Troubleshooter's Case Book for Pacemaker and ICD Follow-Up: The Causes of Pauses

Tom Kenny, RN
Tom Kenny, RN
What happens when you see a patient who should be paced, but whose ECG or EGM reveals pauses? Clearly, a patient receiving optimal pacing support is going to be paced regularly. Pauses indicate trouble. (Figure 1) The best approach to troubleshooting is to be systematic and to proceed, step-by-step, through possible scenarios. This annotated ECG shows long pauses on the strip. However, before addressing that, the clinician should check programmed parameter settings and overall function of the device. In this case, the patient has a single-chamber pacemaker programmed to a base rate of 70 ppm with no rate response function. The measured values, including battery status, do not indicate anything unusual. The next step in the system is to confirm capture that is, that every ventricular output leads to a ventricular depolarization. The annotations on the bottom of the strip clearly show that the device is delivering ventricular output (marked with a V). However, no single V spike on the strip leads to a ventricular depolarization. In fact, the only ventricular depolarizations on this strip are sensed events, annotated with an R. This patient is relying on his own underlying ventricular escape rhythm (and a pretty slow rate), and is not being paced at all. Without careful examination of the annotations below the tracing on the surface ECG, it would be easy to mistake all of these QRS complexes for paced beats. A closer inspection of these QRS complexes shows that they are wide and bizarre, another indication that the patient is relying on his idioventricular or ventricular escape rhythm. Since the device is pacing, that is, pacing outputs are being delivered at the proper time, the device is failing to capture the heart. This means that the output pulse is not delivering sufficient energy to capture the cardiac tissue. This sometimes perplexes clinicians, particularly if the device had captured the heart previously and no settings had been changed. However, the fact is that capture thresholds or the amount of energy required to reliably and consistently capture the heart are not static value. They fluctuate, and sometimes considerably. Capture thresholds can change with disease progression and drug interaction, and sometimes there are variations over the course of a single day. Non-capture can be addressed effectively by increasing the pacemaker s output values. Output is the common "pacer lingo" for the amount of energy the device delivers in a single pulse to stimulate the heart. Output is governed by two distinct parameters: pulse amplitude (or voltage) and pulse duration (or milliseconds). Increasing either one of these will increase the amount of energy in the output pulse, but increasing pulse amplitude is the more efficient way. However, rather than just randomly dialing up the output settings, the best next step in this scenario is to run a capture test. Today s pacemakers offer automatic, semi-automatic, and manual capture testing options, but all of them work on the premise that output is programmed temporarily to a very high value and then stepped down gradually, in tiny increments, until capture is lost. The lowest amount of energy which still assures regular, consistent capture is called the capture threshold. (Figure 2) Once the capture threshold is determined, the output settings should incorporate a safety margin. This safety margin provides the "margin for error" to accommodate threshold variations that may occur. As a rule of thumb, a 2:1 safety margin (twice the voltage) is preferred. Since the threshold in this case is 0.75 V and 0.4 ms, the safety margin should be at least 1.5 V at 0.4 ms. Some clinicians routinely program a 3:1 safety margin, particularly when the patient s threshold is relatively low, as is the case here. When would you use the higher safety margin? That is always a matter of clinical judgment, but here are the key points to consider: Is the patient pacemaker dependent? The more the patient needs the device to function, the higher the safety margin ought to be. Has the patient experienced non-capture before? Is this an ongoing problem? The more frequent the complaint, the higher the safety margin ought to be. Is the patient taking new drugs or been diagnosed with a new condition that might impact his capture threshold? If so, it is better to increase the safety margin until the patient s condition is more stabilized. How high is the capture threshold? If the capture threshold is relatively low, there is not a lot of economy in setting a strict 2:1 safety margin. On the other hand, if the patient s capture threshold is 3.0 V, 0.4 ms, then it would drain the battery considerably to provide a 3:1 safety margin. This may be necessary, but it should never be done without careful consideration. Since this patient has a low capture threshold, is clearly not pacemaker dependent, and has not had capture problems before, a 2:1 safety margin might be adequate. In this case, the patient reports he has just been diagnosed as being in the early stages of heart failure, and his physician has recently prescribed several new drugs for him (he can t remember what they re called). Based on that, I would program higher output settings (around 2 or 2.5 V at 0.4 ms). Had the patient told me nothing in his medical condition had changed, I would have used output settings of 1.5 to 2.0 V at 0.4 ms. Another cause for pauses has nothing at all to do with the device s output. (Figure 3) Using the systematic approach, first interview the patient about any symptoms or particular complaints and confirm through the measured values that the device has adequate battery energy. Look at the programmed pacemaker settings and see if the device is pacing as one