Final answer:
A third-degree AV block on an ECG is marked by no correlation between P waves and QRS complexes, leading to a lowered heart rate due to the reliance on a slower secondary pacemaker.
Step-by-step explanation:
On an ECG, a third-degree AV block (complete heart block or complete AV block) is characterized by a complete disconnection between the atrial and ventricular activities which means there is no correlation between the P waves and the QRS complexes. The P waves, which represent atrial depolarization, continue at their own inherent rate, while the QRS complexes, representing ventricular depolarization, also occur at a separate rate, commonly initiated by an escape rhythm from the AV node or lower in the conduction system.
Third-degree AV block often results in a lowered heart rate (bradycardia), as the escape rhythm that drives the ventricles is slower than the normal rhythm initiated by the SA node. In the event of a total SA block, the AV node can act as a pacemaker, but its intrinsic rate is typically between 40-60 beats per minute, which is lower than the normal heart rate of 60-100 beats per minute. Subsequently, the pulse in a third-degree block may feel slow and potentially less strong than a normal heartbeat.