AF with slow ventricular response in a patient with VVI pacemaker with intermittent loss of capture and intermittent undersensing

Interpretation courtesy of Dr. Raed Abu Sham'a:

The underlying rhythm is AF [no clear P waves]. There are QRSs of 4 different morphologies. The ventricular rhythm is irregular which excludes underlying CAVB in AF patient. There is at irregular pacing  spikes. If you have completely irregular pacing spikes like this, then there is a combined problem!!! There is no capture after the spikes which indicates non-capture problem and the pacing spikes fall randomly that  indicate undersensing. Furthermore, they are irregular which indicates intermittent undersensing. Example of this is QRS 1 and 2 are undersensed and QRS 3 is sensed and the following spike is delayed to the lower pacing limit. There are two paced beats with capture [8, 12]. Here are all beats: QRS 1=US, 2=US, 3=S, 4=S, 5=S, 6=US, 7=S, 8=P, 9=S, 10=S, 11=S, 12=P, 13=S, 14=S, 15=S. Interestingly after the two paced beats there are two beats [9 and 13] of the same morphology and coupling intervals. These are called bundle branch reentrant beats. In conclusion: AF with slow ventricular response in a patient with VVI pacemaker [temporary or permanent] with intermittent loss of capture and intermittent undersensing.



  1. Hi, sry for the stupid question, but i would like to know why the QRS 1 and QRS 2 are undersensed ? It seems like the spike after the QRS is at the lower pacing limit (1000ms after QRS 1) ?
    Thanks in advance

  2. Thank you so much for your comments. There is not such thing as a stupid question. I think your observation was right that QRS my have been a sensed QRS but QRS is an undersensed event. If you look closely, the pacer spike after QRS 2 is about 1000 ms after the pacer spike before it. So R1 could be S and R2 is US. Thanks again for that observation.

  3. Thanks, later i saw that QRS 2 is undersensed, but i only measured the QRS 1! Have a nice day