Battery Status

Although not all systems provide the information in the same manner (some are graphic and others are numeric), data concerning battery function often include a measure of battery voltage, impedance, and current drain.^7–11 All three parameters are interrelated. The battery current drain is a measure of the average current being drawn from the battery, assuming 100% pacing at the programmed rate and output settings. Inhibition or pacing at higher rates, as might occur with intrinsic rhythm or sensor drive, respectively, directly affects the functional battery current drain, as do changes in lead or stimulation impedance. The reported current drain is only an estimate of the actual battery current drain. Newer devices are able to estimate remaining longevity based on true current drain measurements, giving a much more accurate assessment (see Fig. 29-1, C).

The measured current drain of the battery at a known set of parameters can be used to qualitatively assess the effect of programming changes on the longevity of the system. After the rate or output (or both) has been changed, the measured data can be reassessed and the effect of the programming change on longevity estimated. A simple calculation may be used to obtain an estimate of battery longevity when the pacemaker is relatively new. Most manufacturers provide the usable battery capacity reported in ampere-hours (A-hr). For this calculation, the battery’s capacity is converted to microampere-hours (µA-hr) and then divided by the measured battery current drain. The result is the anticipated number of hours of normal function at the programmed rate and output. This number, divided by 8760 (the number of hours in 1 year), yields an estimate of the longevity of the system in years. This calculation is best performed with a new system, before there has been significant battery depletion. Many newer pacemakers perform these calculations and display the remaining longevity in years or months, providing a relatively good estimate of remaining longevity.

Virtually every pacemaker incorporates a change in either the magnet or the demand rate (and sometimes in mode or other functions) to signal a level of battery depletion that warrants pulse generator replacement. Most of these changes occur when the battery voltage reaches a predefined level, which is specific for each manufacturer and even for each model of pacemaker. These changes are termed elective replacement time (ERT), recommended replacement time (RRT), or elective replacement indicator (ERI), with further changes in rate or function associated with continued battery depletion labeled either end of life (EOL) or end of service (EOS). Although these changes may occur abruptly, usually a 3- to 6-month period exists between activation of the ERT indicator and erratic behavior or device nonfunction caused by continued battery depletion. A marker to determine when to increase the frequency of pacing system surveillance (either transtelephonically or in the office) is included in some systems and is typically a magnet rate intermediate between a fully functional battery and the ERT indicator. Alternatively, this indication may be given to the clinician during interrogation of the device by a message on the programmer screen or on the programmer report. This is termed an intensified follow-up indicator and provides an indication of changing battery status before the ERT is reached.

Given the degree of programmability of most current pacemakers, a dual-chamber pacemaker programmed to pace all the time at maximal output on each channel may last only 2 years. An identical model programmed to outputs of 2.5 V or lower on each channel that are inhibited a significant portion of the time may be anticipated to function properly for 10 years or longer. It would be inappropriate to follow the second unit on a monthly basis beginning 1 year after implantation, whereas it would be equally inappropriate to plan only annual checks on a pacing system programmed with high outputs and 100% pacing. By following some measure of the status of the battery, either directly or indirectly, the clinician can achieve a qualitative assessment about when to increase the frequency of pacing system follow-up.¹²

There are two primary indicators of battery status: battery voltage and battery impedance. Battery voltage progressively decreases over time. The lithium/iodine (Li/I₂) power cell is the most common power source in pacemakers, whereas the power source in an ICD is lithium/silver vanadium oxide (Li/VSO). Some of the newer pacemakers are now using Li/VSO–polycarbon monofluoride (LVSO/P). The nominal unloaded output voltage of the Li/I₂ cell is 2.8 V, with each manufacturer triggering the RRT indicator at a specified battery voltage based on circuit design considerations. LSVO/P in pacemaker application has an initial voltage of 3.20, with an early drop to 2.90 V, where it plateaus, then drops to an elective replacement voltage of 2.60 V. The battery voltage and programmed output voltage are not identical. The “programmed output voltage” is achieved by modifying the 2.8-V battery output with either voltage multipliers or charge pumps to provide a higher voltage. The battery voltage can also be divided to deliver a voltage lower than 2.8 V. As energy is consumed, the battery voltage progressively decreases because of increased internal impedance. Even with decreasing battery voltage, the device circuitry is designed to maintain the stability of the programmed voltage delivered to the patient. The ability to track the progressive decrease in battery voltage and increase in battery impedance provides an effective guide to the clinician regarding the frequency with which the pacing system should be routinely checked for signs of battery depletion (Fig. 29-2). In addition, the measured battery voltage is affected by the battery current drain: the higher the current drain, the lower the battery voltage.

Figure 29-2 Pacemaker magnet rate, battery voltage, and battery impedance.

Graph over time demonstrates the inverse relationship between battery voltage and battery impedance. For this particular St. Jude Medical pacemaker, the magnet rate drops proportionally to the battery voltage. kOhm, killiohms; M, months; ppm, Pulses per minute; V, volts;

Battery impedance is inversely related to battery voltage. In the Li/I₂ power cell, the release of electrons is generated by the combination of the lithium ion with two iodine atoms. The result of this chemical reaction is the release of two electrons and the formation of lithium iodide. As it accumulates, the lithium iodide forms a resistive barrier between the anode and the cathode. With progressive cell depletion, the increasing amount of lithium iodide results in a rise in the internal impedance of the battery, which some pacemakers can measure and report in the telemetry data. The higher the battery impedance, the lower is the cell voltage.

