Final answer:
In a third-degree AV heart block, there is no correlation between the atrial P waves and the ventricular QRS complexes on the ECG, leading to a slower and independent heartbeat of the ventricles. This disconnection significantly affects the heart's efficiency and the overall heart rate.
Step-by-step explanation:
The third-degree AV heart block is a significant abnormality observable on an electrocardiogram (ECG). In this condition, there is a total disconnection between the atrial and ventricular activities, which means no impulses from the atria reach the ventricles through the AV node. The ECG changes characteristic of a third-degree block include P waves not followed by QRS complexes, resulting in no correlation between the P wave and the QRS complex. Despite the lack of atrioventricular conduction, the AV node or other pacemaker cells distal to the block can maintain a ventricular rate of 40-60 contractions per minute, thus ensuring circulation that is often sufficient to maintain consciousness.
In terms of heart rate (pulse), one would expect it to be slower than normal because the ventricles are beating independently of the atria, often at a slower intrinsic rate. Without coordination between the atrial contraction and ventricular contraction, the efficiency of the heart's function can be markedly reduced.