What is the first-line medication in the Emergency Room (ER) for a patient with narrow QRS tachycardia, a heart rate (HR) of 135 beats per minute (bpm), and no visible P waves on an electrocardiogram (ECG)?

First-Line Medication for Narrow QRS Tachycardia in the ER

Adenosine is the first-line medication for narrow QRS tachycardia with HR 135 and no P waves in the Emergency Room setting. 1

Diagnostic Considerations

When encountering a patient with narrow QRS tachycardia (QRS <120 ms) with no visible P waves and a heart rate of 135 bpm, the most likely diagnosis is supraventricular tachycardia (SVT), specifically atrioventricular nodal reentrant tachycardia (AVNRT). 2

Key diagnostic features:

• Narrow QRS complex (<120 ms) indicates supraventricular origin
• Absence of P waves is characteristic of AVNRT
• Regular rhythm at 135 bpm is consistent with SVT

Treatment Algorithm

Step 1: Initial Approach

• Ensure patient is hemodynamically stable
• If unstable (hypotension, altered mental status, signs of shock, severe chest pain), proceed directly to synchronized cardioversion 1
• If stable, proceed with pharmacological management

Step 2: First-Line Treatment

1. Vagal maneuvers (first attempt)

   • Valsalva maneuver or carotid sinus massage 2
   • Success rate is variable but worth attempting as non-pharmacological option

2. Adenosine (if vagal maneuvers fail)

   • Initial dose: 6 mg IV rapid bolus followed by 20 mL saline flush 2
   • If ineffective, may repeat with 12 mg IV push 2
   • Administration via proximal IV access for maximum efficacy
   • Success rate: 90-95% for SVT termination 1

Step 3: Second-Line Options (if adenosine fails)

• Calcium channel blockers:

  • Diltiazem: 15-20 mg (0.25 mg/kg) IV over 2 minutes 2
  • Verapamil: 2.5-5 mg IV over 2 minutes 2

• Beta blockers:

  • Metoprolol: 5 mg IV over 1-2 minutes, may repeat up to 15 mg 2
  • Esmolol: 500 mcg/kg over 1 minute, followed by infusion 2

Step 4: Refractory Cases

• If medication fails, proceed to synchronized cardioversion 2

Rationale for Adenosine as First-Line

Adenosine is recommended as first-line therapy for several reasons:

1. Rapid onset of action (within seconds)
2. Short half-life (<10 seconds), minimizing prolonged side effects
3. High efficacy in terminating SVT (90-95% success rate) 1
4. Diagnostic value - can help differentiate the type of SVT by revealing underlying atrial activity 3
5. Recommended by multiple guidelines including AHA and ACC 2, 1

Important Precautions

• Have a defibrillator immediately available when administering adenosine due to potential for inducing atrial fibrillation, especially in patients with concealed accessory pathways 1, 4
• Monitor for transient side effects: dyspnea, chest pain, flushing, and headache are common but brief 3
• Watch for ventricular pauses: can exceed 2 seconds in some patients 3
• Contraindicated in patients with:
  • Asthma (relative contraindication)
  • Second or third-degree AV block
  • Sick sinus syndrome without pacemaker

Special Considerations

• Reduce adenosine dose in patients taking dipyridamole or carbamazepine and when administered via central venous access 2
• Increase dose in patients taking theophylline or caffeine
• In patients with suspected Wolff-Parkinson-White syndrome, avoid verapamil and diltiazem as they can accelerate conduction through accessory pathways 2, 4

By following this approach, most cases of narrow complex tachycardia in the ER setting can be effectively managed, with adenosine serving as the cornerstone of initial pharmacological therapy.