Vagally induced block and delayed conduction as a mechanism for circus movement tachycardia in frog atria.

Episodes of tachycardia induced by strong vagal stimulation in spontaneously beating isolated atria of frog (Rana temporaria) were studied with multielectrode mapping technique. These episodes were inducible in 19 of 39 preparations. The arrhythmia started several seconds after cessation of vagal stimulation strong enough to cause sinus arrest, without electrical stimulation of the myocardium. The arrhythmia consisted of two to 20 beats (6 +/- 4, mean +/- SD, n = 42) with a cycle length of 100-500 msec. Recording from 32 sites with spatial resolution of 1-2 mm showed that the arrhythmia was due to intra-atrial circus movement. The estimated perimeter of the reentrant circuit ranged from 6 to 20 mm. In circuits of the minimal size, the average conduction velocity along the circuit was as low as 2-3 cm/sec. Paroxysms of the tachycardia were always preceded by vagally induced nonuniform depression of conduction, with some areas of atria being completely blocked. As the vagal influence decreased, the blocked ...