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Taking advantage of sophisticated pacemaker diagnostics
Taking Advantage of Sophisticated Pacemaker Diagnostics Bernd Nowak,
MD
The ever-increasing complexity of pacing systems, combined with functions that vary from one manufacturer to another, can pose challenges during analysis of device function. Standard pacemaker diagnostics are measured data, electrogram telemetry, marker annotations and event counters, albeit with their current limitations. New diagnostic features discussed include time-based diagnostics, histograms of sensed amplitudes, pacing thresholds, or impedance trending. Mode-switching algorithms, combined with diagnostic features, facilitate the use of dual-chamber devices in patients with parox-
ysmal atrial tachyarrhythmias. The introduction of electrogram storage into pacemakers further improves diagnostic capabilities and allows a permanent validation and optimization of diagnostic and therapeutic algorithms. External diagnostic devices, which provide Holter recordings with continuous marker annotations and patient-triggered diagnostics, are additional features that will become increasingly important. Q1999 by Excerpta Medica, Inc. Am J Cardiol 1999;83:172D–179D
acemaker therapy has evolved over the last 40 years from the first nonprogrammable, singleP chamber, asynchronous VOO devices to multipro-
provide information about battery and lead status and help to optimize the output programming to save energy. The measured data allow early detection of potential hardware problems before the patient becomes symptomatic or health is jeopardized. Patientrelated diagnostics such as intracardiac electrograms and marker annotations, help to identify the device’s response to the patient’s intrinsic rhythm and also provide troubleshooting assistance. Pacemaker diagnostics facilitate the monitoring of underlying arrhythmia or disease and of the therapeutic effects resulting from modified programmed settings or medications. Additionally, the function of sensors and pacemaker algorithms can be assessed and fine tuned.2 All these applications of pacemaker diagnostics are thereby helpful to ensure the patient’s safety and quality of life.
grammable, dual-chamber, rate-adaptive rate-responsive pacing devices. Still, to date, the development of single- and dual-chamber devices is far from complete.1 New indications and applications of special stimulation techniques are being evaluated, such as bi-atrial stimulation for prevention of paroxysmal atrial arrhythmias and multichamber pacing for patients with congestive heart failure. Advances in lead technology are aimed at developing floating leads, designed to stimulate the atrium, and at thinner leads with improved handling characteristics, to reach special pacing sites. New algorithms are another main emphasis of pacemaker development. Such algorithms are designed to diagnose and treat many kinds of arrhythmias and make the pacemaker function as physiologically as possible. In addition, modern devices are becoming more and more automated and are now able to adjust their programmed settings to sensed or measured parameters. This increasing sophistication and complexity of pacing can complicate the analysis of pacemaker function. Therefore, a variety of new pacemaker diagnostic functions has become available. A programmer can perform various measurements, reflect the actual behavior of the device during a visit, and obtain data on pacemaker function during the past days, weeks, or months. Usually, several diagnostic features, supplementing each other, are necessary to verify optimal device function.
USE OF PACEMAKER DIAGNOSTICS Modern pacemakers include 2 types of diagnostic function: system-related diagnostics and patient-related diagnostics. System diagnostics are obtained through device interrogation and measured data that From the II. Medical Clinic, University Mainz, Mainz, Germany. Address for reprints: Bernd Nowak, MD, II. Medical Clinic, University Mainz, D-55101 Mainz, Germany.
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©1999 by Excerpta Medica, Inc. All rights reserved.
SYSTEM DIAGNOSTICS System diagnostics provide information about the battery status and the lead impedance. Battery status information includes cell voltage, internal impedance, and current drain, which allow an estimation of device longevity. In addition, the effectiveness of changing output amplitudes and pulse widths can be assessed to optimize energy consumption without impairing patient safety. Based on these measured data, a new method for output optimization has been proposed that uses the determination of the minimum charge delivered per pulse at the pacing threshold to perform low output programming with steroid-eluting low threshold leads.3 This concept uses a 100% safety margin, in terms of charge measured by real-time telemetry, to achieve stimulation at an output amplitude of 1.0 V. Based on the system diagnostics, this method has the advantage of being less impacted by polarization effects compared with the widely used voltage threshold. Lead impedance is used to judge the integrity of the lead. Insulation defects result in decreased impedance values, whereas conductor fractures or loose connections translate into an increase in lead impedance. In some cases, when the lead failure is not yet 0002-9149/99/$20.00 PII S0002-9149(99)01021-2
complete, the defects and the corresponding variations in impedance may occur only intermittently. In these cases, a single impedance measurement during follow-up might be normal, even when the measurements are taken during provocative maneuvers and lead manipulation. In such cases of a suspected malfunction, it is helpful to have devices that provide beat-to-beat lead impedance values or that regularly store lead impedance measurements at given intervals. Intracardiac electrograms offer the unique opportunity to view the electrical activities of the heart with the eyes of the device, (which is blind to a 12-lead surface electrocardiogram and bases all signal interpretations on a single channel from each chamber). The electrogram is useful to identify signals sensed by the device but not visible on the surface electrocardiogram. Because the amplitude of the electrogram is directly proportional to the amplitude of the signal sensed by the device, frequent fluctuations of the intracardiac signals are, thus, easy to assess (e.g., the amplitudes during atrial fibrillation or P-wave amplitudes in single-lead VDD systems). The morphology of the electrogram also reveals important information concerning the type of signal and may help to detect retrograde P waves, to diagnose an underlying rhythm or arrhythmia, or to confirm atrial capture. The diagnostic value of the electrogram is improved if used in combination with marker annotations. However, the real-time electrograms restrict the diagnosis to patient’s activities that take place during follow-up, thus, limiting their use. Therefore, Holter recordings combined with marker annotations have been developed recently (see below). Other innovations have resulted in the capability to store electrograms for later retrieval. It must be kept in mind that in some devices, the amplifiers and filters used to record the electrogram may be different from those used for sensing. Thus, the signals on the electrogram might not be completely identical to the signals sensed by the pacemaker.
MARKER ANNOTATIONS The interpretation of pacemaker electrocardiograms, especially in dual-chamber devices, has become increasingly difficult with the use of sensors and/or single or multiple algorithms. This applies to 12-lead surface electrocardiograms, as well as to Holter recordings. Even with detailed knowledge of programmed parameters, algorithms, and timing cycles, it might be impossible to perform an unequivocal electrocardiographic interpretation. Marker annotations, which are transmitted from the pacemaker to the programmer, provide online information about paced and sensed events, refractory periods, sensed events inside these refractory periods that are ignored by the device, or the activation of algorithms and sensors. These annotations should be used with a simultaneously recorded surface electrocardiogram to determine appropriate sensing and pacing. Oversensing of signals not visible on the surface electrocardiogram can be diagnosed, and small P waves, which might be barely visible on a low-quality electrocardiogram, are
annotated. Even when the surface electrocardiogram seems to reveal normal pacemaker function, the annotations might reveal inappropriate behavior (Figure 1). One example is atrial undersensing in the presence of intrinsic atrioventricular conduction, which may look like correct inhibition of the device, but is annotated as a premature ventricular contraction. Atrial undersensing in a VDD system with pacing at the lower rate limit may be mistaken for atrioventricular synchronous stimulation, if the atrial and the programmed lower rate are nearly the same, and if the P waves precede the ventricular stimulus. Therefore, the detection of sensing problems is especially facilitated by marker annotations. In addition, pacing spikes can be identified, even when not clearly visible on the surface electrocardiogram. Marker annotations also help identify activation of algorithms such as rate smoothing or mode switching. However, the use of marker annotations requires a programmer and only reports information about the current function of the device. Exercise testing, which is often helpful for troubleshooting, becomes difficult if permanent telemetry has to be established. To overcome these limitations, new approaches have recently been introduced. They combine surface electrocardiogram in Holter recordings with continuous marker annotations, and have been performed to assess pacemaker function on a 24-hour basis. For this purpose, a miniaturized telemetry-receiving coil, which is connected to a miniaturized programmer (telemetry data logger), is attached to the patient’s skin over the pacemaker. The pacemaker is then enabled to transmit marker annotations continuously. The telemetry data logger converts the annotations to positive and negative analog voltage signals, which can be recorded on 1 channel of a conventional Holter recorder, together with the surface electrocardiogram.4 This approach was used successfully to analyze pacemaker function with continuous marker annotation (Figure 2). It facilitated detection of inappropriate pacemaker function (even in the presence of apparently normal electrocardiogram recordings), to help interpret poor-quality electrocardiograms and to annotate the onset of algorithms.4 Such diagnostic tools will become increasingly important, not only for troubleshooting, but also for the validation of new, complex algorithms.
