Supraventricular tachycardia (SVT) encompasses any tachyarrhythmia that originates above the bundle of His or that requires supraventricular tissue for its initiation and maintenance. The term is used both as a broad anatomic descriptor and, in clinical shorthand, to refer specifically to the paroxysmal regular narrow-complex tachycardias, principally AVNRT and AVRT, that present acutely and respond to AV nodal blockade. Atrial fibrillation and atrial flutter, though supraventricular by origin, are typically classified separately given their distinct management frameworks.1
An anatomic classification framework organizes SVTs by the tissue required for the tachycardia circuit. Atrial tachycardias (focal AT, atrial flutter, atrial fibrillation) originate entirely within atrial tissue and do not require the AV node for maintenance, though AV nodal conduction determines the ventricular rate. AV nodal-dependent tachycardias (AVNRT, AVRT) require participation of the AV node or an accessory pathway as an obligate limb of the re-entrant circuit, which is why AV nodal blockade terminates them rather than merely slowing the ventricular rate.1,2
The initial evaluation of a regular narrow-complex tachycardia (QRS duration below 120 ms, rate above 100 bpm) proceeds in three steps: clinical context and vagal maneuver response; adenosine administration with continuous rhythm recording; and systematic 12-lead ECG analysis focusing on P-wave morphology, RP interval, and rate.2
Vagal maneuvers (Valsalva with the modified technique (supine position, 40 mmHg intrathoracic pressure for 15 seconds followed by leg elevation) or carotid sinus massage in appropriate patients) transiently increase vagal tone and slow AV nodal conduction. Their diagnostic and therapeutic value is greatest when performed correctly and recorded continuously. Three response patterns are possible: termination of the tachycardia (indicating AV nodal dependence: AVNRT or AVRT); transient slowing of the ventricular rate with continuation of the tachycardia (indicating an atrial origin: atrial tachycardia or flutter, where the AV node is not a required circuit limb); or no response (less diagnostically useful but does not exclude any diagnosis).2
The RP interval, defined as the interval from the onset of the QRS to the onset of the subsequent P wave, is the most clinically useful ECG measurement for distinguishing SVT subtypes. A short RP interval (RP below PR, P wave within or immediately after the QRS) indicates that atrial activation follows ventricular activation rapidly, as occurs in typical AVNRT (retrograde conduction via the fast pathway produces near-simultaneous atrial and ventricular activation) and in orthodromic AVRT with a fast retrograde accessory pathway. A long RP interval (RP above PR, P wave closer to the next QRS than to the preceding one) indicates slow retrograde atrial activation, as occurs in atypical AVNRT, AVRT with a slowly conducting accessory pathway (permanent junctional reciprocating tachycardia, PJRT), or focal atrial tachycardia.1,2
P-wave morphology during tachycardia provides additional localizing information. In typical AVNRT, retrograde conduction activates both atria simultaneously from the AV node, producing a narrow, retrograde P wave that is often buried within or immediately following the QRS complex, appearing as a pseudo-R' in V1 or pseudo-S in the inferior leads, a finding absent on the baseline ECG. In orthodromic AVRT, the accessory pathway determines the retrograde P-wave axis: a left-sided pathway produces a negative P wave in leads I and aVL; a right-sided pathway produces a positive P wave in V1. In focal atrial tachycardia, the P-wave morphology reflects the anatomic location of the ectopic focus: positive P waves in inferior leads suggest a superior origin; negative P waves in inferior leads suggest an inferior or coronary sinus origin.3
AVNRT is the most common paroxysmal SVT, accounting for approximately 60% of cases presenting to the emergency department. It affects women more frequently than men (roughly 2:1) and typically presents in the second through fourth decades of life, though it can occur at any age.2
The electrophysiologic substrate of AVNRT is the presence of two functionally distinct pathways within or immediately adjacent to the AV node. The slow pathway has a longer conduction time but a shorter effective refractory period (ERP). The fast pathway has a shorter conduction time but a longer ERP. Under normal sinus conditions, the impulse conducts preferentially down the fast pathway (shorter conduction time to His) while the slow pathway is in a refractory state from simultaneous activation.3
A critically timed atrial premature beat can find the fast pathway refractory (longer ERP) but the slow pathway recovered (shorter ERP). The impulse then conducts antegradely via the slow pathway while the fast pathway recovers. If the impulse then returns retrogradely via the fast pathway and finds the slow pathway recovered, a sustained re-entrant circuit is established: slow pathway antegrade, fast pathway retrograde. This is typical AVNRT, producing near-simultaneous atrial and ventricular activation and a short RP interval.3
In atypical AVNRT (approximately 5 to 10% of cases), the circuit runs in the reverse direction: antegrade via the fast pathway, retrograde via the slow pathway. The prolonged retrograde conduction produces a long RP interval and a P wave that precedes the next QRS rather than following the current one. Atypical AVNRT can be difficult to distinguish from focal atrial tachycardia and PJRT on surface ECG alone; electrophysiology study is often required for definitive diagnosis.
