Paroxysmal Atrial Tachycardia with Block

Abstract

Disorders in atrial rhythm are relatively common and increasingly frequent accompaniments of digitalis intoxication. The usual ecg pattern of the atrial arrhythmias is paroxysmal atrial tachycardia (PAT) with block. The emergence of a change in rhythm after increasing doses of digitalis or active diuresis affords a clue to the possible presence of this mechanism. Confirmation rests upon the following ecg features: an atrial rate of 200 or less, a change in contour of the P waves, isoelectric baseline between atrial complexes and variable degrees of atrioventricular block. In the phases of its development and recession PAT with block may simulate any of the atrial arrhythmias. Potassium depletion favors the provocation of PAT with block by digitalis. The serum potassium level, however, is not abnormal. Restoration of a normal mechanism requires omission of digitalis, suspension of diuretic measures, and supplementation with potassium or procaine amide, either singly or in combination. PAT with block is a unifocal atrial arrhythmia. The ectopic site is lodged in the cephalic part of the atrium, probably in proximity to the sinus node. It has been conjectured that losses of potassium interfere with the vagus effect upon the atrium but not upon the auriculoventricular (A-V) node. In the absence of vagus action upon the atrium, digitalis enhances atrial automaticity, provoking premature beats and tachycardia. Simultaneously both the direct and indirect (vagus) effect of digitalis on the A-V node is to delay conduction. The end result is PAT with block.