EVENT COUNTERS The cumulative number of paced and sensed events is stored in event counters. In dual-chamber devices, the counters tally the absolute number or percentage of atrial and ventricular paced and sensed events. This function also provides the number of paced and sensed beats in each chamber, the number of premature ventricular contractions or nonsustained ventricular tachycardia, stimulation at the upper rate limit, or the number of activations of special algorithms such as mode switching. The counters allow clinicians to judge the pacemaker’s overall performance over the time period since the last interrogation or counter reset. As a prerequisite for interpreting the counters, however, one must determine the correct pacing and
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FIGURE 1. Surface electrocardiogram (ECG, top) with atrial (middle) and ventricular (bottom) marker channels. The surface ECG (artifacts below the curve) shows sinus rhythm, and there seems to be regular pacemaker function with complete inhibition of the VDD pacemaker. However, the marker annotations reveal undersensing of the P wave; only the R wave is sensed (S). This occurs at nearly the same time in the atrial and the ventricular channels. From this pattern, it is possible to diagnose a dislocation of the atrial dipole of the VDD lead into the right ventricle.
FIGURE 2. Holter recording with continuous marker annotation (top). In the first 2 cycles of the surface electrocardiogram (ECG), P waves trigger a ventricular stimulus. Thereafter, a premature ventricular contraction (PVC) seems to inhibit the pacemaker, followed by 2 other atrioventricular (AV) synchronous stimulations. However, the marker annotations reveal that the PVC was preceded by a P wave, which was sensed by the device. This triggered a ventricular stimulus that was delivered simultaneously with the PVC and is not visible on the ECG. Thus, 2 important pieces of information are provided by these marker annotations. First, the annotations revealed the appearance of the PVC just after P-wave and AV interval, which prevented its detection by the pacemaker. Second, the pacemaker’s stimulation was discernible only with the marker annotation. Marker spikes: As 5 atrial sense; PVARP 5 end of postventricular atrial refractory period; Vp 5 ventricular pace; VRP 5 end of ventricular refractory period.
sensing function and have knowledge of the underlying rhythm. If there is a marked shift in the pacing counter values, a change in the underlying rhythm or 174D THE AMERICAN JOURNAL OF CARDIOLOGYT
a pacemaker malfunction must be suspected. An example is a patient whose pacing indication was isolated high-degree atrioventricular block. In this case, predominant atrial sense/ventricular pace events are to be expected. If the counters reveal a significant percentage of atrial pace/ventricular pace events, it can point to the development of sinus node disease or atrial undersensing. Other explanations may be an aggressively programmed sensor, or a lower rate interval being programmed too high, which prevents atrial tracking at lower sinus rates. Such patterns of event counter data, combined with knowledge about the patient’s status, allow the identification of problems that can be addressed at the follow-up visit. Event counters are also useful to evaluate the effects of programming changes during long-term follow-up. Consider the example of a patient with a dual-chamber pacemaker and spontaneous atrioventricular conduction with frequent ventricular fusion beats. Increasing the atrioventricular interval or activating atrioventricular hysteresis should result in an increase of atrial events followed by ventricular sensed events and a decrease of ventricular paced events. The diagnostic value of event counters is improved if information about the rate distribution of the various pacing states is provided. This allows clinicians to assess the patient’s underlying rhythm and chronotropic function at various rates. For example, it is possible to judge the appropriateness of the programmed lower and upper pacing rate or of the rate response slope of a sensor. To facilitate interpretation, these data are presented not only in table format, with absolute counts or percentages, but also in the graphic form, which allows a quick review of the data. To avoid misinterpretation of diagnostic data, however, it is important to know the definition of each type of counter, which may vary between manufacturers. Limitations of event counters: Event counters can reflect only what the pacemaker has done; they do not take into account the clinical situation and possible inappropriate pacemaker behavior or programming. Therefore, special caution must be taken when analyzing and interpreting the counters. The counter is not able to distinguish whether a sensed event is due to a properly sensed cardiac event or due to oversensing of far-field signals, myopotentials, crosstalk, or electromagnetic interference.2 An example is a sinus tachycardia above the upper rate limit with spontaneous conduction to the ventricle. In this situation, consecutive P waves may occur inside the postventricular atrial refractory period and will not be sensed. Thus, a ventricular tachycardia is stored, or atrial flutter, with every second flutter wave falling inside the postventricular atrial refractory period, may be stored in the event counter as atrioventricular-synchronous stimulation. In addition, calculations based on these values may be misleading. An example is the calculation of atrioventricular synchrony, where the same counter may indicate different results in the same patient, depending on whether premature ventricular contractions are included in the calculation.5 Event counters merely store cumulative data. Identifying specific
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FIGURE 3. Time-based trend histograms for sensed amplitudes and lead impedance. The values on the right show the daily measurements for the last 7 days. The values on the left display a weekly average for the last 52 weeks. The ventricular lead (squares) shows stable values. The atrial lead (diamonds) documented a decrease in impedance and sensed amplitude, a result of an insulation failure.
events within the data and correlating them to the patient’s symptoms is almost impossible, especially if the counters have not been reset for some time.
TIME-BASED DIAGNOSTICS The first event counters that provided time-based data were used to document the patient’s heart rate.6 Now, many additional parameters are collected as time-based data. Because the amount of device memory is fixed, the time frame that can be stored in the device depends directly on the selected sampling rate. High sampling rates shorten the time frame, whereas low sampling rates prolong it. When low sampling rates are selected, an average value of sampled data is calculated and stored. To interpret these data, it is important to know the sampling rate that was used to store the data, because short events can be missed. The duration of time-based event recordings is programmable in most devices and allows clinicians to choose an appropriate resolution of data. Long-time recordings are used to monitor the patient’s heart rate for a period of time $24 hours, whereas short-time recordings are valuable to monitor a specific activity or to calibrate the pacemaker sensor. Advanced pacemakers even allow clinicians to store sensor data in addition to the patient’s intrinsic rate. These data can be used to calibrate a sensor without the need for repeated exercise tests, because programmable sensor parameters can be modified and the resulting change in rate response can be simulated. Time-based counters are initialized with the programmer collecting data for a chosen period of time. After this time has elapsed, the counter either will be frozen or, depending on the programming, will continue to store data, thereby overwriting the oldest records in a first-in–first-out fashion. Whereas the former is useful to perform planned testing, the latter is useful for routine follow-up or to retrieve retrospective information if a patient returns due to symptoms that occurred within
the storage period of the counter. Some of the latest devices also have the capability to activate the data collection at a predefined date and time. This allows a physician to monitor a specific patient activity at a certain moment in time. In addition to time-based rate and/or sensor data collection, some devices can record other parameters over time, such as lead impedance, sensed amplitude, or even pacing threshold. Lead impedance values are usually measured only once during follow-up, but this can be insufficient to detect a developing lead problem. In some cases, a lead problem may first manifest itself with marked impedance changes (.200 V or .50%), whereas the absolute value of the impedance measurement stays within the normal range. An intensified follow-up will therefore only be initiated if the previous values are available, which is not always the case. To detect such possibly intermittent and mostly silent lead problems, pacemakers are able to measure lead impedance automatically in regular time intervals. The results are displayed as a trend graph or table, allowing one to detect impedance variations early, before a complete lead failure occurs (Figure 3). However, impedance variations of up to 450 V can be measured in properly functioning leads, as demonstrated in a recently published study.7 The sensed amplitude and the pacing threshold give important additional information about lead status. Automatic measurement functions have been designed that determine the sensed amplitudes or the pacing threshold in regular time intervals and store the results in diagnostic counters, providing a timebased trend.8 Thanks to these new technologies, the pacing threshold of a lead can be closely monitored, helping to ensure patient safety. For example, threshold increases— especially those occurring intermittently due to a lead problem or as a result of extrinsic factors such as administration of antiarrhythmic drugs—are easily detected. The time stamp on the threshold trend also facilitates the correlation between
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FIGURE 4. P-wave amplitude histogram (left) in a VDD pacemaker with 98% of sensed P-wave amplitudes >0.6 mV. No events with amplitudes between 0.3 and 0.6 mV have been detected. Two percent of atrial events have been sensed with an amplitude of 0.1 and 0.2 mV, arousing suspicion of atrial sensing of ventricular far-field signals. This is confirmed by the findings of a VA-interval histogram (right) performed over a period of approximately 1 hour: 3% of atrial sensed events were detected <198 msec after the ventricular events, thus suggesting far-field sensing. Further confirmation was provided by marker annotations (not shown) demonstrating occasional atrial sensing of ventricular far-field signals (original printout in German). Anzahl Ereignisse 5 number of events; Atrium detektiert 5 atrium detected; BI 5 bipolar; programmierte Empfindlichkeit 5 programmed sensitivity; prozent 5 percentage; sampled P-Wellen Amplitude Histogram 5 sampled P-wave amplitude histogram; Sense Polarita¨t 5 sense polarity; Speicherperiode 5 storage period; Tage 5 days; V Ereignis 5 V event.
