Wolff Parkinson White Syndrome: Diagnosis and Treatment

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Wolff-Parkinson-White Syndrome: What every physician needs to know.

The term Wolff-Parkinson-White (WPW) syndrome is used to refer to the combination of supraventricular arrhythmias and an electrocardiographic pattern of preexcitation. This syndrome was first described in 1930 in an article by Louis Wolff, Sir John Parkinson, and Paul Dudley White. The authors described eleven patients with recurrent tachycardia associated with an ECG pattern of “Bundle Branch Block (BBB) with short PR interval.” Since publication of this initial report, our understanding of the anatomic and pathophysiologic features of preexcitation syndromes has improved enormously. The purpose of this brief review is to review the pathophysiologic basis of the WPW syndrome, and the approach to diagnosis and management.

In a normal heart, the electrical impulse is initiated by the sinoatrial node (SAN) and conducted to the atrioventricular node (AVN) to subsequently propagate to the ventricles through an efficient conduction system, His-Purkinje fibers. The slow propagation of the electrical impulse through the AV node is reflected in the PR interval. Because the conduction through the His-Purkinje system is brisk, the resulting QRS complex has narrow morphology.

WPW syndrome occurs when there is an antegradely conducting accessory pathway (AP), as well as one or more types of supraventricular arrhythmias. The term accessory pathway refers to the presence of an anomalous pathway that connects the atria and ventricles, thereby short-circuiting the normal AV node and His-Purkinje system. A pattern of preexcitation on a 12-lead ECG reflects the presence of an antegradely conducting accessory pathway and consists of the constellation of a short PR interval, a widened QRS complex, and a delta wave – which is a slow upstroke of the QRS complex. (Figure 1, Figure 2).

Figure 2.

APs result from failure of fibrous separation between the atria and the ventricles during the embryological development. These muscle bundles typically connect the epicardial surfaces of the atria and the ventricles along the AV groove outside of the regular atrioventricular conduction system. It is present in 0.15% to 0.25% of the general population and a higher prevalence (0.55%) has been reported in first-degree relatives of patients with WPW syndrome. APs may conduct antegradely from the atria to the ventricles, retrogradely, or both. APs which conduct antegradely are termed “manifest” whereas those that conduct only from the ventricles to the atria are termed “concealed”.

The degree of shortening of the PR interval and the extent of ventricular preexcitation depend on several factors, including location of the AP, the relationship between antegrade conduction times and refractory periods of the AP, and the properties of the normal AV conduction system. An AP that crosses the AV groove in the left lateral region may also result in inapparent preexcitation and minimal PR interval shortening during sinus rhythm because of greater interatrial distance for impulse propagation from the sinus node to the site of atrial input of the AP. Conversely, an AP on the right side is more likely to demonstrate marked preexcitation.

Preexcitation may be less apparent during sinus tachycardia, when sympathetic tone is high and vagal tone is low, resulting in faster AV node conduction time than that in the AP. On the other extreme, during conditions of slowed conduction through the AV node by intrinsic nodal factors, withdrawal of sympathetic tone, or increased vagal tone, the degree of preexcitation apparent on the 12-lead ECG is maximized because of relatively greater conduction through the AP. Rapid intravenous administration of adenosine causing blockage or slowing of AV nodal conduction and exposing the anterograde AP conduction has been used as a diagnostic maneuver.

The degree of preexcitation can also be enhanced with atrial pacing directly over the AP, eliminating the intra-atrial conduction delay from the sinus node to the atrial insertion site of AP (Figure 3). Intermittent preexcitation is characterized by abrupt loss of delta wave, normalization of the QRS duration, and an increase in the PR interval during a continuous ECG recording, often despite only minor variations in resting sinus rhythm heart rate. This should be distinguished from day-to-day variability in preexcitation or inapparent preexcitation caused by factors described above.

The presence of intermittent preexcitation has been considered to suggest that the antegrade refractory period of the AP is long, making them very unlikely to mediate a rapid, preexcited ventricular response during atrial fibrillation which is known to be a risk factor for sudden cardiac death. On the other hand, if the preexcitation persists despite significant increase in sinus rate (eg, during exercise), the AP is thought to have short antegrade refractory period, putting the patient at risk for faster antegrade conduction during atrial fibrillation.

Are there different types of accessory pathways?

APs can be classified based on their site of origin and insertion, location along the mitral or tricuspid annulus, type of conduction, and properties of conduction APs that connect the atrium to the distal AVN or His-Purkinje bundle are rare and their physiologic significance is uncertain. APs usually exhibit rapid, non-decremental conduction, similar to His-Purkinje tissue and atrial or ventricular myocardium. Approximately 8% of APs display decremental antegrade or retrograde conduction.

As mentioned previously, the APs that are capable of retrograde conduction only are referred to as concealed and those capable of antegrade conduction are referred to as manifest, demonstrating preexcitation on a standard ECG (Figure 1). About 60% of APs conduct both antegrade and retrograde. Antegrade-only APs are particularly uncommon (<5%). When present, they are usually right-sided and frequently demonstrate decremental conduction. Concealed APs account for approximately 17%-37% of all APs.

Patients with Ebstein’s anomaly who also have WPW syndrome frequently have more than one AP. Variant APs include those that connect the atrium to the distal or compact AV node (James fibers), the atrium to the His bundle (Brechenmacher fibers), and the AV node or His bundle to the distal Purkinje fibers or ventricular myocardium (Figure 4). In 1937, Mahaim reported the pathology specimens that showed a group of fibers that connect the His bundle and the ventricular tissue. These fibers were named as Mahaim fibers and soon after, other APs such as nodoventricular and atriofascicular were also included in this group. Therefore, the use of the term Mahaim fibers should be avoided as it could be a source of confusion due to lack of anatomic specificity.

Figure 4.

Classification of Accessory Pathways

• Based on site of origin and insertion

  Atrioventricular

  Right-sided: Anteroseptal, right ventricle free wall, posteroseptal

  Left-sided: Anteroseptal, left ventricle free wall, posteroseptal

  Atriofascicular

  Nodofascicular

  Nodoventricular

  Fasciculoventricular

• Based on direction of the conduction

  Antegrade only

  Retrograde only (concealed)

  Bidirectional

• Based on the conduction property

  Slow, decremental

  Fast, nondecremental

• Based on number

  Single

  Multiple

Are there any specific tachycardias associated with accessory pathways?

