Concise Review for Primary-Care Physicians

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1 Concise Review for Primary-Care Physicians Narrow QRS Complex Tachycardias STEPHEN J. PIEPER, M.D., AND MARSHALL S. STANTON, M.D. Regular narrow QRS complex tachycardias are a common problem encountered by general internists or family practitioners. Although such tachycardias often occur in patients with a normal heart and seldom represent life-threatening conditions, they may cause bothersome symptoms. The key to approaching the diagnosis of these arrhythmias is identifying atrial activity (P waves) on the surface electrocardiogram and classifying the tachycardia as long RP or short RP. On the basis of that information, a differential diagnosis can be generated, logical therapy can be delivered for termination of the tachycardia, and a Narrow QRS complex tachycardias are fast cardiac rhythms (generally more than 100 beats/min) with a QRS duration of 100 ms or less. I Although rarely life-threatening, these arrhythmias are a common source of morbidity. The purpose of this review is to present (l) an overview of the responsible mechanisms, (2) a systematic approach to the differential diagnosis, and (3) the treatment options for these tachycardias. Atrial flutter will be briefly mentioned. Atrial fibrillation, multifocal atrial tachycardia, and wandering atrial pacemaker do not constitute regular narrow QRS complex tachycardias and will not be discussed. MECHANISMS A basic understanding of the mechanisms of tachycardias can be used to assist in the systematic evaluation of narrow QRS complex tachycardia as it appears on the surface electrocardiogram. Such an evaluation is important because knowing the type of tachycardia (for example, intra-atrial versus atrioventricular [AV] nodal) and the mechanism (for example, reentry versus triggered) aids the physician in logically choosing therapy. The differential diagnosis of narrow QRS complex tachycardia is best deduced by first assessing the relationship of atrial activity (P waves) to the QRS complex.' P-wave morphologic features are related to the atrial activation sequence. Therefore, P waves with a morphologic From the Division of Cardiovascular Diseases and Internal Medicine, Mayo Clinic Rochester, Rochester, Minnesota. Address reprint requests to Dr. M. S. Stanton, Division of Cardiovascular Diseases, Mayo Clinic Rochester, 200 First Street SW, Rochester, MN Mayo Clin Proc 1995; 70: plan can be developed to prevent recurrence. Because intravenously administered adenosine alleviates 90% of the episodes of supraventricular tachycardias and has minimal side effects, it has become the drug of choice for termination of most types of narrow QRS complex tachycardias. (Mayo Clin Proc 1995; 70: ) AV = atrioventricular; AVNRT = atrioventricular nodal reentrant tachycardia; AVRT = atrioventricular reentrant tachycardia; NPJT = nonparoxysmal junctional tachycardia; ORT = orthodromic reciprocating tachycardia; PJRT = permanent junctional reciprocating tachycardia appearance identical to that of the sinus P waves are probably due to tachycardias initiated in (or near) the sinus node, regardless of mechanism.>" In general, three situations are possible. First, atrial activity can precede the QRS complex with a normal delay. In such cases, the RP intervals are longer than the PR intervals, and the tachycardia is designated a "long RP tachycardia." Second, P waves can occur simultaneously with or shortly after the QRS complex (that is, in the ST segment or T wave) and thus yield a "short RP tachycardia." Finally, no relationship may exist between atrial activity and the QRS complex, in which case AV dissociation is present. Examples of short RP and long RP tachycardias are depicted in Figure 1. The differential diagnoses for short RP and long RP tachycardias are outlined in Table 1. The most common mechanism that underlies narrow QRS complex tachycardias (excluding sinus tachycardia) is reentry.' In general, this term is used to describe a discrete, self-propagating pattern of conduction around a specific circuit. Such circuits can be small (occurring within nodal or atrial tissues themselves) or large (involving large portions of the atria and ventricles). 2 Reentry accounts for more than 90% of all narrow QRS tachycardias studied in many electrophysiology laboratories.? Other mechanisms of tachycardia include "triggered activity" and "automaticity.'?' Sinus tachycardia, for example, is due to increased sinus node automaticity. Triggered activity is less common and may be operative in arrhythmias due to digitalis-related intoxication.' Further elaboration on these mechanisms of tachycardia is outside the scope of this review, and interested readers are referred elsewhere.p Mayo Foundation for Medical Education and Research