threshold variations and changes (e.g., metabolic changes) in the patient’s status. Based on the results of these threshold measurements, some devices automatically adjust their output close to the pacing threshold to prolong the battery longevity.8
SPECIAL DIAGNOSTIC HISTOGRAMS Today’s devices are able to measure sensing thresholds in both chambers automatically and at regular time intervals. Some pacemakers, which perform daily measurements, provide the results as a trend, as discussed above. However, sensed signals are known to vary frequently and 1 measurement per day gives only a rough orientation about general trends in sensing. Other devices measure the P-wave amplitude every 6 minutes and store this data in P-wave amplitude histograms. Such data allow a better assessment 176D THE AMERICAN JOURNAL OF CARDIOLOGYT
of signal fluctuations thanks to cumulative data, but they do not provide any information about the time course. These amplitude histograms have proved useful in optimizing the programmed sensitivity based on multiple measurements, rather than based on a single or a few measurements performed during 1 visit.9,10 In patients with paroxysmal atrial fibrillation, the histograms can be used to determine the amplitude of the fibrillation waves as sensed by the device.11 The Pwave amplitude histogram may also be very useful in detecting ventricular far-field sensing in the atrial channel. In this case, the histogram may reveal high amplitudes for the great majority of P waves, very low amplitudes for a few events, and no events of intermediate amplitudes. To confirm this finding, a clinician can use the intracardiac electrogram with marker
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annotations, or a VA-interval histogram, which stores the interval between a sensed or stimulated ventricular event and the consecutive atrial sensed event. In the case of atrial sensing of ventricular far-field signals, one would expect sensed events to occur 100 –200 msec after the ventricular event (Figure 4).
RESPONSE TO ATRIAL TACHYCARDIAS Pacemaker therapy to treat patients with paroxysmal atrial fibrillation or flutter has attracted widespread interest in the last several years. Pacing indications in these patients include symptomatic bradycardias, atrioventricular nodal ablation, or pace prevention of atrial arrhythmias.12–14 Until a few years ago, paroxysmal atrial arrhythmias were a contraindication for dual-chamber pacemakers, because the high atrial rates would trigger high-rate ventricular pacing. But with the development of reliable mode-switching algorithms, these patients are now often treated with dual-chamber rate-responsive pacing. Mode-switching algorithms provide the number of activations, date and time of activation, duration of the episodes, and the cumulative number of times the device was mode switched. These data are useful to diagnose the presence of tachyarrhythmias (if not previously known), and to offer information about the number and duration of arrhythmias, which define the course of the disease and the efficacy of therapeutic interventions. To optimize detection of low-amplitude tachyarrhythmias, the atrial sensing threshold has to be programmed to highly sensitive values. Atrioventricular intervals and atrial blanking periods are kept short to provide more time for tachyarrhythmia detection.15 However, this type of programming does not rule out intermittent undersensing of low-amplitude atrial fi-
brillation, and it also makes the devices susceptible to atrial oversensing of far-field QRS complexes or T waves. The counters tally the resulting false-positive or false-negative mode switches, so it is impossible to know if all of the mode changes were appropriate. Additionally, the programmed detection criteria of the mode-switching algorithm have recently been shown to be crucial for correct diagnostic function16; with standard programming (detection rate 160 beats/min, 40 detection beats, 10 termination beats) and with simultaneous Holter monitoring, sinus rhythm was classified as an atrial arrhythmia for 12 episodes in 4 of 30 patients (13%) with a duration of 12 of 720 hours (1.6%). These false-positive results were caused by atrial oversensing of R and T waves. On the other hand, 32 of 125 tachyarrhythmia episodes (25.6%) were not detected. This was mostly due to a short duration of the episodes, but atrial undersensing and blanking of every second atrial flutter wave was also observed. With optimized programming (detection rate 220 beats/min, 10 detection beats, 20 termination beats), sensitivity was increased from 74% to 98% and specificity from 87% to 100%.16 These results emphasize the importance of optimized programmed parameters to achieve appropriate mode switching, and the importance of obtaining meaningful diagnostic data that enable programming to be optimized. To enhance the accuracy of diagnostic data in conjunction with the tachycardia response of the devices, the heart rate before, during, and after a mode-switch episode can be stored in diagnostic counters. More detailed information can be retrieved if diagnostic algorithms trigger the storage of event marker chains from the atrial and ventricular channel, facilitating the diagnosis of atrioventricular association or dissociation.17 This infor-
FIGURE 5. Detailed onset report showing the events before the start of a short supraventricular run: the first 9 events show atrioventricular (AV) sequential stimulation. Beats 9 –14 are the first beats of the supraventricular run, which trigger 3 ventricular stimulations. Thereafter, the atrial rate remains above the tachycardia detection rate of 150 beats/min and initiates a mode switch. However, there is spontaneous conduction to the ventricle. After 10 more beats, the tachycardia is spontaneously terminated. (Courtesy Dr. T. Lewalter, Bonn; original printout in German.) AP 5 atrial pace; AS 5 atrial sense; Ereignis 5 event; Frequenz-Profil-Diagramm 5 rate profile diagram; VP 5 ventricular pace; VS 5 ventricular sense. A SYMPOSIUM: ELECTRICAL MANAGEMENT OF CARDIAC DISORDERS
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mation, however, displays the events as they are seen by the pacemaker and may not allow a complete interpretation and analysis of the mode-switching episodes, a situation vastly improved with the advent of stored electrograms (see below). Special interest is currently focused on pacing to prevent atrial tachyarrhythmias, either with specific pacing sites or specialized algorithms. This requires detailed knowledge of the tachyarrhythmia onset mechanisms. Toward this end, diagnostic counters have been developed that report not only date, time, duration, and end of detected episodes, but also provide additional information about events in the minutes preceding the tachyarrhythmia. These are marker diagrams with the sequence of atrial and ventricular events, rate profiles, and the number of premature atrial contractions before arrhythmia onset (Figure 5). With the expanded insight offered by such diagnostic counters, it might be possible to successfully apply specific therapies for preventing paroxysmal atrial tachyarrhythmias.18 Nevertheless, all these sophisticated diagnostic functions are susceptible to the collection of erroneous data as a result of over- or undersensing. Some of these sensing problems can be diagnosed by certain patterns of stored events, for instance, atrial sensed events occurring at a stable interval shortly after a ventricular event, thereby suggesting atrial sensing of ventricular far-field signals.
STORED ELECTROGRAMS Although pacemaker function is accurately reflected by data stored in diagnostic counters, all information included can be distorted by inappropriate device behavior. In implantable cardioverter defibrillators, the storage of intracardiac electrograms has been a milestone for detecting appropriate versus inappropriate device behavior and for troubleshooting.19 Stored intracardiac electrograms are now also available with the latest generations of pacemakers (Figure 6).20,21 With this tool, the clinician is, for the first time, able to perform a reliable retrospective evaluation of events that triggered algorithms. Dual-channel recordings, with a duration of #40 seconds for each channel, are possible. The number and duration of recorded electrograms is programmable from two 20-second episodes to twenty 2-second episodes. Electrogram storage is initiated when certain events are detected, such as atrial arrhythmias identified by the modeswitching algorithm, ventricular tachycardias that meet the programmed detection criteria, or nonsustained ventricular tachycardias defined as $3 consecutive premature ventricular contractions. A patienttriggered recording is also available, enabled when a magnet is held over the device. Clinical results with these stored intracardiac electrograms have recently been presented with an analysis of 192 stored electrograms in 73 patients20: the electrograms were triggered by mode-switching episodes in 28%, by ventricular tachycardias in 14%, and by nonsustained ventricular tachycardias in 58% of episodes. Beside the confirmation of the detected events, the stored electrograms have been shown to be extremely useful 178D THE AMERICAN JOURNAL OF CARDIOLOGYT
FIGURE 6. Stored electrogram (EGM) with atrial (A) and ventricular (V) channels, showing an atrial arrhythmia.