Battery voltage and impedance may be tracked together or individually. If these measurements are provided to the manufacturer’s technical service engineers, along with the programmed parameters and an estimate of the percentage of pacing versus inhibition, the remaining longevity of that pacing system can be estimated with reasonable accuracy, assuming that the various parameters remain stable. These calculations have more recently been incorporated into the programmer interface to provide an online estimate of longevity in years or months. These estimates are sometimes shown as a range based on the tolerances of the battery measurements (i.e., giving minimum and maximum ranges of longevity), as well as an estimate based on present current drain and actual device use based on data acquired from the event counter diagnostics (see Fig. 29-1, B).

Lead Status

Although many systems provide data on lead function, including pulse voltage, current, charge, and energy, the most common measurement is lead impedance (stimulation resistance). Changes in lead impedance directly affect the other measures of lead function. The terms resistance and impedance, although technically different, are often used interchangeably by the clinical community. Impedance is a complex concept that reflects a changing environment involving a variety of factors. This results in fluctuations in the moment-to-moment resistance. The resistance to electron flow in a pacing system progressively rises during delivery of the stimulation pulse as a result of polarization at the electrode-myocardium interface and, as a continuously changing variable, it is appropriately termed impedance. Many pulse generators and pacing system analyzers make this measurement at a specific point within the pacing stimulus; this is a resistance. Other devices report the resistance as an average of the impedance throughout the pulse. The actual resistance to current flow imparted by the conductor coil is fixed and represents a small portion of the total stimulation resistance. The polarization at the electrode-myocardium interface, partly related to the surface area and geometry of the electrode, and the impedance associated with conduction of the pulse through the body’s tissues play a larger role in the overall resistance of the system. All these factors are incorporated in the single measurement termed lead impedance or, more accurately, stimulation impedance.

Stimulation impedance is affected by many factors, but primarily by the design of the electrode, including size, configuration, and materials. Manufacturers have designed some electrodes with high impedance values. For any given output, a high-impedance system reduces the overall current drain of the battery and effectively increases the longevity of the device. Other leads have been designed specifically for low polarization to allow for detection of capture with each pace stimulus. Polarization and impedance are not the same phenomenon, although one affects the other. For any given lead model, there may be a broad range of normal impedance values; for a specific lead within that model series, however, the impedance should fall within a relatively narrow range.

The clinician can use knowledge of lead impedance to monitor and identify a developing mechanical problem with the lead. This requires baseline and historical data to recognize subtle changes that may reflect conductor fracture or an insulation breach. It is essential to know what device is being used to make these measurements. As noted previously, different devices may obtain these data at different points of the pacing stimulus. Because of these differences, the impedance measurement obtained with a pacing system analyzer at implantation may be significantly different from that obtained by telemetry from the implanted pacemaker moments, if not years, later. This difference does not necessarily imply a problem. Furthermore, impedance may naturally evolve over time, with a fall in impedance occurring during the days to weeks after implantation, followed by a gradual rise toward the initial measurements. Multiple factors may affect impedance, particularly in a unipolar system. For example, measurements obtained during deep inspiration may significantly differ from those obtained during maximal exhalation. In the same patient, impedance measurements based on a single-output pulse have been reported to vary by 100 ohms (Ω), or even more, during the same follow-up evaluation, while remaining consistent with normal function. If a marked change in lead impedance from previous measurements (e.g., >200 Ω) is encountered during a routine follow-up evaluation, the pacing system should be further evaluated.^13,14 If the patient has no clinical symptoms and has stable capture and sensing thresholds, surgical intervention would be premature, although more frequent follow-up in the office or by remote interrogation would be prudent. However, a dramatic change in telemetered lead impedance in the presence of a clinical problem directs the physician toward the likely source of the difficulty (Fig. 29-3).

Figure 29-3 Measured data telemetry.

Rate-modulated dual-chamber pacemaker data report a low impedance on the atrial lead of 200 to 220 ohms (Ω) (Boston Scientific 1297). Although a unipolar lead, this impedance is very low and may be the result of an insulation failure. This also produces an increase in the battery or cell current, caused by a higher-output current, charge, and energy being delivered by the pacemaker (compare energy to that of the ventricular lead), whether or not it reaches the heart.

A dramatic fall in impedance usually reflects a break in the insulation.^15–17 In a unipolar system, an insulation problem provides an alternative pathway for current flow starting closer to the pulse generator, resulting in less energy reaching the heart. An outer insulation break in a bipolar lead tends to result in “unipolarization” of the lead. The amplitude of the stimulus artifact as recorded by an analog electrocardiogram (ECG) is determined by the distance the current travels in the tissues from the cathode (tip electrode) to the anode (ring electrode or housing of the pulse generator). A normally functioning bipolar pacing system in which both active electrodes are inside the heart, separated only 1 to 2 cm, results in a small stimulus artifact. In a unipolar system, the current travels from the tip electrode to the housing of the pulse generator, and therefore the stimulus artifact is large despite equivalent output settings. Some of the newer digital ECG designs result in a marked signal-to-signal variation in amplitude as a result of the digital sampling rate of the recording system. Other recording systems create an artificial uniform amplitude artifact with any high-frequency electrical transient, thereby precluding differentiation of a bipolar from a unipolar pacing system based on the analysis of the ECG recording.

