Complete heart block is diagnosed when there are independent atrial and ventricular rhythms, the atrial rate exceeds the ventricular rate, and the atrial beat does not conduct to the ventricles although there is clear opportunity for it to do so ( Fig, 4). The first heart sound varies in intensity and atrial sounds may be heard during long diastoles. The arterial pulse pressure is wide. Complete heart block may occur in the atrioventricular node or at the level of the bundle of His (narrow QRS), or it may be below the bundle of His or in the Purkinje system (prolonged QRS). When the atrial and ventricular rates are similar, atrioventricular dissociation may be present without complete heart block. When the atrial and ventricular rates are almost identical but still independent, isorhythmic dissociation is present. Complete heart block can be caused by digoxin, b-blockers, antiarrhythmic drugs, calcium-channel blockers, infiltrate conduction system diseases, electrolyte disturbances, endocarditis, calcific aortic stenosis, and myxedema. Complete heart block can complicate acute inferior or anterior myocardial infarction.
Fig. 4 Complete heart block in a patient with underlying right bundle branch block. Atrial rhythm and ventricular rhythm are independent, atrial rate is faster than ventricular rate, and the atrial beat does not conduct to the ventricle although clear opportunity for it to do so is noted.
Manifestations of digoxin toxicity include varying degrees of atrioventricular block, often in conjunction with atrial dysrhythmias. The pathognomonic conduction abnormality associated with digoxin toxicity is paroxysmal atrial tachycardia with varying block. Asystole is not uncommon in patients in the intensive care unit setting.
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