Even experienced ECG technicians can sometimes be puzzled by paced ECGs. Paced ECG analysis is an important component of device therapy, and it is one of the things that we emphasize in our course.

This is an artist’s rendition of a paced ECG. You can tell it’s a “textbook ECG” because it is so crisp and clear. In real-world clinical practice, ECGs can be a little more indistinct. But this artist’s version is useful for educational purposes.

The small upward vertical lines are pacemaker spikes. They show when the pacemaker delivered an output pulse, a small amount of electrical energy aimed at causing the heart to depolarize and contract. Look at the very first spike in this ECG. It occurs right before a large event (the up-and-down of a QRS complex). As far as a device specialist would tell, it looks like this pacing spike caused the QRS complex, in other words, the pacemaker sent the heart a small amount of energy and, as a result, the heart depolarized and a QRS complex appeared on the ECG. The QRS complex represents the contraction of the ventricles. The rounded event following the QRS complex is the T-wave, which represents the relaxation of the ventricles following contraction.

Find the next pacing spike. The same thing occurs. The spike is followed by the QRS complex and then a T-wave occurs.

These first two pacing spikes look pretty good. It shows the pacemaker and the heart are interacting and working together. Everything seems normal.

Now look at the next two pacemaker spikes. The ECG goes flat, indicating that the pacemaker sent an output pulse to the heart, but the heart did not respond. What would cause this? The most likely explanation is that the output pulse was delivered correctly but it was not sufficiently large to cause the heart to respond. In other words, the output pulse was not adequate to “capture” the heart. Furthermore, the lack of cardiac activity reveals that this patient’s heart does not beat on its own fast enough to keep the patient feeling comfortable.

The fix for this is that the pacemaker’s output settings (programmable parameters) need to be adjusted. In specific, the output pulse needs to be deliver more energy to get the heart to respond.