You are on call and your page goes off. You call the number back and find out that a patient you are covering is complaining of the sensation that his heart is "skipping beats". He is in the hospital for Community Acquired Pneumonia, and has a history of DM, HTN, Rheumatoid arthritis. This is his only complaint. Since the patient is not on a telemetry floor you ask the nurse to get an EKG and that you will be right down. The following is the lead v1 rhythm strip of the 12 lead EKG that was taken. Why is he feeling like this?

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Looking at this rhythm strip, the first thing that jumps out at you is the long pause between the 4th and 5th beats. However what is not so obvious is the shortened P-R interval found in the last complex. So what is going on in the last complex?

We know from the length of the last qrs that this rhythm is originating somewhere above the ventricles. So why the short P-R? There are two reasons why we can have a perceived shortened PR in this case. First, we could have a sinus pause long enough to allow a high junctional escape (high his bundle) to get a retrograde p wave to trace before the qrs giving the appearance of a shortened pr. But the problem with that is that if this where a retrograde conducted p wave than the morphology would have to be different that the other p waves in the same lead.

That is not the case here. In fact the morphology is the same suggesting that the p wave in the last complex originated from the sinus node. The only way to account for this is the following. The patient had a sinus pause, the sinus node eventually fired late, however before the impulse from the sinus node reached the av node, the delay was long enough to have a junctional escape fire as well. This way you have a perceived shortened pr but in actuallity the p wave found the av node refractory and never conducted down because it had already fired from the junctional escape.