Christopher P. Ingelmo and P. Peter Borek
Supraventricular tachycardias (SVTs) continue to be the most common as well as most diverse type of cardiac arrhythmias. SVTs occur in all age-groups and are associated with a wide range of etiologies, heart rates, frequency, and severity of heart disease. The numerous potential mechanisms and variety of descriptive adjectives for these arrhythmias make establishing a succinct classification system difficult. Approaches to classification of SVT include (a) clinical behavior (i.e., paroxysmal, persistent, permanent, sustained, nonsustained and chronic), (b) mechanism (i.e., ectopic, reentrant, reciprocating, slow/fast, fast/slow, orthodromic, and antidromic), (c) electrocardiographic appearance (i.e., narrow or wide QRS), and finally (d) location (i.e., sinus, atrial, and atrioventricular [AV] nodal/junctional). These types of classifications may provide important information in the diagnosis and treatment of SVTs. Clinicians often have difficulty determining a consistent and inclusive comprehensive management algorithm that may be applied to all forms of SVTs. This chapter briefly reviews the most common SVTs and also provides a concise approach to diagnosis that incorporates all four methods of classification.
Symptoms of SVT may range from none to profoundly disabling. SVT generally presents with a variety of symptoms, the most common of which is palpitations. Palpitations may be accompanied by shortness of breath, chest pain, lightheadedness, near-syncope, and/or syncope. The cardiac evaluation of palpitations may include electrocardiography, ambulatory Holter monitoring, cardiac event recording, transtelephonic monitoring, and, less commonly, an implantable loop recorder. The goals of this diagnostic process are twofold. First, it is necessary to document the arrhythmia. Second, it is helpful to correlate symptoms with the documented arrhythmia. Although ambulatory Holter monitoring is easy to obtain and thus frequently ordered for patients with suspected SVT, its diagnostic yield is very low. Symptoms and simultaneous electrocardiographic abnormalities are observed in only 2% to 13% of 24-hour ambulatory Holter monitors. In contrast, patient-activated cardiac event recorders provide a greater yield and are more cost-effective than ambulatory Holter monitoring in the correlation of arrhythmia with transient symptoms.1
Implantable loop recorders are placed subcutaneously at a parasternal chest site, similar to a pacemaker. These monitors can record arrhythmias for more than a year. They record when activated by an external trigger or following automatic programmed parameters within the device. Because implantation of this device is considered invasive, it is reserved for patients with severe symptoms associated with presumed tachyarrhythmic episodes or those with very rare occurrences that are difficult to diagnose with less invasive testing modalities.
APPROACHING SUPRAVENTRICULAR TACHYCARDIA: AV NODE DEPENDENT VERSUS ATRIAL AND SINUS NODE DEPENDENT
In terms of evaluating arrhythmias based on site of origin, there are three types of SVT: (a) sinus node dependent (sinus tachycardia, inappropriate sinus tachycardia, and sinus node reentry), (b) atrial dependent (atrial tachycardia, atrial flutter, and atrial fibrillation), and (c) AV node/junction dependent (atrioventricular node reentry tachycardia [AVNRT] and atrioventricular reciprocating tachycardia [AVRT]). The first step in classifying SVTs is to differentiate sinus and atrial tachycardias from AV node/junctional tachycardias.2,3 This difference is easiest to see if the tachycardia is observed during changes in AV node conduction. AV node conduction can be altered by a change in vagal tone or with medications such as calcium channel blockers or adenosine. If spontaneous changes in vagal tone do not change AV node conduction, maneuvers may be performed to prolong AV node conduction and refractoriness. Physiologic maneuvers include deep breathing, carotid sinus massage, the strain phase of the Valsalva maneuver, and facial immersion. Carotid sinus massage increases vagal tone and is easily performed at the bedside. In older patients, it is important to exclude the presence of significant carotid artery atherosclerotic disease before performing carotid sinus massage. The “diving reflex” may be applied with pediatric patients. This can be done by placing a plastic bag filled with ice water on the patient ’s face for 15 to 20 seconds. Though this maneuver may be effective, it is less well tolerated and therefore not frequently used.
If physiologic maneuvers are unsuccessful, drugs may be used to alter AV node conduction. Edrophonium chloride (Tensilon) was used classically because its potent vagotonic effect creates temporary AV node block; however, it is often poorly tolerated and therefore is no longer commonly used. Verapamil and diltiazem can also be used to create temporary AV node block. These calcium channel–blocking medications have a slow onset, allowing for a “gentler” diagnosis than sudden AV node block. However, calcium channel blockers have potential for side effects such as hypotension. Adenosine is the most commonly used drug for this purpose because of its extremely short half-life. Adenosine provides a very transient AV nodal block. This effect is maximized when it is administered in a rapid intravenous bolus via a central vein. Care must be taken when administered in patients with bronchospasm as it may exacerbate their reactive airway disease.
