Abstract Body (Do not enter title and authors here): Introduction:
Management of tachyarrhythmias during pregnancy, particularly atrioventricular nodal reentrant tachycardia (AVNRT), presents unique challenges due to concerns regarding maternal and fetal safety. Electrical cardioversion poses risks such as fetal distress and preterm labor, while pharmacologic therapies must be cautiously administered to avoid potential fetal harm. Intravenous adenosine remains first-line therapy, however, experience with doses exceeding 12 mg during pregnancy is limited. We present a case demonstrating that administration of high-dose adenosine (24 mg) can safely and effectively enhance subsequent beta-blocker efficacy in termination of refractory AVNRT during late-term pregnancy.

Case:
A 29-year-old woman at 37 weeks of gestation presented with sustained narrow-complex tachycardia (~180 bpm) consistent with AVNRT, accompanied by palpitations and lightheadedness. Initial heart rate was ~187 bpm with blood pressure of 105/83 mmHg. Labs, including electrolytes and cardiac biomarkers, were normal. ECG confirmed AVNRT with retrograde P-waves.

Methods:
Initial management with vagal maneuvers and intravenous metoprolol (two doses of 2.5 mg) failed to convert the AVNRT and resulted in transient hypotension (BP ~85/60 mmHg). Intravenous adenosine was then administered in escalating doses (6, 12, 18, and 24 mg) under continuous maternal and fetal monitoring. While high-dose adenosine administration produced transient AV block and slowed the heart rate, sustained conversion was not achieved. A subsequent IV dose of metoprolol (5 mg) was given immediately thereafter.

Results:
Immediately following administration of metoprolol, the tachycardia terminated and sinus rhythm was immediately restored (HR ~103 bpm). Blood pressure promptly stabilized, fetal monitoring remained reassuring. Post-conversion echocardiogram revealed normal cardiac structure and function (EF 60–65%). Patient’s hospitalization and delivery were uncomplicated.

Discussion:
High-dose adenosine administration up to 24 mg was safely tolerated during late-term pregnancy and significantly enhanced the effectiveness of subsequent beta-blocker therapy in refractory AVNRT. Adenosine's transient AV nodal suppression likely conditioned the reentrant circuit, facilitating effective termination with beta-blocker therapy. This sequential pharmacologic strategy is a valuable pharmacologic alternative to terminate AVNRT in late-term pregnancy and avoid electrical cardioversion.