ECG from the limb leads during sinus rhythm in a 64-year-old patient who has come for catheter ablation treatment of atrial fibrillation. P wave is biphasic (marked with an oval) and prolonged to 170 ms (normal upper threshold 100 ms). This is a sign of prolonged conduction time within the atrial tissue and between the atria. This is considered to predispose to atrial fibrillation. The PQ interval is also prolonged to 270 ms.