A 28-year-old woman presents to the emergency department with palpitations and chest discomfort lasting 2 hours. She reports sudden onset while at rest. Vital signs: HR 180 bpm, BP 110/70 mmHg, RR 18/min. Physical examination reveals a regular tachycardia with no murmurs. ECG shows a narrow complex tachycardia with a rate of 180 bpm; P waves are not clearly visible. After vagal maneuvers fail, intravenous adenosine 6 mg is administered. The rhythm abruptly terminates, followed by sinus rhythm at 72 bpm with a normal PR interval and QRS duration. What is the most likely diagnosis?

Explanation

## Diagnosis: Atrioventricular Nodal Reentrant Tachycardia (AVNRT) ### Clinical Presentation - **Sudden onset** of narrow complex tachycardia at rest in a young woman - **Regular rhythm** at ~180 bpm - **Hemodynamically stable** (BP well-maintained) - **Abrupt termination** with adenosine, followed by normal sinus rhythm ### Key Diagnostic Features **Key Point:** AVNRT is the most common SVT (60% of all SVTs) and occurs due to a reentry circuit within the AV node itself, typically involving two pathways with different conduction properties (fast and slow). **High-Yield:** The adenosine response is **pathognomonic** — sudden termination with resumption of sinus rhythm indicates AV nodal or AV reentrant involvement. The normal PR interval post-termination excludes an accessory pathway as the anterograde limb. ### Mechanism 1. Dual AV nodal pathways (fast and slow) create a reentry circuit 2. Typically: anterograde conduction via slow pathway, retrograde via fast pathway 3. P waves are buried in or immediately after the QRS (not visible on surface ECG) 4. Adenosine blocks AV node conduction → circuit breaks → sinus rhythm resumes ### Distinguishing Features from AVRT | Feature | AVNRT | AVRT | |---------|-------|------| | **Mechanism** | Reentry within AV node | Reentry via accessory pathway (Kent bundle) | | **P wave location** | Buried in QRS or immediately after (pseudo-S in II, pseudo-R' in V1) | Visible, separated from QRS | | **Post-adenosine PR interval** | Normal | May show pre-excitation (short PR) if pathway conducts anterograde | | **Incidence** | Most common SVT (60%) | Second most common (30%) | | **Age of onset** | Any age, peak 40–50 years | Often younger; associated with WPW | **Clinical Pearl:** In AVNRT, the retrograde P wave is often **buried within the QRS complex**, making it invisible on standard ECG. This is why the ECG shows "no visible P waves" — they are actually there but superimposed on the QRS. ### Adenosine Mechanism Adenosine transiently blocks AV nodal conduction by activating adenosine A1 receptors → hyperpolarization of AV nodal cells → interruption of reentry circuit → sinus rhythm resumes. **Mnemonic: AVNRT = AV Node ReenTry** — the circuit is *within* the node, not *bypassing* it. ### Management 1. **Acute termination:** Vagal maneuvers (Valsalva, carotid massage) → adenosine IV (6–12 mg) → verapamil/diltiazem if adenosine unavailable 2. **Long-term:** Beta-blockers or calcium channel blockers for rate control; radiofrequency ablation for refractory cases (curative, >95% success) [cite:Harrison 21e Ch 231]