Observing tachycardia behavior during slowed AV nodal conduction or AV block may help differentiate between sinus/atrial and AV nodal/junctional tachycardias. Perpetuation of the tachycardia, despite AV block occurs primarily with atrial/sinus tachycardias (Fig. 29.1). Termination of the tachycardia as a result of AV block often implicates the AV node as an essential part of the tachycardia circuit (thus an AV node/junctional-dependent tachycardia). There are rare exceptions to this rule. First, some atrial tachycardias terminate with adenosine administration. However, these atrial tachycardias often slow gradually before terminating, as compared to AV node–dependent arrhythmias, which terminate suddenly. Additionally, if the tachycardia breaks spontaneously, examining the termination can provide insight as to whether the tachycardia is atrial/sinus or AV nodal/junctional. If the tachycardia terminates with a P wave that is not followed by a QRS (Fig. 29.2), it is most consistent with an AV nodal/junctional-dependent tachycardia. This is best explained by understanding that if the rhythm had been an atrial tachycardia, it would have had to terminate at the atrial focus and develop AV block simultaneously, which is very unlikely.
FIGURE 29.1 Atrial tachycardia terminating following the administration of adenosine demonstrating continuation of the atrial tachycardia despite block in the AV node. This would imply that the tachycardia is not AV nodal dependent.
FIGURE 29.2 AV nodal/junctional-dependent tachycardia.
Other electrocardiographic findings are helpful in distinguishing sinus/atrial-dependent from AV nodal/junctional-dependent tachycardia. The P-wave axis indicates the origin of atrial depolarization, which may help differentiate sinus/atrial from AV junction tachycardia. A “high to low” activation sequence is manifested by a surface electrocardiogram positive P-wave deflection in the inferior leads. This is most consistent with a sinus or high atrial tachycardia. AV junctional tachycardias must activate the atria from the area at the AV ring (often near the AV node), leading to “low to high” activation and negative P-waves in the inferior leads.
If the tachycardia appears to have a 1:1 P:QRS relationship, examining the relationship of the R wave to the following P wave may provide additional information to differentiate between sinus/atrial versus AV nodal/junctional tachycardias. If the distance from the R to the following P wave is >50% of the R–R distance, the tachycardia is termed “short RP”. If the distance from the R to the following P is <50% of the R—R distance, the tachycardia is termed “long RP” A short RP interval is more often seen in AV nodal/junctional-dependent tachycardias (AVNRT and AVRT) and a long RP interval is more often seen in sinus/atrial tachycardias. However, this rule also has exceptions that should be noted. Long RP tachycardias, though typically indicative of sinus/atrial dependence, can be seen in atypical (fast–slow) AVNRT or in an unusual form of AVRT that occurs with an accessory pathway (AP) that displays decremental properties. This is seen in the permanent form of junctional reciprocating tachycardia (PJRT). Also, sinus or ectopic tachycardias with a long first-degree AV block may present with a short R–P interval.
In <50% of patients, an appreciable P wave may not be clearly distinct from the QRS. This is because the P wave is hidden within the QRS complex or because the rate and artifact of the tachycardia mask the P wave. In typicalAV node reentrant tachycardia the P wave generally occurs simultaneously with the QRS. In this arrhythmia, the P wave is often inscribed in the terminal portion of the QRS and results in pseudo-R ’ deflection in V1 or S wave in II, III, and aVF. However, appreciation of this change may require comparison of this QRS during tachycardia with the QRS when in normal sinus rhythm (Fig. 29.3).
FIGURE 29.3 A: ECG in typical AV node re-entrant tachycardia. B: ECG in normal sinus rhythm.
AV NODE-DEPENDENT TACHYCARDIAS: AVNRT VERSUS AVRT
Once the above methods lead to establishing that the rhythm is AV nodal dependent, further evaluation is needed to determine what type of AV nodal tachycardia is present. AV node–dependent tachycardias include (a) AVNRT and (b) AVRT using an AP.
AV Nodal Reentry Tachycardia
Clinical Presentation and Diagnosis
In patients without ventricular pre-excitation in sinus rhythm, typical AVNRT is the most common mechanism of SVT, accounting for >60% of presenting SVTs.4–6 As shown in Table 29.1, AVNRT is seen in children as well as in the elderly; however, it is most common in the fourth decade. This arrhythmia affects women more often than men, as women represent two-thirds of the patients with AVNRT. This arrhythmia is not associated with structural heart disease. Although palpitations are the primary symptom of this arrhythmia, syncope and near-syncope have been observed, most notably in elderly patients. Because the right atrium is activated nearly simultaneously with the right ventricle, it is often contracting against a closed tricuspid valve, causing many patients to feel neck fullness. Canon A waves can be seen on physical exam. Because this is an AV node–dependent arrhythmia, termination can be achieved with AV nodal blockade. Onset and termination are characteristically abrupt. AVNRT is often triggered by a premature atrial beat which finds the fast pathway refractory and conducts down the slow pathway with associated PR prolongation consistent with dual AV node physiology AVNRT typically terminates with a retrograde P wave (negative P-wave axis in the inferior leads) without a subsequent QRS indicating AV block and dependence of the tachycardia on AV nodal conduction. Because the majority of the reentry circuit is within the AV node, the atrial and ventricular activation are nearly simultaneous; thus, this is a “short RP” tachycardia. The retrograde P wave is frequently hidden within the QRS complex.
TABLE
29.1 Narrow-QRS Tachycardia: Clinical and Electrophysiologic Characteristics
Diagnostic categories listed in the table represent the final diagnosis made at electrophysiology study. P polarity was not identifiable in AV node reentry and orthodromic tachycardia.
