Variations in Atrial Flutter Cycle Length.Introduction: The purpose of this investigation was to study the mechanisms responsible for small variations in atrial flutter cycle lengths.Methods and Results: In a study group of 11 patients with common atrial flutter, atrial electrograms were recorded from an intraesophageal lead together with a surface lead (V1). Upon the onset of the QRS complex, atrial flutter intervals consistently increased by an average of 1.8% (SD± 0.9; P<0.01) and subsequently decreased by 2.1% (SD ± 0.8; P<0.01) before returning to the average flutter rate. Carotid sinus massage, which temporally prevented ventricular activation, markedly reduced the variations in atrial flutter intervals. Ventricular pacing at different rates clearly demonstrated that the pattern in atrial flutter intervals was coupled to the moment of ventricular contraction. The hypothesis was formulated that these periodic variations in atrial flutter intervals following a ventricular contraction were caused by the influence of stretch of the atrial myocardium on the conduction properties of a circulating impulse in the atrium. The secondary decrease in flutter rate could be explained if a partial excitable gap is assumed between head and tail of the circus movement. This hypothesis was tested in a simulation study, which revealed that the alternation in intervals as found in patients could only be reproduced if the excitable gap in the circus movement was partially excitable.Conclusion: In conclusion, the analysis of variations in atrial flutter cycle lengths points to a mechanism of circus movement with a partially excitable gap in common atrial flutte