Although patients with WPW syndrome can have any type of arrhythmia as others (eg, sinus tachycardia, atrial tachycardia, atrial flutter, atrial fibrillation, or ventricular tachycardia), there are two specific tachycardias that are associated with APs and these can be divided in reentrant and non-reentrant:

• Atrioventricular reentrant tachycardia (reentrant)
• Preexcited atrial fibrillation (non-reentrant)

Atrioventricular reentrant tachycardia (AVRT) is a macroreentrant tachycardia involving the atrium, the AP, the AV node, and the ventricle. AVRT is further subclassified into orthodromic- and antidromic-reciprocating tachycardia (ORT and ART, respectively). During ORT, the reentrant impulse uses the AVN-His-Purkinje system for antegrade conduction and uses the AP for retrograde conduction. Conversely, during ART, the reentrant impulse uses the AP for antegrade conduction and the AVN-His-Purkinje for retrograde conduction.

ORT can be initiated by atrial or ventricular premature depolarizations (APDs and VPDs, respectively). APDs initiating the tachycardia blocks antegradely in the AP and conducts relatively slowly over the AV nodal tissue to the ventricles. The impulse then retrogradely reaches the AP and finds it nonrefractory as the premature beat allowed more time to recover; hence, this impulse can reenter the atrium at the atrial insertion site of the pathway completing the reentrant loop. A VPD, conversely, blocks in the His-Purkinje system and reaches the AP through the ventricular insertion site. If the AP is not refractory by then and the AP has the ability to conduct retrogradely, it can conduct to the atrium and subsequently to the AVN completing the circuit.

As the ORT uses the His-Purkinje system for antegrade conduction, the QRS complex during tachycardia is narrow (Figure 5) unless there is underlying bundle branch block or intraventricular conduction delay. On the other hand, during ART, the reentrant impulse travels in the reverse direction, with conduction from the atrium to the ventricle occurring via the AP. As the usual ventricular insertion site of the AP rests on the ventricular myocardium, the QRS complex is wide during the tachycardia (Figure 6). ART is rare, occurring in only 5% to 10% of patients with WPW syndrome.

Figure 5.

Figure 6.

APDs that occur at a coupling interval that is longer than the refractory period of the AP but shorter than the AV nodal refractory period can initiate the ART; the converse is true with a VPD. Susceptibility of ART appears to depend on the existence of adequate separation between the AP and the AV nodal tissue. Hence, most of the antidromic AVRTs seem to occur with left-sided APs.

Other forms of SVTs, including atrial tachycardia, junctional tachycardia, AVNRT, and even ventricular tachycardia may occur in patients with APs. Dual AV nodal physiology has been noted in nearly 12% of patients with WPW syndrome. Coexisting ventricular tachycardia is less likely because patients with WPW syndrome tend to present at a younger age and have less structural heart disease. Atrial fibrillation is a less common but potentially more serious arrhythmia in patients with the WPW syndrome. If an AP has a short antegrade refractory period, atrial fibrillation may result in a rapid ventricular response with subsequent degeneration to ventricular fibrillation.

A number of risk factors have been identified to be associated with a higher risk of sudden cardiac death. These include antegrade AP refractory time of less than 250 ms, Shortest Preexcited RR Interval (SPERRI) during preexcited AF of less than 250 ms, presence of multiple APs, and inducible AVRT and sustained AF during the electrophysiologic study (EPS). APs appear to play a pathophysiologic role in the development of atrial fibrillation in these patients because most are young and do not have structural heart disease. Importantly, surgical or catheter ablation of APs usually results in elimination of atrial fibrillation as well.

Prevalence, symptoms and prognosis of WPW syndrome

An electrocardiographic pattern of preexcitation occurs in the general population at a frequency of around 1.5 per 1000. Of these, 50% to 60% of patients become symptomatic. Approximately one-third of all patients with paroxysmal supraventricular tachycardia (PSVT) are diagnosed as having an AP-mediated tachycardia. Patients with AP-mediated tachycardias most commonly present with the syndrome of PSVT.

Population-based studies have demonstrated a bimodal distribution of symptoms for patients with preexcitation, with a peak in early childhood followed by a second peak in young adulthood. Nearly 25% of infants who demonstrate preexcitation and/or have AP-mediated arrhythmias will lose evidence of preexcitation and/or become asymptomatic over time as the conduction property of the AP can degenerate with time.

Pappone et al reported that during a mean follow-up of 37.7 months, 18.2% and 30% of noninducible patients have lost the anterograde and retrograde conduction, respectively. The mean age of these patients was 33.6 ± 14.3. Compared to others who had persistent conductibility through the AP, these patients were asymptomatic, noninducible, and had longer minimal 1:1 conduction cycle length through the AP during the baseline EPS.

Symptoms range from palpitations to syncope to sudden death. As in other supraventricular tachycardias, episodes of tachycardia may be associated with dyspnea, chest pain, decreased exercise tolerance, anxiety, dizziness, or syncope. Although syncope is often considered a bad prognostic sign, the evidence is not clear. One study evaluated consecutive patients with WPW syndrome, thirty-six of whom had syncope. Although a higher percentage of patients who had syncope had a history of aborted sudden death (28% vs 18%), the difference was not significant.

Another study investigated the mechanism of syncope in patients with PSVT and found that most had AVRT. These authors reported that syncope during SVT might, in fact, be caused by a vasodepressor mechanism and not by a rapid rate of tachycardia. Physical examination demonstrates a fast, regular pulse with a constant intensity first heart sound. The jugular venous pressure waveform is usually constant, but it can sometimes be elevated. The incidence of sudden cardiac death in patients with WPW syndrome has been estimated to range from 0.15% to 0.39%. It is distinctly unusual for cardiac arrest to be the first symptomatic manifestation of WPW syndrome.

Patients with functioning AP have a slightly increased likelihood of having one of several congenital abnormalities, particularly Ebstein’s anomaly of the tricuspid valve. Nearly 10% of patients with Ebstein’s anomaly have preexcitation. APs also commonly occur in patients with congenitally corrected transposition of great vessels. In this case, the Ebstein’s anomaly of the left (tricuspid) valve is associated with APs to the functioning systemic ventricle (anatomic right ventricle). In addition, multiple APs are frequently seen in patients with Ebstein’s anomaly. Conversely, of patients with WPW syndrome presenting with SVT early in childhood, only 5% have Ebstein’s anomaly even though it is the most common form of congenital heart disease associated with WPW syndrome.

