Final answer:
A second-degree AV block can progress to a third-degree heart block, with the latter showing complete dissociation between atrial and ventricular activities on an ECG and posing serious risks.
Step-by-step explanation:
In the context of heart block, a second-degree AV block can progress to a third-degree heart block, which is also known as a complete heart block. Of the common types of AV nodal and infra-Hisian blocks, which are categorized by degrees of severity, the second-degree block is particularly concerning. A first-degree block is a partial block with a delay between the SA node and AV node, noticeable by an extended PR interval on an ECG. A second-degree block occurs when only some impulses from the SA node pass through to the AV node, resulting in an ECG where some P waves are not followed by a QRS complex. If this condition worsens, it can progress to a third-degree block, where no impulses from the SA node reach the ventricles, leading to a complete dissociation between atrial and ventricular activities and posing serious risks to the patient. This is evident as there is no correlation between the P waves and QRS complexes on an ECG.