['The importance of anatomic barriers in the atrial flutter reentry circuit has been well demonstrated in canine models. It has been shown previously that the crista terminalis and its continuation as the eustachian ridge form a posterior barrier. In this study we tested the hypothesis that the tricuspid annulus forms the continuous anterior barrier to the flutter circuit.', 'Thirteen patients with typical atrial flutter were studied. A 20-pole halo catheter was situated around the tricuspid annulus. A mapping catheter was used for activation and entrainment mapping from seven sequential sites around the tricuspid annulus and from three additional sites including the tip of the right atrial appendage, at the fossa ovalis, and in the distal coronary sinus. Sites were considered to be within the circuit when the postpacing interval minus the flutter cycle length and the stimulus time minus the activation time were < or = 10 ms; sites were considered to be outside the circuit when these intervals were > 10 ms. All seven annular sites were within the circuit; activation occurred sequentially around the annulus and accounted for 100% of the flutter cycle length. The fossa ovalis, the distal coronary sinus, and the right atrial appendage were outside the circuit.'] Does activation and entrainment mapping define the tricuspid annulus as the anterior barrier in typical atrial flutter?