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Figure 2 Upper tracing: extracellular electrogram from the His bundle region. Lower tracings are action potentials recorded from the same endocardial site after the last basic stimulus (S1) and an early coupled stimulus (S2). The tracing marked superficial shows the action potential from a cell located in a superficial layer (directly beneath the endocardium). The tracing marked deep shows the action potential from a cell located at a deeper level. Both tracings show double components during basic stimulation (S1), but the notch in the superficial tracing (bold arrow) becomes more distinct after the premature stimulus (S2). The main (first) component of the action potentials of the superficial tracing has a low upstroke velocity. The timing corresponds with the first, low amplitude deflection of the action potentials from the deeper tracing (open arrow). The main (second) component of the deeper tracing has a high amplitude and fast upstroke velocity. The timing corresponds with the notches marked by the bold arrow in the superficial tracing. The inset shows the endocardial location of the microelectrode. The dips following the bold arrows are artifacts caused by the reference of the microelectrode. Abbreviations as in Figure 1.
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