In an adult, how is tachycardia severity defined by heart‑rate ranges, QRS width, and hemodynamic status, and what are the appropriate acute management steps?

Tachycardia Severity Parameters and Acute Management

Tachycardia is defined as a heart rate >100 bpm, but clinical significance typically occurs at ≥150 bpm, where symptoms are more likely attributable to the arrhythmia itself rather than a physiologic response—unless ventricular function is impaired. 1, 2

Heart Rate Thresholds

• >100 bpm: Technical definition of tachycardia 1, 2
• <150 bpm: Symptoms of instability are unlikely to be caused primarily by the tachycardia unless impaired ventricular function exists 1, 2
• ≥150 bpm: Rate becomes clinically significant and more likely represents a primary arrhythmia requiring intervention 1, 2
• 180-190 bpm (cycle length 320-330 ms): Highly suggestive of paroxysmal SVT mechanisms, particularly AVNRT or AVRT 3

QRS Width Classification

QRS duration is the critical parameter that determines the differential diagnosis and management pathway. 4

Narrow-Complex Tachycardia (QRS <120 ms)

• Always supraventricular in origin 4, 5
• Includes: sinus tachycardia, atrial fibrillation, atrial flutter, AVNRT, AVRT, atrial tachycardia, multifocal atrial tachycardia, junctional tachycardia 1, 4
• Critical pitfall: Verify QRS width in multiple leads, as it may appear falsely narrow in one or two leads 6

Wide-Complex Tachycardia (QRS ≥120 ms)

• Most are ventricular tachycardia—when uncertain, treat as VT 1, 4
• Differential includes: VT/VF, SVT with aberrancy, pre-excited tachycardias (WPW), ventricular paced rhythms 1, 4
• QRS ≥120 ms requires differentiation from VT with strength of evidence IIa 3

Hemodynamic Status Assessment

Hemodynamic stability is the single most important factor determining immediate management. 4

Unstable Tachycardia

Immediate synchronized DC cardioversion is indicated for any hemodynamically unstable tachycardia, regardless of QRS width or mechanism. 3, 4

Signs of instability include:

• Hypotension
• Altered mental status
• Chest pain/acute coronary syndrome
• Acute heart failure
• Syncope during episodes (warrants urgent cardiology referral) 3

Stable Tachycardia

Management proceeds algorithmically based on QRS width and regularity 1, 4

Acute Management Algorithm

For Stable Narrow-Complex SVT

1. Vagal maneuvers first (Valsalva, carotid massage)—Class I recommendation 4

2. Adenosine if vagal maneuvers fail:

   • First dose: 6 mg rapid IV push with NS flush 1, 4
   • Second dose: 12 mg if needed 1, 4
   • Caution: Avoid in cardiac transplant recipients due to paradoxical effects 4

3. Alternative agents if adenosine fails or contraindicated:

   • IV diltiazem or verapamil (preferred, Class I) 4
   • IV beta-blockers (reasonable alternative) 4
   • IV metoprolol: 5 mg over 1-2 minutes, repeat every 5 minutes to maximum 15 mg 2

4. Synchronized cardioversion if pharmacologic therapy fails 4

For Stable Wide-Complex Tachycardia

When diagnosis is uncertain, treat as ventricular tachycardia. 1, 4

1. Procainamide IV: 20-50 mg/min until arrhythmia suppressed, hypotension develops, QRS widens >50%, or maximum 17 mg/kg reached 1, 4

   • Maintenance: 1-4 mg/min 1
   • Avoid if prolonged QT or CHF 1

2. Amiodarone IV: 150 mg over 10 minutes, repeat if VT recurs 1, 4

   • Maintenance: 1 mg/min for first 6 hours 1

3. Adenosine may be used as a diagnostic test if cause cannot be determined 6

For Irregular Narrow-Complex Tachycardia

• Likely atrial fibrillation or multifocal atrial tachycardia 1
• Occasionally atrial flutter with variable conduction 1
• Focus on rate control and anticoagulation assessment

Critical Pitfalls to Avoid

• Never assume narrow QRS from one lead—verify in multiple leads to avoid misclassifying wide-complex tachycardia 6
• Do not use adenosine or atropine in cardiac transplant recipients—vagal innervation is absent 4
• Avoid beta-blockers in pre-excited atrial fibrillation/flutter (WPW with AF)—can precipitate VF 2
• Do not use verapamil/diltiazem for wide-complex tachycardia of uncertain origin—can cause cardiovascular collapse if VT 4
• Always correct underlying causes (hypoxemia, fever, hypovolemia, electrolyte abnormalities) before attributing symptoms solely to heart rate 2, 4

Special Considerations

• Sinus tachycardia: Treat underlying cause (fever, dehydration, anemia, infection, stimulants); beta-blockade effective for stress-related physiologic sinus tachycardia 4
• Atrial flutter: Synchronized cardioversion is Class I for acute rhythm control; pharmacologic options include IV ibutilide or oral dofetilide 4
• P-wave analysis: P waves buried in QRS (RP <90 ms) indicate typical AVNRT; P waves in early ST segment suggest AVRT or atypical AVNRT 3
• "Neck pounding" sensation: Highly specific for AVNRT due to cannon A-waves 3