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Backrround Atrial tachycardia (AT) with cycle length (CL) alteans is uncommon and conventional mapping of this AT remains challenging.We used an ultrahigh density mapping system to rapidly map complicated circuits with sufficient spatial resolution and electrogram quality to elucidate the precise mechanism of this special ATs.Methods Of 210 consecutive patients with clinical ATs who underwent catheter ablation with the ultrahigh density mapping system,4 patients (1.9％) with CL alteans were identified.The AT alteating cycles mapped by the Rhythmia mapping system for long CL were 317±51 (range 245-355)ms and for short CL were 282±51 (range 235-333) ms.Both long and short cycles followed in 1 : 1 sequence in all 4 patients (longshort-long-short).Results By comparing the separate maps with long and short CL,we classified ATs with CL alteans into 2 types.In type 1,CL alteans resulted from an intermittent 2: 1 conduction block through the slow conduction channel in the small circuit.In type 2,CL alteans caused by the alteated conduction velocity passing through the conduction gap were manifested.Ablation at the fractionated potentials contributes to the termination of AT in 3 of the 4 patients.Conclusions Ultrahigh density mapping system can accurately identify the mechanism of complex ATs with alteating CL.The CL alteans may be related to the intermittent conduction block within the channel of the small circuitor different conduction velocity through the identical channel.Fractionated electrogram recorded in the common isthmus or some "conduction gaps" may be a reasonable approach to terminate these ATs.