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.

Once you confirm proper sensing, resume the programmed pacing rate. If you still see spikes on top of intrinsic events, this is fusion. Fusion is a timing problem. It occurs because the heart s natural rhythm competes with the pacemaker s setting. In a single-chamber system, you can fix this by decreasing the base rate slightly. In a dual-chamber system, the best fix is to extend the AV/PV delay slightly.

One danger in any sort of device troubleshooting is overcompensating. Sometimes we figure that if a slight rate decrease or delay extension is good, then a bigger change ought to be even better. Actually, with pacing, you should make small changes. Major changes to any of the settings may solve your present problems but introduce new ones!

It's important to realize that fusion is not only a timing issue, it represents the expected and programmed behavior of the device. When fusion occurs, the parameter settings should be changed. The device is only doing exactly what you told it to do. Resetting the parameter values, even slightly, will clear up fusion.

Another related phenomenon occurs when the pacemaker spikes literally fall right on top of an intrinsic depolarization (Figure 5). When this occurs, it is hard to discern whether you re dealing with a sensing problem (why didn't the device see the intrinsic depolarization and inhibit the spike?), a capture problem (maybe the spike did not cause the depolarization), fusion (is the spike contributing to an already-occurring intrinsic event?), or something else. That something else is called "pseudofusion," partly because it is often mistaken for fusion.

Seeing pseudofusion on a rhythm strip should make you suspect possible sensing or capture problems. Fusion confirms capture, but pseudofusion has to make you wonder if the spike was capable of depolarizing the heart. Since the most fundamental activity of any pacemaker or ICD with pacing capability is capturethe ability to reliably depolarize the heart in response to an output pulseyour first step has to be to confirm capture. Even if your clinician s instinct tells you that you re dealing with pseudofusion, you must verify proper capture first.

The easiest way to do that is to temporarily increase the pacing rate slightly, so that the base rate is faster than the patient s underlying rhythm. This will force a series of paced events, which you should evaluate for proper capture. If there is a capture problem, conduct a capture threshold test and adjust the output parameters (pulse width and pulse amplitude) to deliver more energy. On the other hand, if a higher base rate confirms proper capture, you can move on to the next step of your differential diagnosis. Is the device sensing appropriately?

Just as with fusion, decrease the base rate temporarily. If the device is sensing properly, the phenomenon (the suspected pseudofusion) should disappear. If you still see spikes on top of intrinsic events, then you have a sensing problem which can be corrected by adjusting the sensitivity settings.
But if a slower rate makes the phenomenon go away, you can now assume you re dealing with pseudofusion. Pseudofusion is wasteful, in that the spike plays no part in the native depolarization. It can make rhythm strips confusing, and it is not the way the pacemaker works best. Pseudofusion, like fusion, is nothing "wrong" with the device; the device is only doing what it was programmed to do. Pseudofusion and fusion are timing problems which can be corrected by making changes to the timing parameters (reduce the ventricular rate slightly in a single-chamber device or increase the AV/PV delay slightly in a dual-chamber system).

While both fusion and pseudofusion may seem similar, there are important distinctions (Table 1).

Fusion and pseudofusion can occur whenever a device paces the heart. It can even occur in the atrium (Figure 6). When you see a potentially fused beat, be sure to rule out basic device problems (such as sensing); when you see a potentially pseudofused beat, be sure to rule out both capture and sensing problems.

Fusion and pseudofusion are not uncommon. They often occur in the same patient, and it is not unusual for a clinician to see lots of such confusing activity in one surface ECG (Figure 7). While I do recommend learning how to distinguish typical fusion and pseudofusion morphologies, use this information as a starting point, not as your final diagnosis. Whenever you see potential fusion and pseudofusion, you should first confirm proper capture (pseudofusion) and sensing (fusion and pseudofusion) before deciding that it is a timing problem. Capture and sensing problems can t be fixed with new timing parameters!