What is the relationship between the atrial and ventricular rates in a third-degree AV block?

In a third-degree AV block, also known as complete heart block, there is a complete dissociation between the electrical signals generated in the atria and those in the ventricles. This means that the atrial and ventricular contractions occur independently of each other. The atrial rate, which is determined by the sinoatrial (SA) node or other atrial pacemakers, typically remains consistent and can be relatively faster compared to the ventricular rate.

Meanwhile, the ventricular rate depends on secondary pacemaker sites such as the AV node or bundle of His, which will typically fire at a much slower rate than the atria, often in the range of 20-40 beats per minute (bpm). Because of this dissociation, the two rates do not align, leading to a complete lack of coordination between atrial and ventricular contractions. This independence is a hallmark characteristic of a third-degree AV block, distinguishing it from other types of heart blocks where the atrial and ventricular rhythms might still show some degree of relationship.