A 28-year-old woman presents to the emergency department with palpitations and dyspnea for 2 hours. She reports a history of similar episodes since childhood, often triggered by emotional stress or caffeine. Vital signs: HR 180 bpm, BP 110/70 mmHg, RR 20/min. Physical examination reveals no structural abnormalities. 12-lead ECG shows a regular narrow-complex tachycardia with a rate of 180 bpm; P waves are buried within or immediately after the QRS complex. Adenosine 6 mg IV is administered, resulting in immediate termination of the tachycardia and restoration of normal sinus rhythm. What is the most likely diagnosis?

Explanation

## Diagnosis: Atrioventricular Nodal Reentrant Tachycardia (AVNRT) ### Clinical Presentation This patient presents with a classic SVT scenario: young female, long history of episodic palpitations triggered by stress/caffeine, and sudden onset. The immediate adenosine response is pathognomonic for AVNRT or AVRT. ### ECG Findings **Key Point:** The buried or immediately post-QRS P wave is the hallmark of AVNRT. In AVNRT, the P wave is typically inscribed at or very near the QRS complex (pseudo-R' in V1, pseudo-S in II/III) because atrial and ventricular depolarization occur nearly simultaneously. ### Mechanism 1. AVNRT involves a reentrant circuit entirely within or immediately adjacent to the AV node 2. The AV node has dual pathways: a fast pathway (short refractory period) and a slow pathway (long refractory period) 3. Most commonly, the circuit conducts anterograde via the slow pathway and retrograde via the fast pathway 4. This produces a regular narrow-complex tachycardia at 140–250 bpm ### Why Adenosine Works **Clinical Pearl:** Adenosine blocks conduction through the AV node by increasing K+ efflux and decreasing cAMP. It terminates AVNRT and AVRT by interrupting the reentrant circuit. Abrupt termination (not gradual slowing) is typical. ### Differential Features | Feature | AVNRT | AVRT | Atrial Flutter | |---------|-------|------|----------------| | P wave timing | Buried/post-QRS | Early, visible | Sawtooth pattern | | Accessory pathway | No | Yes (WPW) | No | | Adenosine response | Abrupt termination | Abrupt termination | Gradual AV block → flutter waves visible | | Incidence | 50–60% of SVTs | 30–40% of SVTs | Structural heart disease common | **High-Yield:** AVNRT is the most common form of SVT in patients without pre-excitation or structural heart disease. ### Management - Acute: Adenosine, verapamil, or diltiazem - Chronic: Beta-blockers, calcium channel blockers, or radiofrequency ablation (definitive) - This patient is a candidate for ablation given frequent, symptomatic episodes since childhood [cite:Harrison 21e Ch 233]