A patient in atrial flutter with rapid ventricular response receives adenosine IV. The expected immediate cardiac effect and its mechanism are:

• A Transient complete AV block by activating A1 receptors, increasing K+ conductance and hyperpolarizing AV nodal cells ✓
• B Conversion to sinus rhythm by blocking sodium channels in atrial tissue
• C Slowing of sinus rate by direct vagomimetic action on the SA node
• D Termination of flutter by prolonging AV nodal refractory period through calcium channel blockade

Explanation

Adenosine activates cardiac A1 receptors coupled to Gi proteins, which open inward-rectifying K+ (IKAdo/IKAch) channels in the AV node and SA node. This hyperpolarizes AV nodal cells and dramatically reduces their firing rate, causing transient AV block (3-15 seconds). In atrial flutter, adenosine will not cardiovert the atrial arrhythmia but will produce a momentary AV block that slows ventricular rate and may allow better identification of flutter waves on ECG. Adenosine is diagnostic and therapeutic for AVNRT/AVRT but not for atrial flutter/fibrillation conversion.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.