Adenosine is first-line for acute termination of AVNRT, with success rates of 90 to 95% when administered correctly as a rapid IV bolus with immediate saline flush via a proximal vein. Its mechanism, dosing, and adverse effects are detailed in Part 5. If adenosine fails or is contraindicated (symptomatic asthma, significant caffeine intake reducing efficacy, high-degree AV block), IV verapamil 5 mg over 2 to 3 minutes (repeat 5 to 10 mg at 15 minutes if needed) or IV diltiazem 0.25 mg/kg over 2 minutes are effective second-line agents, terminating AVNRT in approximately 90% of cases by blocking the slow AV nodal pathway or the retrograde fast pathway. IV metoprolol (2.5 to 5 mg, up to 3 doses) or IV esmolol are alternatives, though generally less rapidly effective than calcium channel blockers for acute SVT termination.2,4
DC cardioversion (synchronized, 50 to 100 J biphasic) is immediately available and appropriate if the patient is hemodynamically compromised or if pharmacologic termination fails. It is nearly universally effective for AV nodal-dependent tachycardias.
For patients with infrequent, well-tolerated episodes, no pharmacologic prophylaxis is required and episodic management (including patient-administered vagal maneuvers or pill-in-the-pocket adenosine equivalent where available) is appropriate. For patients with frequent or symptomatic recurrences, the choice is between chronic antiarrhythmic drug therapy and catheter ablation.4
Pharmacologic options for chronic AVNRT suppression include: beta-blockers (metoprolol, atenolol) or non-DHP CCBs (verapamil, diltiazem) as first-line, which reduce the frequency of initiating atrial premature beats and slow AV nodal conduction. Class Ic agents (flecainide, propafenone) are effective second-line options in patients without structural heart disease by slowing slow pathway conduction and raising the threshold for circuit initiation. These drugs suppress AVNRT in 60 to 80% of patients but require indefinite therapy and carry drug-specific adverse effect profiles.4
Catheter ablation of the slow pathway (detailed in Section 6) achieves cure rates exceeding 95% with a recurrence rate of 1 to 3% and a risk of inadvertent complete AV block requiring permanent pacemaker of approximately 0.5 to 1%. Given this efficacy and safety profile, ablation is the preferred approach in patients with frequent symptomatic AVNRT who prefer a curative strategy over lifelong drug therapy, and it is the first-line recommendation in the 2019 ESC SVT guidelines for symptomatic recurrent AVNRT.4
AVRT accounts for approximately 30% of paroxysmal SVT presentations. It requires an accessory pathway (an anomalous muscular connection between atrium and ventricle that bypasses the AV node) as an obligate limb of the re-entrant circuit. The Wolff-Parkinson-White (WPW) syndrome is defined by the combination of a pre-excitation ECG pattern (delta wave, short PR interval, widened QRS during sinus rhythm) and symptomatic tachyarrhythmias.5
Accessory pathways conduct impulses rapidly without the decremental (rate-dependent slowing) properties of the AV node. In sinus rhythm, pathways with antegrade conduction pre-excite the ventricle directly, producing the delta wave, a slurred initial QRS deflection representing the fusion of pre-excited ventricular activation via the pathway and normal activation via the AV node and His-Purkinje system. The PR interval is short because conduction begins immediately without AV nodal delay. The total QRS duration is widened by the additional delta wave component. Pathways with only retrograde conduction (concealed pathways) produce no delta wave in sinus rhythm and are detectable only during tachycardia when they serve as the retrograde limb.5
The accessory pathway location determines the delta wave polarity on 12-lead ECG and predicts ablation approach. Left lateral pathways (the most common location) produce a negative delta wave in lead I and aVL. Right free wall pathways produce a positive delta wave in V1. Posteroseptal pathways produce negative delta waves in the inferior leads. This localization guides the electrophysiologist to the appropriate approach (transseptal for left-sided pathways, right-sided approach for right free wall pathways).