Interestingly, one study found that structural heart disease is less likely in patients with left-sided APs than in those with right-sided pathways (5% vs 45%). In this study, structural heart disease was present in only 20% of the patients presenting with WPW syndrome in the first 4 months of life.

How can we tell the location of the AP based on the superficial 12-lead ECG?

The ECG hallmark of an antegradely conducting AP is the delta wave along with a shorter than usual PR interval and a widened QRS complex. Conversely, the presence of retrograde conduction only in an AP will not be apparent on a surface ECG during sinus rhythm (concealed pathway). Whereas ECG during ORT has a normal QRS complex with retrogradely conducting P wave after the completion of the QRS complex in the ST segment or early in the T wave, the QRS during ART is fully preexcited.

Numerous algorithms have been described to localize the site of the AP using the axis of the delta wave and QRS morphology. The location of the AP along the AV ring is classified variously into five or ten regions, which can be broadly divided into those on the left and the right of the AV groove. Distribution along these lines is not homogenous. Some 46% to 60% of the pathways are found on the left free wall space. Nearly 25% are within the posteroseptal and midseptal spaces, 15% to 20% in the right free wall space, and 2% in the anteroseptal space.

The positive predictive value of these algorithms is better when the delta wave polarity is included and when algorithms involve fewer than six locations. Two simple algorithms that include both the delta wave axis and the QRS axis are shown (Figure 7). For the purpose of localization of the APs, delta wave is defined as the first 20 ms of the earliest QRS deflection.

QRS alternans is observed in approximately one-third of patients with circus movement tachycardia involving an AP. Although this phenomenon was previously felt to be strongly predictive of an AP-mediated tachycardia, subsequent studies have revealed that QRS alternans can also be seen in patients with AVNRT at rapid rates.

ST-segment depression may occur during ORT. Although this pattern of ST-segment depression may at first appear strongly predictive of myocardial ischemia, multiple studies have been performed which demonstrate that the presence of ST-segment depression during episodes of narrow complex tachycardia are very unlikely to be predictive of coronary artery disease. Because of this, the threshold to perform further evaluations including nuclear stress tests and cardiac catheterization in these patients would be high.

What are the typical electrophysiologic findings of WPW syndrome?

Electrophysiology study (EPS) in patients with WPW syndrome can help to confirm the presence of an AP, differentiate this condition from other forms of SVT, and to localize the pathway participating in the tachycardia for ablative therapy.

By definition, if an AP is present and conducting antegradely, some part of the ventricle begins activation earlier than expected, so that the HV interval is less than normal at rest . Because the QRS complex is a fusion complex of conduction down both the AV node and the AP, slowing of conduction down the normal pathway results in an increasing degree of preexcitation. Eccentric atrial activation with ventricular pacing makes it easy to identify the presence of an AP (Figure 8).

Figure 8.

Retrograde conduction over most APs is nondecremental. Hence, in the absence of intraventricular conduction delay or the presence of multiple bypass tracts, the VA conduction time is the same over a range of pace cycle lengths. The exception to this is the slowly conducting decremental posteroseptal pathway found in the permanent form of junctional reciprocating tachycardia, in which the VA conduction time increases with increasing ventricular pacing rate.

It is important and often challenging to differentiate retrograde conduction over septal pathway from conduction over the normal AV system. One maneuver that can make this differentiation is differential pacing (ie, pacing from the right ventricular apex and the RV base) and measuring the VA conduction time.

Retrograde conduction over the normal AV conduction system is fastest when pacing from the apex because conduction can occur rapidly over the His-Purkinje system. VA intervals are longer when the pacing site is moved from the apex to the base. The converse is true in the presence of an AP, with VA intervals shortest when pacing from the base, closer to the site of pathway insertion than from the apex. The technique of para-Hisian pacing is useful in differentiating the septal pathway from AVNRT.

Development of bundle branch block (BBB) aberration during tachycardia can be useful in determining both presence of and participation of an AP in tachycardia (Figure 9). An increase in tachycardia cycle length caused by an increase in VA conduction time with functional BBB is consistent with the presence of an AP ipsilateral to the BBB.

Management

Management of patients with AP can vary depending on the symptoms, prognosis, and patient’s preference. While observation and close follow-up can be an option, particularly in the asymptomatic patient, most patients with WPW syndrome and/or PSVT involving an AP prefer curative treatment with catheter ablation.

Management of Patients with an Asymptomatic Preexcitation Pattern on ECG

Most patients with asymptomatic preexcitation have a good prognosis. Because of the small but real risks associated with invasive procedures, EPS is not mandated for risk stratification or ablative therapy. The ACC/AHA/ESC Guidelines for Management of Patients with Supraventricular Arrhythmias gives catheter ablation a 2A classification for treatment of patients with asymptomatic preexcitation. A 2A designation means that it is reasonable to offer EPS with or without ablation in selected patients after a thorough discussion about the risks and benefits of the procedure.

Several noninvasive and invasive tests have been proposed as useful in stratifying patients for the risk of sudden death. The detection of intermittent preexcitation—which is characterized by an abrupt loss of the delta wave, normalization of the QRS complex, and an increase in the PR interval during a continuous ECG recording—is evidence that an AP has a relatively long refractory period and is unlikely to precipitate ventricular fibrillation. The loss of preexcitation after administration of antiarrhythmic drugs such as procainamide or ajmaline has also been used to indicate a low-risk subgroup. These noninvasive tests are generally considered inferior to EPS in the assessment of risk of sudden cardiac death. Because of this, they play little role in patient management at present.

When screening studies are performed in patients with asymptomatic preexcitation, a significant proportion of patients demonstrate the presence of one or more features associated with an increased risk of sudden death. As noted earlier, these include (1) a Shortest Preexcited RR Interval (SPERRI) less than 250 ms during spontaneous or induced atrial fibrillation (Figure 10), (2) a history of symptomatic tachycardia, (3) multiple APs, (4) Epstein’s anomaly, and (5) inducible persistent AVRT or AF. A SPERRI during atrial fibrillation above 250 ms has been reported to have a negative predictive value greater than 95%. More recent evidence makes a stronger case for use of EPS in risk stratifying all asymptomatic patients with preexcitation.