Mechanism
The electrophysiologic circuit of AVNRT uses regions of tissue within or adjacent to the AV node that possess different electrophysiologic properties. Patients with AVNRT have dual AV nodal physiology.Within or near the AV node there is a fast conducting pathway with a long refractory period and a slower conducting pathway with a shorter refractory period. In typical AVNRT (slow–fast form) there is antegrade conduction over the slower pathway and retrograde conduction over the fast pathway (Fig. 29.4). AVNRT typically initiates with a premature atrial beat that arrives at the AV node when the fast pathway is still refractory (longer refractory time), but the slow pathway is able to conduct antegrade. However, because of the slower conduction, by the time the impulse arrives to the compact AV node, the fast pathway has recovered and is able to conduct retrograde back up to the atrium. Thus, typical AVNRT is down the slow pathway and up the fast pathway. Because the atria are activated via the fast pathway, the retrograde P wave is very close to (or buried within) the QRS. As mentioned previously, ECG lead V1 may reveal a pseudo-R prime pattern due to deformation of the terminal portion of the QRS complex by the retrograde P wave. This ECG finding is very specific for AVNRT.
FIGURE 29.4 Typical AVNRT denoting superimposed P waves and retrograde atrial depolarization (arrows).
In the unusual variety or “atypical” AVNRT, antegrade conduction occurs over the fast pathway and retrograde conduction over the slow or intermediate pathway. Though this “atypical” form uses a similar mechanism (dual AV node physiology), although in reverse, it results in a long RP (retrograde slow conduction) and a short PR (fast antegrade conduction) interval. This atypical AVNRT is still AV node dependent and thus may terminate with adenosine or during any change in AV nodal conduction properties.
Treatment
Because both typical and atypical AVNRT are manifestations of dual AV node physiology, they are treated in the same fashion. Patients with unstable hemodynamics should undergo immediate cardioversion. In the acute setting, a hemodynamically stable patient is typically responsive to vagal maneuvers or adenosine. If those are not successful at terminating the tachycardia then an intravenous formulation of beta-blockers and calcium channel blockers may be utilized. If the patient has recurrent arrhythmic episodes, longer-acting medications that alter AV node conduction (including beta-blockers and calcium antagonists) may be used. Though medications can be successful, catheter ablation is considered the treatment of choice. Ablation is a Class I recommendation for patients with one or more episodes of AVNRT who desire resolution of the arrhythmia and patients with recurrent symptomatic or poorly tolerated AVNRT episodes. Catheter ablation of either form of AV node reentry consists of an anatomic ablation of the slow pathway located at the posterior input to the AV node. This area is most often bordered by the coronary sinus and the tendon of Todaro posteriorly and the tricuspid annulus anteriorly. Once identified, radiofrequency or cryotherapy ablation or modification of the slow pathway is performed to the point where tachycardia is no longer inducible.
AV Reciprocating Tachycardia and Ventricular Pre-excitation (Wolff–Parkinson–White Syndrome)
Clinical Presentation and Diagnosis
Accessory AV connections or pathways may also participate in AV nodal–dependent reentrant arrhythmias. The presence of such an AP may be apparent by surface ECG. Because of the rapid and nondecremental conduction properties of these pathways, they may “pre-excite” the ventricle. Ventricular pre-excitation results in a short PR interval and slurring of the upstroke of the QRS ("delta wave"). This ECG abnormality is termed Wolff–Parkinson–White (WPW) pattern. The term WPW syndrome is reserved for patients who also have clinical SVT. It is estimated that 1 to 1.5 in 1,000 ECGs show a WPW pattern. Population-based studies have indicated that 50% to 60% of patients with WPW pattern demonstrate symptoms that may include palpitations or more severe symptoms such as syncope.7Approximately 85% of such symptomatic patients have AV reentrant tachycardia using the AV node as the antegrade limb and the AP as the retrograde limb of the circuit.8,9 Approximately 30% to 40% of patients with WPW are capable of sustaining atrial fibrillation.
This type of tachycardia is not only dependent on the AV node but requires the presence of an AP. This pathway provides an additional connection between the atria and ventricles, other than the AV node. The term “pre-excitation” stems from the fact that the ventricle receives electrical activation from both the AV node-His-Purkinje system (AVN-HPS) and the AP Because antegrade conduction via the AP is not decremental as with the AV node, the ventricle is activated via the pathway before the AVN-HPS activation. This pre-excitation is represented by the delta wave on the ECG. When there is evidence of ventricular pre-excitation by ECG at baseline, the AP is described as “manifest.” This pre-excitation tends to be more evident at more rapid atrial rates, when AV nodal conduction is slowed, making preexcitation even more apparent. Occasionally, conduction through the AV node is preferred to that over the AP. This may be secondary to slow or poor antegrade conduction in the AP or to the location of the pathway. In these patients there may be minimal or no delta wave on the ECG, as the ventricle is predominantly or completely activated via the AV node. The AP is still present and may be able to conduct retrograde (and thus be able to cause tachycardia), but it is not evident in sinus rhythm. These APs are termed “concealed.”
