Final answer:
In a third-degree AV block, the pulse rate is expected to be slower as the ventricles rely on a backup pacemaker. Artificial pacemakers can be used to normalize heart rhythm when natural pacemaking is inadequate. Abnormal patterns in electrical activity, such as atrial fibrillation or ventricular tachycardia, can result in variable effects on heart rate.
Step-by-step explanation:
The atrial contraction, which is started by the SA node, and the ventricular contraction, which ordinarily follows the atrial contraction as shown by the P wave and QRS complex in an ECG, completely disconnect when a person has a third-degree atrioventricular (AV) block. Due to this lack of coordination, the ventricles will rely on a backup pacemaker, which will cause a slower heart rate (junctional rhythm). Usually, this backup pacemaker is the AV node or other subsidiary pace-making cells located lower in the heart.
The AV node is the source of a junctional rhythm, which keeps the heart rate in the 40–60 beats per minute range—much lower than the SA node's typical range of 60–100 beats per minute when at rest. An artificial pacemaker may be placed to create a regulated and dependable heart rate, guaranteeing efficient blood circulation by atrial pacing, in situations where the heart's natural pacemaker system is unable to sustain an appropriate heart rate. The electrical activity preceding the QRS complexes in the conditions mentioned, such as atrial fibrillation or ventricular tachycardia, is aberrant, and the frequency of the QRS complexes might fluctuate, influencing the heart rate. A total absence of electrical activity, as in ventricular fibrillation, would cause the blood supply to stop and there would be no functional pulse—a potentially fatal situation.