Supernormal Conduction in the Human Atria

A supernormal period of intra- and interatrial conduction was observed in five patients during premature stimulation of various atrial sites with driving cycle lengths ranging between 500 and 600 msec. Electrograms were recorded with filtered, 1-mm apart, bipolar catheter electrodes placed in the high right atrium, coronary sinus, and midleft atrium. The supernormal period, which lasted from 90 to 140 msec, was located at the end of the relative refractory period. During this part of the cycle, the response1-response2 (R1-R2) intervals were shorter than the corresponding St1-St2 intervals. As in experiments performed with plunge electrodes, the conduction time of premature atrial responses was shorter than in late diastolic or driven beats. Although a mechanical origin (due to inevitable catheter movement produced with cardiac motion) can be invoked in the genesis of these changes, it is highly probable that supernormality was a true electrophysiologic event involving, predominantly, the specialized atrial tracts.