FIGURE 7. Stored electrogram (EGM) with atrial (A) and ventricular (V) channel. Due to hardware and software blanking, the atrial pacing spike (Ap) is visible only in the ventricular channel and vice versa (Vp 5 ventricular pacing spike). Short runs of premature ventricular complexes (PVC) can be seen. The fourth and fifth atrial events (*) with slightly differing morphologies may be P waves, retrograde P waves, or ectopic atrial beats. The variations in the pacing rate are due to activation of a ratesmoothing algorithm.
for detecting false-positive diagnostic data, such as atrial undersensing and oversensing (myopotentials and ventricular far-field signals) and noise sensing. Our personal experience with stored electrograms in single-lead VDD systems has demonstrated the particular value of this diagnostic tool for the examination of atrial sensing. For example, atrial undersensing combined with intrinsic atrioventricular conduction, led to stored electrograms of ventricular tachycardias, and atrial sensing of ventricular far-field signals triggered mode-switching episodes with electrogram storage. Reviewing the stored electrograms facilitated diagnosis of both sensing problems, and the programmed settings for atrial sensitivities and the atrial blanking period were optimized accordingly. Thereafter, the stored electrograms have been used to monitor the effectiveness of the changed parameters as none of the previously recorded problems recurred. All these results emphasize the value of stored electrograms for troubleshooting and for optimizing programmed parameters. The diagnosis and documentation of arrhythmias is facilitated. Furthermore, the stored electrograms are available whenever an episode occurs, without requiring further diagnostic tools. The reliability of conventional event counters, which do not provide such detailed information, nevertheless remains to be established. As with every new feature, a certain learning curve must be anticipated for the interpretation of electro-
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grams. The morphologies of the stored signals are significantly different from those seen on the surface electrocardiogram, which can complicate the classification of signals as intracardiac potentials, pacing spikes, or artifacts (Figure 7). Additionally, abrupt changes in intrinsic amplitudes or altered morphologies resulting from bundle-branch blocks may lead to potential misdiagnosis.19 In some cases, the stored electrograms did not confirm the arrhythmic events that should have been detected. For example, an event may be long enough to trigger the detection algorithm but too short to be stored as an electrogram, because the rhythm is stored only after the detection criteria have been fulfilled. Such nonconfirmed episodes will also include false-positives, in which the detection algorithms were triggered by short periods of over- or undersensing. Future devices, currently under development, include the recording of onset electrograms and of annotations, thus facilitating the interpretation of the tracings and elucidating nonconfirmed episodes. When implemented, this feature will be another important improvement in the diagnostic capabilities of pacemakers.
PATIENT-TRIGGERED DIAGNOSTICS During follow-up visits, pacemaker patients sometimes report a variety of symptoms, which can be pacemaker related, or due to underlying arrhythmias, cardiac disease, or extracardiac causes. In patients with infrequent and paroxysmal symptoms, the diagnostic evaluation can be challenging. Even if date- and time-stamped event counters are available, the correlation with the patient’s complaint is often poor. Particularly difficult is the diagnosis of symptoms that are independent from arrhythmias. Patient-triggered diagnostics have become available to help evaluate the heart rhythm during symptoms. With this function the patient, when experiencing symptoms, can trigger the recording of episodes with a magnet or a miniature programmer. The data stored in the devices are marker annotations from a time period before and after the activation. Stimulation parameters and electrograms can also be provided by some devices.2 In a recent study, results of such patient-triggered event recordings were compared with those from a digital loop event recorder. Clinically relevant information was obtained by patient-triggered event recordings in 97.7% of episodes, compared with 81.4% with the loop event recorder.22 Such patient-triggered diagnostics are a useful tool for patients with infrequent symptoms, thereby avoiding the need for repeated Holter recordings or external monitoring.
CONCLUSIONS Pacemaker diagnostic functions accurately reflect the activities of the device, allowing fine-tuning of advanced therapies and diagnostics. In many cases, the appropriateness of the device behavior can be assessed only with the help of real-time diagnostics. To take advantage of
diagnostic features, one must always take into account the possibility of inappropriate device function or programming, although certain patterns of diagnostic data may strongly suggest such a problem. The introduction of stored electrograms improves diagnostic capabilities and facilitates their validation. Patient-triggered diagnostics offer the potential to classify infrequent patient symptoms. With today’s complex pacemakers, sophisticated diagnostic features have become vital to achieve maximum patient benefit. 1. Jeffrey K, Parsonnet V. Cardiac pacing, 1960 –1985: a quarter century of
medical and industrial innovation. Circulation 1998;7:1978 –1991. 2. Waktare JEP, Malik M. Holter, loop recorder, and event counter capabilities of
implanted devices. PACE Pacing Clin Electrophysiol 1997;20:2658 –2669. 3. Schwaab B, Schwerdt H, Heisel A, Fro¨hlig G, Schieffer H. Chronic ventricular pacing using an output amplitude of 1.0 Volt. PACE Pacing Clin Electrophysiol 1997;20:2171–2178. 4. Nowak B, Middeldorf T, Housworth CM, Bru¨ls A, Liebrich A, Rosocha S, Voigtla¨nder T, Himmrich E, Meyer J. Holter recordings with continuous marker annotations: a new tool in pacemaker diagnostics. PACE Pacing Clin Electrophysiol 1996;19:1791–1795. 5. Israel CW, Bo¨ckenfo¨rde JB. Pacemaker event counters: possible sources of error in calculation of AV synchrony in VDD single lead systems as an example for present limitations. PACE Pacing Clin Electrophysiol 1998;21:489 – 493. 6. Begeman MJS, Boute W. Heart rate monitoring in implanted pacemakers. PACE Pacing Clin Electrophysiol 1988;11:1687–1692. 7. Danilovic D, Ohm OJ. Pacing impedance variability in tined steroid eluting leads. PACE Pacing Clin Electrophysiol 1998;21:1356 –1363. 8. Clarke M, Liu B, Schu¨ller H, Binner L, Kennergren C, Guerola M, Weinmann P, Ohm OJ. Automatic adjustment of pacemaker stimulation output correlated with continuously monitored capture thresholds: a multicenter study. PACE Pacing Clin Electrophysiol 1998;21:1567–1575. 9. Boute W, Albers BA, Giele V. Avoiding atrial undersensing by assessment of P-wave amplitude histogram data. PACE Pacing Clin Electrophysiol 1994;17: 1878 –1882. 10. Nowak B, Voigtla¨nder T, Rosocha S, Liebrich A, Wagner S, Geil S, Przibille O, Zellerhoff C, Himmrich E, Meyer J. How to programme the atrial sensing threshold in single-lead VDD pacing: sensing tests, safety margins or most sensitive value. Eur J Cardiac Pacing Electrophysiol 1997;7:168 –173. 11. Nowak B, Voigtla¨nder T, Rosocha S, Liebrich A, Zellerhoff C, Przibille O, Geil S, Himmrich E, Meyer J. Paroxysmal atrial fibrillation and high degree AV block: use of single-lead VDDR pacing with mode switching. PACE Pacing Clin Electrophysiol 1998;21:1927–1933. 12. Brignole M, Gianfranchi L, Menozzi C, Alboni P, Musso G, Bongiorni MG, Gasparini M, Raviele A, Lolli G, Paparella N, Aquarone S. Assessment of atrioventricular junction ablation and DDDR mode-switching pacemaker versus pharmacological treatment in patients with severely symptomatic paroxysmal atrial fibrillation: a randomized study. Circulation 1997;96:2617–2624. 13. Saksena S, Prakash A, Hill M, Krol RB, Munsif AN, Mathew PP, Mehra R. Prevention of recurrent atrial fibrillation with chronic dual-site right atrial pacing. J Am Coll Cardiol 1996;28:687– 694. 14. Allessie M. How might pacing prevent atrial fibrillation? (Abstr.) Arch Mal Coeur 1998;91:57. 15. Palma EC, Kedarnath V, Vankawalla V, Andrews CA, Hanson S, Furman S, Gross JN. Effect of varying atrial sensitivity, AV interval, and detection algorithm on automatic mode switching. PACE Pacing Clin Electrophysiol 1996;19:1734 –1739. 16. Seidl K, Meisel E, VanAgt E, Ottenhoff F, Hess M, Hauer B, Zahn R, Senges J. Is the atrial high rate episode diagnostic feature reliable in detecting paroxysmal episodes of atrial tachyarrhythmias? PACE Pacing Clin Electrophysiol 1998;21:694 –700. 17. Lascault G, Barnay C, Cazeau S, Frank R, Medvedowsky JL. Preliminary evaluation of a dual chamber pacemaker with bradycardia diagnostic functions. PACE Pacing Clin Electrophysiol 1995;18:1636 –1643. 18. Sopher SM, Waktare JEP, Camm AJ. Pacing algorithms to prevent atrial fibrillation. (Abstr.) Arch Mal Coeur 1998;91:58. 19. Sarter BH, Callans DJ, Gottlieb CD, Schwartzman DS, Marchlinski FE. Implantable defibrillator diagnostic storage capabilities: evolution, current status, and future utilization. PACE Pacing Clin Electrophysiol 1998;21:1287–1298. 20. Charles R, Rauscha F, Waucquez JL, Olbrich HG, Holt P, Hartung W. Storage of intracardiac electrograms in pacing systems: a new tool in arrhythmia diagnosis. (Abstr.) Arch Mal Coeur 1998;91:230. 21. Mabo P, Geroux L, Limousin M. Usefulness of stored EGM in DDDR device to assess clinical diagnosis. (Abstr.) Arch Mal Coeur 1998;91:230. 22. Machado C, Johnson D, Thacker JR, Duncan JL. Pacemaker patient-triggered event recording: accuracy, utility, and cost for the pacemaker follow-up clinic. PACE Pacing Clin Electrophysiol 1996;19:1813–1818.