In a previously stable system, a mechanical problem developing with the lead—either a breach in the insulation or a conductor fracture—results in a change in the stimulation impedance, which may be reflected by a change in the ECG-recorded stimulus artifact (Fig. 29-4). In a bipolar pacing system, an insulation defect between the proximal conductor and the tissues of the body is not likely to affect capture thresholds, but it results in a larger stimulus artifact, making it appear unipolar. Depending on the actual location of the insulation failure in either the bipolar or the unipolar lead, stimulation of the extracardiac muscle contiguous to the insulation defect may occur. Insulation failures may also attenuate the electrical signal reaching the pacemaker, possibly resulting in sensing failure.

Figure 29-4 Simultaneous electrocardiogram (ECG) rhythm strip and event markers.

Single-chamber, bipolar, rate-modulated pacemaker with an internal insulation failure of the lead. There are both oversensing (VS and VR) and functional undersensing when the true native complex coincides with the refractory period initiated by the oversensing. During the intervention for lead replacement, the measured lead impedance was 125 Ω. The event markers are both labeled (VP, ventricular pacing; VR, sensed event occurring during the refractory period; VS, ventricular sensing) and identified by different amplitudes of the marker artifacts. The surface ECG should also be examined. This recording system generates a large-amplitude “artifact” to simulate the paced output, with any high-frequency transient that is detected independent of the output configuration of the pulse. If no stimulus is detected, no artifact is generated. There are paced QRS complexes identified by the event markers that have no “pacing stimulus” on the ECG, whereas others have a large “unipolar” pacing stimulus visible. The identification of the pacing system malfunction was facilitated by the simultaneous event markers.

In a bipolar lead, an insulation defect developing between the proximal and distal conductors can present as a variety of clinical manifestations. Intermittent contact between the two conductors generates a voltage “transient” that the pacemaker can sense, resulting in inhibition or triggering, depending on the programmed sensing mode and the channel on which the problem has occurred. This small electrical signal is not seen on the surface ECG and is appropriately termed oversensing. If the two conductors remain in contact, current flowing down the distal conductor is short-circuited to the proximal conductor and never reaches the electrodes. This may result in a loss of capture and a further reduction in amplitude of the normally diminutive bipolar stimulus artifact on the ECG. In addition, loss of appropriate sensing may occur.

Programming of the output configuration to unipolar (most pacing systems, but not ICDs, now have this capability) may prevent the loss of capture and possibly undersensing from an internal short circuit. This does not prevent the oversensing behavior associated with make-break electrical transients generated by intermittent contact between the two conductors, as is caused by failed inner insulation. Typically, a bipolar lead with failed internal insulation exhibits a higher impedance when pacing is programmed to the unipolar rather than the bipolar output configuration.^18,19 This is opposite to the expected higher impedance of a bipolar system. Although programming to a unipolar configuration may result in an apparent resolution of a malfunction, it is not a cure and should be considered only as a temporary measure, one that allows the observed problem to be managed on an elective rather than emergent basis. The malfunctioning lead should be replaced expeditiously.

An increase in lead impedance may be the result of a conductor fracture or a connector problem.²⁰ When this occurs, the lead impedance often rises to high levels. It is inappropriate, however, to assume that a normal lead impedance is 500 Ω. Leads are available that are designed to have high impedance, with values ranging from 1500 to 2500 Ω. Other leads, at implantation, may have an impedance in the range of 250 to 300 Ω. Therefore, it is essential to look for a trend in serial lead impedance measurements rather than a single measurement. Impedance, taken in conjunction with the stability or changes in capture and sensing thresholds, is the best way to determine the condition of a pacing lead. A mechanical problem with the lead—either a conductor fracture resulting in high impedance or an insulation failure resulting in low impedance—eventually results in an overt clinical problem that can be identified by telemetric measurement of the stimulation impedance. If the impedance is sufficiently high, there is no current flow and no effective output. It must be understood that, although the telemetered event markers indicate a pace output, there may be loss of capture. The reduced current flow also results in a fall in the measured current drain of the battery. Note that any problem may be intermittent. This typically occurs when the two broken ends make contact at times but are separated at other times, or in the case of an insulation failure, when lead movement either opens the compromised area or pushes the edges of the break together, resulting in intermittent normal function.

Before the availability of telemetry for lead impedance measurements, physicians could obtain similar information by oscilloscopic analysis of the pacemaker stimulus. In the 1970s, some manufacturers included a photograph of the pulse wave contour at a variety of different impedance values with the technical manual accompanying each pacemaker. Some computer-based pacemaker ECG follow-up systems are able to record and display the pacemaker pulse at an expanded scale. This allows the pace waveform to be recorded and tracked as a routine part of the periodic pacing system evaluation. Although this feature is available, it is not routinely used, given the ready access to lead impedance measurements provided by the implanted pacing system.

