Isorhythmic AV dissociation

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.


#327