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Figure 2 (A–E) Comparison of the mean amplitude of P waves produced by pacing at six different sites in four pulmonary veins (PVs). (A) In lead I, amplitudes of P waves produced by left PV pacing were significantly lower than those of the right PVs. (B) All six pacing sites produced negative P waves in lead aVR. Pacing in superior sites yielded P waves with more negative amplitudes. (C) In lead aVL, amplitude of P waves increased in a stepwise manner from TLS to RI, and all pacing sites except RI produced negative P waves. (D) In lead II, pacing in superior sites produced P waves with higher amplitudes, whereas P waves paced from similar heights were not significantly different between right and left. Conversely, P waves in lead III during pacing of right PVs were lower in amplitude than those of left PVs paced from a similar height (E). (F) The ratio between the amplitude of leads II and III (III/II ratio) showed values of about 1.0 during left PV pacing, whereas each of the three right PV pacings produced a mean value below 0.8. The dotted horizontal line represents a value of 0.8. *p < 0.05; **p < 0.01. BLS = bottom of the left superior PV; BRS = bottom of the right superior; LI = left inferior PV; RI = right inferior PV; TLS = top of the left superior; TRS = top of the right superior PV.