A 62-year-old woman with diabetes and chronic kidney disease presents with palpitations and dyspnea. Her ECG shows a regular narrow-complex tachycardia at 160 bpm with no visible P waves, and the PR interval cannot be determined. The QRS duration is 0.08 seconds. Carotid sinus massage does not terminate the arrhythmia. What is the most appropriate immediate next step in management?

Explanation

## ECG Analysis & Arrhythmia Identification The clinical presentation describes: - Regular narrow-complex tachycardia (QRS <0.12 seconds) - Heart rate 160 bpm - Absent or buried P waves (likely buried in the QRS or T wave) - No response to carotid sinus massage - Hemodynamically stable (palpitations and dyspnea suggest compensated state) This pattern is consistent with **atrioventricular nodal reentrant tachycardia (AVNRT)** or **atrioventricular reentrant tachycardia (AVRT)**, both of which are supraventricular tachycardias (SVT) with narrow QRS complexes. ## Management Algorithm for Stable SVT ```mermaid flowchart TD A[Narrow-Complex Tachycardia<br/>Rate >150 bpm]:::outcome --> B{Hemodynamically Stable?}:::decision B -->|No| C[Synchronized Cardioversion<br/>100 J]:::urgent B -->|Yes| D[Vagal Maneuvers<br/>Carotid Massage/Valsalva]:::action D --> E{Terminated?}:::decision E -->|Yes| F[Diagnosis Confirmed<br/>Observe]:::outcome E -->|No| G[IV Adenosine 6 mg<br/>Rapid Push]:::action G --> H{Converted?}:::decision H -->|Yes| I[SVT Terminated<br/>Observe/Arrange EP Study]:::outcome H -->|No| J[IV Adenosine 12 mg<br/>or IV Verapamil]:::action ``` ## Key Point: **Adenosine is the first-line pharmacological agent for terminating stable, hemodynamically compensated SVT** because it: 1. Blocks AV nodal conduction transiently 2. Has a very short half-life (~10 seconds) 3. Terminates AVNRT and AVRT with high efficacy (>90%) 4. Allows diagnostic clarification of the underlying mechanism 5. Has minimal systemic hemodynamic effects [cite:Harrison 21e Ch 298]. ## High-Yield: **Adenosine dosing:** Start with 6 mg IV rapid push (over 1–3 seconds) via large-bore IV or central line, followed immediately by 12 mL normal saline flush. If unsuccessful within 1–2 minutes, repeat with 12 mg. Success rate is 90–95% for AVNRT/AVRT. Warn patient of transient chest discomfort, dyspnea, and flushing [cite:KD Tripathi 8e Ch 12]. ## Clinical Pearl: **Why carotid sinus massage failed:** This patient's arrhythmia did not terminate with vagal maneuvers, indicating the need for pharmacological intervention. Adenosine is more reliable than calcium channel blockers in this scenario because it has faster onset and shorter duration of action, allowing rapid assessment of response.