In orthodromic AVRT (approximately 95% of AVRT cases), conduction proceeds antegradely via the normal AV node and His-Purkinje system, producing a narrow QRS, and retrogradely via the accessory pathway, producing a P wave after the QRS with an RP interval determined by the retrograde pathway conduction time. Because the AV node is the antegrade limb, AV nodal blocking agents (adenosine, verapamil, beta-blockers) terminate orthodromic AVRT by blocking the obligate antegrade limb, the same mechanism as in AVNRT.5
In antidromic AVRT (approximately 5% of AVRT cases), conduction proceeds antegradely via the accessory pathway and retrogradely via the AV node or a second accessory pathway, producing a fully pre-excited, maximally wide QRS that can be mistaken for ventricular tachycardia on first inspection. Antidromic AVRT is a wide-complex tachycardia by definition and requires careful diagnosis before AV nodal agents are administered.
The most immediately life-threatening arrhythmia in WPW is atrial fibrillation with rapid antegrade conduction over the accessory pathway. Unlike the AV node, accessory pathways lack decremental conduction properties: they transmit impulses at a fixed rapid rate determined only by their refractory period. In pre-excited AF, atrial impulses at 400 to 600 per minute are conducted to the ventricle at rates limited only by the pathway's ERP, potentially 200 to 300 bpm or faster, producing an irregularly irregular wide-complex tachycardia with variable QRS morphology (reflecting variable degrees of pre-excitation) that can degenerate to ventricular fibrillation.5,6
The ECG appearance of pre-excited AF is distinctive and must be recognized immediately: an irregularly irregular rhythm (distinguishing it from antidromic AVRT, which is regular), wide and variable QRS morphology, and very rapid ventricular rates frequently exceeding 200 bpm. This is an emergency.
AV nodal blocking agents are absolutely contraindicated in pre-excited AF. Adenosine, verapamil, diltiazem, and digoxin all block the AV node but have no effect on accessory pathway conduction and may paradoxically accelerate conduction over the pathway by removing competing wavefront collisions from AV nodal conduction. The resulting acceleration of the ventricular rate can precipitate VF. This contraindication applies even in the context of apparently stable hemodynamics, as the margin between 250 bpm pre-excited AF and VF is narrow and unpredictable.6
The correct pharmacologic management of hemodynamically stable pre-excited AF is IV procainamide (10 to 17 mg/kg at up to 50 mg/min), which blocks accessory pathway conduction directly through Class Ia sodium channel blockade, slowing or terminating the rapid pathway conduction. IV ibutilide (1 mg over 10 minutes) is an alternative with high efficacy for pre-excited AF cardioversion. If the patient is hemodynamically unstable at any point, immediate synchronized DC cardioversion is the treatment of choice and should not be delayed for pharmacologic trials.6
Not all patients with WPW carry equivalent risk. The risk of sudden cardiac death (SCD) from pre-excited AF with VF degeneration is estimated at 0.15 to 0.39% per year in symptomatic patients but varies substantially with accessory pathway properties. High-risk features include: shortest pre-excited RR interval (SPERRI) below 250 ms during spontaneous or induced AF (indicating a very short pathway ERP allowing extremely rapid conduction); multiple accessory pathways; anteroseptal pathway location (higher risk of rapid conduction); and history of symptomatic tachycardias or syncope. Intermittent pre-excitation on the resting ECG (delta wave appearing and disappearing) or abrupt loss of pre-excitation during exercise testing suggests a relatively long pathway ERP and lower risk.5
Electrophysiology (EP) study is recommended for risk stratification in symptomatic WPW patients and in asymptomatic patients engaged in high-risk occupations (pilots, competitive athletes, professional drivers) or who have high-risk features on non-invasive testing. The EP study measures the pathway ERP and SPERRI during induced AF, directly assessing the risk of rapid ventricular conduction.
