Whether you’re new to device therapy or have been seeing pacemaker and ICD patients for a while, you will encounter puzzling rhythm strips. One of the biggest areas for confusion involves phenomena that have been named "fusion" and "pseudofusion" (there’s even something called pseudo-pseudofusion, but that’s another article). The problem with fusion and pseudofusion is that they appear on the surface ECG in such a way that it is often unclear whether you’re dealing with a sensing problem, a capture problem, or just normal device behavior.

Figure 1.

A textbook unpaced ECG from a healthy patient. Notice that the QRS complexes are narrow, tall, and sharp. (photo permission courtesy of St. Jude Medical, Inc.)

       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.

Figure 2.

A ventricular single-chamber pacemaker captures the heart (a depolarization follows immediately after the large, unipolar pacing spikes). Note that the QRS is wider, notched, and more "rounded" than an unpaced complex. (photo permission courtesy of St. Jude Medical, Inc.)

       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).

Figure 3.

A dual-chamber paced surface ECG as seen on a programmer. The annotations (A and V for paced atrial and paced ventricular events) and timing cycles report what the device is thinking. In this case, the device is pacing at 1,000 ms intervals (which translates to 60 ppm) with good capture. (photo permission courtesy of St. Jude Medical, Inc.)

       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.

Figure 4.

Fusion in a single-chamber paced ECG is indicated in the circled complexes. Notice that the fused complexes do not look like unpaced QRS complexes (which here are small, sharp, and narrow) nor do they look like paced QRS complexes (which are wider with pronounced notches). Instead, the fused complex has a unique appearance. (photo permission courtesy of St. Jude Medical, Inc.)

       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 problem…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.

Figure 5.

The circled complex represents pseudofusion, where a pacemaker spike falls on top of an intrinsic beat. Unlike fusion (where spike and intrinsic activity both contribute to the depolarization), the spike makes no contribution at all to a pseudofusion beat. That’s why the QRS complex in pseudofusion looks much like an unpaced QRS complex. The spike occurs, but plays no part in depolarization. When you see something like this, you cannot immediately rule out the possibility that there is a capture or sensing problem involved. (photo permission courtesy of St. Jude Medical, Inc.)

       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 capture—the ability to reliably depolarize the heart in response to an output pulse—your 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?

Sidebar Image:

Ready to Take the Fusion and Pseudofusion Challenge? There are eight complexes on this VVI rhythm strip. Which complexes make you think you might be dealing with fusion, and which make you think that you might be dealing with pseudofusion? What do you do next?

Ready to Take the Fusion and Pseudofusion Challenge?

- There are eight complexes on this VVI rhythm strip. Which complexes make you think you might be dealing with fusion, and which make you think that you might be dealing with pseudofusion? What do you do next? Answer: Complexes 2, 3, and 4 are fusion. Complexes 5, 6, and 7 are pseudofusion. The first complex is a paced QRS complex and the last complex is an intrinsic QRS complex. This is a good example of why you need to view a strip with many events before making clear determinations about fusion, pseudofusion and normal paced activity. The first complex might be confused for fusion; there is a pacing spike and a wide, notched QRS complex. It is only when contrasted with the appearance of other complexes that it becomes apparent that this is not a fusion beat but a normal ventricular capture event. The reason for this is that ventricular capture QRS complexes are going to be the widest, most notched, and most rounded complexes on the strip. The next three complexes are clearly similar to each other in morphology. Looking further down the strip, it’s clear they are different from the sharp, narrow QRS complexes of intrinsic activity. Since they don’t look like paced QRS complexes (the widest and most rounded), nor do they resemble intrinsic QRS complexes (the narrower and sharper complexes), they have to be fused beats. The last four complexes are narrow, intrinsic events. But unlike the last event — which has no pacing spike — the fifth, sixth, and seventh complexes all have a characteristic pacing spike. The pacing spike did not contribute to the ventricular contraction: this is pseudofusion. If an output pulse plays a role in depolarization (fusion), and therefore changes the QRS morphology, it will also affect repolarization. Therefore, the QRS is changed in fusion and so is the T wave. In pseudofusion, if the pacing pulse plays no role in depolarization (therefore no change to QRS morphology), then it will not alter the T wave or repolarization. While these beats should definitely make you think about fusion and pseudofusion, don’t succumb to the confusion! Since both fusion and pseudofusion make an appearance in this rhythm strip, you should first confirm capture and second confirm sensing. Once those fundamentals are verified, you can rule out more serious problems and turn to making timing adjustments that will clear up the fusion and pseudofusion.

       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).

Figure 6.

This stylized ECG shows a fused atrial beat in the last complex. Like ventricular activity, a fused atrial beat has a unique morphology that is unlike an intrinsic or paced p-wave. (photo permission courtesy of St. Jude Medical, Inc.)

       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.

Figure 7.

This surface ECG shows seven complexes. The first complex reveals a paced QRS complex with a characteristic wide, notched morphology. The second complex looks suspiciously like fusion, mainly because of the variation in QRS morphology. The third, fourth, fifth and sixth QRS complexes look like intrinsic depolarizations, except that there are pacing spikes. Something is going on here; a paced beat should look more like the first complex. This should make you suspect pseudofusion. The last beat could be fusion; it does not look like the first paced complex. While this strip should make you think of fusion and pseudofusion, your first step is to be sure that you’re not dealing with more fundamental issues (capture and sensing) before you revise device timing parameter settings. (photo permission courtesy of St. Jude Medical, Inc.)

       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!