2.2 Acute Tachyarrhythmias: Narrow- and Wide-Complex Management

2.2 Acute Tachyarrhythmias: Narrow- and Wide-Complex Management

Acute tachyarrhythmias encompass a broad spectrum of electrophysiologic disorders that challenge emergency medicine clinicians. Rapid decision-making is essential to distinguish hemodynamically stable patients from those experiencing cardiovascular collapse, differentiate narrow-complex from wide-complex morphologies, and identify rhythm-specific contraindications before administering antiarrhythmic or nodal-blocking pharmacotherapy.

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Initial Assessment: Stable vs. Unstable Tachycardia

The fundamental branch point in acute tachyarrhythmia management is assessing hemodynamic stability. The presence of any one of the following five cardinal signs indicates clinical instability caused by the rapid heart rate:

1. Hypotension: Systolic blood pressure $<90\text{ mmHg}$ or mean arterial pressure (MAP) $<65\text{ mmHg}$.
2. Acutely Altered Mental Status: Confusion, lethargy, encephalopathy, or agitation secondary to cerebral hypoperfusion.
3. Signs of Shock: Delayed capillary refill ($>3\text{ seconds}$), cool, mottled, or clammy extremities, and pallor.
4. Ischemic Chest Discomfort: Anginal symptoms, subendocardial ischemia, or dynamic ST-segment depressions driven by severe rate-related myocardial oxygen supply-demand mismatch.
5. Acute Heart Failure: Acute pulmonary edema, bilateral pulmonary rales, and jugular venous distention.

Management of Unstable Tachyarrhythmias

Unstable patients with a pulse require immediate synchronized cardioversion. Energy selections based on rhythm morphology include:

• Atrial Fibrillation: the 2025 AHA guidelines call an initial biphasic setting of at least 200 J reasonable, increased after a failed shock. Lower first shocks (120 J) failed more often in trials.
• Atrial Flutter: an initial setting of 200 J may be reasonable; flutter often converts at 50–100 J, but starting at 200 J shortens the procedure and avoids under-treating atrial fibrillation mistaken for flutter.
• Other Regular Narrow-Complex SVT: commonly 50 to 100 J biphasic when adenosine and vagal maneuvers fail or the patient is unstable.
• Wide Regular (Monomorphic VT with pulse): synchronized cardioversion, commonly starting around 100 J biphasic and escalating.
• Polymorphic VT / Torsades: the 2025 AHA guidelines treat polymorphic VT as always unstable. Give unsynchronized defibrillation immediately, because the defibrillator cannot synchronize to a chaotic waveform.

If the patient is conscious and time permits, administer rapid procedural sedation (e.g., etomidate 0.1 to 0.15 mg/kg IV, midazolam 1 to 2 mg IV, or ketamine 0.5 to 1 mg/kg IV) without delaying emergent electrical cardioversion.

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Narrow-Complex Tachycardias (QRS $<120\text{ ms}$)

1. Regular Narrow-Complex: Paroxysmal Supraventricular Tachycardia (PSVT)

PSVT most commonly arises from atrioventricular nodal re-entrant tachycardia (AVNRT) or atrioventricular re-entrant tachycardia (AVRT).

Non-Pharmacologic: Vagal Maneuvers

Initial therapy in hemodynamically stable patients involves vagal stimulation. The modified Valsalva maneuver (demonstrated in the REVERT trial) is significantly more effective than standard Valsalva (43% vs. 17% conversion rate). The technique involves having the patient perform a 40 mmHg forced exhalation strain for 15 seconds in a semi-recumbent position, immediately followed by laying the patient supine and elevating their legs to 45 degrees for 15 seconds.

Adenosine Pharmacotherapy

Adenosine stimulates purinergic A1 receptors on AV nodal tissue, activating outward potassium channels and inhibiting calcium influx. This causes profound, transient hyperpolarization and complete heart block (half-life $<10\text{ seconds}$ secondary to rapid enzymatic degradation by adenosine deaminase in erythrocytes and vascular endothelial cells).

• Standard Dosing:
  • First Dose: 6 mg rapid IV push over 1 to 2 seconds via an antecubital vein or proximal port, immediately followed by a rapid 20 mL 0.9% NaCl flush and limb elevation.
  • Second Dose: 12 mg rapid IV push with a 20 mL flush if conversion does not occur within 1 to 2 minutes.
• Critical Pharmacokinetic Modifications:
  • Reduce Initial Dose by 50% (3 mg): When administering via a central venous line (direct atrial delivery avoids peripheral dilution); in patients taking dipyridamole (inhibits cellular reuptake of adenosine); in patients taking carbamazepine (potentiates AV block); and in orthotopic heart transplant recipients (denervated myocardium possesses upregulated, hypersensitive purinergic receptors predisposing to prolonged sinus arrest).
  • Increase Dose / Anticipate Resistance: In patients consuming high amounts of caffeine or taking theophylline / aminophylline (competitive methylxanthine antagonists at purinergic A1 receptors).
  • Contraindications: Severe active bronchospastic asthma (purinergic A2B/A3 receptor activation can precipitate severe, bronchospasm).

