Pace/Off: Pacemaker Basics with Nicolas Cage

A pacemaker is an implanted device that provides electrical impulses to the heart when intrinsic heart pacing is inadequate. Pacemaker leads are typically placed in the right atrium and right ventricle, however pacemakers can be bi-ventricular as well. The rhythm from a paced right ventricle will appear as a left bundle branch block on ECG (as the left ventricle is depolarized after the right).

The main indications for pacemakers are symptomatic bradycardia or high-grade AV block.

An ICD (or AICD) is different from a pacemaker in that it does not provide constant electrical impulses to the heart, but provides cardioversion or defibrillation when tachyarrythmias are detected. Indications for ICD placement include previous ventricular fibrillation (VF), unstable ventricular tachycardia, structural heart disease, or advanced heart failure.

Both devices appear similar on X-ray, however, ICDs tend to have thicker leads.

Pacemaker

ICD

There is a five-letter code for every pacemaker and the first three letters are the most important. The first letter indicates the chamber(s) paced: A=atrium, V=ventricle, D=dual (atrium + ventricle). The second letter indicates the chamber sensed, using the same A, V, D description. The third letter indicates the response the pacemaker will give to a sensed signal: T = if an intrinsic complex is sensed it will trigger an impulse from the pacemaker, I = if an intrinsic complex is sensed it will not trigger (or it will inhibit) a paced impulse, D = the pacemaker can both trigger and inhibit paced impulses depending on what is sensed or not sensed.

For example, in D mode if both a native atrial complex and ventricular complex are sensed, the pacemaker will inhibit both the atrial and ventricular pacemaker impulses because they are not needed, the heart is already electrically functioning on its own. If an atrial complex is sensed but no ventricular complex is sensed, the pacemaker will not pace the atrium (because the atrium already has its native electrical impulse), but it will trigger the ventricle (because the ventricle does not have a native impulse). If the pacemaker senses no atrial or ventricular native rhythm it will pace both. Most devices are designed or programmed VVI or DDD.

The most common problems with pacemakers are complications from surgical implantation, such as site infections or hematomas. Also, pneumothorax or vessel thrombosis may occur soon after placement, but these are rare.

Any potential problems with a pacemaker’s function can be identified with an ECG:

1. Failure to pace – The pacemaker does not trigger any impulses at all and no pacemaker spikes are seen on ECG. The heart will be reliant on its native rhythm.

2. Failure to capture – The pacemaker is giving off electrical impulses, but they do not cause myocardial depolarization. In this case, pacemaker spikes are seen on the ECG, but they are not immediately followed by an atrial or ventricular complex and therefore are not stimulating a heartbeat. Again, the heart will be reliant on its native rhythm.

3. Failure to sense – The pacemaker triggers despite the presence of intrinsic cardiac activity. The pacemaker is basically just pacing away with no regard to what the heart is doing. The ECG will show pacemaker spikes before, during, or after intrinsic cardiac activity with no organized relationship between the two. 

4. Pacemaker-mediated tachycardia – The pacemaker acts as one arm of a re-entry circuit in which a ventricular impulse travels retrograde through AV node triggering the atrial sensing lead in the pacemaker which then triggers another ventricular complex. This will look like a wide-complex tachycardia. The heart rate will be limited by the upper limit of normal in the pacemaker’s settings.

The first step will be to examine the pacemaker pocket itself for infection of hematoma. Next, get a chest x-ray to confirm correct lead placement and lead integrity. A chest x-ray can also help identify which device is in place – an ICD or a pacemaker. The next step will be to check electrolytes and cardiac enzymes, then perform device interrogation.

One helpful hint – placing a device-specific magnet on top of a pacemaker will temporarily disable its sensing ability and the pacemaker will revert to a fixed-rate (asynchronous) pacing. This means it will pace at its preset rhythm, typically 70 bpm, no matter what intrinsic electrical activity is occurring in the heart, similar to what happens in “failure to sense”. This could be dangerous if the pacemaker triggers a ventricular impulse during the repolarization of an intrinsic cardiac cycle – causing a so-called R on T phenomenon – leading to ventricular tachycardia, ventricular fibrillation, and death.

Magnets should only be used in unstable patients with possible pacemaker-mediated tachycardia.