It is important to remember that the presence of an AP does not always indicate that this pathway is a critical part of the presenting SVT. For this reason it is important first to determine the role of the AV node (dependent or independent) in the presenting SVT. APs provide only an additional connection between the atria and ventricles. Patients with an AP and even a history of WPW syndrome may also have other SVTs (i.e., AVNRT) that have no relationship to the AP.
Accessory Pathways
APs may be located along either the mitral or tricuspid annuli. These pathways consist of a bundle of myocardial muscle that bypasses the AV groove with direct insertion on the ventricular myocardium of the right or left ventricle. On rare occasions, the lower insertion is in proximity or attached to the branches of the right bundle. As shown in Table 29.2, of all APs, almost half are located on the left side of the heart along the mitral annulus, nearly one-third are in the posteroseptal region, and the remainder are at the right anteroseptal region or the right lateral wall of the tricuspid annulus.
TABLE
29.2 Distribution of Most APs
Multiple electrocardiographic algorithms have been proposed to diagnose the location of the AP from surface 12-lead ECGs (Fig. 29.5).9–14 Unfortunately, no algorithm has proven entirely reliable. A general and easy classification proposed by Rosenbaum and Hecht15 describes a “type A pattern,” with a positive delta wave and QRS in the precordial leads (Fig. 29.6), which is usually associated with a left lateral AP, and a “type B pattern,” with a left bundle branch block-type QRS morphology in the precordial leads, which is usually associated with a right-sided pathway (Fig. 29.7).
FIGURE 29.5 Milstein ’s algorithm for localization of APs. LL, left lateral; PS, posteroseptal; RAS, right anteroseptal; RL, right lateral. *LBBB, +QRS LL, rS V, and V2. (Reprinted from Milstein S, Sharma AD, Guiraudon GM, et al. An algorithm for the electrocardiographic localization of accessory pathways in the Wolff–Parkinson–White syndrome. Pacing Clin Electrophysiol. 1987;10(3 pt 1): 555–563, with permission from John Wiley and Sons.)
FIGURE 29.6 Type A accessory pathway.
FIGURE 29.7 Type B accessory pathway.
Mechanism
AVRT is a reentrant arrhythmia. In AVRT, one limb of the reentry circuit is the AV node and the other limb is the AP. The clinical importance of APs and pre-excitation reside primarily in their predisposition to tachyarrhythmias. The conduction properties of the AP (speed of conduction and recovery) determine the likelihood of developing a reentrant circuit arrhythmia. The direction of the circuit differentiates the two types of AVRT: orthodromic AVRT and antidromic AVRT. Orthodromic reciprocating tachycardia (ORT) comprises the majority of the reciprocating tachycardias associated with WPW syndrome. The antegrade limb of this reentrant circuit is the AV node, whereas the retrograde limb is the AP. Because the ventricle is activated via the AV node, the QRS complex is narrow. Though it is rare, antidromic reciprocating tachycardia (ART) uses the AP as the antegrade limb and the AV node as the retrograde limb. Because the ventricle is activated via the AP, the QRS complex is wide (maximal pre-excitation).
Electrophysiologic Characteristics and Diagnostic Maneuvers
ORT is typically characterized by a short RP and a long PR interval as the circuit is conducting up the AP and down the AV node. Because conduction from the atria to the ventricles is via the AV node, the QRS morphology during tachycardia should be similar (in the absence of aberration) to the QRS morphology during normal sinus rhythm.
Though algorithms based on the surface ECG are notoriously inconsistent, behavior of the AP during tachycardia can provide clinicians with hints as to the location of the pathway. For example, spontaneous or induced functional bundle branch block during ORT can yield a diagnostically useful phenomenon. As demonstrated in Figure. 29.8A,B, if the bundle branch block is ipsilateral to a free wall bypass tract, the retrograde reentrant impulse is compelled to traverse a greater distance from the His–Purkinje fibers via the AP and to the atrium. As a result, the global VA conduction time during the tachycardia must increase (usually by at least 35 milliseconds). The tachycardia cycle length may also increase such that the rate of the tachycardia may become slower. In contrast, if the bundle branch block is contralateral to a free wall bypass tract, there is no change in the distance the retrograde reentrant impulse has to travel to reach the atrium. Thus there is no effect on the tachycardia cycle length. Therefore, an SVT that slows with the development of bundle branch block should invoke suspicion that the tachycardia is ORT using an AP located ipsilateral to the site of bundle branch block.
FIGURE 29.8 A - A narrow complex tachycardia utilizing the ipsilateral bundle (left bundle) antegrade and accessory pathway retrograde. Note the cycle length of the tachycardia and compare to B where the tachycardia now utilizes the contralateral bundle as the left bundle is blocked. This may result in prolongation of the cycle length of the tachycardia and is diagnostic of the location of the pathway.
Exceptions and Rare Forms of AVRT
Occasionally, patients present with an incessant ORT in which retrograde conduction up the AP is decremental.
Though it is rare, this type of AP is associated with a tachycardia called the PJRT. PJRT typically uses a concealed posteroseptal AP with a long conduction time and decremental AV node–like properties. These decremental properties prolong the time between the R and P, often making it a “long RP” tachycardia rather than the typical “short RP.” Because of the incessant nature of this arrhythmia, it has been associated with tachycardia-induced cardiomyopathy. Tachycardia-induced cardiomyopathy is not related to any one specific SVT, but is associated with any incessant or persistent tachycardia.