Pappone and colleagues studied two hundred twelve consecutive asymptomatic WPW patients after a baseline EPS over 5 years. After a mean follow-up of 37.7 months, thirty-three patients became symptomatic. Of these, twenty-nine had inducible SVT on EPS, and only four were not inducible. More importantly, there were three sudden deaths in the entire population, and all of them occurred in patients in whom AVRT and atrial fibrillation were inducible during EPS.

In a more recent study, Pappone and colleagues examined the role of prophylactic catheter ablation in children with asymptomatic preexcitation. Of the one hundred sixty-five eligible children, sixty were determined to be at high risk of an arrhythmia based on their results of EPS. Of these sixty patients, twenty underwent prophylactic catheter ablation, twenty-seven had no treatment, and thirteen withdrew from the study. During a mean follow-up of 34 months, one child in the ablation group (5%) and twelve in the control group (44%) had arrhythmic events. Among these twelve patients in the control group, two experienced ventricular fibrillation and one died suddenly.

Santinelli and colleagues published two papers describing the natural history of asymptomatic preexcitation. Among two hundred ninety-three adults with asymptomatic preexcitation followed for a median of 67 months, thirty-one patients (10.6%) developed a first arrhythmic event. Among these patients, the event was classified as potentially life-threatening in seventeen patients. One of these patients experienced a cardiac arrest. Multivariate analysis identified inducibility and antegrade effective refractory period of the AP less than 250 ms as predictive of potentially life-threatening arrhythmias.

Among one hundred eighty-four children with asymptomatic preexcitation followed for a median of 57 months, fifty-one patients (28%) developed a first arrhythmic event. Among these patients, the event was classified as potentially life-threatening in nineteen patients. Three of these patients experienced cardiac arrest. Multivariate analysis identified an antegrade effective refractory period of the AP less than 250 ms and the presence of multiple APs as predictive of potentially life-threatening arrhythmias.

Based on the results of these studies, as well as the well-established safety and efficacy of catheter ablation of APs, most electrophysiologists will discuss the option of performing an EPS as a risk stratification tool to all asymptomatic patients with a pattern of preexcitation on an ECG. If high-risk features are found, catheter ablation can be performed in the same setting. Although in the past this approach was generally limited to patients who were competitive athletes or had a high-risk occupation such as a pilot or school bus driver, the threshold for performing an EPS for risk stratification has fallen with the emergence of the new studies cited above.

Management of Symptomatic WPW

Patients with WPW syndrome can present with an acute episode of tachycardia or for an elective consultation because of an incidentally discovered WPW pattern on ECG.

Acute presentation

As stated in the ACLS guideline, the initial assessment has to focus on the hemodynamic stability of the patient. If the tachycardia continues and there are any signs of hemodynamic instability, direct current cardioversion (DCCV) has to be performed without further delay. Please see Figure 11 for further details of the algorithm (Figure 11). The approach to terminate the acute presentation generally differs from that used for long-term suppression and prevention of further episodes of SVT. In general, the approach used to treat acute presentations do not vary based on the specific tachycardia mechanism as it is usually unknown when the patient first presents to an emergency department (ED). Pharmacologic agents are in general more effective in terminating an acute episode of tachycardia than preventing future recurrences.

Also, it is extremely important to recognize the difference between the AVRT and the preexcited AF as there’s a clear difference in their pharmacological management. The former can present as a narrow complex tachycardia (ORT) or wide complex tachycardia (ART) (Figure 10). If there’s any underlying BBB or intraventricular conduction delay, the ORT can present as a wide complex tachycardia. In any case, the AVRT will be regular which is suggestive of a reentrant nature of the rhythm. Conversely, in preexcited AF, the QRS complex will be wide and irregular given the direct antegrade conduction of the AF through the AP (Figure 12).

Figure 12.

AVN blockers: Calcium channel blockers, beta-blockers, and adenosine can be given for acute termination of AVRT but not for preexcited atrial fibrillation as they may not only accelerate conduction through the AP leading to ventricular fibrillation but could also cause hemodynamic instability due to their vasodilatory and negative inotropic effects. It is also important to remember that about 12% of people who are given adenosine can develop atrial fibrillation and this could result in a worse clinical scenario such as preexcited atrial fibrillation that degenerates into ventricular fibrillation. So, it is important to be ready for DCCV when using adenosine for patients with WPW syndrome. Possible mechanisms for adenosine-induced atrial proarrhythmia are shortening of the atrial refractory period, reflex increase in catecholamine levels, and sympathetic nerve traffic.

Procainamide: Mandel et al described the beneficial effects of procainamide in thirteen patients with WPW syndrome and atrial fibrillation. Procainamide, a class 1A antiarrhythmic, increases effective refractory period and reduces impulse conduction velocity and excitability in the atria, His-Purkinje fibers, ventricular muscle, and the AP of the heart. This is caused by an increase in excitability threshold combined with inhibition of ectopic pacemaker activity. In their study, infusion of IV procainamide was used at a rate of 10mg per kilogram. The delta wave was eliminated by procainamide in ten patients and modified in three patients. With the use of procainamide, they were able to prevent tachycardia induction in 66% of the patients. Moreover, they concluded that procainamide will be an ideal agent for the prevention of rapid ventricular rates in patients with the WPW syndrome and atrial fibrillation.

Ibutilide: This class III antiarrhythmic agent prolongs the atrial and ventricular action’s potential refractoriness and duration. These effects have been attributed to the activation of slow inward sodium current I(Na-s), although recent data suggest that blockade of the delayed rectifier potassium current (Ikr) which slows repolarization may contribute in a minor way in a dose-dependent manner. Glatter et al reported the electrophysiological effects of ibutilide in patients with APs without any structural heart disease (except for one patient who had Ebstein’s anomaly). In their series, they administered higher doses of ibutilide more rapidly than in standard practice (2mg IV infusion over 15 minutes if older than 18 years old or 0.03mg per kilogram if younger than 18 years old, as opposed to mg IV infusion in 10 minutes). Ibutilide terminated AF in twenty-one of twenty-two patients (95%) and caused transient loss of the delta wave in one (1/18) patient and abolished inducible tachycardia in two (2/18) patients, although retrograde mapping was still feasible for successful AP ablation. They also mentioned about the theoretical concern regarding ibutilide abolishing the AP conduction, thus delaying the study, and that this finding was never observed in their patients except for one patient (without AF) whose study was delayed as the result of effects of ibutilide. Hence, the use of ibutilide to treat atrial fibrillation in any acute setting or during an EP study in patients with AP seems to be effective and safe, especially if the AF recurs despite DCCV. It is also important to remember that ibutilide can cause torsade de pointes in approximately 8% of the cases. Therefore, the patient should be monitored at least for 4 hours after the infusion of ibutilide or until the QT interval normalizes.

