Documentation of Pacemaker Pulse Generator Battery Depletion

Most bradycardia pacemaker pulse generators provide direct or indirect indicators of battery depletion, documenting the need for enhanced follow-up, elective generator replacement, or incipient battery failure (end of service). Additionally, certain nonspecific indicators may alert the physician to early signs of battery wear (Box 24-3). Because most pacemaker patients are followed by remote monitoring systems more frequently than by full evaluation in the physician’s office, it is not surprising that battery depletion for permanent pacemaker patients is most often detected through remote recordings.^62–65 Remote evaluation of defibrillator systems also allows interrogation of device function and arrhythmic events, as well as battery capacity.^66–73

A change in magnet-activated paced rate remains the most common indicator of reduced battery output voltage for pacemakers (Box 24-4). Some pacemaker pulse generator models respond to declining voltages through a gradual reduction in magnet-activated pacing rate; reduced rates indicate the need for enhanced follow-up, with even slower rates indicating elective or obligatory replacement. Other models demonstrate an abrupt shift in the magnet-activated paced rate at the enhanced follow-up period or at the time of elective replacement. A demand mode switch from DDD to VVI (or a magnet mode switch from DOO to VOO) may occur at the elective replacement time or as an obligate replacement indicator for dual-chamber systems before complete battery failure. Inability to reprogram the device, inaccurate measurement of lead impedances, and an automatically reprogrammed reduction in data storage capabilities to preserve battery life may also occur as the generator approaches end of service.

Other, secondary parameters suggest gradual battery depletion and can be interrogated remotely. The usual battery impedance in a new pulse generator is less than 1000 ohms (Ω). As pulse generator batteries deplete, internal battery impedance increases, providing a secondary indicator of the impending need for replacement (Fig. 24-1). Replacement is not required, however, until more definitive indicators appear, such as a change in magnet-activated pacing rate, a specific elective replacement indicator voltage, or mode switch. The patient’s dependence on the pacing functions of the device needs to be considered when timing the generator replacement. For example, a patient with complete heart block may not tolerate a mode switch to an asynchronous VVI pacing mode at the elective replacement time.

Figure 24-1 Pacemaker battery depletion.

Acquired telemetry data from St. Jude Medical Affinity SR Model 5130 single-chamber (VVI) pacemaker programmed to the unipolar pacing and bipolar sending mode to enable the autothreshold function. Cell impedance has increased to 15,700 Ω, a secondary indicator of battery depletion, concomitant with a reduction in cell voltage to 2.61 volts (V). Pulse amplitude and pulse width are maintained, with the device minimizing ventricular output by reducing pacing voltage through autothreshold to 0.9 V with a threshold of 0.75 V. Ventricular lead impedance is acceptable. Pulse energy is low at 0.9 microjoule (µJ) per impulse, which has enabled its long battery life from implant in March 2000 until the most recent interrogation in September 2010. Magnet rate has dropped to 91.5 beats per minute (bpm) from an initial magnet rate of 98.6 bpm. Elective replacement indicator (ERI) occurs at a magnet rate of 86.3 bpm.

Telemetered battery depletion curves or battery status screens (Fig. 24-2), internal calculations of anticipated device longevity at current programmed settings, and a general knowledge of the expected performance of various generators all assist in anticipating a pacemaker generator’s end of service. Ultimately, loss of sensing and pacing capabilities occurs with battery exhaustion. The rate of battery depletion may accelerate as the device reaches end of service, making timely replacement in dependent patients very important.

Figure 24-2 Battery status indicator.

Visual graphic of the battery status screen from a Boston Scientific pacemaker is clear, giving a “fuel gauge” depiction of remaining battery life, which here is “full.” The screen depicts the measured magnet rate and an estimate of the remaining longevity of the device at current programmed settings. Elective replacement indicator (ERT) and end of life (EOL) magnet rates are also indicated. Changes in programming or use of pacing affect battery longevity.