Antidromic tachycardia is the least common arrhythmia associated with WPW syndrome, occurring in only 5% to 10% of patients. This tachycardia is characterized by a wide QRS complex that is fully pre-excited with a regular R–R interval. If the diagnosis of WPW is not recognized, this tachycardia may be mistaken for ventricular tachycardia.
Uncommonly, patients may have more than one AP. In these cases there are multiple potential reentrant arrhythmia circuits. Of these patients, 33% to 60% will present with antidromic tachycardia. Even less common is a tachycardia involving two APs as both the antegrade and retrograde limbs of the circuit, without any involvement of the AV node. In these very rare cases, the tachycardia does not terminate with AV node blockade. In patients with more than one pathway, more likely scenarios are one pathway involved in either ORT or ART with the AV node as one limb of the circuit, and the other pathways as “bystanders.” The term “bystander” describes accessory AV pathways that exist, but that are not part of the tachycardia circuit. The typical use of the term “bystander conduction” involves a wide-complex pre-excited SVT that is due to typical AVNRT (antegrade fast AV nodal pathway conduction and retrograde slow AV nodal pathway conduction) with concomitant antegrade conduction through the AP producing the wide-complex QRS pattern.
Occasionally, the presence of multiple APs will become apparent only after the dominant pathway has been ablated. The Ebstein anomaly is associated with WPW syndrome in 6% to 26% of patients with this congenital heart defect. In addition, 40% to 55% of patients with WPW and the Ebstein anomaly have multiple APs.16
Treatment
Treatment of a patient with WPW syndrome (presence of an AP and tachycardia) may involve both drug therapy and catheter ablation. In the acute setting, patients with ORT (conduction down the AV node and thus a narrow QRS complex) can be treated with AV nodal-blocking agents such as adenosine and vagal maneuvers. Chronic treatment may be directed at any essential component of the circuit. Administration of calcium channel antagonists or beta-adrenergic blockers affects the AV node, whereas antiarrhythmic drugs including flecainide, propafenone, quinidine, procainamide, and amiodarone may be chosen to target the AP. More invasive options such as catheter ablation therapy of the AP play a more prominent role in management of WPW syndrome. Catheter ablation is a Class I recommendation for patients with WPW syndrome with or without atrial fibrillation as well as patients with no evidence of pre-excitation by Electrocardiogram (EKG) but poorly tolerated AVRT episodes. With newer computerized mapping technologies, the acute success rate nears 100%, with a <1% risk of significant complications. However, pathway location may play a role in risk during ablative procedures in that some right anteroseptal pathways are very close to the AV node, increasing the possibility of disruption of normal conduction during ablation.
Wolff–Parkinson–White Syndrome and Sudden Cardiac Death
In contrast, in patients presenting with pre-excited tachycardia, adenosine and AV node-blocking agents are absolutely contraindicated. In these rhythms, the ventricles are being activated antegrade via the AP Often these rhythms are atrial arrhythmias (atrial tachycardia, atrial fibrillation, and atrial flutter) that are conducting predominantly down the AP (and partially down the AV node). In these scenarios, the “bystander” AP is conducting the tachycardia to the ventricle. If an AV-blocking agent is administered, conduction may be exclusively through the AP Because these pathways typically do not have protective decremental properties as the AV node does, rhythms such as atrial flutter/fibrillation (>300 beats/min [bpm]) may be conducted to the ventricle in a 1:1 fashion (Fig. 29.9). AV conduction at that rate may degenerate rapidly into ventricular fibrillation and subsequent cardiac arrest. In these patients, procainamide, flecainide, propafenone, or amiodarone should be considered. In case of atrial fibrillation/flutter with rapid response and hemodynamic instability, electrical direct-current cardioversion is the treatment of choice.
FIGURE 29.9 Atrial arrhythmia in WPW syndrome.
As shown in Table 29.3, atrial fibrillation may be a coexisting or presenting arrhythmia in about 20% to 40% of patients with WPW syndrome. Atrial fibrillation in the presence of an AP with rapid antegrade conduction can result in degeneration to ventricular fibrillation and subsequently result in sudden cardiac death. The risk of sudden death for patients with WPW syndrome is not clear but is definitely not very high. Population-based studies suggest an incidence of 0.15% per year, and sudden death occurs almost exclusively in previously symptomatic patients.7
TABLE
29.3 Tachyarrhythmias in 161 WPW Patients
RT, reciprocating tachycardia; AF, atrial fibrillation; VF, ventricular fibrillation.