Elective consultation for WPW syndrome

Depending on the severity of the symptoms, presence of risk factors, and patient’s preference, one could opt to pursue either electrophysiologic study with possible ablation or medical treatment (Figure 12). As it was mentioned previously, the latter approach has been increasingly replaced by screening EPS and prophylactic catheter ablation when a high-risk AP is uncovered. The Heart Rhythm Society Policy Statement on Catheter Ablation states that catheter ablation is considered first-line therapy (class 1) and the treatment of choice for patients with WPW syndrome. It is curative in more than 95% of patients and has a low complication rate. It also obviates the unwanted side effects of antiarrhythmic agents.

Catheter ablation is also considered first-line therapy (class 1) for patients with PSVT involving a concealed AP. However, because concealed APs are unlikely to be associated with an increased risk of sudden cardiac death, catheter ablation can be presented as one of the potential therapeutic approaches among others like pharmacologic therapy and clinical follow-up alone. When pharmacologic therapy is selected for patients with concealed APs, it is reasonable to consider a trial of beta-blockers, calcium channel blockers, or class 1C antiarrhythmic agents. It is important to note that beta-blockers and calcium channel blockers are generally not recommended for the management of patients with evidence of preexcitation.

For an easier applicability and better understanding, we have prepared a flowchart for the long-term management of patients with AP-mediated arrhythmias based on the recommendations developed by the ACC/AHA/ESC in 2003. In this statement, the catheter ablation was considered as class 1 therapy for treatment of patients with WPW syndrome and for those with poorly tolerated AVRT in the absence of preexcitation. In contrast, catheter ablation received a class 2a indication for the treatment of patients who have had only a single episode of AVRT and those with asymptomatic preexcitation.

Catheter ablation of accessory pathways

Catheter ablation of APs is performed in conjunction with a diagnostic EPS. After the AP has been localized to a region of the heart, precise mapping and ablation are performed using a steerable electrode catheter. Catheter ablation of APs is routinely performed today on an outpatient basis with conscious sedation. No prospective, randomized clinical trials have evaluated the safety and efficacy of catheter ablation of APs. However, the results of catheter ablation of APs have been reported in a large number of other trials.

The largest prospective, multicenter clinical trial to evaluate the safety and efficacy of RF ablation was reported by Calkins and colleagues in 1999. This study involved analysis of one thousand fifty patients, of whom five hundred had APs. Overall success of catheter ablation in curing APs was 93%. The success rate for catheter ablation of left free wall APs was slightly higher than for catheter ablation of right-sided APs (95% vs 90%, P = 0.03). After an initially successful procedure, recurrence of AP conduction was found in approximately 5% of patients. The recurrence-free interval postablation was also best with left-sided pathways. APs that recur can usually be successfully ablated again.

Complications associated with catheter ablation of APs may result from obtaining vascular access (hematomas, deep venous thrombosis, perforation of the aorta, arteriovenous fistula, or pneumothorax), catheter manipulation (valvular damage, microemboli, perforation of the CS or myocardial wall, coronary dissection, or thrombosis), or delivery of RF energy (AV block, myocardial perforation, coronary artery spasm or occlusion, transient ischemic attacks, or cerebrovascular accidents).

In the same report, Calkins and colleagues mentioned about the incidence of major complications in their trial to be 3% and of minor complications around 8%. The procedure-related mortality associated with catheter ablation of APs has ranged from 0% to 0.2%. The two most common types of major complications reported during catheter ablation of APs are inadvertent complete AV block and cardiac tamponade.

The incidence of inadvertent complete AV block ranges from 0.17% to 1.0%. Most instances of complete AV block occur in the setting of the ablation of septal and posteroseptal APs. The frequency of cardiac tamponade as a result of the ablation of APs varies between 0.13% and 1.1%.A recently published meta-analysis of studies of catheter ablation of SVT, including APs, has reported similar incidence and distribution of complications resulting from catheter ablation of APs.

In the past several years, cryoablation has become available as an alternative energy source for creation of myocardial lesions, which can be used for catheter ablation of APs. The main advantage of cryoenergy compared with RF energy is that the risk of heart block appears to be lower. This potential benefit must be balanced against longer procedure times and lower acute and long-term efficacy. Because of the lower acute and long-term success rates of catheter ablation of APs using cryoenergy, this energy source is generally used only for ablation of APs located in the anteroseptal and para-Hisian locations.

Long-term medical therapy

Although it is less common, medical treatment represents one therapeutic option for management of patients with AP-mediated arrhythmias (Figure 12). The agents that primarily affect conduction through the AV node include non-dihydropyridine calcium channel blockers, beta-blockers, and adenosine. In contrast, the antiarrhythmic drugs, which primarily modify conduction across the AP, consist of class 1 drugs such as procainamide, propafenone, and flecainide as well as class 3 antiarrhythmic drugs such as sotalol and amiodarone.

There have been no randomized controlled trials of drug prophylaxis involving patients with AVRT. However, a number of small, non-randomized trials have been performed. A subset of patients in these studies had AVRT as their underlying arrhythmia. Available data do not allow a comparison of the efficacy of these drugs. Manolis and colleagues evaluated the efficacy of propafenone in eleven adult patients, nine of whom had a manifest AP. During 9 ± 6 months of follow-up, none of the ten patients discharged on a combination of propafenone and a beta-blocker experienced recurrence and no major side effects were reported. In this report, the authors also commented that the effect of propafenone on the AP was reversed by Isuprel infusion. Hence, concomitant use of a beta-blocker was recommended.

Other small trials have evaluated the efficacy of propafenone in the treatment of AVRT in children. The largest of these involved twenty-six young children (younger than age 10 years) with AVRT. Complete arrhythmia control was accomplished in twenty patients and partial control in one additional patient. A number of studies have examined the acute and long-term efficacy of oral and intravenous flecainide in the treatment of patients with AVRT. Helmy and colleagues reported the largest of these, involving twenty patients with AVRT. The oral administration of flecainide resulted in inability to induce sustained tachycardia in seventeen of twenty patients. During 15 ± 7 months of follow-up on oral flecainide treatment, three patients developed recurrence of tachycardia. The addition of a beta- blocker resulted in greater efficacy with more than 90% of patients achieving abolition of symptomatic tachycardia.

