1. Initial approach
   • Is the patient unstable? Hypotension, shock, ischemic chest pain, acute heart failure, altered consciousness.
   • Obtain a 12-lead ECG and classify the rhythm as narrow or wide QRS, and regular or irregular.
   • Look for reversible triggers: hypoxia, fever, pain, anemia, hypovolemia, electrolyte disturbance, stimulant/drug use, thyroid disease, ischemia.
2. Sinus tachycardia
   • Usually a physiologic response, not a primary arrhythmia.
   • Treat the underlying cause rather than attempting rhythm termination.
3. Supraventricular tachyarrhythmias
   • Atrial fibrillation and atrial flutter
   • AV nodal re-entrant tachycardia (AVNRT)
   • AV re-entrant tachycardia (AVRT, including WPW-associated tachycardia)
   • Atrial tachycardia and multifocal atrial tachycardia
   • Stable, regular narrow-complex SVT may respond to vagal maneuvers then adenosine. Unstable tachyarrhythmia needs synchronized cardioversion.
4. Ventricular tachyarrhythmias
   • Monomorphic or polymorphic ventricular tachycardia
   • Torsades de pointes, usually associated with QT prolongation
   • Ventricular fibrillation
   • Treat a wide-complex tachycardia as VT unless there is strong evidence otherwise. Pulseless VT/VF requires immediate defibrillation and CPR.
5. Core emergency distinction
   • Unstable with a pulse: synchronized cardioversion.
   • Pulseless VT/VF: unsynchronized defibrillation plus high-quality CPR.
   • Stable rhythm: ECG-guided treatment based on rhythm mechanism and QRS morphology.

Diagnosis of tachyarrhythmias

1. Assess immediately for instability

• Hypotension or shock
• Acute chest pain/ischemia
• Pulmonary edema or acute heart failure
• Syncope, altered mental status, or signs of poor perfusion

2. ECG-based classification

3. Four practical ECG groups

4. Key diagnostic rules

• Regular narrow-complex tachycardia: distinguish sinus tachycardia from re-entry SVT by onset/offset, P-wave findings, and response to vagal maneuver or adenosine where appropriate.
• Atrial fibrillation: irregularly irregular ventricular rhythm with absent organized P waves and a fluctuating baseline.
• Atrial flutter: regular atrial activity, often “sawtooth” flutter waves, with fixed or variable AV conduction.
• Wide-complex tachycardia: presume ventricular tachycardia if uncertain, especially in structural heart disease or prior myocardial infarction. Clinical appearance alone cannot reliably distinguish VT from SVT with aberrancy; use the 12-lead ECG. Tintinalli’s Emergency Medicine, p. 2033.
• Findings favoring VT include AV dissociation, capture beats, fusion beats, extreme axis deviation, and a history of structural heart disease. Goldman-Cecil Medicine, p. 1756.

5. Tests after the initial ECG

• Continuous cardiac monitoring or telemetry
• Ambulatory ECG: Holter monitor, event monitor, or implantable loop recorder
• Serum potassium, magnesium, calcium, glucose, complete blood count, renal function
• Troponin if ischemia is possible
• Thyroid function testing when clinically indicated
• Echocardiography to assess structural heart disease and ventricular function
• Assessment for ischemia when indicated
• Electrophysiology study for recurrent, uncertain, or ablation-amenable arrhythmias

Treatment of tachyarrhythmias

1. Immediate measures for all patients

• ABC assessment, cardiac monitor, IV access, 12-lead ECG
• Give oxygen only if hypoxemic
• Treat reversible precipitants: hypoxia, fever, pain, dehydration, anemia, sepsis, ischemia, hypokalemia/hypomagnesemia, stimulant or drug toxicity, and hyperthyroidism.
• If there is no pulse, manage as cardiac arrest: CPR plus defibrillation for VF/pulseless VT.

2. Unstable tachyarrhythmia with a pulse

• Immediate synchronized cardioversion
• Sedate if feasible, but do not delay shock in a critically unstable patient.
• Current AHA electrical-cardioversion guidance lists initial biphasic settings of 200 J for atrial fibrillation/flutter and 100 J for narrow-complex SVT or monomorphic VT. Polymorphic VT requires an unsynchronized high-energy shock. AHA 2025 cardioversion algorithm

3. Stable, regular narrow-complex tachycardia

1. Vagal manoeuvres, preferably a modified Valsalva manoeuvre.
2. If unsuccessful: adenosine rapid IV push, typically 6 mg followed by saline flush; if needed, 12 mg.
3. If persistent, consider a beta-blocker or nondihydropyridine calcium-channel blocker such as diltiazem or verapamil, if no contraindication and after confirming the rhythm.
4. Recurrent AVNRT or AVRT: electrophysiology referral and catheter ablation is often definitive.

4. Atrial fibrillation or atrial flutter with rapid ventricular response

• If unstable: synchronized cardioversion.
• If stable: rate control with a beta-blocker or diltiazem/verapamil. Digoxin may have a role in selected patients, particularly with heart failure or low blood pressure, but its onset is slower.
• Assess stroke risk and need for anticoagulation using a validated risk score such as CHA₂DS₂-VASc.
• Rhythm-control options include cardioversion, antiarrhythmic therapy, or catheter ablation, chosen based on symptoms, duration of AF, structural heart disease, and anticoagulation status.

5. Stable wide-complex tachycardia

• Obtain expert help and monitor continuously.
• For regular monomorphic wide-QRS tachycardia, adenosine may be considered only if it is regular and monomorphic.
• IV antiarrhythmics may be used in monitored settings:
  • Procainamide: avoid in prolonged QT or heart failure.
  • Amiodarone
  • Sotalol, in suitable patients
• If drug therapy fails or is contraindicated, synchronized cardioversion is appropriate even in a stable patient. This aligns with the AHA 2025 adult tachycardia guidance.

6. Polymorphic VT and torsades de pointes

• Unstable/pulseless: immediate defibrillation.
• Correct the cause: stop QT-prolonging drugs and correct potassium and magnesium.
• IV magnesium sulfate is first-line for torsades de pointes, even if the serum magnesium is normal.
• In recurrent pause-dependent torsades, increase heart rate with overdrive pacing or isoproterenol under specialist supervision.

7. Pre-excited AF in Wolff-Parkinson-White syndrome

• If unstable: immediate synchronized cardioversion.
• If stable: IV procainamide or ibutilide may be used by experienced clinicians.
• Avoid isolated AV-nodal blockers, including adenosine, beta-blockers, verapamil/diltiazem, digoxin, and usually IV amiodarone, because they can accelerate conduction through the accessory pathway. Harrison’s Principles of Internal Medicine, 22e, Ch. 260.

8. Long-term prevention

• Address structural heart disease, ischemia, cardiomyopathy, sleep apnea, alcohol/stimulant use, and electrolyte disorders.
• Catheter ablation: highly effective for recurrent SVT, typical flutter, many focal atrial tachycardias, and selected VT.
• Implantable cardioverter-defibrillator (ICD): indicated for selected patients at high risk of recurrent sustained VT/VF or sudden cardiac death.
• Antiarrhythmic drugs are selected individually, based on ventricular function, coronary/structural heart disease, kidney function, QT interval, and proarrhythmic risk.