Pharmacologic chronic management of WPW-related tachycardias is generally considered a temporizing strategy. Class Ic agents (flecainide, propafenone) slow accessory pathway conduction and raise the VF threshold in pre-excited AF by increasing the pathway ERP, and they suppress AVRT initiation. They are appropriate in patients without structural heart disease who decline ablation or are awaiting it. Beta-blockers and CCBs can be used for orthodromic AVRT suppression but must be used with caution given the risk of unmasking rapid pathway conduction if AF develops while on these agents alone. Digoxin and verapamil are contraindicated in WPW with antegrade pathway conduction because they shorten accessory pathway ERP while simultaneously slowing AV nodal conduction, the reverse of what is desired.6
Catheter ablation is the definitive treatment for symptomatic WPW and is recommended as first-line in the 2019 ESC SVT guidelines. Ablation success rates exceed 93 to 95% for most pathway locations, with recurrence rates of 3 to 8% (higher for epicardial and anteroseptal pathways). The risk of complete AV block is low for most pathway locations but is higher for midseptal and anteroseptal pathways, which lie close to the His bundle. Ablation eliminates both the arrhythmia substrate and the risk of pre-excited AF, which pharmacologic therapy cannot fully achieve.4
Atrial flutter is a macro-re-entrant atrial tachycardia with a characteristic sawtooth ECG pattern and an atrial rate of approximately 250 to 350 bpm. It is anatomically classified as cavotricuspid isthmus (CTI)-dependent (typical flutter) or non-CTI-dependent (atypical flutter), a distinction with important implications for ablation strategy.7
Typical atrial flutter circulates around the tricuspid annulus, with the cavotricuspid isthmus, the narrow channel of myocardium between the tricuspid annulus and the inferior vena cava, serving as a critical zone of slow conduction that is the obligate vulnerable parameter of the circuit. In counterclockwise typical flutter (by far the most common), activation proceeds up the interatrial septum, across the roof of the right atrium, and down the right atrial free wall, then through the CTI. This produces the characteristic negative sawtooth flutter waves in inferior leads (II, III, aVF) and positive flutter waves in V1. Clockwise typical flutter produces the mirror pattern.7
The atrial rate in typical flutter is approximately 300 bpm. AV nodal conduction typically occurs at 2:1, producing a regular ventricular rate of approximately 150 bpm: a rate that should specifically prompt consideration of flutter whenever an apparently regular tachycardia at 150 bpm is encountered. Variable AV conduction (3:1, 4:1, or irregular) produces a less regular ventricular rate. Class Ic agents, by slowing atrial conduction velocity, can reduce the flutter rate to 200 to 220 bpm and paradoxically permit 1:1 AV conduction, as discussed in Part 2.
Ventricular rate control is considerably more difficult to achieve in atrial flutter than in atrial fibrillation. The organized, high-amplitude atrial impulses of flutter penetrate the AV node more effectively than the disorganized fibrillatory activity of AF, requiring higher doses of AV nodal blocking agents to achieve equivalent rate control. In practice, pharmacologic rate control in flutter frequently results in abrupt transitions between 2:1 conduction (150 bpm) and 3:1 or 4:1 conduction (75 to 100 bpm) as drug effect varies, producing unpredictable and often inadequate rate control. This pharmacologic limitation is one of the strongest arguments for cardioversion or ablation rather than long-term rate control in atrial flutter.7
Beta-blockers and non-DHP CCBs remain the agents of choice for rate control in flutter when cardioversion is not immediately pursued, used at the same doses as for AF rate control. Digoxin is particularly ineffective in flutter given its vagotonic mechanism. Amiodarone can control ventricular rate when other agents are inadequate but carries its own toxicity burden if used chronically for this indication alone.