Amiodarone has been evaluated in several trials for its efficacy in the treatment of patients with AP-mediated tachycardias. However, these studies did not demonstrate the superiority of amiodarone over class 1C antiarrhythmic agents or sotalol. Acute studies have also revealed that amiodarone does not consistently prolong the AP refractory period and, as a result, cannot be considered to be protective against sudden death in all patients with WPW syndrome.

As a result of these findings—combined with the well-recognized organ toxicity associated with amiodarone and the high rate of discontinuation of this drug because of noncardiac adverse effects—amiodarone generally does not play an important role in the treatment of patients with APs. Verapamil, although less studied, can be moderately effective in the prevention of AVRT. No studies have been performed to determine the long-term efficacy of procainamide or quinidine in the treatment of AVRT.

What is the evidence for specific management and treatment recommendations?

What every physician needs to know about WPW syndrome

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Are there different types of Accessory Pathways?

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Are there any specific tachycardias associated with accessory pathways?

Cain, ME, Luke, RA, Lindsay, BD. “Diagnosis and localization of accessory pathways”. Pacing Clin Electrophysiol. vol. 15. 1992. pp. 801-24.

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Klein, GJ, Bashore, TM, Sellers, TD. “Ventricular fibrillation in the Wolff-Parkinson-White syndrome”. N Engl J Med.. vol. 301. 1979. pp. 1080-5.

Dreifus, LS, Haiat, R, Watanabe, Y. “Ventricular fibrillation: a possible mechanism of sudden death in patients with Wolff-Parkinson-White syndrome”. Circulation. vol. 43. 1971. pp. 520-7.

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Campbell, RWF, Smith, R, Gallagher, JJ. “Atrial fibrillation in the preexcitation syndrome”. Am J Cardiol. vol. 40. 1977. pp. 514-20.

Sharma, AD, Klein, GJ, Guiraudon, GM. “Atrial fibrillation in patients with Wolff-Parkinson-White syndrome: incidence after surgical ablation of the accessory pathway”. Circulation. vol. 72. 1985. pp. 161-9.

Dagres, N, Clague, JR, Lottkamp, H. “Impact of radiofrequency catheter ablation of accessory pathways on the frequency of atrial fibrillation during long-term follow-up: high recurrence rate of atrial fibrillation in patients older than 50 years of age”. Eur Heart J. vol. 22. 2001. pp. 423-7.

Pappone, C, Santinelli, V, Rosanio, S. “Usefulness of invasive electrophysiologic testing to stratify the risk of arrhythmic events in asymptomatic patients with Wolff-Parkinson-White pattern: results from a large prospective long-term follow-up study”. J Am Coll Cardiol. vol. 41. 2003. pp. 239-44.

Prevalence, symptoms and prognosis of WPW syndrome

Wellens, HJJ, Durrer, D. “Wolff-Parkinson-White syndrome and atrial fibrillation”. Am J Cardiol. vol. 34. 1974. pp. 777-82.

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Leitch, JW, Klein, GJ, Yee, R. “Syncope associated with supraventricular tachycardia: an expression of tachycardia rate or vasomotor response?”. Circulation. vol. 85. 1992. pp. 1064-71.

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Timmermans, C, Smeets, JL, Rodriguez, LM. “Aborted sudden death in the Wolff-Parkinson-White syndrome”. Am J Cardiol. vol. 76. 1995. pp. 492-4.

Pappone, C, Santinelli, V, Rosanio, S. “Usefulness of invasive electrophysiologic testing to stratify the risk of arrhythmic events in asymptomatic patients with Wolff-Parkinson-White pattern: results from a large prospective long-term follow-up study”. J Am Coll Cardiol. vol. 41. 2003. pp. 239-44.

Lundberg, A. “Paroxysmal atrial tachycardia in infancy: long term follow-up study of 49 subjects”. Pediatrics. vol. 70. 1982. pp. 638

Deal, BJ, Keana, JF, Gillette, PC. “Wolff-Parkinson-White syndrome and supraventricular tachycardia during infancy: management and follow-up”. J Am Coll Cardiol. vol. 5. 1985. pp. 130

Lev, M, Givson, S, Miller, RA. “Ebstein’s disease with Wolff-Parkinson-White syndrome”. Am Heart J. vol. 49. 1955. pp. 724

How can we tell the location of the AP based on the superficial 12 lead ECG?

Gallager, JJ, Pritchett, ELC, Sealy, WC. “The preexcitation syndromes”. Prog Cardiovasc Dis. vol. 20. 1978. pp. 285-327.

Ross, DL, Uther, JB. “Diagnosis of concealed accessory pathways in supraventricular tachycardia”. Pacing Clin Electrophysiol. vol. 7. 1984. pp. 1069-85.

Arruda, MS, McClelland, JH, Wang, X. “Development and validation of an ECG algorithm for identifying accessory pathway ablation site in Wolff-Parkinson-White syndrome”. J Cardiovasc Electrophysiol. vol. 9. 1998. pp. 2-12.

Fitzpatrick, AP, Gonzales, RP, Lesh, MD. “New algorithm for the localization of accessory atrioventricular connections using a baseline electrocardiogram”. J Am Coll Cardiol. vol. 23. 1994. pp. 107-16.

Lindsay, BD, Crossen, KJ, Cain, ME. “Concordance of distinguishing electrocardiographic features during sinus rhythm with the location of accessory pathways in the Wolff-Parkinson-White syndrome”. Am J Cardiol. vol. 59. 1987. pp. 1093-1102.

Cain, ME, Luke, RA, Lindsay, BD. “Diagnosis and localization of accessory pathway”. Pacing Clin Electrophysiol. vol. 15. 1992. pp. 801-24.

D’Avila, A, Brugada, J, Skeberis, V. “A fast and reliable algorithm to localize accessory pathways based on the polarity of the QRS complex on the surface ECG during sinus rhythm”. Pacing Clin Electrophysiol. vol. 18. 1995. pp. 1615-27.

Milstein, S, Sharma, AD, Guiraudon, GM. “An algorithm for the electrocardiographic localization of accessory pathways in the Wolff-Parkinson-White syndrome”. Pacing Clin Electrophysiol. vol. 10. 1987. pp. 555-63.

Xie, B, Heald, SC, Bashir, Y. “Localization of accessory pathways from the 12-lead electrocardiogram using a new algorithm”. Am J Cardiol. vol. 74. 1994. pp. 161-5.