Indicators for Replacement of ICD Generators

Precise longevity of ICD battery systems is more difficult to predict because the rate of defibrillator generator depletion depends on the (1) frequency of bradycardia pacing, (2) delivery of high-energy shocks to terminate tachyarrhythmias, and (3) data storage. Nevertheless, documentation of ICD battery drain remains crucial to evaluation of the safe function of the defibrillator system.

Manufacturers have developed relatively straightforward methods to document impending ICD battery depletion, which fall into three major categories: (1) a measurable reduction in battery voltage (V) or remaining charge (mAh, milliamphours) that can be acquired through telemetry, (2) an increase in measured charge time to a level that indicates the need for elective replacement, and (3) various device-specific markers that indicate a particular degree of pulse generator depletion. Measurement of charge time to a maximal voltage on the device capacitors is the earliest method used for estimating the remaining longevity of an ICD. Most early systems included such markers. However, this method of determining elective replacement time requires fully charging the capacitors, which may necessitate an office visit, or determining the charge time from a capacitor recharge that may have occurred weeks earlier because of programmed automatic capacitor maintenance. Charge times can also be obtained if the device spontaneously charges and delivers therapy at maximum output, but this process does not always occur opportunely between office visits.

Reduced battery voltage provides a readily useful method of determining generator depletion; the value can be obtained telemetrically. This method is now the most common for marking elective replacement time and end of service for ICDs. Specifically, telemetered voltage can be obtained on initial interrogation of the device and is remotely retrievable in most ICD devices.

Some specific ICD models use battery voltage to produce labels that indicate the beginning, middle, or end of service for pulse generators. These labels can be obtained directly through interrogation of the device in the office. End of service may also be clearly indicated graphically (see Fig. 24-2).

Regardless of the method used by the device to document end of service, the clinician has the responsibility to increase the frequency of follow-up visits as the unit nears elective replacement time, to ensure continued safe function of the system, protection of the patient from tachyarrhythmias, and bradycardia support if required. In the event that the patient receives frequent shocks just before the anticipated need for device replacement, elective replacement may occur earlier than originally planned.

Documentation of Lead Malfunction

A variety of causes of lead malfunction require reoperation,^51–5774 from primary lead dysfunction to premature battery depletion as a result of excessive current drain (see Box 24-1). Primary lead malfunction may result from outer insulation break,^75–81 inner insulation break in a bipolar coaxial lead,^82,83 lead conductor fracture,^{17,19,84–86} or lead dislodgment.^{49,50,59,60,87} Current drain may be increased by (1) high pacing thresholds^46,88–90 through a need to increase output voltage, (2) failure to optimize generator output for long-term pacing after lead maturation, or (3) inner insulation break with a resultant low pacing impedance. All these scenarios can result in premature battery depletion. Before reoperation for lead malfunction is performed in these patients with primary lead malfunction, the physician should consider upgrading or replacing the pulse generator, especially if the battery is old.

Lead malfunction can usually be documented by noninvasive telemetric evaluation or remote monitoring.^{63,66–73,91} A measured bipolar pacing lead impedance less than 200 Ω suggests an inner insulation break between the two coaxial pacing coils. An outer insulation break may be the result of lead wear or may have been inadvertently caused during surgery, especially with generator replacement; an inner insulation break between the two coils of a bipolar system occurs most often at the subclavian insertion site as the result of crush injury to the lead, especially with leads inserted into the subclavian vein and tied securely in a medial position in patients with a tight clavicular–first rib space. Telemetry for lead diagnostics may demonstrate markedly low bipolar pacing impedance in patients with an inner lead insulation break. The impedance may vary with manipulation of the pacemaker, which causes intermittent short-circuiting of the two lead conductors (Fig. 24-3).

Figure 24-3 Inner insulation fracture.

Acquired telemetry data from an early, Intermedics Intertach 262-14 VVICP antitachycardia pacemaker, connected through a bipolar coaxial lead to the right ventricular apex. Battery voltage and cell impedance are normal; however, measured lead impedance of 41 Ω is extremely low. In this patient, measured lead impedance was normal in the supine position but decreased with sitting or when the device was pulled inferiorly This behavior indicates a break in the inner insulator between the coaxial conductor strands in the area of the clavicle. With movement, the conductors contact each other, resulting in low impedance and preventing delivery of electric current to the heart. Because output voltage is fixed, the low lead impedance results in a high delivered current (96.5 mA) and high delivered energy and charge per pulse.

