[详情]

答案：A

讨论

II导联中，P1下传，P2阻滞；也就是说，该患者的房室结在R1P2间期不能传导。P3正在下传，房室结在R3P6、R5P9、R7P12间期可以传导，短于R1P3间期。因此，P3、P5、P8或P11能够下传。

R1P3间期稍长于房室交界性逸搏间期和房室交界性逸搏（R2）。因为P3和R2发生在彼此的不应期，P3和R2是分离的。P4与P2阻滞的原因相同。P5与P3的情况相似、将要下传，周期重复。

因此，原发的传导问题是2：1房室传导阻滞（图2-上图）；这是数分钟前的心电图，窦性心律较快（90 beats/min vs 80 beats/min in the lower tracing）。在2：1房室传导阻滞的患者中，若两个PP间期变得比房室交界性逸搏间期更长，那么这个房室交界性搏动将逃逸，正如这名患者的心电图表现。

图中QRS波不规律出现，可排除三度房室传导阻滞。三度房室传导阻滞中，逸搏心律是很有规律的。

病例来源：K. Wang, MD. What Is the Primary AV Conduction Disorder? Medscape. January 15, 2014

推荐阅读：

图文并茂|三度房室传导阻滞

刘仁光：房室传导阻滞诊断难点与解析

房室传导阻滞心电图诊断需注意的一些问题

讨论原文

In the lower tracing (lead II), P1 is conducted. P2 is blocked; in other words, this patient's AV node cannot conduct at the R1P2 interval.

P3 was going to be conducted. How do I know that? This patient's AV node can conduct at the R3P6, R5P9, and R7P12 intervals, which are shorter than R1P3 interval. Therefore, P3 as well as P5, P8, or P11 should be able to conduct.

The R1P3 interval is slightly longer than the AV junctional escape interval, and AV junction escapes (R2). Because P3 and R2 occur close together during each other's refractory period, P3 and R2 are dissociated. P4 is blocked for the same reason that P2 is blocked. P5 was going to be conducted, but the same event happens as with P3, and the cycle repeats.

Thus, the primary AV conduction problem is 2:1 AV block, as demonstrated in the upper tracing; that tracing was obtained a few minutes earlier, when the sinus rate was faster (90 beats/min vs 80 beats/min in the lower tracing). In a patient with 2:1 AV block, if 2 PP intervals become longer than the AV junctional escape interval, that AV junction will escape, as happened in this patient. Thus, the AV junction behaves like a demand pacemaker -- what I would call a "natural" demand pacemaker that we are all born with!

Thus, a given tracing can be dissected into primary disorder and secondary manifestations. AV junctional escape and AV dissociation in the lower tracing are both inevitable, obligatory secondary responses to the primary disorder: 2:1 AV block. Many times, these secondary responses make the tracing look more complicated and troublesome. The R3 is taller than other R waves because it is superimposed with P5.

Complete (third-degree) AV block is ruled out right away by QRSs not occurring regularly. In complete AV block, the escape rhythm is very regular.