Antiarrhythmic Drugs

The most clinically important classification divides supraventricular tachycardias by whether the atrioventricular node is a required component of the circuit.

Atrioventricular nodal-dependent tachycardias -- atrioventricular nodal reentrant tachycardia and atrioventricular reentrant tachycardia -- require the atrioventricular node as an obligate limb of the re-entrant circuit. Blocking the atrioventricular node with adenosine or calcium channel blockers terminates these arrhythmias rather than merely slowing the ventricular rate.

Atrial tachycardias -- focal atrial tachycardia, atrial flutter, atrial fibrillation -- originate entirely within atrial tissue and do not require the atrioventricular node for circuit maintenance. Atrioventricular nodal blockade slows the ventricular rate but does not terminate the arrhythmia; it continues in the atria and resumes its ventricular conduction when the drug wears off.

Adenosine is first-line for acute management of regular narrow-complex tachycardia and is also the most powerful bedside diagnostic tool. When given as a rapid intravenous bolus with continuous rhythm recording, three response patterns are possible:

Termination of the tachycardia confirms atrioventricular nodal dependence -- the arrhythmia was atrioventricular nodal reentrant tachycardia or orthodromic atrioventricular reentrant tachycardia. This is both therapeutic and diagnostic in one step.

Transient slowing of the ventricular rate without termination reveals an atrial origin -- the atrioventricular node is slowed but the atrial circuit continues. This pattern diagnoses atrial tachycardia, atrial flutter, or atrial fibrillation.

No response is less diagnostically useful but can occur with inadequate dose, peripheral vein delivery, or prior caffeine consumption blocking the adenosine receptor.

For a hemodynamically stable regular narrow-complex tachycardia, the sequence is: vagal maneuvers first (Valsalva, carotid sinus massage); if unsuccessful, adenosine rapid intravenous bolus; if unsuccessful or contraindicated, intravenous verapamil or diltiazem as second-line. Beta-blockers (intravenous metoprolol or esmolol) are an alternative second-line option. For any hemodynamic instability -- hypotension, altered consciousness, chest pain -- immediate synchronized direct-current cardioversion without waiting for drug therapy.

Atrial flutter is a macro-re-entrant atrial tachycardia with a characteristic sawtooth electrocardiogram pattern. The typical circuit circulates around the tricuspid valve annulus, with conduction through the cavotricuspid isthmus -- the narrow channel between the tricuspid annulus and the inferior vena cava -- serving as the critical slow zone. The atrial rate is approximately 300 beats per minute. Atrioventricular nodal conduction typically occurs at 2:1, producing a regular ventricular rate of approximately 150 beats per minute. A regular tachycardia at 150 beats per minute should always prompt consideration of atrial flutter.

Rate control is considerably more difficult in atrial flutter than in atrial fibrillation. The organized, high-amplitude flutter impulses penetrate the atrioventricular node more effectively than disorganized fibrillatory activity, requiring higher doses of atrioventricular nodal blocking agents and often producing unpredictable swings between 2:1 and 4:1 conduction rather than smooth rate control. This pharmacologic limitation makes cardioversion or catheter ablation the preferred approach for most patients with symptomatic flutter rather than long-term rate control.

For pharmacologic cardioversion of recent-onset atrial flutter, ibutilide is the most effective agent (cardioversion rates approximately 65 to 70%, higher than for atrial fibrillation). Dofetilide is an effective oral option. Direct-current cardioversion is highly effective and requires lower energy than for atrial fibrillation. Cavotricuspid isthmus catheter ablation is curative in over 95 percent of cases and is the preferred definitive treatment for symptomatic typical flutter.

Focal atrial tachycardia arises from a discrete atrial site and spreads centrifugally from that point. It accounts for approximately 10 to 15 percent of supraventricular tachycardia presentations. The electrocardiogram shows a P wave that differs in morphology from the sinus P wave, preceding the QRS with a normal PR interval. The tachycardia is typically regular at 130 to 250 beats per minute. A gradual onset with progressive rate acceleration at initiation suggests an automatic mechanism driven by enhanced automaticity of the focus.

Beta-blockers and non-dihydropyridine calcium channel blockers are first-line, providing rate control and partial suppression of catecholamine-driven automatic foci. Class Ic agents (flecainide, propafenone) suppress triggered and re-entrant focal atrial tachycardia in patients without structural heart disease. Adenosine transiently suppresses or slows automatic focal atrial tachycardia -- useful diagnostically (termination rules in atrioventricular nodal reentrant tachycardia/atrioventricular reentrant tachycardia; slowing or brief suppression with resumption suggests atrial origin). Catheter ablation is curative when the focus is localized and accessible, with success rates varying by anatomic location.

Radiofrequency ablation delivers energy through the catheter tip, generating heat at the contact point that creates a permanent coagulation lesion. It is the dominant modality for most supraventricular tachycardia procedures. Cryoablation freezes the contact tissue to very low temperatures. Its key advantage is cryomapping -- at an intermediate temperature the tissue is temporarily rendered non-conductive without permanent injury, allowing the operator to confirm that atrioventricular conduction is preserved before committing to a permanent freeze. This makes cryoablation the preferred approach for atrioventricular nodal reentrant tachycardia ablation near the His bundle, where the risk of inadvertent complete atrioventricular block is highest, reducing that complication rate to below 0.1 percent compared to 0.5 to 1 percent with radiofrequency ablation at the same site.

Atrioventricular nodal reentrant tachycardia (slow pathway modification): acute success over 95 percent; recurrence 1 to 3 percent at one year. Guideline Class I for symptomatic recurrent atrioventricular nodal reentrant tachycardia. Major risk: complete atrioventricular block requiring pacemaker (0.5 to 1 percent with radiofrequency; below 0.1 percent with cryoablation).

Atrioventricular reentrant tachycardia and Wolff-Parkinson-White: acute success 93 to 95 percent overall; higher for left free wall pathways (95 to 97 percent), lower for septal pathways near the His bundle. Guideline Class I for symptomatic Wolff-Parkinson-White. Ablation eliminates the risk of pre-excited atrial fibrillation, which drug therapy cannot reliably achieve.

Typical atrial flutter (cavotricuspid isthmus ablation): highest success rate of any ablation procedure -- 95 to 97 percent acute success; 90 to 95 percent long-term freedom from flutter. Guideline Class I for symptomatic typical flutter. Complication rates low.

Paroxysmal atrial fibrillation (pulmonary vein isolation): 60 to 80 percent freedom from atrial fibrillation at one year after a single procedure in paroxysmal atrial fibrillation without significant structural disease. Repeat procedures improve success to 75 to 90 percent. Persistent atrial fibrillation has lower single-procedure success (40 to 60 percent). The CASTLE-AF trial established ablation as first-line rhythm control in atrial fibrillation with heart failure with reduced ejection fraction.

For atrioventricular nodal reentrant tachycardia and atrioventricular reentrant tachycardia in structurally normal hearts, ablation is preferred 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 (Wolff-Parkinson-White with short accessory pathway refractory period). Drug therapy is reasonable for infrequent well-tolerated episodes or patient preference for medical management.

For typical atrial flutter, the high ablation success rate and the pharmacologic difficulty of rate control in flutter favor ablation as the preferred strategy in most symptomatic patients. For atrial fibrillation, the decision integrates symptom burden, heart failure status, patient preference, and whether early rhythm control is indicated -- with ablation increasingly favored as first-line in younger symptomatic patients with paroxysmal atrial fibrillation and in atrial fibrillation with heart failure with reduced ejection fraction.