High pacing lead impedance (generally >1200 Ω) may be the result of lead conductor fracture or an incomplete circuit caused by a loose lead pin–pulse generator connection. Rarely, this may occur with fractures of conductors inside the pulse generator header. The introduction of high-impedance leads makes it essential to compare the impedance at implantation with follow-up impedance measurements and with established acceptable impedance ranges for each lead. Depending on the point of discontinuity, lead impedance may vary with manipulation of the pulse generator or with respiration. Lead conductor fractures may be evident on chest radiographs or fluoroscopy; however, absence of visual evidence does not exclude conductor fracture. A break in the connection of the lead to the generator, or within the lead itself, can produce intermittent loss of energy delivery to the heart, which in turn results in absence of pacemaker spikes. Undersensing, or oversensing caused by lead “chatter,” may also occur with lead conductor fracture (Fig. 24-4). Most recently, this scenario has occurred with a recalled high-energy ICD lead system.⁹²

Figure 24-4 Oversensing in antitachycardia device.

Sensing of diaphragmatic myopotentials during periods of deep inspiration can lead to inappropriate triggering of the antitachycardia functions of an implantable cardioverter-defibrillator (ICD). These tracings are recorded from an ICD placed with a passive-fixation Endotak endocardial lead that incorporates integrated bipolar sensing and pacing between the right ventricular apical (RVA) high-energy shocking coil and the lead tip (Boston Scientific). Surface electrocardiogram (ECG), rate-sensing electrograms (EGMs), and marker channels all record spurious signals that represent inappropriate sensing of extracardiac electrical potentials. The frequency of these signals is high, and they occur after paced events as well as after sensed events. Pacing increases the gain of the device to avoid undersensing of low-amplitude signals of ventricular fibrillation, but it also increases the possibility of sensing extraneous noise. An underlying paced rhythm exists at a cycle length of 857 msec, but even this is altered by oversensing. The first paced complex (VP 857) is followed by two inappropriately sensed events (VS 650 and VS 648) that inhibit ventricular pacing output. Because the next native QRS complex occurs close to an inappropriately sensed signal, it is interpreted by the device to represent sensing in the ventricular fibrillation zone (VF 176). After that, another myopotential is inappropriately sensed (VS 729), and the native QRS is again sensed in the VF zone (VF 146). Finally, three sequential paced events occur at intervals of 857 msec, despite the presence of spurious electrical signals, which are not of sufficient amplitude to trigger sensing. Repetitive events such as these could lead to inappropriate antitachycardia therapies or prolonged periods of inhibition of pacing. This lead was extracted and replaced with an active-fixation endocardial Defibrillation (DF) lead positioned distally on the lower interventricular septum.

Lead dislodgment produces intermittent noncapture or failure to sense that may be related to respiration. Pacing thresholds needed to achieve consistent capture may rise significantly. Lead impedance increases or remains unchanged. Fluoroscopy may demonstrate a loose or displaced lead tip but is not always diagnostic.

Special Issues for ICD Leads

The ICD lead remains the weak link in the ICD system. Oversensing caused by diaphragmatic impulses or extraneous signals may inhibit pacing therapy or lead to inappropriate delivery of “treatment” for presumed ventricular tachyarrhythmias that actually represent noise sensing.^93,94 Further, although fracture and degradation of leads have become less common with transvenous (vs. epicardial) ICD lead systems, these problems nevertheless occur with some frequency, necessitating reprogramming or reoperation.⁹⁵ Conductor fractures resulting from specific lead design issues have led to recalls and the need for reoperation in patients who experience a lead fracture⁹² (Fig. 24-5).

Figure 24-5 Lead fracture and inappropriate shock.

