Final answer:
A first-degree heart block is a delay in conduction between the SA and AV nodes, resulting in an abnormally long PR interval on an ECG. It can be caused by aging, heart disease, medications, and electrolyte imbalances.
Step-by-step explanation:
A first-degree heart block refers to a delay in conduction between the SA and AV nodes, which can be recognized on an ECG as an abnormally long PR interval. It is a partial blockage that allows all impulses from the SA node to eventually reach the AV node, although at a slower pace. Common causes of first-degree heart block include aging, heart disease, medications, and electrolyte imbalances.