-White syndrome can develop atrial fibrillation and atrial flutter (see Media file 13). The rapid nondecremental conduction via the accessory pathways can result in extremely rapid rates, which can degenerate to ventricular fibrillation and cause sudden death. Patients with preexcitation syndromes with atrial fibrillation must not be administered an AV nodal blocking agent; these agents can further increase conduction via the accessory pathway, which increases the risk of ventricular fibrillation and death.11,46,28,6,49,32,35

Junctional ectopic tachycardia and nonparoxysmal junctional tachycardia


JET and NPJT are rare and presumably arise because of increased automaticity, triggered activity, or both. They are usually observed following valvular surgery, after myocardial infarction, during active rheumatic carditis, or with digoxin toxicity. These tachycardias are also observed in children following congenital heart surgery. ECG findings include a regular narrow QRS complex, although P waves may not be visible. Patients with AV dissociation have also been described.20,39,48
Frequency
International

Paroxysmal supraventricular tachycardia incidence is approximately 1-3 cases per 1000 persons. The incidence rate of the WPW pattern on ECG tracings is 0.1-0.3% in the general population, although not all patients develop SVT.28,32,20,55,4 In a population-based study, the prevalence of paroxysmal supraventricular tachycardia was 2.25 cases per 1000 persons, with an incidence of 35 cases per 100,000 person-years.36 AVNRT is more common in patients who are of middle age or older, while adolescents are more likely to have SVT mediated by an accessory pathway. Paroxysmal supraventricular tachycardia is not only observed in healthy individuals, it is also common in patients with previous myocardial infarction, mitral valve prolapse, rheumatic heart disease, pericarditis, pneumonia, chronic lung disease, and current alcohol intoxication.28,32,20,55 Digoxin toxicity also may be associated with paroxysmal supraventricular tachycardia.20,55,24
Mortality/Morbidity
Paroxysmal supraventricular tachycardia may start suddenly and last for seconds or days. Patients may or may not be symptomatic, depending on their hemodynamic reserve and their heart rate, the duration of the paroxysmal supraventricular tachycardia, and coexisting diseases. Paroxysmal supraventricular tachycardia can result in heart failure, pulmonary edema, myocardial ischemia, and/or myocardial infarction secondary to an increased heart rate in patients with poor left ventricular function.20,55,24 In fact, one study found that one third of patients with SVT experienced syncope, required cardioversion, or had an episode of sudden death.53 Incessant SVT can cause tachycardia-induced cardiomyopathy.
Patients with WPW syndrome may be at risk for cardiac arrest if they develop atrial fibrillation or atrial flutter in the presence of a rapidly conducting (ie, short anterograde refractory period) accessory pathway. Extremely rapid ventricular rates during atrial fibrillation or atrial flutter can cause deterioration to ventricular fibrillation. This complication is unusual and occurs primarily in patients who have had prior symptoms due to WPW syndrome. In rare cases, sudden death may be the initial presentation of WPW syndrome.
In the absence of manifest preexcitation (ie, WPW syndrome), the risk of sudden death with paroxysmal supraventricular tachycardia is extremely small.
Race

No known racial differences exist regarding the incidence or presentation of paroxysmal supraventricular tachycardia.
Sex
Most series of catheter ablation reflect a higher proportion of female patients with AVNRT than male patients. This may reflect a true higher incidence in women, or it may reflect the sample of patients who are referred (or choose) to undergo extensive evaluation and/or catheter ablation.
In a population-based study, the risk of developing paroxysmal supraventricular tachycardia was twice as high in women compared to men.36
Age
The prevalence of paroxysmal supraventricular tachycardia increases with age.36
The relative frequency of tachycardia mediated by an accessory pathway decreases with age.
Clinical
History
Because symptom severity depends on the presence of structural heart disease and on the hemodynamic reserve of the patient, individuals with paroxysmal supraventricular tachycardia may present with mild symptoms or severe cardiopulmonary complaints. Some common presenting symptoms are listed below.53,4 Palpitations and dizziness are the most common symptoms reported by patients with SVT. Chest discomfort may be secondary to a rapid heart rate, and it frequently subsides with the termination of the tachycardia. Persistent SVT may lead to tachycardia-induced cardiomyopathy.
Common presenting symptoms of paroxysmal supraventricular tachycardia and their frequency rates are as follows:
Palpitation - Greater than 96%
Dizziness - 75%
Shortness of breath - 47%
Syncope - 20%
Chest pain - 35%
Fatigue - 23%
Diaphoresis - 17%
Nausea - 13%
History should include time of onset, any triggers, any previous episodes or arrhythmia, and previous treatment. A detailed past medical and cardiac history and a complete list of all medications should be obtained.
Patients who are hemodynamically unstable should be resuscitated immediately with cardioversion. An ECG should be performed as soon as possible.
Many patients with frequent episodes of paroxysmal supraventricular tachycardia tend to avoid activities such as exercising and driving due to past episodes of syncope or near-syncope.
Physical
Pertinent findings are generally limited to cardiovascular and respiratory systems. Patients often appear quite distressed. Tachycardia may be the only finding in patients who are otherwise healthy and have significant hemodynamic reserve.
Patients who have limited hemodynamic reserve may be tachypneic and hypotensive. Crackles may be auscultated secondary to heart failure. An S3 may be present, and large jugular venous pulsations may also be visualized.20,53,55
Causes

Supraventricular tachycardia or paroxysmal supraventricular tachycardia are triggered by reentry mechanism. This may be induced by premature atrial or ventricular ectopic beats. Other triggers include hyperthyroidism and stimulants, including caffeine, drugs, and alcohol.

http://emedicine.medscape.com/article/156670-overview