Final answer:
A third-degree atrioventricular (AV) heart block is the condition where the atrial and ventricular rhythms are regular but independent, characterized by a lack of correlation between the P waves and QRS complexes on an ECG.
Step-by-step explanation:
The heart rhythm where the atrial and ventricular rhythms are both regular but independent of each other is known as a third-degree atrioventricular (AV) heart block. In third-degree AV block, there is no correlation between atrial activity, represented by the P wave, and ventricular activity, represented by the QRS complex. This block can be observed on an Electrocardiogram (ECG) where the impulses from the sinoatrial (SA) node do not reach the atrioventricular (AV) node, and thus the P waves are not followed by the QRS complex. The AV node can take over as a pacemaker, but the resulting ventricular contractions are slower and less coordinated than the normal rhythm led by the SA node.