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Figure 5 Ectopic P-wave morphology before (A) and after (B) QRST subtraction in cases with ectopics from superior vena cava (SVC) (a) and inferior vena cava (IVC) (b). Subtraction results showed a P-wave taller than sinus rhythm in inferior leads but with similar morphology in V1 for ectopy originating from SVC (a). For IVC ectopics, a tall P-wave in lead I and negative P waves in the inferior leads with deeper negativity in lead III than in lead II was observed (b). a’ and b’ are P-wave morphologies during pacemapping from the medial aspect of the SVC and of the IVC, respectively.
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