A woman in her 60s was evaluated for palpitations. She had a history of supraventricular tachycardia (SVT) for 20 years. The episodes were previously controlled with verapamil, but sustained symptoms associated with shortness of breath recurred 6 months before presentation. Her symptoms did not respond to amiodarone therapy. On presentation, she had a regular heart rate of 122 beats per minute and blood pressure of 135/85 mm Hg. Cardiovascular examination demonstrated regular rhythm with cannon A waves in the jugular venous waveform. The 12-lead electrocardiogram (ECG) is shown in Figure 1A. A 24-hour Holter monitor showed sustained tachycardia throughout the monitoring period. Her echocardiogram results were within normal limits. Question: What is the most likely mechanism of the tachycardia?

中文翻译：

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一位有 20 年室上性心动过速病史的女性出现窄复合波心动过速

一名 60 多岁的女性接受了心悸评估。她有 20 年的室上性心动过速 (SVT) 病史。这些发作以前用维拉帕米控制过，但与呼吸短促相关的持续症状在就诊前 6 个月复发。她的症状对胺碘酮治疗没有反应。就诊时，她的心率为每分钟 122 次，血压为 135/85 毫米汞柱。心血管检查显示有规律的节律，颈静脉波形中有炮 A 波。12 导联心电图 (ECG) 如图 1A 所示。24 小时动态心电图监测显示在整个监测期间持续性心动过速。她的超声心动图结果在正常范围内。问题：心动过速最可能的机制是什么？