Abstract
Tachycardias may be associated with the pre-excitation syndrome, a short PR interval or a prolonged QT interval. This report describes seven patients whose electrocardiograms (ECGs) did not reveal any of the above-named conditions. All of the patients presented with syncopal episodes, and the history of palpitations was elicited in five of the patients. His bundle electrogram studies in all of the patients showed an essentially unchanged AH interval with atrial pacing rates up to 180 beats/min. This can only be explained by a total bypass of the AV node. Two of the patients were also paced from the ventricle, and an unchanged pacing stimulus to A wave interval was observed. This signifies in these patients a functioning bypass pathway in the retrograde direction as well. Thus it would appear that pre-excitation cannot by eliminated as a diagnostic possibility even if the ECG is normal.
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