This ECG is from a patient admitted due to dizziness. Work-up revealed a posterior circulation stroke. During routine monitoring this "strange" ECG strip was captured.
Image 1 - Long lead II
This is sinus rhythm. The first 2 QRS complexes were conducted with a normal PR interval of about 0.20 sec. After R3 (PVC), the PRI became prolonged at around 0.60 sec. The PRI remained prolonged up to R12. After R13 (PVC), the PRI became normal again.
This strange ECG behavior can be explained by the existence of 2 pathways in the AV node. One pathway conducts fast and creates a short or normal PRI and the other pathway conducts slow and creating a long PRI.
This is better appreciated by a ladder diagram (courtesy of a friend Jason R).
Image 2 - Ladder diagram
The PVC's made one pathway refractory and allowed preference of conduction to the other pathway. This is just one of many manifestations of dual AV node conduction during sinus rhythm.
The existence of 2 pathways in the AV node is also called dual AV node physiology. Some individuals have this. This particular strip manifested dual AV node conduction during sinus rhythm. Another instance where we can see dual AV node physiology is during AV nodal reentry tachycardia (AVNRT).
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