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MD
The ever-increasing complexity of pacing systems, combined with functions that vary from one manufacturer to another, can pose challenges during analysis of device function. Standard pacemaker diagnostics are measured data, electrogram telemetry, marker annotations and event counters, albeit with their current limitations. New diagnostic features discussed include time-based diagnostics, histograms of sensed amplitudes, pacing thresholds, or impedance trending. Mode-switching algorithms, combined with diagnostic features, facilitate the use of dual-chamber devices in patients with parox-
ysmal atrial tachyarrhythmias. The introduction of electrogram storage into pacemakers further improves diagnostic capabilities and allows a permanent validation and optimization of diagnostic and therapeutic algorithms. External diagnostic devices, which provide Holter recordings with continuous marker annotations and patient-triggered diagnostics, are additional features that will become increasingly important. Q1999 by Excerpta Medica, Inc. Am J Cardiol 1999;83:172D–179D
acemaker therapy has evolved over the last 40 years from the first nonprogrammable, singleP chamber, asynchronous VOO devices to multipro-
provide information about battery and lead status and help to optimize the output programming to save energy. The measured data allow early detection of potential hardware problems before the patient becomes symptomatic or health is jeopardized. Patientrelated diagnostics such as intracardiac electrograms and marker annotations, help to identify the device’s response to the patient’s intrinsic rhythm and also provide troubleshooting assistance. Pacemaker diagnostics facilitate the monitoring of underlying arrhythmia or disease and of the therapeutic effects resulting from modified programmed settings or medications. Additionally, the function of sensors and pacemaker algorithms can be assessed and fine tuned.2 All these applications of pacemaker diagnostics are thereby helpful to ensure the patient’s safety and quality of life.
grammable, dual-chamber, rate-adaptive rate-responsive pacing devices. Still, to date, the development of single- and dual-chamber devices is far from complete.1 New indications and applications of special stimulation techniques are being evaluated, such as bi-atrial stimulation for prevention of paroxysmal atrial arrhythmias and multichamber pacing for patients with congestive heart failure. Advances in lead technology are aimed at developing floating leads, designed to stimulate the atrium, and at thinner leads with improved handling characteristics, to reach special pacing sites. New algorithms are another main emphasis of pacemaker development. Such algorithms are designed to diagnose and treat many kinds of arrhythmias and make the pacemaker function as physiologically as possible. In addition, modern devices are becoming more and more automated and are now able to adjust their programmed settings to sensed or measured parameters. This increasing sophistication and complexity of pacing can complicate the analysis of pacemaker function. Therefore, a variety of new pacemaker diagnostic functions has become available. A programmer can perform various measurements, reflect the actual behavior of the device during a visit, and obtain data on pacemaker function during the past days, weeks, or months. Usually, several diagnostic features, supplementing each other, are necessary to verify optimal device function.
USE OF PACEMAKER DIAGNOSTICS Modern pacemakers include 2 types of diagnostic function: system-related diagnostics and patient-related diagnostics. System diagnostics are obtained through device interrogation and measured data that From the II. Medical Clinic, University Mainz, Mainz, Germany. Address for reprints: Bernd Nowak, MD, II. Medical Clinic, University Mainz, D-55101 Mainz, Germany.
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©1999 by Excerpta Medica, Inc. All rights reserved.
SYSTEM DIAGNOSTICS System diagnostics provide information about the battery status and the lead impedance. Battery status information includes cell voltage, internal impedance, and current drain, which allow an estimation of device longevity. In addition, the effectiveness of changing output amplitudes and pulse widths can be assessed to optimize energy consumption without impairing patient safety. Based on these measured data, a new method for output optimization has been proposed that uses the determination of the minimum charge delivered per pulse at the pacing threshold to perform low output programming with steroid-eluting low threshold leads.3 This concept uses a 100% safety margin, in terms of charge measured by real-time telemetry, to achieve stimulation at an output amplitude of 1.0 V. Based on the system diagnostics, this method has the advantage of being less impacted by polarization effects compared with the widely used voltage threshold. Lead impedance is used to judge the integrity of the lead. Insulation defects result in decreased impedance values, whereas conductor fractures or loose connections translate into an increase in lead impedance. In some cases, when the lead failure is not yet 0002-9149/99/$20.00 PII S0002-9149(99)01021-2
complete, the defects and the corresponding variations in impedance may occur only intermittently. In these cases, a single impedance measurement during follow-up might be normal, even when the measurements are taken during provocative maneuvers and lead manipulation. In such cases of a suspected malfunction, it is helpful to have devices that provide beat-to-beat lead impedance values or that regularly store lead impedance measurements at given intervals. Intracardiac electrograms offer the unique opportunity to view the electrical activities of the heart with the eyes of the device, (which is blind to a 12-lead surface electrocardiogram and bases all signal interpretations on a single channel from each chamber). The electrogram is useful to identify signals sensed by the device but not visible on the surface electrocardiogram. Because the amplitude of the electrogram is directly proportional to the amplitude of the signal sensed by the device, frequent fluctuations of the intracardiac signals are, thus, easy to assess (e.g., the amplitudes during atrial fibrillation or P-wave amplitudes in single-lead VDD systems). The morphology of the electrogram also reveals important information concerning the type of signal and may help to detect retrograde P waves, to diagnose an underlying rhythm or arrhythmia, or to confirm atrial capture. The diagnostic value of the electrogram is improved if used in combination with marker annotations. However, the real-time electrograms restrict the diagnosis to patient’s activities that take place during follow-up, thus, limiting their use. Therefore, Holter recordings combined with marker annotations have been developed recently (see below). Other innovations have resulted in the capability to store electrograms for later retrieval. It must be kept in mind that in some devices, the amplifiers and filters used to record the electrogram may be different from those used for sensing. Thus, the signals on the electrogram might not be completely identical to the signals sensed by the pacemaker.
MARKER ANNOTATIONS The interpretation of pacemaker electrocardiograms, especially in dual-chamber devices, has become increasingly difficult with the use of sensors and/or single or multiple algorithms. This applies to 12-lead surface electrocardiograms, as well as to Holter recordings. Even with detailed knowledge of programmed parameters, algorithms, and timing cycles, it might be impossible to perform an unequivocal electrocardiographic interpretation. Marker annotations, which are transmitted from the pacemaker to the programmer, provide online information about paced and sensed events, refractory periods, sensed events inside these refractory periods that are ignored by the device, or the activation of algorithms and sensors. These annotations should be used with a simultaneously recorded surface electrocardiogram to determine appropriate sensing and pacing. Oversensing of signals not visible on the surface electrocardiogram can be diagnosed, and small P waves, which might be barely visible on a low-quality electrocardiogram, are
annotated. Even when the surface electrocardiogram seems to reveal normal pacemaker function, the annotations might reveal inappropriate behavior (Figure 1). One example is atrial undersensing in the presence of intrinsic atrioventricular conduction, which may look like correct inhibition of the device, but is annotated as a premature ventricular contraction. Atrial undersensing in a VDD system with pacing at the lower rate limit may be mistaken for atrioventricular synchronous stimulation, if the atrial and the programmed lower rate are nearly the same, and if the P waves precede the ventricular stimulus. Therefore, the detection of sensing problems is especially facilitated by marker annotations. In addition, pacing spikes can be identified, even when not clearly visible on the surface electrocardiogram. Marker annotations also help identify activation of algorithms such as rate smoothing or mode switching. However, the use of marker annotations requires a programmer and only reports information about the current function of the device. Exercise testing, which is often helpful for troubleshooting, becomes difficult if permanent telemetry has to be established. To overcome these limitations, new approaches have recently been introduced. They combine surface electrocardiogram in Holter recordings with continuous marker annotations, and have been performed to assess pacemaker function on a 24-hour basis. For this purpose, a miniaturized telemetry-receiving coil, which is connected to a miniaturized programmer (telemetry data logger), is attached to the patient’s skin over the pacemaker. The pacemaker is then enabled to transmit marker annotations continuously. The telemetry data logger converts the annotations to positive and negative analog voltage signals, which can be recorded on 1 channel of a conventional Holter recorder, together with the surface electrocardiogram.4 This approach was used successfully to analyze pacemaker function with continuous marker annotation (Figure 2). It facilitated detection of inappropriate pacemaker function (even in the presence of apparently normal electrocardiogram recordings), to help interpret poor-quality electrocardiograms and to annotate the onset of algorithms.4 Such diagnostic tools will become increasingly important, not only for troubleshooting, but also for the validation of new, complex algorithms.