Conductor fracture with intermittent contact of the broken ends of the lead wire may result in inappropriate sensing of noise chatter, as demonstrated here in a Medtronic 6949 ICD lead with a fracture of the rate/sense conductor. Underlying native QRS complexes are easy to discern in the far-field intracardiac recordings in the top two tracings. Despite regular sinus rhythm, marker channels show a large amount of noise sensing; intracardiac recordings of noise are also evident on the right side of the tracing. Noise sense intervals vary from 120 to 500 msec. The high degree of variability of sensed intervals, as well as frequent nonphysiologic intervals shorter than 200 msec, lead to the diagnosis of noise sensing. In this example, the number of sensed intervals in the ventricular fibrillation zone is great enough to trigger the VF detection algorithm of the device. Ventricular fibrillation therapy was inappropriately delivered (VF Rx 1 Defib, 21 J). Lead replacement is recommended; alternatively, placement of a new high-energy lead may be indicated in some instances where extraction is not feasible.

Depleted battery status is readily evident on routine ICD follow-up both in the office and remotely (as described previously), and integrity of the ICD high energy coils can be evaluated easily in most current devices. High shocking-electrode impedance measurements may indicate lead conductor fracture or a lead-generator interface problem.

Measuring high-energy electrode impedance in early devices required delivery of a shock, either to treat a clinical tachyarrhythmia or as part of a noninvasive testing protocol. As a result, about 10% of patients undergoing ICD generator replacement because of battery depletion were found to have a previously undetected sensing or defibrillation system failure.^96,97

Noninvasive indicators of lead malfunction almost always occur before surgery; nevertheless, the operator should always test the lead system carefully during a battery replacement procedure and must be prepared to deal with malfunctioning leads at the time of generator change. Manipulation of a lead in a pocket may uncover a previously undetected conductor fracture. Newer systems automatically measure high-energy lead impedance at office or remote device interrogation, similar to standard pacing and sensing electrodes. The lower-energy impulses delivered by these devices may be more sensitive to the detection of microfractures than higher-energy shocks.

Determination of Pulse Generator-Lead Interface Malfunction

Pulse generator–lead interface problems may be grouped into three categories: (1) loose, incomplete, or uninsulated connections; (2) reversal of atrial and ventricular leads in the pulse generator connector block (for ICDs, reversal of shocking electrode polarity may also occur); and (3) pulse generator–lead mismatch.^98,99

A loose pace/sense lead connection is apparent with noninvasive testing. The device may fail to deliver pacing spikes when appropriate, may intermittently fail to sense, or may oversense as a result of chatter caused by intermittent contact with the set screw. Oversensing can result in inappropriately high tracking rates or inhibition of ventricular output. Capture or sensing problems may be exacerbated by manipulation of the device. An uninsulated connection most often produces current leakage (an electrical short circuit in the system) that inhibits pacing or sensing. Leakage can occur if a set screw is not properly insulated or tightened, or if sealing rings on the lead header do not prevent body fluid from oozing into the pulse generator connector block around a loosely fitting lead. Leakage around lead header sealing rings may result from a loose lead connection or lead–pulse generator mismatch.

Lead impedance in pulse generator–lead interface problems varies, depending on the specific situation. A loose, unconnected lead that remains in the pulse generator connector block, so that lead header sealing rings prevent fluid from entering, causes a break in the electric circuit and a very high impedance. If fluid enters the pulse generator connector block around a loose lead or at the level of a set screw and maintains contact with body fluids, the short circuit can produce very low measured impedance. As with lead fractures, impedance can vary with manipulation of the device.

Reversed lead connections (atrial lead in ventricular port, or vice versa) should be evident before the patient leaves the implantation laboratory, allowing immediate correction. Some atrial and ventricular leads are marked to enable easy identification, especially preformed, passive atrial J leads and straight, passive ventricular leads; however, “generic” leads may be placed into both chambers, especially straight screw-in leads, which may not be so labeled. Likewise, most atrial, left ventricular (LV), and right ventricular (RV) lead pace/sense headers for insertion into ICD ports are all of the International Standard IS-1, so the implanter must exercise care in placing these leads properly into the appropriate locations in the ICD generator connector block. It is also possible, in patients whose pacemaker or ICD remains inhibited because of native electrical activity, to see no pacing spikes initially after implantation. To be certain that the pulse generator–lead system functions appropriately immediately after implantation, the device should be programmed to an atrioventricular (AV) delay shorter than the intrinsic P-R interval, and the device should be checked with a programmer after the leads are attached to document appropriate function. Caution exercised at implantation should avoid reversed leads; for example, we always connect the ventricular lead first to ensure pacing in the proper chamber.