Lead impedance measurements have an indeterminate incidence of false-negative results when the lead problem is only intermittently manifested. The system forces the pacemaker to function asynchronously in an effort to obtain the measurements. The measurements, however, are made over a few cycles. If there is a true lead problem, but it is intermittent and was not present when the measurements were being made, the telemetered lead impedance may be normal. Therefore, if a problem is suspected, repeated measurements should be performed. Use of provocative maneuvers, such as placing manual pressure along the subcutaneous course of the lead, or having the patient raise or in other ways manipulate the ipsilateral arm, may be required to unmask a problem.

Some pacemakers are able to report lead impedance measurements on a beat-by-beat basis, allowing the clinician to observe the digital readout of lead impedance on the programmer screen over many cycles. As with the oscilloscopic monitoring of the pulse wave morphology, this technique reduces but does not eliminate the incidence of false-negative results. A further enhancement is the “automatic periodic monitoring” of lead impedance, which results in a graphic display reporting the history of lead impedance performance. Measurements can be obtained continuously or daily. This increases the frequency of measurement, increasing the likelihood of diagnosing an intermittent lead problem and possibly intervening on an automatic basis (Fig. 29-5). Some devices respond by automatically programming the pacemaker from bipolar polarity to unipolar in the presence of either a marked rise in impedance, reflecting a conductor fracture,²¹ or a marked drop in impedance, consistent with an insulation failure.

Figure 29-5 Pacemaker lead impedance and intrinsic signal amplitude.

Graphic display (top) and tabular summary (bottom) of serial automatic atrial and ventricular lead impedance measurements as well as intrinsic signal amplitude from an implanted Boston Scientific pacemaker. This event counter reports measurements made on a daily basis for the past 7 days and the mean weekly measurements before that time, with the potential for extending back a full year. These data were retrieved at a follow-up evaluation.

Event Marker Displays

A variety of different displays have been used over the years. Although not intended for this purpose, the simplest event marker for sensed events was achieved with triggered mode pacing, particularly if the output configuration was unipolar. A sensed event would be marked by the simultaneous release of a pacing stimulus coincident with sensing. This was easily recorded on the ECG, because the stimulus artifact distorted the morphology of the native sensed complex. The next evolutionary step in this technology involved the pacemaker emitting a series of subthreshold stimuli of varying amplitudes to represent the release of an output pulse, a sensed event, and the end of a refractory period. Both these systems, the triggered mode and the series of markers, were limited by the signals requiring an intact lead system, to allow the pulse to be delivered to the heart through the lead and subsequently recorded for display by the ECG. With a bipolar output pulse, the small artifact might not be readily visible, particularly in the triggered mode, because it would be obscured by the larger native complexes. In addition, if there were an open circuit (most often a conductor fracture) or an internal insulation failure involving a bipolar lead, no marker would be visible.

The next improvement was to transmit coded signals representing paced and sensed events from the pacemaker to the programmer. The programmer reconstituted these signals into a series of varying-amplitude pulses or alphanumeric labels representing paced or sensed events. These could be either displayed on a monitor screen or recorded using a printer integral to the programming system, which allowed for the simultaneous recording of a surface ECG acquired by way of a separate set of cables. The simultaneous ECG and markers allowed the clinician to correlate the behavior of the pacemaker directly with the patient’s rhythm, to determine whether the system is functioning properly. A calibration signal composed of a series of different amplitude pulses or a legend of the cryptic labels allowed the clinician to interpret the various markers (Fig. 29-7). In an early series of markers, the largest pulse represented a pacing output, the intermediate one indicated a sensed event, and the smallest represented either the end of the refractory period or a sensed complex that occurred during the refractory period. With the advent of dual-chamber pacing systems, a marker pulse extending above the baseline identified atrial events, and ventricular events were labeled with a pulse extending below the baseline. This type of marker system has become obsolete, and there are very few devices remaining as active implants that use this scheme.

Figure 29-7 Example of formerly used event marker annotation.

Lines of various amplitude are used to designate paced, sensed, and refractory events. Lines above the baseline are used to describe atrial events, and lines below the baseline to describe ventricular events. This type of annotation has been made obsolete by the current annotation systems.

A further refinement to this system was the addition of alphanumeric annotations, which made the simple (yet cryptic) system just described obsolete. Two common sets of notations have been used and are summarized later. These are not the only possibilities, because some manufacturers use a unique set of symbols to identify paced and sensed events, with the location of the symbol on the recording identifying whether it represents an atrial or a ventricular event. Others have expanded on the set of labels and provide, on command, a detailed explanation.

One method for single-chamber pacing systems with event marker telemetry uses the letter P to reflect a paced event and S to represent a sensed event. This may cause confusion with other dual-chamber systems in which the letter P reflects a sensed atrial event indicative of a native P wave (with which most medical personnel are familiar, based on their knowledge of the standard ECG). The interpretation of the alphanumeric event markers is often product specific, and the clinic personnel who evaluate the pacing system must take this into account (Fig. 29-8).

Figure 29-8 ECG and EGM tracings with event marker legend.

Lower ECG rhythm strip (top) with simultaneously recorded atrial (AEGM, middle) and ventricular (bottom) electrograms. Labels across bottom identify specific events reflecting the behavior of this Boston Scientific pacemaker. In these tracings, the atrial refractory sensed events (AS) are caused by far-field R-wave sensing that is also visible on the AEGM channel.

