In a third degree AV block, what paces the atria?

In a third degree AV block, what paces the atria?

• A. Ventricular foci
• B. SA node
• C. AV junction
• D. Cardiac pacemaker

Answer: (B)

In a third degree AV block, the atria are typically paced by the SA node, which is the natural pacemaker of the heart located in the right atrium. In this type of block, there is a complete dissociation between the atrial and ventricular activity due to the failure of the electrical signals to be conducted from the atria to the ventricles through the AV node. This means that while the atria continue to receive impulses from the SA node and contract, the ventricles do not respond to these signals, as they are not being activated by the same input.

The SA node generates impulses at a regular rhythm which helps maintain atrial contraction even when the communication to the ventricles is blocked. This is why the atrial activity is still present and is primarily driven by the signals from the SA node during a third degree AV block. Understanding this relationship is crucial in recognizing the effects of the block on heart rhythm and function.