This occasional occurrence of ventricular fibrillation as the initial manifestation of WPW syndrome has stimulated interest in the possibility of identifying asymptomatic patients who may be at risk for this complication. These patients possess antegrade conducting APs (pre-excitation during normal sinus rhythm) and no symptoms of SVT. Screening these patients involves evaluating the conduction properties of the AP. An easy and noninvasive method is to observe for intermittent ventricular pre-excitation by electrocardiogram, which may involve supervised treadmill stress testing or simply ambulatory Holter monitoring during the patient ’s daily activities. This intermittent pre-excitation refers to the abrupt loss of pre-excitation or the delta wave from one beat to the next. This phenomenon suggests an AP that is incapable of extremely rapid antegrade conduction and therefore carries a low risk for sudden cardiac death.17
If normalization of the QRS is not observed during Holter monitoring or during exercise, electrophysiologic evaluation must be considered to assess the conduction property of the AP. The most direct method for such risk stratification is the induction of atrial fibrillation in an electrophysiologic laboratory setting and the determination of the shortest R–R interval. This provides information as to the antegrade conduction capabilities of the AP. Studies have shown that patients with WPW syndrome who experienced and survived an episode of sudden cardiac death had the shortest R–R intervals.18 A shortest R–R interval of >250 milliseconds indicates an AP that is incapable of dangerously rapid antegrade conduction. However, caution should be exercised about drawing conclusions if the shortest R–R interval is <250 milliseconds, as this does not necessarily confer a high risk of sudden death for a patient with WPW syndrome. The positive predictive value of this finding is only 20%. Therefore, the finding of a shortest R–R interval <250 milliseconds may mean simply that the patient cannot be told that he or she is excluded from the small subset of patients with WPW syndrome who will ultimately experience sudden cardiac death.
In addition, because of the very high cure rates achievable with catheter ablation, this approach may be the best treatment option for patients involved in high-performance physical activity or with specific jobs, such as pilots or bus/truck drivers. In symptomatic patients with WPW syndrome, particularly those with frequent SVT, catheter ablation may be considered as the first-line treatment option.
In general, chronic treatment of AV nodal–dependent SVTs includes AV nodal–blocking medications (beta-adrenergic blockers and/or calcium channel antagonists) and catheter ablation. In general, it is recommended that the more invasive strategy of catheter ablation be reserved for those patients in whom medical therapy has failed or is poorly tolerated. However, there is evidence to support the consideration of catheter ablation as a first-line therapy for patients with frequent episodes of SVT, as catheter ablation in these patients is more effective and more cost-effective than medical therapy over time.
Radiofrequency catheter ablation of SVT has been shown to be cost-effective and to improve quality of life for patients with WPW syndrome who survive cardiac arrest or who experience SVT or atrial fibrillation19 and for highly symptomatic patients with AV nodal reentrant tachycardia or AV reentrant tachycardia using a concealed AP.20
ATRIAL-DEPENDENT ARRHYTHMIAS
There are three predominant types of atrial-dependent arrhythmias: (a) atrial tachycardia, (b) atrial flutter, and (c) atrial fibrillation. This section discusses the first two; the third, atrial fibrillation, is discussed elsewhere in this textbook.
Atrial Tachycardia
Clinical Presentation and Diagnosis
Atrial tachycardias require only the atrium for the initiation and maintenance of the arrhythmias. It is found in all age-groups and has a wide range of potential mechanisms and clinical presentation. Atrial tachycardia can be completely asymptomatic in some individuals versus recurrent and disabling in others. Short episodes of atrial tachycardia may be seen on ambulatory Holter monitoring in 2% to 6% of normal young subjects and in up to 29% of older subjects. The majority of these episodes are asymptomatic and do not require treatment. However, in about 10% of patients, atrial tachycardia is associated with significant symptoms. As shown in Table 29.1, atrial tachycardia is more prevalent in older patients and is common in patients with underlying heart disease.
Mechanisms
There are several possible mechanisms for atrial tachycardia. Automatic or focal atrial tachycardias typically originate from a discrete area within the atria. Most commonly they originate from the right atrium in the crista terminalis region, but they can be located anywhere within the atria. They are often incessant and unresponsive to medical therapy. In fact, the incessant nature of automatic atrial tachycardia may predispose these patients to tachycardia-mediated cardiomyopathy.
Another distinct mechanism is multifocal atrial tachycardia (MAT). MAT is a SVT characterized by multiple P-wave morphologies, a varying PR interval, and an irregular ventricular response. Though the mechanism is not well understood, many feel that it is the result of multiple automatic or triggered foci driving the atria at different rates. It is distinct from atrial fibrillation in that there are well-defined P waves. By definition, there are at least three distinct P-wave morphologies and thus at least three different sources of atrial depolarization.
Treatment
As with most arrhythmias, treatment approaches for atrial tachycardia are twofold: (a) medication and (b) catheter ablation. Medications such as beta-adrenergic blockers and calcium channel antagonists can be used, although most ectopic atrial tachycardias and reentrant atrial tachycardias are not terminated or prevented by these drugs. These medications can be considered in the acute setting to control the ventricular response by decreasing AV conduction. Unfortunately, they often do little to slow or control the actual atrial focus. Other agents, including Class IA, IC, and Class III drugs are more effective in maintaining sinus rhythm or terminating the arrhythmia. Catheter ablative procedures are an alternative treatment strategy for patients in whom medical therapy fails.
MAT is most often observed in chronically ill patients, frequently those with respiratory failure or chronic pulmonary or cardiac disease. It has also been associated with digoxin toxicity, electrolytic imbalance (hypokalemia and hypomagnesemia), acute myocardial infarction, and mitral valve disease. Intravenous verapamil, intravenous potassium and magnesium have been used in the acute treatment of this arrhythmia, but with limited success. Low doses of beta-adrenergic antagonists have also been suggested but may be problematic in the majority of patients with chronic lung disease. Long-term treatment usually involves the use of a calcium channel antagonist, often requiring relatively high doses. However, the most effective therapy for MAT is treatment of the underlying medical condition or abnormality, such as treatment of bronchospasm and hypoxemia and correction of electrolyte disturbances. Cardioversion is generally ineffective for this arrhythmia disorder. Targeted catheter ablation is not effective. AV node ablation and permanent pacing may be considered in cases that are refractory to conventional medical therapy.