In the case of dual-chamber pacing or when the pacemaker knows that it is providing atrial pacing and sensing, a native atrial depolarization may be identified as either the letter P or the combination AS. P is taken from the standard ECG identifier for an atrial depolarization. AS is one abbreviation for an atrial sensed event. An atrial stimulus may be identified by either the letter A or the combination AP for atrial paced event.

Analogous lettering is used on the ventricular channel. Again, single-chamber pacing in some systems might still use the letters P to represent a paced event and S to refer to a sensed event. Other systems use R to refer to a native ventricular depolarization, which is generically called an R wave on the standard ECG. This may also be identified by the letter combination VS for a ventricular sensed event. The letter V in one system might be used to represent a ventricular pacing stimulus, whereas the identical event may be labeled VP (for ventricular paced event) in other systems.

Other symbols may be used for a paced or sensed event and are usually displayed with an identifying key on the resultant printout. Each paced or sensed event may also be identified by a vertical line, going up in the case of atrial events and down for ventricular events. In some cases, a difference in the amplitude of these lines has been retained in conjunction with the alphanumeric lettering. Indeed, as the complexity of the devices increases, the variety of symbols and letters identifying specific events and behaviors is also increasing (see Fig. 29-8).

In many systems, interval measurements between the various events are either calculated automatically and displayed, or measured after cursors are aligned with specific events. There may be a time delay between the actual sensed or paced event and the release of the event marker. This is caused by the finite period that is required for the pacemaker to recognize the event and then transmit the appropriate information to the programmer through the telemetry channel. In most cases, priority is given to the normal function of the pacemaker, with the diagnostic marker feature being delayed. Differences of up to 40 msec between the actual event and the resultant marker have been noted.

Laddergramming

Laddergramming is an advanced adjunct to the interpretation of clinical arrhythmias. It was incorporated in some programming systems by using the known programmed parameters of the pacemaker combined with the event marker information telemetry from the implanted pacemaker^23–26 (Fig. 29-9). For all the elegance of these graphic interpretations of the pacemaker’s behavior, they still require the simultaneously recorded ECG rhythms. Although the pacemaker may be functioning normally, in that it is behaving properly with respect to its programmed parameters, failure to capture or properly sense would not be diagnosed from the various diagrams and markers without the simultaneously recorded surface ECG.²⁷ Laddergramming has become a teaching tool at this time, because no programmers currently provide this level of graphic description.

Figure 29-9 Laddergram from rate-modulated dual-chamber pacing system.

The event markers are identified by a variety of symbols displayed in a key to the left of the tracing. In addition, battery (cell) impedance and projected longevity are reported in the upper right portion of the tracing, and selected programmed parameters are shown in the upper left quadrant. AV, Atrioventricular; RRF, rate response factor.

Data from a META DDDR Model 1250 Telectronics Pacing Systems, now Pacesetter, St. Jude Medical CRMD, Sylmar, Calif.), displayed with a Model 9600 programmer.

Event Marker Limitations

Several limitations are associated with event marker telemetry. First, used alone, markers report the behavior of the pacemaker but do not allow the clinician to determine whether this behavior is appropriate for the patient.²⁷ In this regard, markers are analogous to the hand-held digital counters that report pacing rates and intervals. The counters only detect and report the pacing stimuli, not whether these output pulses are effective in causing a cardiac depolarization, or whether native events are properly and consistently sensed. A long interval between consecutive pacing stimuli could reflect normal function because the pacemaker responded appropriately to a native complex, or it could represent a system malfunction with oversensing or no output (e.g., from intermittent lead fracture). Neither event marker telemetry nor the digital counters should be used independently of a simultaneously monitored ECG rhythm.

The second limitation is that the report of a stimulus output does not mean that there was appropriate capture. Confirmation of capture requires an ECG or concomitant hemodynamic pulse monitoring. In addition, the markers may report that the pacemaker is sensing a signal, but if this signal is not visible on the surface ECG, there is cause for concern. Is it an appropriate signal that the pacemaker should be sensing (some intrinsic atrial rhythms may not be visible on the specific lead that is being monitored), or is the system responding to an inappropriate or nonphysiologic signal, such as the make-break potentials associated with a breach of the internal insulation in a coaxial bipolar lead?

Most pacemaker event marker telemetry is limited to real-time recordings. The markers must be telemetered from the pacemaker to the programmer or another display system while the rhythm is being actively monitored. Neither the pacemaker nor the programmer can retrospectively provide markers for a previously recorded rhythm. If the pacemaker is responding to events that are not visible on the surface ECG, the event marker simply confirms this fact but does not identify the specific signal. An evaluation of sensed but otherwise invisible events requires intracardiac electrogram telemetry or an invasive procedure to record the signal from the implanted lead. Many premium-level pacemakers can store event markers with or without electrograms. Markers may be stored using a patient trigger (e.g., magnet application),²⁸ a simple battery-operated radiofrequency transmitter, or on spontaneous activation of predefined algorithms or sequences of sensed or paced events.

