P waves are caused by atrial depolarization. In normal sinus rhythm, the SA node acts as the pacemaker. The electrical impulse from the SA node spreads over the right and left atria to cause atrial depolarization. The P wave contour is usually smooth, entirely positive and of uniform size. The P wave duration is normally less than 0.12 sec and the amplitude is normally less than 0.25 mV. A negative P-wave can indicate depolarization arising from the AV node.
- Note that the P wave corresponds to electrical impulses not mechanical atria contraction. Atrial contraction begins at about the middle of the P wave and continues during the PR segment. A Wiggers diagram can be used to illustrate that the left atrial pressure beginning to rise at about half way through the P-wave. Increase in atrial pressure indicates atrial contraction
- The animation below shows conduction during a P wave to illustrate the impulses (red dots) traveling through the heart, and the EKG recording. The electrical impulse causes atrial contraction, pumping blood into the ventricles. Note the time that atrial contraction (blinking stars) and pumping of blood (blue and red arrows) occurs begins relative to the EKG. The animation had been slowed about 10 times normal heart rate to show the details.