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Figure 1 Upper tracing: extracellular recording from the His bundle region. Middle tracing: action potentials with a double upstroke recorded in the posterior aspect of the AV node during basic stimulation (S1) with a cycle length of 500 ms followed by an early coupled stimulus (S2). The heart was stimulated from a posterior site. Lower tracings: Action potentials recorded after a premature stimulus with a coupling interval of 1) 310, 2) 290 and 3) 280 ms, respectively. Delay between the first and the second component of the action potential increased and the amplitude of the second component decreased with prematurity. Delay between the stimulus and the first upstroke remained virtually the same. Activation evoked after the extra of 280 ms was blocked toward His. This presents a rather exceptional case. Inset: schematic drawing of the AV junctional area, showing the recording site. A = atrial deflection; CFB = central fibrous body; CS = orifice of coronary sinus; H = His deflection; ME = microelectrode; TT = Tendon of Todaro; TVA = tricuspid valve annulus; V = ventricular deflection.
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