Complete heart block, also referred to as third-degree heart block, or third-degree atrioventricular (AV) block, is a disorder of the cardiac conduction system. In this case, there is no conduction through the AV node.

There is a complete disassociation of the atrial and ventricular activity of the heart. The ventricular escape mechanism can occur anywhere from the AV node to the bundle-branch Purkinje system. In a case of complete AV block, the p-waves (atria) will be moving independent and at a regular rate, from the QRS complexes (ventricles), which will have their own intrinsic and regular rate. Basically, both poles of the heart will be working independent of each other because of a short in the electrical system. No atrial impulses will ever reach the ventricles therefore the ventricular rhythm is maintained by a secondary pacemaker, lower in the HIS-Purkinje system. The junctional pacemaker rate is usually faster (40–80 beats/min) compared with the peripheral Purkinje network (20–40 beats/min). The lower in the system that the block is, the worse the prognosis if pacing is not initiated. Common symptoms of a complete heart block include: fatigue, dizziness, impaired exercise tolerance, chest pain, syncope, confusion, dyspnea, severe chest pain, and sudden death. Patients may also exhibit tachypnea or respiratory distress, rales, jugular venous distention, altered mental status, hypotension, and lethargy; signs of anxiety such as agitation or unease, diaphoresis, pale or pasty complexion and tachypnea.
Causes of third degree heart blocks are many. They can include overdoses (even therapeutic doses) of medicines or drugs, infective processes, myocardial ischemia, myocardial infarctions, CHF, and electrolyte imbalances. Congenital AV dissociation has been seen, and maternal lupus has been indicted as one of the culprits.

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