Adult patient admitted due to abdominal pain. What is the rhythm?
Figure 1
Isorhythmic AV dissociation.
Figure 2
The rhythm is sinus with some P waves merging with the QRS (R1 – R6 highlighted with red oval). As we move from the left to the right, you will notice a gradual increase in the P to P rate. A distinct P wave is seen before R7 with a change in QRS morphology (R7-R10 and highlighted by a blue oval). What does it mean? This mean that from R7 to R10, the ventricle is depolarized by the SAN and R1-R6 is depolarized by the AV junction.
This strip is a nice example on how to recognize origin of an impulse by looking at QRS morphology/shape.
As the dominant pacemaker (SA node) slow down the subsidiary pacemaker (AV junction) kicks-in and capture the ventricle. This created AV dissociation. When the rate of the dominant pacemaker and the subsidiary pacemaker is the same, it is called isorhythmic AV dissociation.
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