Final answer:
PVCs deviate from NSR by presenting as extra beats with wide, aberrant QRS complexes and are often followed by a compensatory pause. In contrast, a third-degree AV block, which shows no correlation between atrial and ventricular activity, leads to a lower heart rate.
Step-by-step explanation:
Premature Ventricular Contractions (PVCs) are a type of cardiac arrhythmia characterized by early heartbeats originating in the ventricles. They deviate from a normal sinus rhythm (NSR), which is the regular, coordinated rhythm of heartbeats driven by the sinoatrial (SA) node. PVCs disrupt this sequence, and three main ways they can differ from NSR include:
- The PVC is an extra, abnormal beat that occurs before the next expected sinus beat. It can lead to a feeling of a skipped beat or palpitations.
- The QRS complex during a PVC is usually wide and aberrant in appearance compared to the narrow QRS complex of a regular sinus beat. This is because the ventricles are not depolarizing in the usual manner dictated by the SA node.
- There is often a compensatory pause following a PVC, meaning there is a longer interval before the next normal sinus beat occurs, as the heart's electrical system resets.
In the case of a third-degree AV block (which is different from PVC), where there is no correlation between atrial and ventricular activity on an ECG, you would expect the heart rate (pulse) to decrease. This is because, in a third-degree block, the impulses from the SA node do not reach the AV node, leading the AV node or other lower pacemaker sites to take over at a slower inherent rate.