## Diagnosis: Atrioventricular Nodal Reentrant Tachycardia (AVNRT) ### Clinical Presentation A young woman with sudden-onset palpitations, a narrow complex tachycardia at 180 bpm, and no structural heart disease is a classic presentation of AVNRT — the **most common** paroxysmal SVT in adults, particularly young women. ### Key ECG Findings **Key Point:** In AVNRT, the reentrant circuit is entirely (or nearly entirely) within the AV node, using a slow pathway (antegrade) and a fast pathway (retrograde). Because retrograde atrial activation occurs almost simultaneously with ventricular activation, **P waves are buried within or at the very end of the QRS/T wave** — producing the characteristic "pseudo-R'" in V1 or "pseudo-S" in inferior leads. This is the hallmark of **typical (slow-fast) AVNRT**. In AVRT (accessory pathway–mediated), retrograde conduction via the accessory pathway takes slightly longer, so P waves typically appear **just after** the QRS (in the ST segment), giving a short but **measurable** RP interval (RP < 70–80 ms in AVNRT vs. RP > 70 ms in AVRT). The stem describes P waves "buried in the T wave of the preceding beat," which is most consistent with AVNRT. ### Why Not AVRT? **High-Yield:** AVRT requires an accessory pathway (e.g., WPW syndrome or a concealed pathway). In orthodromic AVRT, retrograde conduction via the accessory pathway produces a P wave that is visible **after** the QRS in the ST segment — not buried within the T wave. Additionally, there is no mention of a delta wave on baseline ECG, pre-excitation, or prior WPW diagnosis. While AVRT can also terminate with adenosine, the P-wave timing described is more consistent with AVNRT. ### Mechanism of Termination **High-Yield:** Adenosine causes transient AV nodal block, interrupting the reentrant circuit (which depends entirely on AV nodal tissue in AVNRT). This results in abrupt termination and restoration of sinus rhythm — a classic response confirming AV nodal dependence of the tachycardia. ### Differential Features | Feature | AVNRT | AVRT | Atrial Flutter | |---------|-------|------|----------------| | **P wave timing** | Buried in QRS/T wave (RP < 70 ms) | Just after QRS in ST segment (RP 70–150 ms) | Saw-tooth, visible between QRS | | **Rate** | 140–250 bpm | 140–250 bpm | 250–350 bpm atrial | | **Adenosine response** | Abrupt termination | Abrupt termination | Slows AV conduction, reveals flutter waves | | **Accessory pathway** | No | Yes (WPW or concealed) | No | | **Most common in** | Young women | Younger patients, WPW | Older, structural disease | **Clinical Pearl:** The single most important distinguishing feature between AVNRT and AVRT on ECG is the **RP interval**: AVNRT has RP < 70 ms (P buried in QRS/T), while orthodromic AVRT has RP > 70 ms (P visible in ST segment). Definitive diagnosis requires electrophysiology study (EPS). ### Why Other Options Are Wrong - **AVRT (A):** Requires accessory pathway; P waves appear after QRS in ST segment, not buried in T wave; no delta wave or WPW history mentioned. - **Atrial flutter with 1:1 (C):** Atrial rate 250–350 bpm; saw-tooth flutter waves; 1:1 conduction is rare and usually seen with accessory pathways or sympathomimetics. - **Sinus tachycardia (D):** Gradual onset/offset; P waves precede QRS; rate rarely exceeds 150–160 bpm; adenosine would not terminate it abruptly. [cite: Harrison's Principles of Internal Medicine, 21e, Ch. 226; Braunwald's Heart Disease, 12e]
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