2 372 NARROW QRS COMPLEX TACHYCARDIAS Mayo Clin Proc, April 1995, Vol 70 I Fig. 1. Top Panel, Schematic representation of short RP tachycardia. Arrows =P waves occurring in ST segment shortly after QRScomplex. Distancefromonsetof QRS to P waveis much less than distance from P wave to subsequent QRS; thus, the designation "short RP tachycardia" is used. Bottom Panel, Schematicrepresentation of long RP tachycardia. In this case, P waves (arrows) precedeeach QRScomplexby a shortinterval. Therefore, distance from onset of QRS to P wave considerably exceeds distance from P wave to subsequentqrs. Hence, the label "long RP tachycardia" is appropriate. SHORT RP TACHYCARDIAS AV Nodal Reentrant Tachycardia.-If atrial fibrillation, atrial flutter, and sinus tachycardia are excluded from consideration, AV nodal reentrant tachycardia (AVNRT) is the most common cause of supraventricular tachycardia. As the I name implies, it is a type of reentrant tachycardia that involves the AV node (Fig. 2).2.3 In some series, AVNRT accounts for 50 to 60% of all regular narrow QRS complex tachycardias.v'> Because retrograde conduction typically occurs in a "fast" pathway, the atria are activated either simultaneously with or just after the ventricles. Thus, the P wave during typical AVNRT may be identified in the ST segment or is buried within the QRS complex. Careful comparison with the patient's baseline electrocardiogram should identify P waves in approximately 50% of cases of AVNRT. 7 Particular attention should be focused on the terminal portion of the QRS complex. In the typical AVNRT, excitation conducts antegrade down a "slow" pathway, while it blocks antegrade in a fast pathway. As excitation from the slow pathway reaches the final common pathway, it retrogradely enters the fast pathway distal to the site of block. If the fast pathway has recovered at its previous site of block, conduction may continue retrograde through this site until it reenters the slow pathway-thus propagating around the circuit (Fig. 2). Rarely, patients have AVNRT with the antegrade and retrograde limbs of the circuit reversed.i-? In such cases, retrogradeconduction is slow, and this "atypical" AVNRTis classified as a long RP tachycardia. The onset of AVNRT is abrupt and frequently is initiated by an atrial premature complex; 1,5-7 Termination is also abrupt, although some slowing may occur before termination. The rate is not useful in distinguishing AVNRT from other short RP narrow QRS tachycardias because it can range from 100 to 280 beats/min," In general, a 1:1 AV Table I.-Regular Narrow QRS Complex Tachycardia: Differential Diagnosis*t Short RP (RP < PR) AV nodal reentrant tachycardia AV reentrant tachycardia Nonparoxysmal junctional tachycardia Long RP (RP> PR) Sinus tachycardia Sinus nodal reentranttachycardia Atrial tachycardia Permanent junctional reciprocating tachycardia Nonparoxysmal junctional tachycardia Unusualtype of AV nodal reentry AtypicalAV reentrant tachycardia *The most important arrhythmias for the clinician are in italic type. AV = atrioventricular. t Additional notes: 1. Atrialflutteris often a regularnarrowqrstachycardia, but it is not included in this table because P waves cannot be designated as short or long RP tachycardia. 2. The RP relationship in nonparoxysmal junctional tachycardiais unpredictable; patients may have retrogradeblock and resultant AV dissociation, or the atria may be capturedretrogradelywith 1:1 or variableconduction. 3. Occasionally, atrial tachycardia may have a long PR interval and be confused with the other short RP tachycardias.