Ibutilide is the most effective pharmacologic agent for cardioversion of recent-onset atrial flutter, with cardioversion rates of 65 to 70%: substantially higher than for AF (40 to 60%). Its mechanism (rapid delayed rectifier potassium current (IKr) blockade plus activation of slow inward sodium current) produces rapid, potent action potential duration (APD) prolongation that terminates the flutter circuit. Dofetilide is an effective oral option for flutter cardioversion and maintenance of sinus rhythm with the same in-hospital initiation requirements as for AF. Class Ic agents (flecainide, propafenone), by slowing atrial conduction, can convert flutter but carry the risk of 1:1 conduction during the process and must be co-prescribed with an AV nodal blocking agent. IV amiodarone is less effective than ibutilide for acute flutter cardioversion but is appropriate in structural heart disease where other options are limited.7
DC cardioversion is highly effective for atrial flutter at lower energy requirements than for AF (50 to 100 J biphasic in most cases). Anticoagulation principles before cardioversion of flutter of unknown duration mirror those for AF: three weeks of therapeutic anticoagulation or transesophageal echocardiogram (TEE) exclusion of left atrial thrombus before cardioversion, with four weeks of anticoagulation post-cardioversion regardless of CHA2DS2-VASc score.
Ablation of the cavotricuspid isthmus achieves bidirectional conduction block across the isthmus, eliminating the critical slow-conduction zone required for the flutter circuit. CTI ablation is the most successful ablation procedure in cardiac electrophysiology, with acute success rates exceeding 95% and long-term freedom from typical flutter of 90 to 95% at one year. Recurrence is usually attributable to recovery of conduction across the ablation line rather than a new circuit.7
The 2019 ESC SVT guidelines recommend CTI ablation as a Class I indication (recommended) for symptomatic typical flutter, particularly when flutter recurs after cardioversion or when rate control is difficult. In patients with both AF and typical flutter (a common combination given the shared substrate of atrial enlargement and fibrosis), CTI ablation addresses the flutter component but does not prevent AF recurrence. The anticoagulation decision after CTI ablation follows the CHA2DS2-VASc framework, as atrial thrombus risk from ongoing atrial disease is independent of flutter suppression.
Focal atrial tachycardia (AT) arises from a discrete atrial site and spreads centrifugally from that focus, in contrast to the macro-re-entrant circuits of flutter and AVRT. It accounts for approximately 10 to 15% of SVT presentations and has three distinct underlying mechanisms: enhanced automaticity, triggered activity (DAD-mediated), and micro-re-entry at the focal site.8
The hallmark of focal AT is the presence of a discrete P wave that differs in morphology from the sinus P wave, with a consistent PR interval shorter than the tachycardia cycle length: indicating that atrial activation precedes AV nodal conduction rather than following it as in AVNRT and AVRT. The tachycardia is typically regular at 130 to 250 bpm. A gradual onset with a "warm-up" phenomenon (progressive rate acceleration at initiation) and gradual offset suggest automatic rather than triggered or re-entrant mechanisms.8
P-wave morphology predicts the focus location with reasonable accuracy and guides ablation planning. A positive P wave in leads I and aVL with a negative P in V1 suggests a right atrial origin. A negative P in leads I and aVL suggests a left atrial origin. P waves negative in inferior leads (II, III, aVF) suggest a low atrial or coronary sinus origin. Specific anatomic locations have characteristic patterns: the crista terminalis and right pulmonary veins are the most common right and left atrial focal sites respectively, followed by the coronary sinus ostium, the tricuspid and mitral valve annuli, and the pulmonary vein ostia.
Beta-blockers and non-DHP CCBs are first-line pharmacologic treatment for focal AT, providing ventricular rate control and partial suppression of automatic foci through reduction of sympathetic tone and AV nodal slowing. They are most effective in catecholamine-dependent automatic AT (exercise-induced or stress-related) where sympathetic withdrawal directly suppresses the focus. Class Ic agents (flecainide, propafenone) suppress triggered and re-entrant focal AT by reducing abnormal automaticity and micro-re-entry at the site. Their use follows the same structural heart disease restrictions as for all Class Ic applications.8
Digoxin and adenosine transiently suppress automatic focal AT through vagotonic and direct automaticity-suppressing effects, respectively, but are generally insufficient for sustained management. Adenosine's response in focal AT is diagnostically useful: transient suppression or slowing during adenosine administration followed by resumption of tachycardia suggests an adenosine-sensitive cAMP-mediated triggered mechanism, as seen in right ventricular outflow tract (RVOT) tachycardia and some focal ATs arising near nodal tissue.