EVENT COUNTERS The cumulative number of paced and sensed events is stored in event counters. In dual-chamber devices, the counters tally the absolute number or percentage of atrial and ventricular paced and sensed events. This function also provides the number of paced and sensed beats in each chamber, the number of premature ventricular contractions or nonsustained ventricular tachycardia, stimulation at the upper rate limit, or the number of activations of special algorithms such as mode switching. The counters allow clinicians to judge the pacemaker’s overall performance over the time period since the last interrogation or counter reset. As a prerequisite for interpreting the counters, however, one must determine the correct pacing and
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FIGURE 1. Surface electrocardiogram (ECG, top) with atrial (middle) and ventricular (bottom) marker channels. The surface ECG (artifacts below the curve) shows sinus rhythm, and there seems to be regular pacemaker function with complete inhibition of the VDD pacemaker. However, the marker annotations reveal undersensing of the P wave; only the R wave is sensed (S). This occurs at nearly the same time in the atrial and the ventricular channels. From this pattern, it is possible to diagnose a dislocation of the atrial dipole of the VDD lead into the right ventricle.
FIGURE 2. Holter recording with continuous marker annotation (top). In the first 2 cycles of the surface electrocardiogram (ECG), P waves trigger a ventricular stimulus. Thereafter, a premature ventricular contraction (PVC) seems to inhibit the pacemaker, followed by 2 other atrioventricular (AV) synchronous stimulations. However, the marker annotations reveal that the PVC was preceded by a P wave, which was sensed by the device. This triggered a ventricular stimulus that was delivered simultaneously with the PVC and is not visible on the ECG. Thus, 2 important pieces of information are provided by these marker annotations. First, the annotations revealed the appearance of the PVC just after P-wave and AV interval, which prevented its detection by the pacemaker. Second, the pacemaker’s stimulation was discernible only with the marker annotation. Marker spikes: As 5 atrial sense; PVARP 5 end of postventricular atrial refractory period; Vp 5 ventricular pace; VRP 5 end of ventricular refractory period.
sensing function and have knowledge of the underlying rhythm. If there is a marked shift in the pacing counter values, a change in the underlying rhythm or 174D THE AMERICAN JOURNAL OF CARDIOLOGYT
a pacemaker malfunction must be suspected. An example is a patient whose pacing indication was isolated high-degree atrioventricular block. In this case, predominant atrial sense/ventricular pace events are to be expected. If the counters reveal a significant percentage of atrial pace/ventricular pace events, it can point to the development of sinus node disease or atrial undersensing. Other explanations may be an aggressively programmed sensor, or a lower rate interval being programmed too high, which prevents atrial tracking at lower sinus rates. Such patterns of event counter data, combined with knowledge about the patient’s status, allow the identification of problems that can be addressed at the follow-up visit. Event counters are also useful to evaluate the effects of programming changes during long-term follow-up. Consider the example of a patient with a dual-chamber pacemaker and spontaneous atrioventricular conduction with frequent ventricular fusion beats. Increasing the atrioventricular interval or activating atrioventricular hysteresis should result in an increase of atrial events followed by ventricular sensed events and a decrease of ventricular paced events. The diagnostic value of event counters is improved if information about the rate distribution of the various pacing states is provided. This allows clinicians to assess the patient’s underlying rhythm and chronotropic function at various rates. For example, it is possible to judge the appropriateness of the programmed lower and upper pacing rate or of the rate response slope of a sensor. To facilitate interpretation, these data are presented not only in table format, with absolute counts or percentages, but also in the graphic form, which allows a quick review of the data. To avoid misinterpretation of diagnostic data, however, it is important to know the definition of each type of counter, which may vary between manufacturers. Limitations of event counters: Event counters can reflect only what the pacemaker has done; they do not take into account the clinical situation and possible inappropriate pacemaker behavior or programming. Therefore, special caution must be taken when analyzing and interpreting the counters. The counter is not able to distinguish whether a sensed event is due to a properly sensed cardiac event or due to oversensing of far-field signals, myopotentials, crosstalk, or electromagnetic interference.2 An example is a sinus tachycardia above the upper rate limit with spontaneous conduction to the ventricle. In this situation, consecutive P waves may occur inside the postventricular atrial refractory period and will not be sensed. Thus, a ventricular tachycardia is stored, or atrial flutter, with every second flutter wave falling inside the postventricular atrial refractory period, may be stored in the event counter as atrioventricular-synchronous stimulation. In addition, calculations based on these values may be misleading. An example is the calculation of atrioventricular synchrony, where the same counter may indicate different results in the same patient, depending on whether premature ventricular contractions are included in the calculation.5 Event counters merely store cumulative data. Identifying specific
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FIGURE 3. Time-based trend histograms for sensed amplitudes and lead impedance. The values on the right show the daily measurements for the last 7 days. The values on the left display a weekly average for the last 52 weeks. The ventricular lead (squares) shows stable values. The atrial lead (diamonds) documented a decrease in impedance and sensed amplitude, a result of an insulation failure.
events within the data and correlating them to the patient’s symptoms is almost impossible, especially if the counters have not been reset for some time.
TIME-BASED DIAGNOSTICS The first event counters that provided time-based data were used to document the patient’s heart rate.6 Now, many additional parameters are collected as time-based data. Because the amount of device memory is fixed, the time frame that can be stored in the device depends directly on the selected sampling rate. High sampling rates shorten the time frame, whereas low sampling rates prolong it. When low sampling rates are selected, an average value of sampled data is calculated and stored. To interpret these data, it is important to know the sampling rate that was used to store the data, because short events can be missed. The duration of time-based event recordings is programmable in most devices and allows clinicians to choose an appropriate resolution of data. Long-time recordings are used to monitor the patient’s heart rate for a period of time $24 hours, whereas short-time recordings are valuable to monitor a specific activity or to calibrate the pacemaker sensor. Advanced pacemakers even allow clinicians to store sensor data in addition to the patient’s intrinsic rate. These data can be used to calibrate a sensor without the need for repeated exercise tests, because programmable sensor parameters can be modified and the resulting change in rate response can be simulated. Time-based counters are initialized with the programmer collecting data for a chosen period of time. After this time has elapsed, the counter either will be frozen or, depending on the programming, will continue to store data, thereby overwriting the oldest records in a first-in–first-out fashion. Whereas the former is useful to perform planned testing, the latter is useful for routine follow-up or to retrieve retrospective information if a patient returns due to symptoms that occurred within
the storage period of the counter. Some of the latest devices also have the capability to activate the data collection at a predefined date and time. This allows a physician to monitor a specific patient activity at a certain moment in time. In addition to time-based rate and/or sensor data collection, some devices can record other parameters over time, such as lead impedance, sensed amplitude, or even pacing threshold. Lead impedance values are usually measured only once during follow-up, but this can be insufficient to detect a developing lead problem. In some cases, a lead problem may first manifest itself with marked impedance changes (.200 V or .50%), whereas the absolute value of the impedance measurement stays within the normal range. An intensified follow-up will therefore only be initiated if the previous values are available, which is not always the case. To detect such possibly intermittent and mostly silent lead problems, pacemakers are able to measure lead impedance automatically in regular time intervals. The results are displayed as a trend graph or table, allowing one to detect impedance variations early, before a complete lead failure occurs (Figure 3). However, impedance variations of up to 450 V can be measured in properly functioning leads, as demonstrated in a recently published study.7 The sensed amplitude and the pacing threshold give important additional information about lead status. Automatic measurement functions have been designed that determine the sensed amplitudes or the pacing threshold in regular time intervals and store the results in diagnostic counters, providing a timebased trend.8 Thanks to these new technologies, the pacing threshold of a lead can be closely monitored, helping to ensure patient safety. For example, threshold increases— especially those occurring intermittently due to a lead problem or as a result of extrinsic factors such as administration of antiarrhythmic drugs—are easily detected. The time stamp on the threshold trend also facilitates the correlation between
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FIGURE 4. P-wave amplitude histogram (left) in a VDD pacemaker with 98% of sensed P-wave amplitudes >0.6 mV. No events with amplitudes between 0.3 and 0.6 mV have been detected. Two percent of atrial events have been sensed with an amplitude of 0.1 and 0.2 mV, arousing suspicion of atrial sensing of ventricular far-field signals. This is confirmed by the findings of a VA-interval histogram (right) performed over a period of approximately 1 hour: 3% of atrial sensed events were detected <198 msec after the ventricular events, thus suggesting far-field sensing. Further confirmation was provided by marker annotations (not shown) demonstrating occasional atrial sensing of ventricular far-field signals (original printout in German). Anzahl Ereignisse 5 number of events; Atrium detektiert 5 atrium detected; BI 5 bipolar; programmierte Empfindlichkeit 5 programmed sensitivity; prozent 5 percentage; sampled P-Wellen Amplitude Histogram 5 sampled P-wave amplitude histogram; Sense Polarita¨t 5 sense polarity; Speicherperiode 5 storage period; Tage 5 days; V Ereignis 5 V event.
