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Figure 7 Ladder diagram showing the hypothetical model of variation P-QRS relation during atrioventricular node reentry tachycardia. In this diagram, the atypical form of tachycardia using a fast pathway for anterograde and an intermediate pathway for retrograde conduction is presented. The first beat represents a beat during stable tachycardia. The second beat represents a beat with changes in AH, HA and AH/HA ratio due to decremental conduction in the distal common pathway; the conduction time in anterograde limb and the retrograde limb and, therefore, the cycle length of the tachycardia remain unchanged. The third beat represents a beat with block proximal to the His bundle recording site due to block in the distal common pathway. The fourth beat represents a beat with variation of P-QRS relation due to simultaneous prolongation of the conduction time in the distal common pathway and the retrograde limb. The fifth beat represents a beat with variation of P-QRS relation due to prolongation of the conduction time in the distal common pathway as well as the distal anterograde and retrograde limb. The cycle length of the tachycardia is increased in the fourth and the fifth beats. Note that in the fifth beat the AH/HA ratio is that of the slow-fast form. A = atria; N = compact atrioventricular node; NH = nodo-Hisian zone; H = His bundle. Solid line represents stable conduction; interrupted lines represent decremental conduction.