Haissaguerre, M, Marcus, F, Poquet, F. “Electrocardiographic characteristics and catheter ablation of parahisian accessory pathways”. Circulation. vol. 90. 1994. pp. 1124-8.

Diker, E, Ozdemir, M, Tezcan, UK. “QRS polarity on 12-lead surface ECG: a criterion for the differentiation of right and left posteroseptal accessory atrioventricular pathways”. Cardiology. vol. 88. 1997. pp. 328-32.

Boersma, L, Garcia-Moran, E, Mont, L. “Accessory pathway localization by QRS polarity in children with Wolff-Parkinson-White syndrome”. J Cardiovasc Electrophysiol. vol. 13. 2002. pp. 1222-60.

Basiouny, T, de Chillou, C, Fareh, S. “Accuracy and limitations of published algorithms using the twelve-lead electrocardiogram to localize overt atrioventricular accessory pathways”. J Cardiovasc Electrophysiol. vol. 10. 1999. pp. 1340-9.

Morady, F, DiCarlo, LA, Baerman, JM. “Determinants of QRS alter-nans during narrow QRS tachycardia”. J Am Coll Cardiol. vol. 9. 1987. pp. 489-99.

What are the typical electrophysiologic findings of WPW syndrome?

Lesh, MD, Van Hare, G, Scheinman, MM. “Comparison of the retrograde and transseptal methods for ablation of left free-wall accessory pathways”. J Am Coll Cardiol. vol. 22. 1993. pp. 542-9.

Jackman, WM, Wang, X, Friday, KJ. “Catheter ablation of accessory atrioventricular pathways (Wolff-Parkinson-White syndrome) by radiofrequency current”. N Engl J Med. vol. 324. 1991. pp. 1605-11.

Kuck, KH, Schluter, M, Geiger, M. “Radiofrequency current catheter ablation of accessory atrioventricular pathways”. Lancet. vol. 337. 1991. pp. 1557-1561.

Calkins, H, Langberg, J, Sousa, J. “Radiofrequency catheter ablation of accessory atrioventricular connections in 250 patients: abbreviated therapeutic approach to Wolff-Parkinson-White syndrome”. Circulation. vol. 85. 1992. pp. 1337-46.

Kay, GN, Pressley, JC, Packer, DL. “Value of 12-lead electrocardiogram in discriminating atrioventricular nodal reciprocating tachycardia from circus movement atrioventricular utilizing a retrograde accessory pathway”. Am J Cardiol. vol. 59. 1987. pp. 296-300.

Tchou, PJ, Lehmann, MJ, Donga, J. “Effect of sudden rate acceleration on the human His-Purkinje system: adaptation of refractoriness in a damped oscillatory pattern”. Circulation. vol. 73. 1986. pp. 920-9.

Drago, F, DeSantis, A, Grutter, G, Silverti, MS. “Transvenous cryothermal catheter ablation of re-entry circuit located near the atrioventricular junction in pediatric patients”. J Am Coll Cardiol. vol. 45. 2005. pp. 1096-103.

Manolis, AS, Katsaros, C, Cokkinos, DV. “Electrophysiological and electropharmacological studies in pre-excitation syndromes: results with propafenone therapy and isoproterenol infusion testing”. Eur Heart J. vol. 13. 1992. pp. 1489-95.

Janousek, J, Paul, T, Reimer, A, Kallfelz, H. “Usefulness of propafenone for supraventricular arrhythmias in infants and children”. Am J Cardiol. vol. 72. 1993. pp. 294-300.

Musto, B, D’Onofrio, A, Cavallaro, C, Musto, A. “Electrophysiological effects and clinical efficacy of propafenone in children with recurrent paroxysmal supraventricular tachycardia”. Circulation. vol. 78. 1988. pp. 863-69.

Vignati, G, Figini, M, Figini, A. “The use of propafenone in the treatment of tachyarrhythmias in children”. Eur Heart J. vol. 14. 1993. pp. 546-50.

Vassiliadis, Papoutsakis, P, Kallikazaros, I. “Propafenone in the prevention of non-ventricular arrhythmias associated with the Wolff-Parkinson-White syndrome”. Int J Cardiol. vol. 27. 1990. pp. 63-70.

Helmy, I, Scheinman, MM, Herre, JM. “Electrophysiologic effects of isoproterenol in patients with atrioventricular reentrant tachycardia treated with flecainide”. J Am Coll Cardiol. vol. 16. 1990. pp. 1649-55.

Mason, JW. “Amiodarone”. N Engl J Med. vol. 316. 1987. pp. 455-66.

Rosenbaum, MD, Chiale, PA, Ryba, D. “Control of tachyarrhythmias associated with Wolff-Parkinson-White syndrome by amiodarone hydrochloride”. Am J Cardiol. vol. 34. 1974. pp. 215-23.

Wellens, HJJ, Lie, KI, Bar, FW. “Effect of amiodarone in the Wolff-Parkinson-White syndrome”. Am J Cardiol. vol. 38. 1976. pp. 189-94.

Management of Asymptomatic and Symptomatic Preexcitation

Blomström-Lundqvist, C, Scheinman, MM, Aliot, EM. “ACC/AHA/ESC Guidelines for the Management of Patients With Supraventricular Arrhythmias—Executive Summary: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines and the European Society of Cardiology Committee for Practice Guidelines (Writing Committee to Develop Guidelines for the Management of Patients With Supraventricular Arrhythmias)”. Circulation. vol. 108. 2003. pp. 1871-1909.

Klein, GJ, Gulamhusien, SS. “Intermittent preexcitation in the Wolff-Parkinson-White syndrome”. Am J Cardiol. vol. 52. 1983. pp. 292-6.

Campbell, RWF, Smith, R, Gallagher, JJ. “Atrial fibrillation in the preexcitation syndrome”. Am J Cardiol. vol. 40. 1977. pp. 514-20.

Auricchio, A, Klein, H, Trappe, HJ. “Lack of prognostic value of syncope in patients with Wolff-Parkinson-White syndrome”. J Am Coll Cardiol. vol. 17. 1991. pp. 152-8.

Wellens, HJ, Bar, FW, Gorgels, AP. “Use of ajmaline in patients with the Wolff-Parkinson-White syndrome to disclose short refractory period of the accessory pathway”. Am J Cardiol. vol. 45. 1980. pp. 130-33.

