HEART BLOCK - A conduction disturbance that results in the inappropriate delay or the complete inability of a electrical impulse, generated in the atria, to reach the ventricles (via the AV node). Clinical types are divided into first (non-serious), second, and the most serious, third degree, or complete heart block. Causes include myocardial infarction, chronic heart disease, cardiomyopathy, and digoxin toxicity. Some other drugs may also precipitate an AV block (e.g. clonidine, methyldopa, and verapamil). A permanent pacemaker may be required for higher degree blocks, such as complete heart block.