Atrial Flutter
Clinical Presentation and Diagnosis
Atrial flutter is a reentrant atrial tachycardia. Patients with atrial flutter can be divided into two categories: (a) those with previous ablation or cardiac surgery and (b) those with no previous ablation or cardiac surgery. Atrial flutters typically present with a regular atrial rhythm up to 300 to 350 bpm conducting to the ventricle at various rates but often at a constant multiple of the atrial rate (2:1, 4:1, etc.). Patients may be asymptomatic or with symptoms ranging from palpitations to syncope and even heart failure. The classic EKG for typical atrial flutter is characterized by negative sawtooth waves in the inferior leads with no isoelectric interval, positive flutter waves in V1 and V2 and negative flutter waves in V5 and V6 (Fig. 29.10). Atrial flutter is often a precursor to atrial fibrillation with up to 25% to 35% of patients who undergo atrial flutter ablation developing atrial fibrillation in the future.21
FIGURE 29.10 Typical sawtooth pattern of flutter waves.
Mechanism
Atrial flutter involves a reentrant circuit and is typically dependent on conduction through the caval-tricuspid isthmus. Atrial flutter circuits rotate around areas of nonconducting tissue within the atrium. In patients with no history of previous ablation or cardiac surgery, the area of nonconducting tissue is typically anatomic (nonconducting tissue such as the valve rings or great vessels). Typical right atrial flutter most commonly takes a “counterclockwise” rotation as it courses around the crista terminalis, through the isthmus between the tricuspid valve and inferior vena cava, up the septum, and back around to the crista. This circuit may occur in the opposite direction in a “clockwise” direction.
In patients with a previous history of ablation or cardiac surgery, the area of nonconducting tissue is usually a site of scar from previous incisions or radiofrequency lesions. Common sites include the right atriotomy scar, the septum, perimitral valve, and near areas of previously ablated tissue.
Treatment
Like other atrial-dependent rhythms, atrial flutter is often difficult to manage medically. Beta-adrenergic blockade and calcium channel antagonists are options that can be used in the acute setting to slow conduction via the AV node and thus slow the ventricular response. Antiarrhythmic medications may also be used; however, the use of Class IC agents (flecainide, propafenone) may slow the cycle length of the flutter, permitting 1:1 AV conduction. Because of this risk, AV nodal–blocking agents (beta-blockers or calcium channel antagonists) should always be given in addition to Class IC agents. In contrast, Class III drugs may terminate and prevent atrial flutter by prolonging atrial refractoriness. Catheter ablation may be an option for many patients. With use of the newer computerized mapping technology, improved mapping of the reentrant circuit has increased success rates. During catheter ablation the goal is to interrupt the circuit in such a way that reentry cannot perpetuate. In typical isthmus-dependent right atrial flutter, a line of block with ablative lesions can be employed between two nonconducting structures (e.g., the tricuspid valve and the inferior vena cava). Once this is in place, the milieu for reentry ceases to exist.
SINUS-DEPENDENT ARRHYTHMIAS
Sinus-dependent tachycardias are rare forms of atrial tachycardias that originate at or within the area of the sinus node. These arrhythmias include sinus node reentrant tachycardia and inappropriate sinus tachycardia. Both need to be differentiated from physiologic sinus tachycardia, which is an increase of heart rate secondary to either cardiac or noncardiac etiology. In the latter form, treatment of the underlying disease may result in resolution of the sinus tachycardia. Occasionally, it is desirable to slow sinus tachycardia for symptomatic relief while the underlying etiology is being addressed. For example, beta-adrenergic blockers may be useful for thyrotoxicosis or for sinus tachycardia associated with acute myocardial infarction in the absence of heart failure. However, both sinus node reentrant tachycardia and inappropriate sinus tachycardia are arrhythmias that generally behave differently than normal sinus tachycardia. These tachycardias, characterized by a P-wave morphology similar to that observed during normal sinus rhythm, persist without any physiologic cause. Sinus node reentry is a reentry mechanism within the sinus node and is usually observed in older patients with concomitant heart disease. In contrast, inappropriate sinus tachycardia in both the chronic and paroxysmal forms is observed mostly in young adult women.
Treatment of these arrhythmias is often very difficult. Like most tachycardias, rate control with medications such as beta-blockers and calcium channel antagonists may be used. Antiarrhythmic drug therapy has not been shown to be highly effective for these tachycardias but may be considered. When medical treatment is ineffective, catheter ablation may be considered. However, particularly for inappropriate sinus tachycardia, success with sinus node modification has been limited and should not be considered primary therapy. These arrhythmias are rare, and evaluation should focus on elucidating a possible physiologic mechanism (i.e., thyroid disease). Inappropriate sinus tachycardia should be differentiated from a specific syndrome called postural orthostatic tachycardia syndrome (POTS), which is characterized by orthostatic hypotension and 40 to 50 beats increase in the heart rate within 10 minutes after assuming a standing position. This is important in that POTS is managed medically, with catheter ablation contraindicated.
