Adenosine

The drug that briefly flatlines the heart on purpose — adenosine converts SVT in seconds, but only if you push it the right way. Miss the technique and you miss the window.

Adenosine is an ultra-short-acting endogenous nucleoside that temporarily blocks conduction through the AV node, interrupting re-entrant circuits responsible for supraventricular tachycardia (SVT). Its half-life is under 10 seconds, which means administration technique is everything. The initial dose is 6 mg rapid IV push via the most proximal port (antecubital or above), followed immediately by a 20 mL normal saline flush using a two-syringe stopcock or push-pause method. If the first dose fails within 1–2 minutes, a second dose of 12 mg is given the same way. The drug must reach the heart before it is metabolized by red blood cells and vascular endothelium. Slow administration renders it useless. Expect a brief period of asystole (3–15 seconds) — this is the therapeutic effect, not a complication. The patient often reports transient chest tightness, flushing, and a sense of impending doom. Continuous cardiac monitoring and a crash cart must be at the bedside. Adenosine is used as both a treatment for SVT and a diagnostic tool: if the rhythm does not convert, it may not be SVT, helping differentiate narrow-complex tachycardias.

Key Distinctions

Don't confuse adenosine (converts SVT via transient AV block, seconds-long effect) with amiodarone (broad antiarrhythmic for VT/VF, hours-long effect). Students mistake the expected brief asystole after adenosine for a cardiac arrest — it is the intended response. Adenosine is contraindicated in second- or third-degree heart block and must be dose-reduced in patients on carbamazepine, dipyridamole, or those with heart transplants, where the effect is exaggerated.

Clinical Pearl

Push fast, flush faster. If adenosine doesn't hit the heart in under 10 seconds, it's already gone — the drug dies in the bloodstream before it can work.