Explanation

The P wave corresponds to atrial depolarization. The SA node is located in the right atrium near the superior vena cava, causing the direction of depolarization of normal P waves to be directed leftward and inferior, yielding upright (positive) P waves in most leads except aVR (which is oriented in the exact opposite direction). Because of the rightward placement of V1, the P wave is usually biphasic, with the initial positive deflection representing depolarization of the right atrium (toward the V1 electrode) and the subsequent negative depolarization representing the left atrium (away from the V1 electrode). Leads V1 and II are typically the best two leads for determining the P wave morphology and measurements, although this is not true in all cases.