Limitations of Electrogram Telemetry

The apparently adequate amplitude of the telemetered EGM for sensing does not mean that the pacemaker will sense it. The telemetry amplifier may use filters different from those in the sensing circuit, providing qualitative rather than quantitative data on the EGM (see Fig. 29-11). In most pacing systems, telemetered EGMs also have limitations similar to those of event markers. EGMs are real-time recordings and cannot be retrospectively provided by the programmer to facilitate the interpretation of an earlier ECG.

Real-time telemetry allows the physician to analyze the behavior of the pacing system when the patient is in the physician’s office or clinic while these diagnostics are being accessed with the programmer. This is impractical over a long period. In addition, it does not allow the patient to move around much while these recordings are being obtained. Long-term monitoring of pacing system behavior requires a Holter monitor, a loop memory recorder, or event counter telemetry, depending on the desired degree of precision. Pacemakers with microprocessors and significant random access memory (RAM) can now store select EGMs,^47,48 as well as event markers, when triggered by the patient or by a predefined set of circumstances. This may greatly reduce the need for Holter or other monitoring techniques to evaluate intermittent symptoms in patients with pacemakers.

Total System Performance Counters

The simplest systems keep track of the number of times a pacing stimulus is released or a native complex is sensed. This allows for an assessment of the degree to which the pacemaker was used by calculating the percent pacing in each chamber. A refinement of this ability allows the pacemaker to diagnose bradycardia and, in the dual-chamber modes, to determine whether the bradycardia was the result of sinus node dysfunction or AV block. Event counters with this ability have also been termed diagnostic data, implanted Holter systems, and data logging.^49–55

With the introduction of dual-chamber pacing (specifically the DDD mode), the potential interactions between the pacemaker and the patient increased from two (pacing or sensing in one chamber) to five (pacing or sensing in the atria or ventricles and sensing ventricular activity without preexisting atrial activity). The situation became even more complex with the introduction of multiple-site ventricular and atrial pacing configurations. Knowledge of how the pacing system has behaved over time, combined with the clinician’s knowledge of the patient, provides invaluable information toward assessing the overall performance of the implanted system.

The various annotations described in the event marker section were combined to provide a cryptic description of the different operational states of the DDD mode. These include an atrial sensed event followed by a ventricular sensed event (AS-VS or PR), indicating that the pacemaker is inhibited on both channels. Atrial sensing followed by a ventricular paced event (AS-VP or PV) refers to P-wave synchronous ventricular pacing. Atrial pacing with intact AV nodal conduction or functional single-chamber atrial pacing is indicated by AP-VS or AR. Base-rate pacing in both chambers is represented by AP-VP or AV. A ventricular sensed event not preceded by atrial activity, either paced or sensed, is a premature ventricular event (PVE), identified by the letter R or the combination VS. Some systems label these events premature ventricular contractions, or complexes (PVCs).

This information may be collected in conjunction with events occurring in specific rate bins, allowing for a detailed report of heart rate distributions. Additional data collected include the percentage of pacing in the atrium and ventricle, length of time at the maximum tracking rate, and the number of episodes in which the pacing system reached the programmed upper rate limit (Fig. 29-12). Additional data may be recorded regarding mode-switching events, pacemaker-mediated tachycardia (PMT) termination algorithm use, or the number of times any one of several other special features was activated. The counters are able to continue to accumulate data until one of the pacing state bins is full and cannot accept additional information. At this point, one or more of the counters are frozen. The volume of data that can be stored depends on the memory capacity of the pacemaker that is dedicated to these features.

Figure 29-12 Heart rate, event, and mode switch histograms.

Data represent the performance of the pacing system over the preceding 448 days, with monitoring of every event. Patient had a St. Jude Medical device programmed to DDDR mode. The event histogram is a graphic display of the relative percentage of events within each of the five basic pacing states, whereas the heart rate histogram is a graphic display of the rate distribution based on atrial sensed or atrial paced events. This distribution reflects normal chronotropic function of the sinus node in this patient, whose pacemaker was placed for intermittent atrioventricular block (almost all the atrial paced events are at the base rate). The percentage of time per rate bin represents a normalized display of the relative percentage of atrial paced or atrial sensed events and premature ventricular sensed events in each rate bin during this period. The AMS summary and V Rates During AMS indicate the atrial rates during mode switch, the duration of the mode switch events, and the ventricular response during the mode switch events. These data can be used to manage antiarrhythmia medication as well as AV node–slowing medications.

The following example illustrates the amount of data that can be stored, or the quantity of memory. All data are stored in a binary code, represented by a series of 1s and 0s. A binary unit of memory is called a bit. Eight bits are a byte and result in 256 combinations of 1s and 0s. If each series of combinations represents a single item of data, a total of 256 pieces of information can be stored in a system with eight bits of memory. As the number of bits increases, the amount of data that can be stored increases exponentially. If each pacing state is represented by a single combination of bits, and there are 24 bits in the RAM devoted to these counters, more than 17 million events can be counted and stored in memory for retrieval at the time of routine pacing system follow-up. Reading the counters requires access to the memory banks in the pacemaker through the use of specific coded commands from the programmer. One DDD system introduced in the mid-1980s had sufficient memory to provide a summary of pacing system behavior for a period of about 6 months. As memory capacity has increased and data compression algorithms have evolved, these systems are now able to store much more data for periods longer than 1 year.