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2. Irregular Narrow-Complex: Atrial Fibrillation & Flutter with RVR

In hemodynamically stable atrial fibrillation or flutter with rapid ventricular response (RVR, heart rate $>110\text{ bpm}$), the immediate goal is acute ventricular rate control (target resting HR $<110\text{ bpm}$). Acute rhythm conversion is generally avoided in patients with AF duration $>48\text{ hours}$ (or unknown duration) without prior therapeutic anticoagulation for at least 3 weeks or transesophageal echocardiography (TEE) ruling out left atrial appendage thrombus.

Non-Dihydropyridine Calcium Channel Blockers

• Diltiazem: Blocks L-type calcium channels in the SA and AV nodes, slowing conduction velocity and prolonging refractoriness.
  • Initial Bolus: 0.25 mg/kg IV (actual body weight; typical adult dose 15 to 20 mg) administered over 2 minutes.
  • Second Bolus: 0.35 mg/kg IV (20 to 25 mg) after 15 minutes if heart rate remains elevated.
  • Continuous Infusion: 5 to 15 mg/hour, titrated in 5 mg/h increments every 15 to 30 minutes to target heart rate.
• Verapamil: 2.5 to 5 mg IV over 2 minutes; repeat 5 to 10 mg after 15 to 30 minutes if needed (maximum cumulative dose 20 mg).

Beta-Adrenergic Antagonists

• Metoprolol Tartrate: 2.5 to 5 mg IV push over 2 minutes; repeat every 5 minutes up to a maximum cumulative dose of 15 mg.
• Esmolol: Ultra-short-acting cardioselective beta-1 blocker (elimination half-life ~9 minutes via red blood cell esterases). Loading dose: 500 mcg/kg IV over 1 minute, followed by maintenance infusion of 50 to 200 mcg/kg/min.

Atrial Fibrillation with Heart Failure with Reduced Ejection Fraction (HFrEF)

In patients with acute decompensated heart failure with reduced ejection fraction (LVEF $<40\%$), intravenous non-dihydropyridine CCBs and intravenous beta-blockers are contraindicated. Their potent negative inotropic properties can precipitate acute cardiogenic shock and pulmonary edema.

• Drug of Choice: Intravenous Amiodarone (150 mg IV infused over 10 minutes, followed by 1 mg/min for 6 hours, then 0.5 mg/min for 18 hours). Amiodarone provides rate control without significant negative inotropy.
• Alternative: Intravenous Digoxin (0.25 to 0.5 mg IV loading dose, followed by 0.25 mg every 4 to 6 hours to a maximum of 1.0 to 1.5 mg over 24 hours). Digoxin exhibits delayed onset (1 to 2 hours) and reduced efficacy in high-adrenergic states, making it an adjunctive rather than rapid first-line rate control agent.

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Pre-Excited Atrial Fibrillation (AF + Wolff-Parkinson-White Syndrome)

Pre-excited AF occurs when atrial fibrillation conducts down both the AV node and an accessory bypass tract (bundle of Kent). It manifests on ECG as an irregularly irregular, wide-complex tachycardia with bizarre, varying QRS morphologies and extremely rapid ventricular rates (often $>200\text{ to }250\text{ bpm}$).

• Treatment of Choice: Hemodynamically unstable patients require immediate synchronized cardioversion. In stable patients, administer intravenous Procainamide (15 to 17 mg/kg IV infusion at 20 to 50 mg/min) or Ibutilide (1 mg IV over 10 minutes). These agents prolong refractoriness in the accessory pathway and terminate conduction.

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Wide-Complex Tachycardias (QRS $\ge 120\text{ ms}$)

Any regular wide-complex tachycardia must be treated as Ventricular Tachycardia (VT) until proven otherwise. Attempting to treat presumed SVT with aberrancy using nodal blockers in a patient with monomorphic VT frequently causes catastrophic cardiovascular collapse.

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1. Procainamide (First-Line for Stable Monomorphic VT)

Procainamide is a Class Ia antiarrhythmic that blocks fast sodium channels and inhibits outward potassium currents, prolonging both ventricular refractoriness and conduction time.

• Dosing: 20 to 50 mg/min continuous IV infusion.
• The Four Termination Endpoints (discontinue immediately if any occur):
  1. Arrhythmia suppression / conversion to sinus rhythm.
  2. Onset of significant hypotension.
  3. QRS duration widens by $>50\%$ from baseline.
  4. Maximum cumulative dose of 17 mg/kg is reached.
• Maintenance Infusion: 1 to 4 mg/min continuous IV infusion.
• Evidence (The PROCAMIO Trial): A randomized clinical trial comparing IV procainamide (10 mg/kg over 20 min) against IV amiodarone (5 mg/kg over 20 min) in patients with regular wide-complex tachycardia demonstrated that procainamide achieved significantly higher rates of tachycardia termination (67% vs. 38%) and a lower incidence of severe adverse events (9% vs. 41%).

2. Amiodarone

• Dosing: 150 mg IV diluted in 100 mL D5W infused over 10 minutes. If VT recurs, repeat 150 mg IV over 10 minutes. Follow with a continuous maintenance infusion of 1 mg/min for 6 hours (360 mg), then 0.5 mg/min for 18 hours (540 mg). Total 24-hour target dose is approximately 1050 mg.

3. Sotalol

Class III antiarrhythmic with non-selective beta-blocking activity. Dose: 100 mg (1.5 mg/kg) IV infused over 5 minutes. Avoid in patients with baseline prolonged QTc ($>460\text{ ms}$) or severe renal impairment.

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Tachyarrhythmia Pharmacotherapy Reference Table