What is the P wave?

The P wave is a characteristic waveform observed on an electrocardiogram (ECG) that represents the electrical activity associated with the initial contraction of the atria, which is the first phase of the cardiac cycle. The P wave is generated by the depolarization of the atria, which is the process by which the cardiac muscle cells in the atria are stimulated to contract. This depolarization is initiated by the sinoatrial (SA) node, which is the heart's natural pacemaker located in the right atrium. The wave of depolarization then spreads from the SA node across both atria, causing them to contract and push blood into the ventricles in preparation for the next phase of the cardiac cycle. The sequence of atrial depolarization typically begins in the high right atrium and then proceeds to the rest of the right atrium, followed by the left atrium. This is why the right atrium is often seen to depolarize slightly earlier than the left atrium on an ECG. The shape and duration of the P wave can provide important information about the heart's electrical conduction system and can be indicative of various cardiac conditions.

The P wave is a summation wave generated by the depolarization front as it transits the atria. Normally the right atrium depolarizes slightly earlier than left atrium since the depolarization wave originates in the sinoatrial node, in the high right atrium and then travels to and through the left atrium.