threshold variations and changes (e.g., metabolic changes) in the patient’s status. Based on the results of these threshold measurements, some devices automatically adjust their output close to the pacing threshold to prolong the battery longevity.8
SPECIAL DIAGNOSTIC HISTOGRAMS Today’s devices are able to measure sensing thresholds in both chambers automatically and at regular time intervals. Some pacemakers, which perform daily measurements, provide the results as a trend, as discussed above. However, sensed signals are known to vary frequently and 1 measurement per day gives only a rough orientation about general trends in sensing. Other devices measure the P-wave amplitude every 6 minutes and store this data in P-wave amplitude histograms. Such data allow a better assessment 176D THE AMERICAN JOURNAL OF CARDIOLOGYT
of signal fluctuations thanks to cumulative data, but they do not provide any information about the time course. These amplitude histograms have proved useful in optimizing the programmed sensitivity based on multiple measurements, rather than based on a single or a few measurements performed during 1 visit.9,10 In patients with paroxysmal atrial fibrillation, the histograms can be used to determine the amplitude of the fibrillation waves as sensed by the device.11 The Pwave amplitude histogram may also be very useful in detecting ventricular far-field sensing in the atrial channel. In this case, the histogram may reveal high amplitudes for the great majority of P waves, very low amplitudes for a few events, and no events of intermediate amplitudes. To confirm this finding, a clinician can use the intracardiac electrogram with marker
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annotations, or a VA-interval histogram, which stores the interval between a sensed or stimulated ventricular event and the consecutive atrial sensed event. In the case of atrial sensing of ventricular far-field signals, one would expect sensed events to occur 100 –200 msec after the ventricular event (Figure 4).
RESPONSE TO ATRIAL TACHYCARDIAS Pacemaker therapy to treat patients with paroxysmal atrial fibrillation or flutter has attracted widespread interest in the last several years. Pacing indications in these patients include symptomatic bradycardias, atrioventricular nodal ablation, or pace prevention of atrial arrhythmias.12–14 Until a few years ago, paroxysmal atrial arrhythmias were a contraindication for dual-chamber pacemakers, because the high atrial rates would trigger high-rate ventricular pacing. But with the development of reliable mode-switching algorithms, these patients are now often treated with dual-chamber rate-responsive pacing. Mode-switching algorithms provide the number of activations, date and time of activation, duration of the episodes, and the cumulative number of times the device was mode switched. These data are useful to diagnose the presence of tachyarrhythmias (if not previously known), and to offer information about the number and duration of arrhythmias, which define the course of the disease and the efficacy of therapeutic interventions. To optimize detection of low-amplitude tachyarrhythmias, the atrial sensing threshold has to be programmed to highly sensitive values. Atrioventricular intervals and atrial blanking periods are kept short to provide more time for tachyarrhythmia detection.15 However, this type of programming does not rule out intermittent undersensing of low-amplitude atrial fi-
brillation, and it also makes the devices susceptible to atrial oversensing of far-field QRS complexes or T waves. The counters tally the resulting false-positive or false-negative mode switches, so it is impossible to know if all of the mode changes were appropriate. Additionally, the programmed detection criteria of the mode-switching algorithm have recently been shown to be crucial for correct diagnostic function16; with standard programming (detection rate 160 beats/min, 40 detection beats, 10 termination beats) and with simultaneous Holter monitoring, sinus rhythm was classified as an atrial arrhythmia for 12 episodes in 4 of 30 patients (13%) with a duration of 12 of 720 hours (1.6%). These false-positive results were caused by atrial oversensing of R and T waves. On the other hand, 32 of 125 tachyarrhythmia episodes (25.6%) were not detected. This was mostly due to a short duration of the episodes, but atrial undersensing and blanking of every second atrial flutter wave was also observed. With optimized programming (detection rate 220 beats/min, 10 detection beats, 20 termination beats), sensitivity was increased from 74% to 98% and specificity from 87% to 100%.16 These results emphasize the importance of optimized programmed parameters to achieve appropriate mode switching, and the importance of obtaining meaningful diagnostic data that enable programming to be optimized. To enhance the accuracy of diagnostic data in conjunction with the tachycardia response of the devices, the heart rate before, during, and after a mode-switch episode can be stored in diagnostic counters. More detailed information can be retrieved if diagnostic algorithms trigger the storage of event marker chains from the atrial and ventricular channel, facilitating the diagnosis of atrioventricular association or dissociation.17 This infor-
FIGURE 5. Detailed onset report showing the events before the start of a short supraventricular run: the first 9 events show atrioventricular (AV) sequential stimulation. Beats 9 –14 are the first beats of the supraventricular run, which trigger 3 ventricular stimulations. Thereafter, the atrial rate remains above the tachycardia detection rate of 150 beats/min and initiates a mode switch. However, there is spontaneous conduction to the ventricle. After 10 more beats, the tachycardia is spontaneously terminated. (Courtesy Dr. T. Lewalter, Bonn; original printout in German.) AP 5 atrial pace; AS 5 atrial sense; Ereignis 5 event; Frequenz-Profil-Diagramm 5 rate profile diagram; VP 5 ventricular pace; VS 5 ventricular sense. A SYMPOSIUM: ELECTRICAL MANAGEMENT OF CARDIAC DISORDERS
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mation, however, displays the events as they are seen by the pacemaker and may not allow a complete interpretation and analysis of the mode-switching episodes, a situation vastly improved with the advent of stored electrograms (see below). Special interest is currently focused on pacing to prevent atrial tachyarrhythmias, either with specific pacing sites or specialized algorithms. This requires detailed knowledge of the tachyarrhythmia onset mechanisms. Toward this end, diagnostic counters have been developed that report not only date, time, duration, and end of detected episodes, but also provide additional information about events in the minutes preceding the tachyarrhythmia. These are marker diagrams with the sequence of atrial and ventricular events, rate profiles, and the number of premature atrial contractions before arrhythmia onset (Figure 5). With the expanded insight offered by such diagnostic counters, it might be possible to successfully apply specific therapies for preventing paroxysmal atrial tachyarrhythmias.18 Nevertheless, all these sophisticated diagnostic functions are susceptible to the collection of erroneous data as a result of over- or undersensing. Some of these sensing problems can be diagnosed by certain patterns of stored events, for instance, atrial sensed events occurring at a stable interval shortly after a ventricular event, thereby suggesting atrial sensing of ventricular far-field signals.
STORED ELECTROGRAMS Although pacemaker function is accurately reflected by data stored in diagnostic counters, all information included can be distorted by inappropriate device behavior. In implantable cardioverter defibrillators, the storage of intracardiac electrograms has been a milestone for detecting appropriate versus inappropriate device behavior and for troubleshooting.19 Stored intracardiac electrograms are now also available with the latest generations of pacemakers (Figure 6).20,21 With this tool, the clinician is, for the first time, able to perform a reliable retrospective evaluation of events that triggered algorithms. Dual-channel recordings, with a duration of #40 seconds for each channel, are possible. The number and duration of recorded electrograms is programmable from two 20-second episodes to twenty 2-second episodes. Electrogram storage is initiated when certain events are detected, such as atrial arrhythmias identified by the modeswitching algorithm, ventricular tachycardias that meet the programmed detection criteria, or nonsustained ventricular tachycardias defined as $3 consecutive premature ventricular contractions. A patienttriggered recording is also available, enabled when a magnet is held over the device. Clinical results with these stored intracardiac electrograms have recently been presented with an analysis of 192 stored electrograms in 73 patients20: the electrograms were triggered by mode-switching episodes in 28%, by ventricular tachycardias in 14%, and by nonsustained ventricular tachycardias in 58% of episodes. Beside the confirmation of the detected events, the stored electrograms have been shown to be extremely useful 178D THE AMERICAN JOURNAL OF CARDIOLOGYT
FIGURE 6. Stored electrogram (EGM) with atrial (A) and ventricular (V) channels, showing an atrial arrhythmia.
