AV Block, or Heart Block, is a condition which causes bradycardia. It has nothing to do with "blockages" in the coronary arteries due to atherosclerosis. Instead, it is an electrical block, like a "loose connection," in the wires of the heart. AV block occurs when the AV Node fails to properly conduct the impulses from the atria to the ventricles. The severity or "degree" of AV block varies from mild to life-threatening. First degree AV block simply means that there is an excessive delay in the conduction of the impulse from the atria to the ventricles, but all the impulses are still getting through. By definition, first degree AV block is present if the time it takes for the electrical signal to pass from the atria to the ventricles (the PR interval) is greater than 0.2 seconds. This is a benign condition that does not require treatment. It may be due to disease in the AV node or it may be caused by certain medications (particularly calcium-channel blockers or beta blockers). Second degree AV block occurs when some impulses that originate in the atrium fail to pass through the AV node, so the ventricle doesn't receive the electrical "cue" to beat. This can cause an irregular heart beat and frequent pauses in the heart rhythm. Depending on the actual location of the block (i.e. within the AV node itself vs. below the AV node in the His bundle), this condition may be benign or quite serious. Sometimes AV block causes symptoms of fatigue and lightheadedness because of the relative bradycardia that results. In patients with symptoms from second degree AV block, a permanent pacemaker is indicated for the relief of symptoms. Sometimes (particularly when the location of the block is in the His bundle) a permanent pacemaker must be implanted even though there are no symptoms in order to prevent future problems. Third degree AV block, also called Complete Heart Block, is a condition in which none of the atrial impulses can get through the AV node because of severe disease in cardiac conduction system. This usually results in severe bradycardia, and occasionally the heart can actually stop beating. Complete heart block can be a medical emergency requiring the insertion of a temporary pacemaker wire. This temporary wire is inserted into a large vein in the neck or below the collarbone and is advanced into the right ventricle to keep the heart beating until a permanent pacemaker can be implanted.
In the diagram above, Third Degree AV block is present. The p-waves (atrial activity) "march through" at a faster rate than the QRS complexes (ventricular activity). The ventricles are beating slowly and regularly because of an "escape" rhythm that often arises from the lower part of the AV node (below the level of the block).