Here is the solution to Rhythm Challenge #5.
The rhythm is paced and here’s why.
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.