3 Mayo Clin Proc, April 1995, Vol 70 NARROW QRS COMPLEX TACHYC ARDIAS 373 Atrium AV node A B c His Purkinje Fig. 2. Schematic representation of operative circuit in atrioventricular (A V) nodal reentrant tachycardia. A, Excitation wave front has entered AV node and blocked in the fast pathway; it continues to conduct down a slow pathway. B, Excitation wave front has reached a final common pathway in AV node; it not only exits AV node but also proceeds retrograde through previous site of antegrade block in fast pathway. C, Excitation wave front has reentered the slow pathway and perpetuated excitation around reentrant circuit. relationship exists; however, the ventricle is not part of the circuit, and 2:1 AV block (as well as ventriculoatrial block) has been described," Therefore, AV dissociation or block does not necessarily rule out AVNRT, although it is the exception. AV Reentrant Tachycardia.-In a series reported by Josephson and Kaster, " AV reentrant tachycardia (AVRT) accounted for approximately 40% of narrow QRS tachycardias evaluated electrophysiologically. Therefore, it is generally considered the second most common cause of regular narrow QRS tachycardia (excluding sinus tachycardia). AVRT involves reentry between the atrium and ventricle with use of the AV node as the antegrade, slow pathway and an accessory AV connection as the retrograde, fast path way." This patt ern is al so known as orthodromic reciprocating tachycardia (a RT). Of importance, accessory AV connections are usually fast conducting pathways and do not have many of the conduction properties unique to the AV node (exceptions are ment ioned subsequently). Therefore, retrograde conduction is fast and yields a short RP tachycardia. The following factor s are important differences between AVNRT and AVRT: 1. AI: I relationship is necessary for AVRT because both the atria and the ventricles are part of the circuit (in contradistinction to AVNRT). Therefore, if AV block occurs during tachycardia, AVRT is excluded. 2. If bundle branch block occur s during art and the length of the tachycardia cycle increases (or retrograde conduction time increases), AVNRT is excluded (in AVNRT, the His-Purkinje system is not part of the tachycardia circuit). Therefore, bundle branch block should not affect the length of the tachycard ia cycle. The converse is not necessarily true-that is, the absence of cycle length change with the occurrence of bundle branch block does not exclude AVRT. 3. An RP interval less than 70 ms excludes AVRT. For measurement of this interval, esophageal recording during art is necessary. Althou gh AVNRT can sometimes have an RP interval greater than 70 ms, AVRT will never have an RP interval less than 70 ms. Again, as in many other tachycardias, the use of esophageal recordings to identify atrial activity and to distinguish the relationship of P waves to the QRS complex is extremely helpful and can be diagnostic. 10.1I Nonparoxysmal Junctional Tachycardia.-Nonparoxysmal junction al tachycardia (NPIT) is a tachycardia that arises in the AV junction.' Although often described as a short RP tachycardia, because NPJT causes ventricular activation almo st concurrently with atrial activation, a substantial portion (25%) actuall y show P waves that slightly precede the QRS complex.p" and in some cases, AV dissociation may be present. The P-wave axis will be superior, and P waves will be negative in the inferior electrocardiographic leads II, III, and avf. Because the rates range from 70 to 130 beats/min, NPJT may not be a true "tachycardia," although (because of the normal junctional escape rate of 40 to 60 beats/min) it is tachyarrhythmia. Unlike AVNRT and AVRT, initiation and termination of this tachyarrhythmia are gradual. NPJT is often associated with digitalisintoxication, inferi or myocardial infarction, myocarditis, and mitral valve surgical procedures.

4 374 NARROW QRS COMPLEX TACHYCARDIAS LONG RP TACHYCARDIAS Sinus Tachycardia.-Although generally identifiable by a P wave of normal morphologic features (upright in electrocardiographic leads I, II, III, and avf) that precedes each QRS complex, sinus tachycardia can be difficult to recognize when the P wave begins to fuse with the T wave of the preceding QRS complex. This situation usually occurs at rest only in critically ill patients with heart rates greater than 180 beats/min. Sinus Nodal Reentrant Tachycardia.