This Lead II rhythm strip is from a nine-year-old girl being monitored for an outpatient surgical procedure. She has no known heart disease. Her heart rate is 110 per minute. The PR interval is .12 seconds (120 ms), the QRS is upright and narrow at .06 seconds (60 ms), and the rhythm is regular.
The most noticeable abnormality here is the RETROGRADE P WAVES. In Lead II, normal P waves are upright. In this case, the rhythm is being initiated in the lower atria, or possibly in the AV junction. The impulse is traveling backward, or in a retrograde fashion, toward the SA node. The electrical impulse travels forward, or in an antegrade direction, to produce a NORMAL QRS complex. Retrograde P waves that are very close to the QRS, or within it, are presumed to occur from a junctional rhythm, as the impulse leaves the ectopic pacemaker and travels forward and backward at the same time. When a normal PR interval is present, it is probably more likely that the impulse originated in the lower atrial tissue, and is delayed as it travels through the AV node.
This ECG abnormality is probably of no clinical significance in a healthy child, but should be worked up in a child with cardiac symptoms or complaints. This strip is a very good one for illustrating retrograde and antegrade conduction to beginning students.