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Figure 4 Tracings are intracellular recordings made during cooling of the anterior area of the AV junction (Panel A). This area was the atrial exit during retrograde conduction. Recordings were made during posterior stimulation at three instants during the cooling procedure: 1) baseline, no cooling, 2) 30 s of cooling, 3) 60 s of cooling. A double component is present during baseline (tracing 1). Delay between the first and second components of the action potential increases with cooling. Delay between the stimulus and the first component remains the same. The inset shows the location of the cooling probe and the recording electrode. The distance between the center of the cooling probe and the endocardial site where the microelectrode was impaled was 5 mm. The tracing, marked His, shows the extracellular electrogram from a site overlying the bundle of His and was made 90 s after starting the cooling procedure (Panel B). At this level of cooling Wenckebach periodicity occurred. The lower tracing (ME) shows the microelectrode recording in the posterior region. Note that the two components do not change with the Wenckebach periodicity. Abbreviations as in Figure 1.
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