Inappropriate sinus tachycardia (IST) is a distinct clinical entity characterized by a persistently elevated resting heart rate (above 100 bpm) or an exaggerated heart rate response to minimal exertion, with normal P-wave morphology and axis confirming a sinus origin, in the absence of a secondary cause (fever, anemia, thyrotoxicosis, deconditioning). The mechanism involves enhanced sinus node automaticity and possibly altered autonomic regulation, with increased If current activity playing a central role.8
Ivabradine, which selectively blocks the If channel in the sinus node without affecting AV conduction, contractility, or blood pressure, is the pharmacologic agent most specifically suited to IST. It reduces the intrinsic firing rate of the sinus node without the adverse hemodynamic effects of beta-blockers (which are the alternative first-line agent). Beta-blockers remain a reasonable initial choice in IST, particularly in patients with comorbid hypertension or anxiety-driven sympathetic excess, but they are less well tolerated in younger patients and those with baseline hypotension. Ivabradine is approved for IST (2 mg IV or 5 mg orally twice daily, titrated to effect) and provides effective heart rate reduction in the majority of patients, though long-term symptom relief is variable.
Catheter ablation has transformed the management of SVT over the past three decades, offering curative therapy for arrhythmias that previously required lifelong pharmacologic suppression. Understanding ablation principles, success rates, and complication profiles is essential for appropriate patient counseling and for the pharmacology-to-ablation decision framework.
Radiofrequency (RF) ablation delivers alternating current at 500 kHz through the ablation catheter tip to the target tissue, generating resistive heating to 50 to 70 degrees Celsius in the contact zone. The thermal injury produces a coagulation necrosis lesion of 5 to 7 mm depth. RF ablation is the dominant modality for most SVT ablation procedures due to its precision, controllable lesion size, and efficacy. The main limitation is that thermal lesions are irreversible: if the catheter is inadvertently positioned on the His bundle during AVNRT ablation, complete AV block results.9
Cryoablation delivers nitrous oxide through the catheter tip, cooling the contact tissue to minus 70 to minus 80 degrees Celsius. The critical advantage of cryoablation is the cryomapping capability: at temperatures of minus 30 degrees Celsius, the tissue is rendered temporarily non-conductive without permanent injury, allowing the operator to confirm that AV conduction is preserved before committing to a full freeze. This makes cryoablation particularly valuable for AVNRT ablation near the His bundle, where the risk of inadvertent AV block is highest, reducing the AV block risk to below 0.1% compared to 0.5 to 1% with RF ablation at the same target site. Cryolesions are slightly larger and less precise than RF lesions, making cryoablation less suitable for small, precise targets such as accessory pathways in certain locations.9
AVNRT ablation (slow pathway modification using RF or cryo) achieves acute success rates exceeding 95 to 97% with recurrence rates of 1 to 3% at one year. The procedure is recommended (Class I) for symptomatic recurrent AVNRT in patients who prefer ablation to ongoing drug therapy, and it is reasonable (Class IIa) even for a first episode of poorly tolerated AVNRT. The choice between RF and cryoablation for AVNRT is largely operator-dependent, with similar overall success rates.4
AVRT and WPW ablation achieves acute success rates of 93 to 95% overall, with variation by pathway location: left free wall pathways (transseptal or retrograde aortic approach) have the highest success rates (95 to 97%) and lowest recurrence (3 to 5%). Right free wall pathways have slightly lower success (90 to 95%) due to catheter stability challenges. Anteroseptal and midseptal pathways carry the highest risk of inadvertent AV block (1 to 2%) due to proximity to the His bundle, and cryoablation is preferred at these locations. Ablation is the Class I recommendation for symptomatic WPW, particularly given the SCD risk that pharmacologic therapy cannot fully eliminate.4
Typical atrial flutter (CTI ablation) has the highest success rate of any ablation procedure: 95 to 97% acute success with 90 to 95% long-term freedom from flutter at one year. Complication rates are low: right phrenic nerve injury (rare), vascular access complications, and cardiac tamponade (less than 0.5%). CTI ablation is Class I for symptomatic typical flutter.4
Paroxysmal atrial fibrillation ablation (pulmonary vein isolation, PVI) achieves freedom from AF in approximately 60 to 80% of patients at one year after a single procedure in patients with paroxysmal AF and no significant structural disease. Repeat procedures improve success rates to 75 to 90% at two to three years. Persistent AF ablation has substantially lower single-procedure success rates (40 to 60%) due to the more advanced atrial remodeling and non-pulmonary vein triggers that characterize long-standing persistent AF. The CASTLE-AF trial demonstrated that ablation of AF in HFrEF (mean EF 33%) reduced all-cause mortality and AF burden compared to medical therapy, establishing ablation as a first-line rhythm control option in this population.4
Focal atrial tachycardia ablation success rates vary substantially by focus location. Crista terminalis and right atrial appendage foci have high acute success rates (85 to 95%) with low recurrence. Pulmonary vein-origin foci are often managed with PVI as part of AF ablation strategies. Coronary sinus and parahisian foci are technically challenging, with lower success rates and higher complication risk. Overall long-term cure rates for focal AT are approximately 75 to 90% depending on location.
