Clinical and experimental studies of the effects of atrial extrastimulation and rapid pacing on atrial flutter cycle. Evidence of macro-reentry with an excitable gap.

To investigate the mechanism of atrial flutter in human beings, effects of atrial pacing and extrastimulation on flutter cycle length were studied. In four cases, properly timed extrastimuli shortened the F-F interval after the extrastimulus without affecting the stimulus-encompassing interval. The width of the excitable gap ranged from 14 to 25 percent of the basic flutter cycle length (mean +/- standard deviation 20.4 +/- 4.1). Entrainment to rapid atrial pacing was demonstrated in each case. The width of the entrainment zone nearly coincided with that of the excitable gap in each case. In 16 days, atrial flutter was induced by electrical stimulation after an obstacle was placed between the superior and the inferior venae cavae according to Rosenblueth and Garcia Ramos. In 11 dogs, extrastimuli shortened the F-F interval next to the stimulus-encompassing interval as in the clinical study. The excitable gap, measured in six dogs, ranged from 15 to 24 percent of the basic flutter cycle length (mean 20.6 +/- 3.0). Entrainment was observed in six dogs studied and, during entrainment, the atrial activation sequence was almost the same as that during the basic flutter cycle. It is concluded that shortening of the F-F interval next to the stimulus-encompassing interval that is not affected favors macro-reentry as the mechanism of atrial flutter in human beings.