CONCLUSION
SVT is a descriptive diagnosis with varying pathologies, mechanisms, and treatments. Many of these underlying mechanisms can be determined from the surface electrocardiogram and via utilization of bedside maneuvers. It is important to implement a systematic approach to SVT, not only to define the underlying mechanism but also to provide rapid, effective, and appropriate treatment (Fig. 29.11).
FIGURE 29.11 Algorithm for treatment of SVT.
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QUESTIONS AND ANSWERS
Questions
1. A 44-year-old patient with no previous cardiovascular history, who presents with a wide-QRS, irregular, and fast tachycardia (on ECG) is best treated with:
a. Lidocaine
b. Procainamide
c. Metoprolol
d. Diltiazem
2. Catheter ablation is an established and well-accepted treatment for each of the following tachycardia types except:
a. AV node reentry
b. AV reentry
c. Permanent junctional tachycardia
d. Sinus tachycardia
2. Which of the following forms of congenital heart disease is commonly associated with Wolff–Parkinson–White (WPW) syndrome?
a. Aortic stenosis
b. Ebstein anomaly
c. Pulmonary stenosis
d. Atrial septal defect
2. Which of the following supraventricular tachycardias (SVTs) is associated with tachycardia-induced cardiomyopathy?
a. Permanent junctional tachycardia
b. Incessant atrial tachycardia
c. Atrial flutter with rapid ventricular response
d. All of the choices
2. Which test would you consider for an asymptomatic 31-year-old man with intermittent pre-excitation?
a. Holter monitoring
b. Electrophysiologic study
c. Exercise test
d. Catheter ablation
e. None of the choices
2. Conduction block in the atrioventricular (AV) node without termination of the tachycardia is compatible with all of the following mechanisms except:
a. Atrial tachycardia
b. AV reentry tachycardia
c. Atrial flutter
d. Sinus tachycardia
2. The initial manifestations of WPW syndrome include which of the following?
a. Atrial fibrillation
b. AV reentry tachycardia
c. Ventricular fibrillation
d. Wide-QRS tachycardia
e. All of the choices
2. Administration of metoprolol is more likely to terminate:
a. Sinus tachycardia
b. Atrial tachycardia
c. Atrial fibrillation
d. AV reentry tachycardia
2. Which of the following is the treatment of choice to terminate a narrow-QRS tachycardia?
a. Metoprolol
b. Diltiazem
c. Adenosine
d. Procainamide
e. Cardioversion
2. For a patient with WPW syndrome who presents with a regular wide-QRS tachycardia, all of the following are possible treatment choices except:
a. Procainamide
b. Cardioversion
c. Amiodarone
d. Ibutilide
e. Adenosine
2. Transesophageal recording may help in establishing the diagnosis in which of the following SVTs?
a. Atrial tachycardia
b. AV node reentrant tachycardia
c. AV reentrant tachycardia
d. Atrial flutter
e. All of the choices
2. A decrease in the tachycardia rate with development of bundle branch block is consistent with:
a. Atrial tachycardia
b. AV node reentry
c. AV reentry
d. Sinus tachycardia
2. The presence of an r prime in V1 during narrow-QRS tachycardia is suggestive of:
a. AV reentry
b. AV node reentry
c. Rate-dependent bundle branch block
d. Atrial tachycardia
2. The best therapy for multifocal atrial tachycardia (MAT) is:
a. Digoxin
b. Diltiazem
c. Metoprolol
d. Flecainide
e. Treatment of the underlying disorder
Answers
1. Answer B: This patient has atrial fibrillation with pre-excited QRS and should be treated with procainamide. AV node–blocking agents are absolutely contradicted because they will favor conduction over the accessory pathway with increased risk of degeneration into ventricular fibrillation.
2. Answer D: Catheter ablation is an established therapy for all the arrhythmias listed except sinus tachycardia.
3. Answer B: Ebstein Anomaly is associated with WPW in 6% of patients with this congenital anomaly.
4. Answer D: All tachycardias of incessant nature can cause tachycardia-induced cardiomyopathy.
5. Answer E: No further investigation or treatment is indicated for an asymptomatic patient with intermittent pre-excitation.
6. Answer B: The only tachycardia that cannot sustain with conduction block in the AV node is AV reentry.
7. Answer E: All of the choices are possible arrhythmias in WPW syndrome.
8. Answer D: Sinus tachycardia will slow down but not terminate. Atrial tachycardia and atrial fibrillation will not be affected by metoprolol.
9. Answer C: Adenosine is the best acute treatment for narrow-QRS tachycardia.
10. Answer E: Adenosine and AV node–blocking agents are contraindicated in pre-excited arrhythmias.
11. Answer E: Transesophageal recording can provide information that is helpful in establishing a diagnosis in all of the SVTs listed.
12. Answer C: AV reentry due to an accessory bypass tract ipsilateral to the bundle branch block is the only arrhythmia associated with the above behavior.
13. Answer B: The correct answer is AV node reentry.
14. Answer E: No drug therapy will be effective for MAT if the underlying disorder is not corrected.