Storing a simple marker of the pacing state and rate requires relatively little memory compared with storing waveform data representing the rhythm, as depicted by a consecutive series of EGMs. A theoretical 100-Hz bandwidth with 200 samples per cardiac cycle at eight bits of resolution per sample, recording the entire rhythm during a 24-hour period, would require 140,000,000 bits/day, even if the heart rate was a steady 60 beats per minute (bpm).⁴⁷ Most implantable defibrillator systems store a series of EGMs preceding and following the delivery of antitachycardia therapy. Premium pacing systems use certain triggers to initiate EGM storage. These triggers may be a high atrial or a high ventricular rate, initiation of mode switching, or activation of any of several other specialized algorithms. Storage may also be triggered by application of the magnet or use of a simple transmitter device by the patient when symptoms are present. An early method to save memory was to provide “snapshots” of representative complexes; the trigger to store these data in an ICD was delivery of antitachycardia therapy. Extensive storage of cardiac rhythm data within pacemakers became possible with increasingly sophisticated data compression algorithms and more memory than previously available. Higher-bandwidth data transmission channels are required for this volume of data storage, particularly for long series of rhythms, so that transmission can be accomplished as efficiently and as quickly as possible.

Implanted looping memory event recorder (implantable loop recorder, ILR) capability, complete with stored rhythms, is now available as a stand-alone device, in ICDs, and in high-end pacemakers. Most of the older pacemakers, however, continue to store only data reflecting the total number of times a pacing state was encountered and, in some systems, the distribution of rates or intervals, rate ranges, or mean rates within these pacing states. This information provides the physician with an overview of the function of the system in the patient. Predominant base-rate pacing is expected in patients with marked sinus node dysfunction and in those receiving certain medications such as beta-adrenergic blockers or some calcium channel blockers. On the other hand, predominant base-rate pacing (AP-VP) in patients whose primary indication for pacing was complete heart block suggests either the development of sinus node dysfunction or primary atrial undersensing. In these same patients with high-grade AV block, frequent counts of AS-VS and PVEs, particularly in the absence of known ventricular ectopy, suggests episodes of ventricular oversensing or resolution of the AV block. Large numbers of counts regarding PMTs warrant a reassessment of retrograde conduction and possible adjustment of the programmed PVARP or PMT prevention and termination algorithm. It may also mean only that the maximum tracking rate (MTR) is being reached, as some devices use the MTR as the trigger for PMT intervention. A large number of counts at the MTR suggests that the upper rate limit should be reassessed, because it may be too low for the patient’s physiologic needs. It might also indicate pathologic atrial rhythms. Event counter telemetry can help reveal the overall function of the pacing system; a variation from the clinician’s knowledge of the patient may suggest the need for a further evaluation.

When the additional ability to monitor a series of rates and pacing states is added, chronotropic function can be assessed by the distribution of atrial sensed rates (AS-VS or PR and AS-VP or PV; see Fig. 29-12). Furthermore, atrial pacing at rates above the programmed base rate reflects sensor drive in the DDDR pacing systems, providing an overview of the sensor behavior. The ability to report both rates and pacing states can provide the clinician with a better insight into the cause of PVEs as well as overall pacing system function. True premature ventricular contractions tend to occur at short coupling intervals, which is equivalent to a rapid rate. Large numbers of PVEs might suggest recurrent ventricular arrhythmias.^56,57 If there are large numbers of PVE counts at relatively low rates, the clinician should consider episodes of atrial undersensing or accelerated junctional rhythms, which would also fulfill the pacemaker’s criteria for a PVE (i.e., a sensed R wave not preceded by an atrial event).

Subsystem Performance Counters

An increasing number of specific algorithms and capabilities are used either intermittently or potentially independently of the functional performance of the pacing system. An early counter representative of this capability is the sensor-indicated rate histogram, which reports the distribution of pacing rates that would have occurred had the sensor been totally controlling the pacing rates.^58,59 This counter reports the sensor-defined rates, even if the pacemaker was inhibited by a faster native rhythm or the actual functional rate was being controlled by the sensed atrial activity (Fig. 29-13).

Figure 29-13 Sensor-indicated rate histograms showing heart rates for two patients.

The bars show the actual heart rates and are dark if atrial paced or white if atrial sensed (St. Jude Medical pacemaker). The open circles indicate what the activity sensor would have driven the heart rate to if the sensor-indicated rate was faster than the patient’s intrinsic heart rate. A, This histogram is from a patient with a normal sinoatrial (SA) node, showing that the patient’s heart rate exceeded the sensor-indicated rate in all bins except the base rate bin. B, This histogram is from a patient with sinus node incompetence; thus almost all the atrial events are paced, and at the sensor-indicated rate.

Other subsystem performance counters include automatic mode switch histograms, reports of the cumulative length of time for which the system functioned at MTR, number of times the PMT algorithm was enabled, high atrial or ventricular rate episodes (Fig. 29-14), and sudden rate-drop episodes. These diagnostic counters provide detailed information about the use of a specific algorithm or system behavior that is not activated daily or is not visible on the ECG, making identification difficult using standard recording techniques such as a Holter monitor.