Serious complications of SVT ablation occur in approximately 1 to 3% of all procedures, with the specific risk profile varying by arrhythmia type and approach. Cardiac tamponade (from transseptal puncture or catheter perforation) occurs in 0.5 to 1% of procedures; most are managed with pericardiocentesis without surgical intervention. Complete AV block requiring permanent pacemaker implantation is the most feared complication of AVNRT and His-adjacent accessory pathway ablation, occurring in 0.5 to 1% of RF AVNRT procedures and under 0.1% of cryo procedures. Pulmonary vein stenosis is specific to AF ablation, occurring in 1 to 5% of early-generation ablation procedures; modern wide-area PVI approaches have reduced this substantially. Phrenic nerve injury during cryoablation of right-sided pulmonary veins occurs in 2 to 3% of AF cryo cases; the majority resolve spontaneously within one year. Vascular access complications (hematoma, arteriovenous fistula, pseudoaneurysm) occur in 1 to 2% of cases.9
The decision between ablation and long-term pharmacologic therapy integrates four factors: arrhythmia-specific cure rate and complication risk; patient symptom burden and quality of life impact; drug efficacy and tolerability; and patient preference after informed discussion.
For AVNRT and AVRT in patients without structural heart disease, ablation is the preferred strategy when: symptoms are frequent or poorly tolerated; drugs have failed or produced unacceptable adverse effects; the patient prefers a potentially curative procedure over lifelong medication; or high-risk features are present (WPW with short pathway ERP). Pharmacologic therapy is reasonable for: infrequent, well-tolerated episodes; patient preference for medical management; or significant comorbidities increasing procedural risk.
For typical atrial flutter, the high CTI ablation success rate and the pharmacologic difficulty of rate control in flutter make ablation the preferred strategy in most symptomatic patients, even after a first presentation of poorly tolerated flutter.
For atrial fibrillation, the pharmacology-to-ablation framework is more nuanced: early rhythm control with ablation is increasingly supported by trial evidence (EAST-AFNET 4, CASTLE-AF) and is first-line in AF with HFrEF and in young symptomatic patients with paroxysmal AF. Pharmacologic rhythm control remains appropriate as initial therapy in many patients, with ablation offered after antiarrhythmic drug failure or as first-line when patient preference and anatomy favor it.
