An electrocardiogram (ECG) is a common, non-invasive diagnostic tool that records the electrical activity of the heart. Electrodes placed on the skin capture the signals that govern the heart’s rhythm and pumping action. The resulting tracing is composed of a series of waves, each corresponding to a specific phase of the cardiac cycle. The P wave is the first deflection, representing the electrical impulse moving through the heart’s upper chambers. Variations in the shape or duration of this wave can be important indicators of underlying changes in heart health.
What the P Wave Represents
The P wave marks the electrical activation of the atria, the heart’s two upper chambers. This process is known as atrial depolarization. The impulse originates in the sinoatrial (SA) node, the heart’s natural pacemaker located in the right atrium. In a healthy heart, this depolarization is fast and synchronized, resulting in a small, smooth, and rounded P wave that typically lasts no more than 0.12 seconds (120 milliseconds). The initial portion reflects the electrical activity of the right atrium, while the latter part reflects the electrical activity of the left atrium.
Identifying the Notched P Wave
A notched P wave is an alteration where the normally smooth contour becomes visibly split. This characteristic morphology presents as two distinct peaks, giving the wave an “M-shaped” or bifid appearance, often seen in lead II of the ECG. This shape suggests that the electrical activation of the atria is no longer synchronized. Historically, this finding was referred to as P Mitrale due to its strong association with mitral valve disease. To be considered an abnormality, the P wave duration must be prolonged, exceeding 120 milliseconds, and the notch width—the distance between the two peaks—must be 40 milliseconds or greater.
The Electrical Basis of the Notch
The notch forms due to a delay in the electrical signal traveling between the two atria. Normally, the right and left atria depolarize almost simultaneously, but with a notched P wave, the impulse is significantly slowed as it moves from the right atrium to the left atrium. This delay causes the electrical signals from the two chambers to be registered separately on the ECG, creating two distinct peaks. This asynchronous activation often stems from an issue within the interatrial conduction pathway, such as the Bachmann’s bundle. The first peak of the M-shape represents the right atrium’s depolarization, while the delayed second peak corresponds to the left atrium’s depolarization.
When a Notched P Wave Is a Concern
The presence of a notched P wave points toward left atrial abnormality or left atrial enlargement (LAE). LAE occurs when the left atrium stretches or thickens, usually in response to increased pressure or volume overload. This structural change physically lengthens the path the electrical impulse must travel, exacerbating the conduction delay. Common causes leading to LAE include chronic systemic hypertension and various forms of mitral valve disease. The notched P wave is recognized as an independent predictor of increased risk for developing atrial arrhythmias, especially Atrial Fibrillation (AFib), and is also linked to an increased risk of ischemic stroke.