In a third-degree AV block, which part of the heart is responsible for pacing the atria?

In a third-degree AV block, the pacing of the atria is primarily managed by the sinoatrial (SA) node. The SA node acts as the heart's natural pacemaker, initiating electrical impulses that cause atrial contraction. In cases of third-degree AV block, there is a complete dissociation between the atria and ventricles due to the failure of impulses to pass from the atria to the ventricles through the atrioventricular (AV) node. Consequently, the atria continue to be paced by the SA node, while the ventricles may rely on an escape rhythm from other pacemakers, such as the Bundle of His or Purkinje fibers, but this does not impact the fact that the atrial contractions are still initiated by the SA node. Understanding the role of the SA node in maintaining atrial rhythm despite the block is crucial in comprehending the overall cardiac electrical activity during this condition.