If a third degree AV block is located in the upper AV node, which automaticity focus will start pacing?

If a third degree AV block is located in the upper AV node, which automaticity focus will start pacing?

• A. SA node
• B. Ventricular focus
• C. Junctional focus
• D. Bundle of His focus

Answer: (C)

In the scenario of a third-degree AV block occurring at the level of the upper AV node, communication between the atria and the ventricles is completely severed. This situation leads to the phenomenon known as complete heart block, where the atria and ventricles beat independently of each other.

In cases of complete heart block, when the impulse from the SA node is unable to propagate through the AV node to the ventricles, the body relies on an alternate pacing site, or automaticity focus, to maintain ventricular activity. In the case of a block in the upper AV node, the next most reliable pace-making site is a junctional focus.

A junctional focus typically originates from the AV junction area itself, which can take over pacing if both the SA node and the passage of electrical signals to the ventricles are compromised. This focus usually fires at a rate between 40 to 60 beats per minute, which is sufficient to keep the ventricles contracting, although at a lower rate compared to the normal atrial pacing.

Understanding the levels of block and associated pacing sites allows for better insights into EKG readings and clinical decisions regarding patient management in cases of heart block. Thus, in this type of third-degree block, the junctional focus