This Lead II rhythm strip shows a regular rhythm with narrow QRS complexes and retrograde P waves. The strip was taken from a nine-year-old girl. The rate is about 110 per minute and the PR interval is .12 seconds (120 ms).
When retrograde conduction is seen in the atria, it is often assumed that the rhythm is originating in the junction. When a junctional pacemaker is initiating the rhythm, the atria and ventricles are depolarized almost simultaneously. This can produce a P wave in front of the QRS with a short PR interval, during the QRS, or after the QRS. Sometimes, in junctional rhythm, a block prevents the impulse from entering the atria, producing NO P wave. Junctional rhythms are usually slow "escape" rhythms, but can be accelerated or tachycardic.
The fact that this rate is 110 / minute and the PR interval is normal at .12 seconds, we should consider that this rhythm could also be from an ectopic pacemaker low in the atria. From this low starting point, the impulse will travel backward, in a "retrograde" fashion, through the atria, producing a negatively-deflected P wave in Lead II.
We do not have clinical data on this patient, and so do not know what possible causes of arrhythmia might be present, and what the expected rate should be in this situation.