Analysis of the interpolation phenomenon in man evaluated by premature atrial stimulation

After early premature atrial stimulations, returning cycles shorter than the basic cycle were observed in 25 patients. In 4 of them sinus echoes were seen to occur after premature atrial stimulation coupling intervals shorter than those able to induce completely interpolated beats. Such a sequence is unexpected if we suggest a sino-atrial entrance block for explaining the interpolation phenomenon. An alternative explanation can be postulated assuming that all these early premature beats are sinus node re-entries and that the completely interpolated beat is only a sinus re-entry fortuitously occurring at an appropriate interval.