FIGURE 7. Stored electrogram (EGM) with atrial (A) and ventricular (V) channel. Due to hardware and software blanking, the atrial pacing spike (Ap) is visible only in the ventricular channel and vice versa (Vp 5 ventricular pacing spike). Short runs of premature ventricular complexes (PVC) can be seen. The fourth and fifth atrial events (*) with slightly differing morphologies may be P waves, retrograde P waves, or ectopic atrial beats. The variations in the pacing rate are due to activation of a ratesmoothing algorithm.
for detecting false-positive diagnostic data, such as atrial undersensing and oversensing (myopotentials and ventricular far-field signals) and noise sensing. Our personal experience with stored electrograms in single-lead VDD systems has demonstrated the particular value of this diagnostic tool for the examination of atrial sensing. For example, atrial undersensing combined with intrinsic atrioventricular conduction, led to stored electrograms of ventricular tachycardias, and atrial sensing of ventricular far-field signals triggered mode-switching episodes with electrogram storage. Reviewing the stored electrograms facilitated diagnosis of both sensing problems, and the programmed settings for atrial sensitivities and the atrial blanking period were optimized accordingly. Thereafter, the stored electrograms have been used to monitor the effectiveness of the changed parameters as none of the previously recorded problems recurred. All these results emphasize the value of stored electrograms for troubleshooting and for optimizing programmed parameters. The diagnosis and documentation of arrhythmias is facilitated. Furthermore, the stored electrograms are available whenever an episode occurs, without requiring further diagnostic tools. The reliability of conventional event counters, which do not provide such detailed information, nevertheless remains to be established. As with every new feature, a certain learning curve must be anticipated for the interpretation of electro-
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grams. The morphologies of the stored signals are significantly different from those seen on the surface electrocardiogram, which can complicate the classification of signals as intracardiac potentials, pacing spikes, or artifacts (Figure 7). Additionally, abrupt changes in intrinsic amplitudes or altered morphologies resulting from bundle-branch blocks may lead to potential misdiagnosis.19 In some cases, the stored electrograms did not confirm the arrhythmic events that should have been detected. For example, an event may be long enough to trigger the detection algorithm but too short to be stored as an electrogram, because the rhythm is stored only after the detection criteria have been fulfilled. Such nonconfirmed episodes will also include false-positives, in which the detection algorithms were triggered by short periods of over- or undersensing. Future devices, currently under development, include the recording of onset electrograms and of annotations, thus facilitating the interpretation of the tracings and elucidating nonconfirmed episodes. When implemented, this feature will be another important improvement in the diagnostic capabilities of pacemakers.
PATIENT-TRIGGERED DIAGNOSTICS During follow-up visits, pacemaker patients sometimes report a variety of symptoms, which can be pacemaker related, or due to underlying arrhythmias, cardiac disease, or extracardiac causes. In patients with infrequent and paroxysmal symptoms, the diagnostic evaluation can be challenging. Even if date- and time-stamped event counters are available, the correlation with the patient’s complaint is often poor. Particularly difficult is the diagnosis of symptoms that are independent from arrhythmias. Patient-triggered diagnostics have become available to help evaluate the heart rhythm during symptoms. With this function the patient, when experiencing symptoms, can trigger the recording of episodes with a magnet or a miniature programmer. The data stored in the devices are marker annotations from a time period before and after the activation. Stimulation parameters and electrograms can also be provided by some devices.2 In a recent study, results of such patient-triggered event recordings were compared with those from a digital loop event recorder. Clinically relevant information was obtained by patient-triggered event recordings in 97.7% of episodes, compared with 81.4% with the loop event recorder.22 Such patient-triggered diagnostics are a useful tool for patients with infrequent symptoms, thereby avoiding the need for repeated Holter recordings or external monitoring.
CONCLUSIONS Pacemaker diagnostic functions accurately reflect the activities of the device, allowing fine-tuning of advanced therapies and diagnostics. In many cases, the appropriateness of the device behavior can be assessed only with the help of real-time diagnostics. To take advantage of
diagnostic features, one must always take into account the possibility of inappropriate device function or programming, although certain patterns of diagnostic data may strongly suggest such a problem. The introduction of stored electrograms improves diagnostic capabilities and facilitates their validation. Patient-triggered diagnostics offer the potential to classify infrequent patient symptoms. With today’s complex pacemakers, sophisticated diagnostic features have become vital to achieve maximum patient benefit. 1. Jeffrey K, Parsonnet V. Cardiac pacing, 1960 –1985: a quarter century of
medical and industrial innovation. Circulation 1998;7:1978 –1991. 2. Waktare JEP, Malik M. Holter, loop recorder, and event counter capabilities of
implanted devices. PACE Pacing Clin Electrophysiol 1997;20:2658 –2669. 3. Schwaab B, Schwerdt H, Heisel A, Fro¨hlig G, Schieffer H. Chronic ventricular pacing using an output amplitude of 1.0 Volt. PACE Pacing Clin Electrophysiol 1997;20:2171–2178. 4. Nowak B, Middeldorf T, Housworth CM, Bru¨ls A, Liebrich A, Rosocha S, Voigtla¨nder T, Himmrich E, Meyer J. Holter recordings with continuous marker annotations: a new tool in pacemaker diagnostics. PACE Pacing Clin Electrophysiol 1996;19:1791–1795. 5. Israel CW, Bo¨ckenfo¨rde JB. Pacemaker event counters: possible sources of error in calculation of AV synchrony in VDD single lead systems as an example for present limitations. PACE Pacing Clin Electrophysiol 1998;21:489 – 493. 6. Begeman MJS, Boute W. Heart rate monitoring in implanted pacemakers. PACE Pacing Clin Electrophysiol 1988;11:1687–1692. 7. Danilovic D, Ohm OJ. Pacing impedance variability in tined steroid eluting leads. PACE Pacing Clin Electrophysiol 1998;21:1356 –1363. 8. Clarke M, Liu B, Schu¨ller H, Binner L, Kennergren C, Guerola M, Weinmann P, Ohm OJ. Automatic adjustment of pacemaker stimulation output correlated with continuously monitored capture thresholds: a multicenter study. PACE Pacing Clin Electrophysiol 1998;21:1567–1575. 9. Boute W, Albers BA, Giele V. Avoiding atrial undersensing by assessment of P-wave amplitude histogram data. PACE Pacing Clin Electrophysiol 1994;17: 1878 –1882. 10. Nowak B, Voigtla¨nder T, Rosocha S, Liebrich A, Wagner S, Geil S, Przibille O, Zellerhoff C, Himmrich E, Meyer J. How to programme the atrial sensing threshold in single-lead VDD pacing: sensing tests, safety margins or most sensitive value. Eur J Cardiac Pacing Electrophysiol 1997;7:168 –173. 11. Nowak B, Voigtla¨nder T, Rosocha S, Liebrich A, Zellerhoff C, Przibille O, Geil S, Himmrich E, Meyer J. Paroxysmal atrial fibrillation and high degree AV block: use of single-lead VDDR pacing with mode switching. PACE Pacing Clin Electrophysiol 1998;21:1927–1933. 12. Brignole M, Gianfranchi L, Menozzi C, Alboni P, Musso G, Bongiorni MG, Gasparini M, Raviele A, Lolli G, Paparella N, Aquarone S. Assessment of atrioventricular junction ablation and DDDR mode-switching pacemaker versus pharmacological treatment in patients with severely symptomatic paroxysmal atrial fibrillation: a randomized study. Circulation 1997;96:2617–2624. 13. Saksena S, Prakash A, Hill M, Krol RB, Munsif AN, Mathew PP, Mehra R. Prevention of recurrent atrial fibrillation with chronic dual-site right atrial pacing. J Am Coll Cardiol 1996;28:687– 694. 14. Allessie M. How might pacing prevent atrial fibrillation? (Abstr.) Arch Mal Coeur 1998;91:57. 15. Palma EC, Kedarnath V, Vankawalla V, Andrews CA, Hanson S, Furman S, Gross JN. Effect of varying atrial sensitivity, AV interval, and detection algorithm on automatic mode switching. PACE Pacing Clin Electrophysiol 1996;19:1734 –1739. 16. Seidl K, Meisel E, VanAgt E, Ottenhoff F, Hess M, Hauer B, Zahn R, Senges J. Is the atrial high rate episode diagnostic feature reliable in detecting paroxysmal episodes of atrial tachyarrhythmias? PACE Pacing Clin Electrophysiol 1998;21:694 –700. 17. Lascault G, Barnay C, Cazeau S, Frank R, Medvedowsky JL. Preliminary evaluation of a dual chamber pacemaker with bradycardia diagnostic functions. PACE Pacing Clin Electrophysiol 1995;18:1636 –1643. 18. Sopher SM, Waktare JEP, Camm AJ. Pacing algorithms to prevent atrial fibrillation. (Abstr.) Arch Mal Coeur 1998;91:58. 19. Sarter BH, Callans DJ, Gottlieb CD, Schwartzman DS, Marchlinski FE. Implantable defibrillator diagnostic storage capabilities: evolution, current status, and future utilization. PACE Pacing Clin Electrophysiol 1998;21:1287–1298. 20. Charles R, Rauscha F, Waucquez JL, Olbrich HG, Holt P, Hartung W. Storage of intracardiac electrograms in pacing systems: a new tool in arrhythmia diagnosis. (Abstr.) Arch Mal Coeur 1998;91:230. 21. Mabo P, Geroux L, Limousin M. Usefulness of stored EGM in DDDR device to assess clinical diagnosis. (Abstr.) Arch Mal Coeur 1998;91:230. 22. Machado C, Johnson D, Thacker JR, Duncan JL. Pacemaker patient-triggered event recording: accuracy, utility, and cost for the pacemaker follow-up clinic. PACE Pacing Clin Electrophysiol 1996;19:1813–1818.
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