would expect. In this particular case, you don't even need to get out the calipers to see that the patient is experiencing intermittent pauses. What s going on? Being systematic about troubleshooting, the next step involves confirming proper capture and proper sensing. Capture can be verified on an ECG by looking for pacing output spikes and seeing if there is a resulting depolarization after the output. This patient has a unipolar VVI pacemaker that causes very prominent vertical spikes on the strips. Where these spikes occur, there is a resulting QRS complex. That means the device is capturing. What about sensing, though? Appropriate sensing can be confirmed on an ECG when an intrinsic complex can be seen to inhibit the pacing output. At the very end of this strip, there is an intrinsic ventricular beat and the device does not try to pace too close to it. That shows proper sensing behavior. However, when we look at the pause at the beginning of the strip, we see no pacing spikes and no intrinsic behavior. The pacing function is inhibited, but why? There is no native activity to inhibit it. This scenario is called "oversensing," which is just another way to say "under-pacing." The pacemaker is "seeing" intrinsic activity that isn t there. As a result of this misinterpretation, the pacemaker withholds pacing outputs because it thinks the heart is beating on its own. In reality, the ventricles are not beating and the patient experiences a long pause with no cardiac activity. Oversensing is a problem with device sensitivity which causes the device to sense cardiac activity that is not there. In other words, a pacemaker that oversenses is overly sensitive. Sensitivity problems in pacemakers occur because pacemakers have to filter incoming signals in an effort to recognize true intrinsic cardiac signals and ignore extraneous signals such as myopotential noise. If this filter is set too high, only the largest and most extreme signals get seen, which means the device misses a lot of true intrinsic activity. If the filter is set too low which has happened here the pacemaker is so sensitive that it detects very small signals that do not even show up on the ECG. The key to resolving an oversensing problem is to make the sensitivity less sensitive. Although it may seem counter-intuitive, decreasing sensitivity in a pacemaker means increasing the mV value setting. By increasing the mV setting of the sensitivity parameter, you are essentially saying that signals have to be that large before the device will see them. Signals below that size will be ignored. Most pacemakers offer automatic, semi-automatic, or manual sensitivity tests, all of which work on the premise that sensitivity is set very high (that is, a low mV value) and then is ramped up in small steps until sensing is lost. The lowest mV value at which the device can still reliably sense is the sensing threshold. (Figure 4) The annotations on this sensing threshold test help show what is going on. When sensitivity is very high, the patient s intrinsic rate comes through, annotated on the bottom as P and R activity. At the point where ventricular sensing was lost, the pacemaker starts to pace the ventricle, as shown by the V annotation. Programming sensing is more exacting than programming output settings for capture: when programming output, going too high may waste the battery, but it will not impair capture. In sensing, too much can cause as many problems as too little. For this patient, the device sensitivity needed to be adjusted to around 5.0 mV. Other causes of pauses may be more apparent at the outset. Using the systematic approach, you always check for battery status and lead impedance values from the measured values on the programmer. Problems with either of these items can result in intermittent or no-output from the device, and that can cause pauses on the ECG. If the programmer flags you that the battery status is low or even depleted, the patient needs to be scheduled for an elective replacement of the unit. There are no ways to program around an exhausted device battery. Lead impedance values are not programmable settings, but they are good watchdogs for knowing what is going on with the lead system. Many programmers will alert you if lead impedance values fall out of range. Should that happen, it is likely that you will encounter no-output or erratic output from the device. The impedance values should make you suspect a problem with the lead (which might include knicked insulation, a broken conductor coil, a kinked lead, or in the acute phase a lead that has become dislodged in the heart or unconnected from the device). Patients should receive a chest X-ray to visualize what has gone wrong with the lead. Lead problems usually require the placement of a new lead. In most cases, the old, damaged lead is not extracted but is capped and left in place. Removing a chronic pacing lead is a very complex procedure associated with additive risk to the patient and should only be undertaken by experts experienced in such cases. Today s pacing leads have a very broad range of acceptable impedance values, but any large change in impedance even if the values technically stay in the "normal" range can be an early red flag. If possible, check the patient s current lead impedance values (even if within range) against the impedance values from the last or last few follow-up visits. Any variation of 200 Ohms or more suggests there might be a lead problem. Sometimes, lead damage does not occur all at once, but happens gradually. This can result in intermittent device problems with no obvious causes. That s why monitoring the general trend in lead impedance can be a good tactic. There are many causes for pauses in the paced patient, but a systematic approach is the best way to resolve most cases quickly and easily (Table 1).