A wide-complex tachycardia (WCT) is defined as a tachycardia with QRS duration of 120 ms or greater at a rate above 100 bpm. The differential diagnosis encompasses three distinct mechanisms: ventricular tachycardia (VT), SVT with aberrant conduction (functional or fixed bundle branch block), and pre-excited tachycardia (antidromic AVRT or pre-excited AF). This distinction is clinically critical because the treatments differ: and the wrong treatment can be fatal.10
VT is the cause of wide-complex tachycardia in approximately 80% of all cases presenting to the emergency department, and in over 90% of cases in patients with structural heart disease or prior MI. SVT with aberrancy (typically LBBB or RBBB pattern) accounts for approximately 15 to 20% of cases. Pre-excited tachycardias are less common but particularly dangerous due to the accessory pathway dynamics described in Section 3.3. The clinical implication of these frequencies is fundamental: in any patient with structural heart disease presenting with a wide-complex tachycardia, VT is the presumptive diagnosis until proven otherwise.10
Several ECG criteria have been developed to distinguish VT from SVT with aberrancy, of which the Brugada criteria (1991) remain the most widely used in clinical practice. The Brugada algorithm applies four criteria in sequence, and a single positive criterion diagnoses VT with high specificity.10
The absence of an RS complex in any precordial lead diagnoses VT (sensitivity 21%, specificity 100%). If RS complexes are present, an RS interval (onset of R to nadir of S) exceeding 100 ms in any precordial lead diagnoses VT. If the RS interval is below 100 ms, AV dissociation diagnoses VT. If none of the above criteria are met, morphology criteria in V1 and V6 are applied: specific VT-favoring morphologies in both leads simultaneously confirm VT. When no criterion is met, SVT with aberrancy is diagnosed by exclusion.
Practically, certain ECG features are highly suggestive of VT regardless of the formal algorithm: AV dissociation (P waves marching independently of QRS, visible in some leads as notching or deformation of the QRS baseline); fusion beats (a QRS that is a hybrid of VT and sinus morphology, indicating simultaneous activation from two sources); capture beats (a narrow QRS interrupting the wide-complex tachycardia as the sinus impulse momentarily captures the ventricle). When any of these features are present, VT is effectively confirmed without requiring the full Brugada algorithm.10
A QRS axis of minus 90 to plus 180 degrees (northwest axis) is highly specific for VT, as this axis cannot be produced by any form of supraventricular conduction. Extreme QRS widening (above 200 ms) strongly favors VT or drug-induced sodium channel toxicity over SVT with aberrancy.
IV verapamil is specifically contraindicated in wide-complex tachycardia of unknown origin, and this contraindication should be memorized as a patient safety imperative. The hazard operates through two distinct mechanisms depending on the actual rhythm. If the WCT is VT: verapamil's negative inotropy and vasodilation precipitate hemodynamic collapse without providing any antiarrhythmic benefit, since VT does not depend on AV nodal conduction. Multiple case reports and series document cardiovascular collapse and death from verapamil administration in VT misidentified as SVT with LBBB aberrancy. If the WCT is pre-excited AF or antidromic AVRT: verapamil accelerates accessory pathway conduction by blocking the competing AV nodal pathway, increasing the ventricular rate to levels that precipitate VF.6,10
The clinical rule is absolute: treat any wide-complex tachycardia of uncertain origin as VT. This rule is correct 80 to 90% of the time and produces no harm in the remaining cases where the rhythm is actually SVT with aberrancy: procainamide and DC cardioversion are effective for both VT and SVT.
When a wide-complex tachycardia is identified and the patient is hemodynamically stable, the pharmacologic approach depends on the established diagnosis. For confirmed monomorphic VT: IV procainamide (preferred, Class IIa) or IV amiodarone (preferred in structural heart disease or when procainamide is contraindicated) as detailed in Part 7. For confirmed SVT with LBBB aberrancy: adenosine (diagnostic and therapeutic) or IV verapamil/diltiazem, with the same considerations as for narrow-complex SVT. For confirmed or strongly suspected pre-excited tachycardia: IV procainamide or IV ibutilide, with AV nodal blocking agents specifically contraindicated as discussed above.
When the diagnosis remains uncertain in a hemodynamically stable patient and the clinical pre-test probability of VT is high (structural heart disease, age above 50, prior MI), treat as VT with procainamide or amiodarone pending further diagnostic information. Adenosine may be used cautiously in this setting: it terminates most SVTs with aberrancy and most adenosine-sensitive VTs (RVOT), and its ultra-short half-life means that even if the rhythm is VT, the transient hemodynamic effect resolves within 10 to 20 seconds. However, adenosine in the setting of pre-excited AF can precipitate VF and must be used only when pre-excitation has been excluded. For any hemodynamic instability: immediate synchronized DC cardioversion regardless of diagnosis.
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doi:10.1016/j.jacc.2015.08.856Cardiovascular Pharmacology | Chapter 8
End of Part 9