What is the immediate effect of adenosine on the heart's conduction system?

• A. Prolonged QT interval
• B. Increase AV conduction
• C. Increase SA node rate
• D. Temporary AV nodal block

Answer: (D)

Adenosine acts quickly to slow conduction through the AV node by activating A1 receptors, which opens potassium channels and hyperpolarizes the nodal cells, reducing calcium-dependent conduction. The immediate result is a temporary block of AV nodal conduction, effectively pausing transmission from atria to ventricles for a few seconds. This transient AV nodal block can terminate reentrant narrow-complex SVT, which is why it’s the correct mechanism.

The other possibilities don’t fit because adenosine does not prolong the QT interval, nor does it increase AV conduction or SA node rate. In fact, it often causes bradycardia or a brief pause due to AV nodal suppression.