Complex sentence: An atrioventricular block (or AV block) is a type of heart block involving impairment of the conduction in the atrioventricular node, but it can involve other structures.
Split into simpler sentences: An atrioventricular block (or AV block) is a type of heart block involving impairment of the conduction between the atria and ventricles of the heart. It usually involves the atrioventricular node, but it can involve other structures.