Rhythm Challenge #5 – Answer

Here is the solution to Rhythm Challenge #5.

The rhythm is paced and here’s why.

As I explained in Evaluating the Pacemaker Patient – Part I, most modern pacemakers are DDD pacemakers according to the NBG pacemaker code.

Essentially that means that most pacemakers will “track” P-waves and deliver a paced QRS complex (when no native QRS complex appears) after a prescribed PR interval to take advantage of the “atrial kick” and the associated improved hemodynamics.

However, there is a limit. As you might expect, cardiologists don’t want the device to track P-waves and supply paced QRS complexes when the atrial rate goes up to 300 as it does during atrial flutter. That would not be in the patient’s best interest. So there’s an upper rate limit.

Let’s say the cardiologist wants a pacemaker to track a patient’s P-waves but he doesn’t want the paced rhythm to exceed 136 BPM. How can this be achieved? By a parameter called the PVARP or Post-Ventricular Atrial Refractory Period. That means that a pacemaker will “close its eyes” for a prescribed interval after each QRS complex, whether it’s a native QRS complex or a paced QRS complex. In other words, it will ignore P-waves during that period of time.

All of the ECGs in Rhythm Challenge #5 can be explained by a PVARP set for approximately 440 ms or 11 small blocks, which is a heart rate of about 136.

Let’s look at a graphic to see how this played out from the pacemaker’s point of view.

As you can see, when a P-wave falls outside of the PVARP the device waits for a prescribed PR interval and then creates a paced QRS complex if a native QRS complex does not appear first. P-waves that fall inside the PVARP are ignored by the pacemaker.

In other words, this is normal pacemaker behavior! Having said that, the only way to know for sure is to identify the exact type of pacemaker (the manufacturer and model), the indication for the pacemaker, how the pacemaker is programmed, and to read the report after the device is interrogated.


  • Christopher says:

    I think the rate in the original strips coupled with the complex shape on an LP12 is enough to do a quick pacemaker inspection and put the rhythm question to bed.

    Tom, is the only way to get the less obscured view to pop a quick 12L on the LP12s?

  • Tom B says:

    In theory you could push 12-LEAD and get a 6-LEAD even without the precordial leads. At least you’d be in diagnostic mode. But why not get a 12-lead? If I’m worried enough to put someone on the monitor I can’t think of a good reason not to.

  • Christopher says:

    Interesting, I need to sit down with the LP12 manual and cover-to-cover the thing. We often place a patient on the monitor simply for rhythm interpretation (pretty much any patient seen by a medic in our system receives a 3L). In the case of our paced patients, showing the ED or attaching a 3L w/ gross ST-segment changes and no 12L is frowned upon. Monitor mode vs Diagnostic mode isn’t well understood I guess.

    Although to your point, almost any patient with enough of a complaint to receive continuous monitoring is going to get a 12-lead.

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