First, let's review normal conduction in the healthy heart (Figure 1). The heart s natural pacemaker in the sinoatrial (SA) node delivers an electrical output which conducts down over the atria, regroups in the AV node, and then travels over the ventricles. Because the atria are small, their depolarization is captured on the surface ECG as the relatively small p-wave. The ventricles depolarization is a much larger electrical event and is recorded on the surface ECG with tall, sharp QRS complexes.

A pacemaker changes the surface ECG in several predictable ways (Figure 2). The pacemaker output pulse appears on a paced ECG as a vertical line or "spike," which is typically taller in unipolar systems versus bipolar systems. The pacemaker spike should result in an immediate depolarization. But when a ventricular output "captures" or causes a depolarization, the resulting QRS complex is wider and often exhibits a characteristic notched effect, typical of unpaced patients with left bundle-branch block (LBBB).

Most clinicians, including me, prefer to work with surface ECGs simply because we are more familiar with them. But in device-based therapy, we frequently have to deal with surface ECGs from the device programmer (Figure 3), which present us with a more condensed image. But neither of these is what the device "sees"! The device relies on the intracardiac electrogram or IEGM, which comes from electrodes within the heart. Interpreting the IEGM is probably the most accurate way of understanding what the device is "thinking." The next best thing is to rely on the annotations that appear on the programmer surface ECG. These codes and numbers report how the device interprets events.

Fusion and pseudofusion have a confusing appearance on the surface ECG. Fusion occurs when a device output spike appears to collide with an intrinsic event. When this happens, it is probably unclear as to what is actually going on (Figure 4). For example, you should be wondering if there is a sensing problem. After all, if there was an intrinsic event, shouldn t the device have "seen" it and inhibited the pacemaker output? Then again, when you look closely, maybe the output and the intrinsic event occurred practically simultaneously. Is that a problem?

When a pacemaker output occurs almost simultaneously with an intrinsic event, the result is called fusion. In such cases, both the pacemaker and the intrinsic activity contribute to the depolarization. The result is a unique kind of beat with a unique-looking QRS complex. However, fusion is not the most efficient form of device therapy. Most device experts will tell you that for conventional pacing, it is best for intrinsic behavior to prevail whenever possible; fusion prevents this. Furthermore, fusion is wasteful because an unnecessary pacemaker output is delivered.

While fused beats do have a unique look, you should never rely on "eyeballing" a surface ECG for your diagnosis. First of all, the surface ECG is not what the device "sees," and second, it is easy to be deceived. While you should always suspect fusion in such cases, it is smart to go back to the basics.

When you have fusion, the one thing you know you do have is capture. The spike is followed by a depolarization. Can the device sense intrinsic activity properly? An easy way to confirm proper sensing is to temporarily decrease the pacing rate long enough to confirm that when pacemaker spikes do appear, the device can see them and inhibit the next output.