What does a 3rd Degree AV Block indicate about atrial and ventricular communication?

A 3rd Degree AV Block, also known as complete heart block, indicates that there is a complete disruption in the electrical signals between the atria and ventricles. In this situation, the atria and ventricles no longer communicate with each other; while the atria may still be contracting independently, the impulses generated in the atria do not reach the ventricles. As a result, the ventricular rate is usually much lower than the atrial rate, leading to a dissociation between the two.

In this block, the atria may still produce P waves, but these waves do not correlate with the QRS complexes because the electrical impulses fail to pass through the AV node to the ventricles. This results in two separate rhythms, where the atria might beat at one rate while the ventricles beat at another, typically dictated by an escape rhythm from a subsidiary pacemaker in the ventricles. Therefore, the characteristic feature of a 3rd Degree AV Block is that there is no communication occurring between the atria and the ventricles.