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Typical atrial flutter is a macro-reentrant arrhythmia within the right atrium. Its zone of slow conduction is the so-called right atrial isthmus, representing the area between inferior vena cava, tricuspid annulus, and coronary sinus. Conduction block of this target area can terminate the arrhythmia and prevent reinduction. Isthmus ablation with radiofrequency (RF) energy is a well-established technique to produce conduction block. However, in most cases multiple energy applications are needed to create linear lesions. In individual cases, this results in prolonged procedure duration and, as a possible result of nontransmural lesions, recurrence of the arrhythmia. This article evaluates the feasibility and safety of a new steerable temperature-monitoring microwave ablation catheter in the production of linear lesions in the right atrial isthmus and examines both the dose-response relationship and the energy required for transmural lesions. Ablation was performed in eight open-chest mongrel dogs using a 2.450-MHz microwave generator with 35-50 W and a newly designed AFx 9-F deflectable catheter with a 25-mm antenna on the tip. The operation was performed under general anesthesia using nembutal and supported with room-air artificial respiration throughout the procedure. The study showed that the new deflectable AFx microwave catheter is a feasible and safe tool for creating long transmural linear lesions in the right isthmus. Based on this data, an appropriate catheter position, energy settings between 40 and 50 W, and application duration of approximately 90 s are needed. Also, the development of different antennas and curve shapes is recommended to simplify the ablation procedure and to target more complex arrhythmia substrates with microwave energy. Further studies are needed to determine the optimal anticoagulation procedure after microwave ablation.