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已经证明，心外膜及M层心肌细胞动作电位上1位相终末部位上的切迹，反映心外膜及M层心肌细胞的全或无复极形式，以及2相折返倾向。
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2相折返是1993年由Antzelevitch和Di Diego等提出的，是指在缺血或药物作用下，心外膜及M层心肌细胞表现为全或无的复极形式。动作电位的平台期被抑制或消失，使动作电位时程缩短约40％～70％。此时，动作电位的平台期丢失区和正常区之间电的异质性，导致2相折返。一般的折返激动，都是0相去极化电流所致，而这种2相折返是由2相复极电流折返引起的。2相折返途径可与0相折返一致，亦可与此不同，但肯定不是解剖依赖性。
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因此，体表心电图上的J波，可以作为诊断和预测2相折返倾向的重要指标。最近的资料显示，Brugada综合征的发生机制，可能亦和2相折返有关。心外膜动作电位的平台期的丢失，引起相应部位ST段抬高，这和Brugada综合征的表现相同。事实上，所谓的Brugada综合征的图形和J波亦有明显的相同之处。
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乙酰胆碱由于抑制Ica内向电流，使平台期消失；β受体兴奋剂则增加Ica，使平台恢复；钠阻滞剂使平台期消失，因此钠通道阻滞剂虽然抑制了0相折返，但同时又诱发了2相折返；兴奋迷走神经和Ⅰ类抗心律失常药加重相应部位ST段抬高，因此也有可能诱发2相折返；β受体阻滞剂则使相应部位ST段抬高减少，从而抑制2相折返。但Brugada最近报道，β受体阻滞剂并不能有效预防Brugada综合征病人的猝死。
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【思考题】
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1．试述心脏性猝死的处理流程及心肺复苏新进展。
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2．常用的心脏性猝死相关的心电信息检测手段有哪些？各有何特点。
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3．何谓Brugada综合征。
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