- The morphologic appearance of sinus nodal reentrant tachycardia is identical to that of sinus tachycardia. In contrast to sinus tachycardia, however, initiation and termination are abrupt. In fact, these may be the only distinguishing features when sinus nodal reentrant tachycardia is compared with sinus tachycardia.' The mean rate is typically 130 to 140 beats/min. Carotid sinus massage or Valsalva maneuver (or both) may terminate sinus nodal reentrant tachycardia, and gradual slowing may precede termination. Atrial Tachycardia.-Other than sinus tachycardia, atrial tachycardia is the most common long RP tachycardia.j:" In atrial tachycardia, an atrial source outside the sinoatrial node activates the atria. Accordingly, P-wave morphologic characteristics vary depending on the site of this source. For example, a low right atrial source will produce negative P waves in the inferior electrocardiographic leads II, III, and avf; the resultant activation sequence will be similar to that in AVNRT. The mechanism of atrial tachycardia can be reentrant, automatic, or triggered. Rates vary considerably, and AV block may occur without interrupting the tachycardia because the AV node is not an integral part of the arrhythmia circuit. Paroxysmal atrial tachycardia in conjunction with AV block should suggest the possibility of digitalis toxicity. Permanent Junctional Reciprocating Tachycardia.As discussed with AVRT, certain types of reentrant circuits exist in which the accessory AV connection has AV nodal properties (for example, slow conduction). In permanent junctional reciprocating tachycardia (PJRT), such a connection is present in the posteroseptal region and usually represents the retrograde limb of the tachycardia circuit. Because of its slow conduction properties, retrograde atrial activation is delayed, and a long RP tachycardia results. Often, PJRT is incessant and may be a cause of tachycardia-induced cardiomyopathy. Because of the posteroseptal accessory pathway site in PJRT, P waves will appear negative in the inferior electrocardiographic leads II, III, and avf. This unusual entity is a potentially reversible type of cardiomyopathy. Patients with this type of accessory pathway almost never have preexcitation (a delta wave) on their electrocardiogram during sinus rhythm. Mayo Clin Proc, April 1995, Vol 70 Atrial Flutter.-Atrial flutter is a reentrant rhythm of the right atrium typically with an atrial rate of 300 beats/min. P waves have a characteristic "sawtooth" appearance, and 2:I AV block is common; thus, a regular narrow QRS complex tachycardia occurs.' Atrial flutter, however, is neither a short nor a long RP tachycardia. In the case of 2: I AV conduction, one flutter wave occurs in the ST-T segment and another before each QRS complex; therefore, the designation of short or long RP is meaningless. TREATMENT Short-Term.-In general, when the AV node is part of the tachycardia circuit (that is, AVNRT or AVRT), any agent that depresses AV nodal conduction-for example, adenosine, calcium channel blockers (verapamil or diltiazem hydrochloride), ~-adrenergic blocking agents, and digoxincan be effective in terminating tachycardia. Intravenously administered adenosine terminates 90% of the episodes of supraventricular tachycardia (excluding sinus tachycardia and atrial tachycardias, fibrillation, and flutter).' In addition, its short half-life (approximately 9 seconds) precludes lasting hemodynamic side effects. Consequently, adenosine has become the drug of choice for immediate termination of narrow QRS complex tachycardias. Furthermore, it can be helpful diagnostically. If AV block occurs during administration of adenosine and without termination of tachycardia, AVNRT and AVRT are excluded. If an atrial tachycardia is present, rate control with calcium channel blockers or [3adrenergic blockers is recommended. Once this has been achieved, primary antiarrhythmic drug therapy should be considered if tachycardia persists. Atrial tachycardias may respond to class I or class III antiarrhythmic drugs (those with sodium channel blockade or prolongation of repolarization, respectively, as their main mechanism of action). Long-Term.-Long-term therapy for supraventricular tachycardia is best tailored to the patient. Catheter ablation is 90 to 95% successful in alleviating AVNRT or AVRT, and the procedure is associated with a 1% or less risk of major complications. We recommend this approach for primary therapy in most patients with arrhythmias due to accessory pathways and some patients with symptomatic AVNRT. Class I and class III antiarrhythmic drugs may also be successful in controlling these arrhythmias. AV nodal blocking drugs (calcium channel blockers, ~-adrenergic blockers, and digoxin) can be effective but should be used cautiously in the setting of Wolff-Parkinson-White syndrome. Atrial tachycardias and atrial flutter are often treated initially with AV nodal blockers and class I or class III antiarrhythmic agents. Recurrence rates are equivalent with use of class IA and class IC drugs, although the latter are generally better tolerated. Catheter ablation is increasingly being used for these arrhythmias as well. NPJT is best

