What is the differential diagnosis and management for a patient with tachycardia (heart rate in the 140s)?

Differential Diagnosis and Management of Tachycardia (Heart Rate 140s)

Initial Assessment: QRS Width and Hemodynamic Stability

The first critical step is determining QRS width on 12-lead ECG—if QRS ≥120 ms and the diagnosis is uncertain, treat as ventricular tachycardia (VT) until proven otherwise. 1, 2

Hemodynamically Unstable Patients

• Immediate synchronized DC cardioversion is the definitive treatment for any unstable tachycardia, regardless of QRS width. 1, 2
• Hemodynamic instability includes hypotension, altered mental status, chest pain suggesting ischemia, or acute heart failure 1

Narrow QRS-Complex Tachycardia (<120 ms)

Differential Diagnosis

Atrioventricular Nodal Reentrant Tachycardia (AVNRT):

• Most common SVT mechanism 2
• Regular RR interval with P waves typically hidden within QRS complex 1, 2
• Look for pseudo-R' wave in lead V1 or pseudo-S wave in inferior leads (II, III, aVF) 1, 2

Atrioventricular Reciprocating Tachycardia (AVRT):

• P wave visible in ST segment, separated from QRS by >70 ms 1, 2
• Requires accessory pathway (orthodromic conduction down AV node, retrograde up accessory pathway) 2

Atrial Tachycardia (AT):

• P wave morphology differs from sinus rhythm 2
• Long RP interval (RP > PR) typical 1, 2
• P wave usually near end of or after T wave 2

Atrial Fibrillation/Flutter:

• Irregular ventricular response suggests AF 1
• Atrial rate exceeding ventricular rate indicates flutter or AT 1

Acute Management for Stable Narrow QRS Tachycardia

Step 1: Vagal Maneuvers

• Attempt Valsalva maneuver, carotid sinus massage, or facial immersion in cold water first 1, 2

Step 2: Adenosine (if vagal maneuvers fail)

• Adenosine is the preferred agent due to rapid onset and short half-life 1, 2
• Dose: 6 mg IV rapid push, followed by 12 mg if needed 1
• Contraindicated in severe asthma 1, 2
• Critical warning: Use adenosine with extreme caution if diagnosis uncertain—can precipitate ventricular fibrillation in coronary artery disease or accelerate ventricular rate in pre-excited AF 1

Step 3: Alternative Agents

• Metoprolol: 2.5-5 mg IV bolus over 2 minutes, up to 3 doses 1
• Diltiazem: 0.25 mg/kg IV over 2 minutes, then 5-15 mg/h infusion 1
• Esmolol: 500 mcg/kg IV over 1 minute, then 60-200 mcg/kg/min 1

Wide QRS-Complex Tachycardia (≥120 ms)

Critical Diagnostic Approach

Three possible etiologies:

1. VT (most common and most dangerous)
2. SVT with bundle branch block
3. SVT with accessory pathway conduction 1, 2

ECG Features Diagnostic of VT

AV dissociation (ventricular rate > atrial rate):

• Proves VT diagnosis but only visible in 30% of cases 1, 2

Fusion beats:

• Pathognomonic for VT 1, 2
• Represent merger of supraventricular and ventricular depolarization 2

QRS width criteria:

• QRS >140 ms with RBBB pattern or >160 ms with LBBB pattern strongly favors VT 1, 2

Concordance:

• All precordial leads showing positive or negative deflections suggests VT or pre-excitation 1

RS interval:

• RS interval >100 ms in any precordial lead highly suggestive of VT 1

Management of Wide QRS Tachycardia

If diagnosis uncertain, treat as VT—never give verapamil or diltiazem for wide-complex tachycardia of unknown etiology, as this can cause hemodynamic collapse in VT or accelerated ventricular rate in pre-excited AF. 1, 2

For stable patients with confirmed VT:

• Amiodarone: 150 mg IV over 10 minutes, then 0.5-1 mg/min infusion 1
• Procainamide or lidocaine are alternatives 1

Special Considerations

Pre-excited Tachycardias (WPW Syndrome)

Critical warning: Beta-blockers, calcium channel blockers, and digoxin are contraindicated in pre-excited AF/flutter—they can accelerate ventricular rate and cause ventricular fibrillation. 1

For hemodynamically stable pre-excited tachycardia:

• Type I antiarrhythmics (procainamide) or amiodarone IV 1
• DC cardioversion if unstable 1

Sinus Tachycardia

Inappropriate sinus tachycardia:

• Persistent resting heart rate >100 bpm with excessive response to activity 1
• Predominantly affects young women (90% female, mean age 38 years) 1
• Beta-blockers are first-line therapy 1
• Treat underlying causes (fever, hypovolemia, hyperthyroidism, anemia, pain) 1

Atrial Fibrillation with Rapid Ventricular Response

Rate control agents:

• Metoprolol 2.5-5 mg IV or diltiazem 0.25 mg/kg IV for patients without accessory pathways 1
• Digoxin 0.25 mg IV every 2 hours (up to 1.5 mg) for heart failure patients 1

Critical Pitfalls to Avoid

• Never assume stable vital signs exclude VT—patients with VT can be hemodynamically stable 1
• Always obtain 12-lead ECG during tachycardia before treatment when possible 1, 2
• Avoid adenosine in pre-excited tachycardias—can precipitate rapid ventricular rates 1
• Do not use AV nodal blockers in atrial flutter treated with class IC agents (flecainide/propafenone) without concurrent AV nodal blockade—risk of 1:1 conduction 1
• Compare tachycardia ECG to baseline sinus rhythm ECG when available—identical QRS morphology suggests SVT with pre-existing BBB 1, 2