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Interpretation of a pacemaker ECG

A 12-years old female patient is referred to pediatric cardiology due to dizziness. She wears a trans venous ventricular lead pacemaker due to sinus node disease. Her pacemaker is programmed as VVI with a lower rate limit of 60 bpm and hysteresis to 50 bpm.

The monitor ECG showed the following:

Now, if you are familiar with VVI pacemakers and SND you probably don't find it challenging to interpret this tracing. For everyone else, let's have a closer look.

For one thing, when assessing a pacemaker patients´ ECG, you look for pacing artifacts. These are usually sharp and very narrow deflections. Just like the ones you find in the first line of the tracing.

Next, it is important to know whether the patient has a single or dual chamber device. Dual chamber devices are usually programmed to DDD, which means that P waves are tracked by the pacemaker and atria and ventricles should be synchronized.

In single chamber devices, however, atria and ventricles may be synchronized (if it is an AAI device and the patient has regular AV conduction). In VVI pacemaker atria and ventricles are not synchronized since the pacemaker senses the ventricular electro gram only!

Here is a short translation of pacemaker code: the first letter depicts the site of pacing (A=atrium, V=ventricle, D=dual). The second letter depicts the site of sensing (A=atrium, V=ventricle, D=dual), or in other words "where the pacemaker looks for electrical signals". The third letter gives away what the pacemaker is programmed to do with the gathered information (I=inhibition, T=trigger, D=dual or both).

In our case, the device acts as VVI. So, it paces in the ventricle every time it does not detect a ventricular electrical signal within a certain time following the last noted ventricular signal. The time that the pacemaker waits to detect a ventricular sensed event before pacing in the ventricle, is what the physician sets as the lower rate limit. I.e. the lowest heart rate that the pacemaker will allow. In our case 60 bpm.

Now, back to the tracing above. As you can see there are several pacing spikes within the tracing. Most of them are followed by a wide QRS complex. These are pure paced heartbeats (yellow arrows). Sometimes the pacing spike is followed by a narrow QRS complex with varying delay from the spike to the QRS. These are fusion beats (red arrows). That means that the pacing spike falls together with an intrinsic ventricular activation, so the electrical impulse fuses.

And then there are a few narrow QRS complexes without a pacing spike in front. However, they also do not show any preceding P waves and must therefore be presumed to be junctional beats (purple arrows). Junctional beats originate at the atrioventricular junction region, mostly just below the AV node at the His region. Therefore, they show narrow QRS complexes and are not preceded by P waves.

Such junctional beats are not uncommon in SND, as they resemble an escape rhythm when the sinus node remains silent.

So, all taken together, the pacemaker does what it's supposed to do and the ECG may be interpreted as normal for this patient.

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