5 Mayo Clin Proc, April 1995, Vol 70 NARROW QRS COMPLEX TACHYCARDIAS 375 managed by identifying the underlying cause; often, no further treatment is needed. PJRT, as previously noted, is a tachycardia associated with an accessory pathway and is therefore best definitively treated with ablation. CONCLUSION Narrow QRS complex tachycardias can present interesting diagnostic and therapeutic challenges. Meeting those challenges begins with formulation of a cogent differential diagnosis obtainable from the surface electrocardiogram by classifying the tachycardia as long RP or short RP. Adenosine remains the treatment of choice for terminating most types of regular narrow QRS complex tachycardias except those due to sinus tachycardia, atrial tachycardia, fibrillation, or flutter. REFERENCES 1. Bar FW, Brugada P, Dassen WRM, Wellens HJJ. Differential diagnosis of tachycardia with narrow QRS complex (shorter than 0.12 second). Am J Cardiol 1984; 54: Frame LH, Hoffmann BF. Mechanisms of tachycardia. In: Surawicz B, Reddy CP, Prystowsky EN, editors. Tachycardias. Boston: Nijhoff, 1984: Josephson ME. Clinical Cardiac Electrophysiology: Techniques and Interpretation. Philadelphia: Lea & Febiger, Stanton MS. Programmed electrical stimulation: elucidation of arrhythmiamechanisms. In: Fisch C, Surawicz B, editors. Cardiac Electrophysiology and Arrhythmias. New York: Elsevier, 1991: Zipes DP. Specific arrhythmias: diagnosis and treatment. In: Braunwald E, editor. Heart Disease: A Textbook of Cardiovascular Medicine. 4th ed. Philadelphia: Saunders, 1992: Akhtar M. Supraventricular tachycardias: electrophysiologic mechanisms, diagnosis, and pharmacologic therapy. In: Josephson ME, Wellens HJJ, editors. Tachycardias: Mechanisms, Diagnosis, Treatment. Philadelphia: Lea & Febiger, 1984: Josephson ME, Kastor JA. Supraventricular tachycardia: mechanisms and management. Ann Intern Med 1977; 87: Miles WM, Hubbard JE, Zipes DP, Klein LS. Elimination of AV nodal reentrant tachycardia with 2: 1 VA block by posteroseptal ablation. J Cardiovasc Electrophysiol 1994; 5: Stanton MS. Arrhythmias involving accessory pathways: mechanisms and management. Am Coli Cardiol Curr J Rev 1993 Jul/Aug; Hammill SC, Pritchett EL. Simplified esophageal electrocardiography using bipolar recording leads. Ann Intern Med 1981; 95: Gallagher JJ, Smith WM, Kasell J, Smith WM, Grant AD, Benson DW Jr. Use of the esophageal lead in the diagnosis of mechanisms of reciprocating supraventricular tachycardia. PACE Pacing Clin Electrophysiol 1980; 3: Questions About Narrow QRS Complex Tachycardias (See article, pages 371 to 375) 1. Which one of the following tachycardias would adenosine be expected to terminate? a. Sinus tachycardia b. Atrial tachycardia c. Atrial flutter d. Atrioventricular nodal reentrant tachycardia e. Nonparoxysmal junctional tachycardia 2. Which one of the following conditions should be considered when nonparoxysmal junctional tachycardia has been detected? a. Digitalis toxicity b. An accessory pathway c. Hyperkalemia d. Hypercalcemia e. Acute anterior myocardial infarction 3. Electrocardiography in a 25-year-old woman with palpitations showed a regular narrow QRS tachycardia at a rate of 175 beats/min; P waves were identifiable in the ST segment after each QRS complex. Which one of the following is the most likely cause of this arrhythmia? a. Sinus tachycardia b. Atrial tachycardia c. Atrioventricular nodal reentrant tachycardia d. Atrioventricular reentrant tachycardia e. Nonparoxysmal junctional tachycardia 4. Permanent junctional reentrant tachycardia is an unusual, but important, cause of narrow QRS tachycardia. Which one of the following statements is not true about permanent junctional reentrant tachycardia? a. It can be an incessant tachycardia b. It is a reversible cause of dilated cardiomyopathy c. Electrocardiographically, it is a short RP tachycardia d. It is curable by catheter ablation e. It is caused by an accessory pathway 5. AI:1 atrioventricular association is necessary for the diagnosis of which one of the following? a. Atrioventricular reentrant tachycardia b. Nonparoxysmal junctional tachycardia c. Atrial flutter d. Atrioventricular nodal reentrant tachycardia e. Sinus tachycardia Correct answers: