Final answer:
An AV block is a delay in the conduction of electrical impulses between the SA and AV nodes. First-degree block shows a long PR interval. Second-degree block has some missing QRS complexes, while a third-degree block shows no correlation between P waves and QRS complexes.
Step-by-step explanation:
An AV block refers to a delay in the conduction of electrical impulses between the SA and AV nodes in the heart. It is often described in degrees. A first-degree block shows an abnormally long PR interval on an ECG. A second-degree block has some P waves that are not followed by a QRS complex, while others appear normal. In a third-degree block, there is no correlation between the atrial activity (P wave) and ventricular activity (QRS complex).