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Sinus rhythm with dropped PACs  

Dropped PACs.   There is an underlying first degree block  A dropped PAC occurs after the 5th complex.  The dropped P wave can be seen better in the AVF lead.  This is followed by an atrial escape beat (6th complex) and a junctional escape beat (7th complex).  A PVC (8th complex) is next. The 9th complex is a junctional escape beat.  A dropped PAC follows.  The 10th complex is another junctional escape complex.   

These three rhythms strips belong to the same patient.  If you examine the morphology of the early beats in the first two strips you can seen the same morphological changes in the T wave when a nonconducted PAC occurs.  In the very last strip, those same characteristics are seen in a bigeminal pattern. In this strip, the patient is showing what appears to be aberrantly conducted PACs in a quadrigeminal pattern.  The change in the height of the T wave before each ectopic beat leads me to believe that they are atrial in origin.  I have circled the ectopic P waves for comparison in the next strips. In this second strip.  The early beat of complex 4 shows the same kind of P wave.  Then another nonconducted P wave is seen after complex 5.  If you look at the morphology of these P waves in aVL the look the same.  The same pattern is seen in complexes 9 and the nonconducted beat after complex 10 Keeping in mind the morphological similar...

On this page a dropped PAC is seen following the 5th complex.  If an impulse is initiated during the phase of relative refractory period an impulse may or may not produce a contraction.  Generally, it takes a stronger than normal impulse to initiate a contraction.