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Figure 3 (A) A microelectrode recording from a pulmonary vein (PV) cell shown in conjunction with three electrograms. With norepinephrine (3.2 x 10^–8M) + acetylcholine (10^–7M), a premature beat initiates non-sustained firing. After a 4,600-ms pause, the next paced beat initiated sustained firing. (B) A PV action potential is shown in conjunction with three electrograms. With norepinephrine (3.2 x 10^–8M) + acetylcholine (10^–7M), a single premature beat initiates non-sustained firing. After a 218-ms pause, a spontaneous (non-stimulated) beat (asterisk) initiates sustained firing. The timing of the spontaneous beat is similar to that observed at the same norepinephrine concentration, but at a lower acetylcholine (10^–8M) concentration (C). Firing is rapid (sleeve) and conducts with variable block to other recording sites. Note that a stimulus (st) applied within atrium captures deep vein, microelectrode, and atrial recording sites (arrows) without capturing the arrhythmia focus (sleeve). Termination of a triggered rhythm (18 beats in duration) ending with an early afterdepolarization (EAD) (arrow) is shown (D).