heart block, an interference with the normal conduction of electrical impulses that control activity of the heart muscle. Heart block is usually specified by the location of the block and the type, such as a first-degree atrioventricular (AV) block, in which atrial impulses are delayed by a fraction of a second before being conducted to the ventricles. Heart block can occur in the sinus node, atria, AV node, AV bundle, fascicles, or a combination of these structures. See also atrioventricular block, bundle branch block, cardiac conduction defect, infranodal block, intraatrial block, intraventricular block, sinoatrial (SA) block.
