A basic rhythm strip showing junctional rhythm in Lead II. The junctional pacemaker is located between the atria and the ventricles, and the resulting P wave is caused by retrograde conduction through the atria. This causes the P wave to be negatively deflected in Lead II. In junctional rhythms, the P wave can occur just before the QRS, during the QRS, or after the QRS, or may not be seen at all. If the P wave occurs before the QRS, the PR interval is usually short, reflecting the fact that the atria and the ventricles are depolarized almost simultaneously.