Costantini Marcello, Carbone Vincenzo, Costantini Lorenzo
Scuola di Specialità in Cardiologia, Università degli Studi, Foggia - DREAM - Laboratorio Diffuso di Ricerca Interdisciplinare Applicata alla Medicina, Università del Salento/ASL Lecce.
Cardiologia Ambulatoriale, ASL Napoli 3 Sud e Salerno.
G Ital Cardiol (Rome). 2018 Apr;19(4):222-231. doi: 10.1714/2898.29216.
The atrioventricular (AV) node is an anatomically well-defined structure, conveniently housed in the triangle of Koch. There are two distinct atrial impulse approaches to the AV node, one of which (in the anterior portion of triangle of Koch) has a faster conduction, while the other one (in the posterior portion) has a slower conduction. However, it is not said that such a conductive duality translates into any arrhythmic phenomena. Actually, these arrhythmias are due to an imbalance of the two pathways electrophysiological properties, which does not always exist. In the presence of such an imbalance, the dual AV nodal physiology is, however, the substrate for various arrhythmias and curious electrocardiographic behaviors. Often the fast pathway is characterized by a relatively long refractory period. In contrast, the slow approach is often characterized by shorter refractoriness.The unbalanced refractoriness of the two nodal pathways constitutes the prerequisite for the most common form of paroxysmal supraventricular tachycardia: the AV nodal reentrant tachycardia (AVNRT). In subjects prone to this type of arrhythmia, during sinus rhythm, nodal conduction usually occurs from the anterior approach (fast pathway). However, a premature atrial beat may find this pathway refractory and cross the AV node through the posterior approach (slow pathway), resulting in a sudden prolongation of the AV conduction time ("jump"). This allows the impulse, once it reaches the common end, to excitate the fast pathway in a retrograde direction and to return backwards to the atrium, thus triggering a circus movement that can result in a "slow-fast" AVNRT. More rarely, an AVNRT can take place in an opposite direction of the reentrant impulse ("fast-slow" variety of AVNRT). A paroxysmal supraventricular tachycardia may seldom occur with a regularly alternating RR cycle, if the reentrant mechanism involves retrogradely an accessory AV pathway and, in anterograde direction, a fast and a slow AV nodal pathway, alternately. Among the mechanisms underlying the total RR irregularity during atrial fibrillation, there is probably also the possibility that the AV node may offer to the atrial impulses two distinct pathways to reach the His bundle. Not too rarely, a dual AV nodal physiology can occur during sinus rhythm, through unexpected and sudden changes in the AV conduction time, so that two distinct PR families can be observed. It is likely that the presence of dual AV nodal pathways can facilitate or promote a Wenckebach conduction mechanism at nodal site.
房室（AV）结是一个解剖结构明确的结构，位于科赫三角内，位置便利。有两种不同的心房冲动传导至房室结的途径，其中一种（在科赫三角前部）传导速度较快，而另一种（在后部）传导速度较慢。然而，并没有证据表明这种传导的二元性会转化为任何心律失常现象。实际上，这些心律失常是由于两条传导途径电生理特性的失衡导致的，而这种失衡并非总是存在。在存在这种失衡的情况下，房室结的二元生理特性是各种心律失常和奇特心电图表现的基础。通常，快径路的特点是相对较长的不应期。相比之下，慢径路的特点通常是不应期较短。两条结性传导途径不应期的不平衡是阵发性室上性心动过速最常见形式：房室结折返性心动过速（AVNRT）的先决条件。在易患此类心律失常的患者中，在窦性心律时，结性传导通常通过前向途径（快径路）发生。然而，一个房性早搏可能会发现这条途径处于不应期，于是通过后向途径（慢径路）穿过房室结，导致房室传导时间突然延长（“跳跃”）。这使得冲动一旦到达共同终点，就能逆向激动快径路并返回心房，从而引发一个折返运动，可能导致“慢 - 快”型AVNRT。更罕见的情况下，AVNRT可能以折返冲动的相反方向发生（“快 - 慢”型AVNRT）。如果折返机制涉及逆向的房室旁路以及前向交替的快、慢房室结传导途径，阵发性室上性心动过速可能很少出现RR周期规则交替的情况。在房颤时RR完全不规则的机制中，很可能也存在房室结为心房冲动提供两条不同途径到达希氏束的可能性。在窦性心律时，通过房室传导时间意外且突然的变化而出现房室结二元生理特性的情况并不罕见，从而可以观察到两个不同的PR间期组。很可能房室结双径路的存在会促进或推动结区的文氏传导机制。
