Coexistent Types of Atrioventricular Nodal Re-Entrant Tachycardia

T he mechanism of atrioventricular nodal re-entrant tachy-cardia (AVNRT) remains elusive. Both anatomic and functional models have been proposed. There has been elec-trophysiological evidence that the right and left inferior extensions of the human AV node and the atrionodal inputs they facilitate, which have been identified histologically, might provide the anatomic substrate for the slow pathway. 3–6 Data indicating the potential anatomic site of the fast pathway are sparse. There is a histological evidence of multiple superior atrial inputs to the AV node, 7–11 but the nature of fast pathway conduction, especially during atypical AVNRT of the fast– slow type, is poorly understood. We have previously reported data, suggesting that atypical fast–slow and typical slow–fast AVNRT do not seem to use the same limb for fast conduction. 12 This evidence, however, was derived by observations on typical and atypical tachycardias recorded in different patients. We are not aware of data on patients who have exhibited both typical and atypical tachycardia at the same study. We hypothesized that by comparing conduction intervals during typical and atypical forms that occur in the same patient at the same study, we could gain insight into the properties of the fast and slow pathways, and the mechanisms responsible for atypical AVNRT.