Brembilla-Perrot, B, Ghawi, R. “Electrophysiological characteristics of asymptomatic Wolff-Parkinson-White syndrome”. Eur Heart J. vol. 14. 1993. pp. 511-15.

Leitch, JW, Klein, GJ, Yee, R, Murdock, C. “Prognostic value of electrophysiology testing in asymptomatic patients with Wolff-Parkinson-White pattern”. Circulation. vol. 82. 1990. pp. 1718-23.

Fitzsimmons, PJ, McWhirter, PD, Peterson, DW. “The natural history of Wolff-Parkinson-White syndrome in 228 military aviators: a long-term follow-up of 22 years”. Am Heart J. vol. 142. 2001. pp. 530-6.

Pappone, C, Santinelli, V, Rosanio, S. “Usefulness of invasive electrophysiology testing to stratify the risk of arrhythmic events in asymptomatic patients with Wolff-Parkinson-White pattern: results from a large prospective long-term follow-up study”. J Am Coll Cardiol. vol. 41. 2003. pp. 239-44.

Pappone, C, Manguso, F, Santinelli, R. “Radiofrequency ablation in children with asymptomatic Wolff-Parkinson-White syndrome”. N Engl J Med. vol. 351. 2004. pp. 1197-1205.

Santinelli, V, Radinovic, A, Manguso, F. “Asymptomatic ventricular preexcitation: a long-term prospective follow-up study of 293 adult patients”. Circ Arrhythm Electrophysiol. vol. 2. 2009. pp. 102-7.

Santinelli, V, Radinovic, A, Manguso, F. “The natural history of asymptomatic ventricular pre-excitation a long-term prospective follow-up study of 184 asymptomatic children”. J Am Coll Cardiol 2009. vol. 53. 20. pp. 275-80.

Strickberger, SA, Ching Man, K, Daoud, EG. “Adenosine-induced atrial arrhythmia: a prospective analysis”. Ann Intern Med. vol. 127. 1997. pp. 417-22.

Catheter Ablation of Accessory Pathways

Lesh, MD, Van Hare, G, Scheinman, MM. “Comparison of the retrograde and transseptal methods for ablation of left free-wall accessory pathways”. J Am Coll Cardiol. vol. 22. 1993. pp. 542-9.

Jackman, WM, Wang, X, Friday, KJ. “Catheter ablation of accessory atrioventricular pathways (Wolff-Parkinson-White syndrome) by radiofrequency current”. N Engl J Med. vol. 324. 1991. pp. 1605-11.

Kuck, KH, Schluter, M, Geiger, M. “Radiofrequency current catheter ablation of accessory atrioventricular pathways”. Lancet. vol. 337. 1991. pp. 1557-61.

Calkins, H, Langberg, J, Sousa, J. “Radiofrequency catheter ablation of accessory atrioventricular connections in 250 patients: abbreviated therapeutic approach to Wolff-Parkinson-White syndrome”. Circulation. vol. 85. 1992. pp. 1337-46.

Kay, GN, Pressley, JC, Packer, DL. “Value of 12-lead electrocardiogram in discriminating atrioventricular nodal reciprocating tachycardia from circus movement atrioventricular utilizing a retrograde accessory pathway”. Am J Cardiol. vol. 59. 1987. pp. 296-300.

Tchou, PJ, Lehmann, MJ, Donga, J. “Effect of sudden rate acceleration on the human His-Purkinje system: adaptation of refractoriness in a damped oscillatory pattern”. Circulation. vol. 73. 1986. pp. 920-9.

Drago, F, DeSantis, A, Grutter, G, Silverti, MS. “Transvenous cryothermal catheter ablation of re-entry circuit located near the atrioventricular junction in pediatric patients”. J Am Coll Cardiol. vol. 45. 2005. pp. 1096-103.

Medical Therapy

Manolis, AS, Katsaros, C, Cokkinos, DV. “Electrophysiological and electropharmacological studies in pre-excitation syndromes: results with propafenone therapy and isoproterenol infusion testing”. Eur Heart J. vol. 13. 1992. pp. 1489-95.

Janousek, J, Paul, T, Reimer, A, Kallfelz, H. “Usefulness of propafenone for supraventricular arrhythmias in infants and children”. Am J Cardiol. vol. 72. 1993. pp. 294-300.

Musto, B, D’Onofrio, A, Cavallaro, C, Musto, A. “Electrophysiological effects and clinical efficacy of propafenone in children with recurrent paroxysmal supraventricular tachycardia”. Circulation. vol. 78. 1988. pp. 863-9.

Vignati, G, Figini, M, Figini, A. “The use of propafenone in the treatment of tachyarrhythmias in children”. Eur Heart J. vol. 14. 1993. pp. 546-50.

Vassiliadis, Papoutsakis, P, Kallikazaros, I. “Propafenone in the prevention of non-ventricular arrhythmias associated with the Wolff-Parkinson-White syndrome”. Int J Cardiol. vol. 27. 1990. pp. 63-70.

Helmy, I, Scheinman, MM, Herre, JM. “Electrophysiologic effects of isoproterenol in patients with atrioventricular reentrant tachycardia treated with flecainide”. J Am Coll Cardiol. vol. 16. 1990. pp. 1649-55.

Mandel, WJ, Laks, MM, Obayashi, K, Hayakawa, H, Daley, W. “The Wolff-Parkinson-White syndrome: pharmacologic effects of procaine amide”. Am Heart J. vol. 90. 1975 Dec. pp. 744-54.

Glatter, KA, Dorostkar, PC, Yang, Y. “Electrophysiological Effects of Ibutilide in Patients With Accessory Pathways”. Circulation. vol. 104. 2001. pp. 1933-39.

Ellenbogen, KA, Stambler, BS, Wood, MA. “Efficacy of intravenous ibutilide for rapid termination of atrial fibrillation and atrial flutter: a dose-response study”. J Am Coll Cardiol. vol. 28. 1996. pp. 130-6.

Stambler, BS, Wood, MA, Ellenbogen, KA. “Efficacy and safety of repeated intravenous doses of ibutilide for rapid conversion of atrial flutter or fibrillation”. Circulation. vol. 94. 1996. pp. 1613-21.

Volgman, AS, Carberry, PA, Stambler, B. “Conversion efficacy and safety of intravenous ibutilide compared with intravenous procainamide in patients with atrial flutter or fibrillation”. J Am Coll Cardiol. vol. 31. 1998. pp. 1414-19.

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