Final answer:
A second-degree AV block occurs when some impulses from the SA node reach the AV node and continue, while others do not. Ischemia to the heart muscle, diseases of the heart vessels or valves, metabolic disorders, pathologies of the endocrine system often involving the thyroid, and electrolyte imbalances can be important causes of second-degree AV blocks.
Step-by-step explanation:
A second-degree AV block occurs when some impulses from the SA node reach the AV node and continue, while others do not. This can be recognized on an ECG as some P waves not followed by a QRS complex, while others appear normal. There are important causes of second-degree AV blocks such as ischemia to the heart muscle, diseases of the heart vessels or valves, metabolic disorders, pathologies of the endocrine system often involving the thyroid, and electrolyte imbalances.