Figure 29-14 High-rate episodes on AEGM.

High-rate episode summary (top) and high-rate episode graphic with stored atrial electrogram (AEGM; bottom). These data facilitate the clinician’s ability to assess the behavior of the pacing system between office visits. In this case, the specific episode associated with the AEGM reported atrial paced events occurring above the trigger level for this event counter, which was also above the maximum sensor rate. The electrogram demonstrates atrial pacing with a large, far-field R-paced ventricular event; presumably the labeled high-rate episode was caused by far-field sensing. This knowledge may be helpful in further programming of the pacemaker.

Data from Medtronic Thera DR Model 7940.

The principal limitation associated with the two types of counters so far described is that counts are placed in a bin, either the pacing state alone or the pacing state and rate, which provides a one-dimensional view of the system’s behavior. If the period of monitoring is short, as during a casual or brisk walk, the clinician can reasonably assess the behavior of the pacing system. Longer periods of monitoring accumulate and overlap the results of many activities. This precludes an assessment of the pacing system’s behavior during a specific activity, or the identification of a symptomatic episode occurring at an earlier time. The ability of the system to store pacing state and rate data with respect to time (i.e., time-based event counters) has been variably termed event record or pacemaker Holter systems.^60–62 Technically, this is not yet a true Holter monitor, because continuous rhythm recordings are not retained in memory, even though rate data may be available.

Time-Based System Performance Counters

Time-based event counters require an extensive amount of memory. Not only are the pacing state and rate stored, but these data must be stored with respect to preceding and subsequent events. The data cannot be simply dumped into a rate or pacing state bin. Each piece of data (i.e., every cardiac event) must have a time reference, which takes significantly more memory than simple histogram-type counters.

Two techniques are available for storing real-time data. One is to continue to accumulate the data until the memory is full and then to freeze the recorded events. Freezing all the counters at the same time maintains the ratios between the events. A clearing function is usually provided to reset the counters. In some devices, reprogramming of any parameter or specific parameters (e.g., rate or pacing mode) can result in the automatic clearing of these data. This “initial events” or “frozen” recording technique is especially useful for short-term monitoring, as in office-based exercise evaluations. It also allows the system’s response to exercise to be evaluated, often without the need for simultaneous ECG monitoring.

The other option is to collect the data continuously. As the counters fill, new data are added at the expense of the oldest data (Fig. 29-15). This has been called rolling trend, final trend, or continuous data storage and is based on the “first in, first out” (FIFO) principle. When the patient is seen in follow-up, the data acquired over the time immediately preceding the interrogation of the system are available for review. The patient can often recall symptoms and activities during this period. These can be correlated with the behavior of the pacing system at the time of the symptoms. In this manner, the clinical staff caring for the patient may further assess chronotropic function, the behavior of the pacemaker during a variety of special or usual activities, and whether the sensor and other algorithms are behaving appropriately. The clinician may gain insight into the cause of palpitations and other symptoms that may have occurred during this time by correlation of the recorded data and the patient’s complaints or activities.^51,58–62

Figure 29-15 Event record display.

The sampling interval was 26 seconds, which provides a detailed overview of the behavior of the pacing system for the preceding 60 hours. The top display is an overview of the moment-to-moment behavior of the pacing system, with the vertical lines representing the maximum and minimum rates during each time period (the total displayed interval is divided into 40 equal time segments) and with the cross-bar representing the mean heart rate. The vertical line is displayed if there are two or more complexes in the specific time bin. The display scale can be expanded to display individual events, in which case a specific alphanumeric label is shown, consistent with the legend in the upper left. The same data may be displayed as an event bar graph (equivalent to the event histogram) or as a rate bar graph (equivalent to the heart rate histogram). The solid bars represent atrial paced events; thus atrial rates greater than the programmed base rate represent sensor drive. Atrial rates lower than the programmed base rate represent “sleep mode” behavior that was also programmed. The time graphs automatically divide the overall monitoring period into six equal segments.

Display is from a Pacesetter Trilogy DR+ Model 2364 (St. Jude Medical).

Sensor-input data have been combined with the actual rates achieved to facilitate reprogramming of the sensor parameters. Once acquired, the data from an exercise session are retained in the memory of the programmer, uploaded to the programmer, and then displayed on a screen. The system’s performance is based on a given set of sensor parameters that were in effect at that time. Because the pacemaker has the actual sensor-signal data, it can calculate how the unit would perform in response to a different set of sensor parameters. If a new set of sensor parameters is entered into the programmer, the curves that were based on the original input data are redrawn to display the system’s projected behavior in response to the new set of parameters. The clinical staff can then determine the best sensor parameters for the individual patient without having the patient perform repeated activities (Fig. 29-16). This feature has been termed redraw,⁶³ exercise test, or prediction model.⁶⁴

Figure 29-16 Exercise test report.

The lower line on the graph represents the actual behavior of the pacing system during a hall walk with the settings as noted to the right of the graph as “tested.” Patient has St. Jude Verity pacemaker. The thicker line above represents the behavior of the pacemaker under sensor drive at the proposed or “modeled” values. By using this type of feature, the patient only needs to take a single walk to set up the activity response of the device.

Limitations of Event Counter Telemetry