Beyond ensuring the presence of adequate and appropriate lead connections to the pulse generator, the battery connector block and leads must be compatible (see later).^98–102 This issue is less important with the standardization of new lead models used for device upgrades or generator replacements. However, incompatibility can result in fluid leakage or loose connections, with resultant loss of pace/sense or shocking capabilities, requiring reoperation.

Detection of Need for Reoperation for Other Reasons

Other indications for pacemaker or ICD generator replacement or lead revision generally become apparent through careful patient evaluation (see Box 24-1). Abrupt pulse generator failure with no antecedent sign of battery depletion is rare but can occur, producing symptoms in pacemaker-dependent patients. In others, abnormal pacing output or rate, lack of pacing output, or inappropriate sensing from generator malfunction may be detected by remote interrogation or in the office.²⁷ Of particular importance to patients with ICDs are the risks of no output when required to terminate tachyarrhythmias, inappropriate shocks from oversensing of diaphragmatic or lead chatter artifact (see Fig. 24-5), and oversensing of extraneous electromagnetic signals, such as surveillance systems or high-voltage generators, which can be sensed as ventricular fibrillation or can inhibit ventricular pacing output. Cellular telephones rarely present substantial interference because of variations in signal frequency.^103–106

Development of pacemaker syndrome in patients with implanted ventricular demand (VVI), ventricular rate-responsive (VVIR), or atrial rate-responsive (AAIR) pacemakers presents another indication for device revision. This need should be apparent from history and physical examination, although confirmatory blood pressure or cardiac output measurements may be required. Pacemaker syndrome occurring with an implanted functioning dual-chamber pacemaker with normal lead function must be managed by reprogramming.^107–110

Interchangeability of Products from Different Manufacturers

Early ICD leads from different manufacturers were compatible only with ICD pulse generators from the same manufacturer. This is especially evident when replacing generators that used LV-1 leads (Fig. 24-6). For later models, manufacturers have adhered to standard header designs for ICDs, including IS-1 ports for both atrial and ventricular pace/sense leads. Defibrillation ports now also follow a standard for defibrillation lead headers, DF-1, a 3.2-mm unipolar lead head with sealing rings (Fig. 24-7). The newest agreed-on IS-4 standard, which provides four electrical connections combining the functions of a bipolar pace/sense connection with up to two high-voltage connections, should reduce some of the confusion. However, there are two approved IS-4 connections with limited distribution at this time, one for devices with and one for devices without high-voltage (defibrillation) capacity. Eventually, this situation should simplify connections, except when extra leads for defibrillation are required in individual patients, which would require an adapter to connect to the IS-4 lead¹¹¹ (Fig. 24-8).

Figure 24-6 Lead upgrade to adapter.

Right, An LV-1 lead (Boston Scientific), which remains in a coronary sinus branch at generator replacement. Left, An adapter to upsize the LV-1 lead to an IS-1 standard header to fit into the new pulse generator. The LV-1 lead header contains a 3.2-mm pin with no sealing rings and would not fit into an IS-1 generator header without this upsizing adapter. The end of the adapter that connects to the LV-1 lead is bulky, containing electrical contacts, sealing rings, and a set screw.

Figure 24-7 Typical trifurcated high-energy ICD lead header at generator replacement.

The most inferior electrode is the coaxial IS-1 standard bipolar rate/sense electrode, with the proximal and distal defibrillation (DF-1) standard electrodes above that, carrying red insulation over the high-energy wires (Boston Scientific). The structural integrity of the lead headers and the trifurcation can be readily examined at generator replacement. The bulk of the trifurcation and the three lead headers are replaced by a single lead head in the DF-4 configuration (see Fig. 24-8).

Figure 24-8 Schematics of quadripolar high-voltage and low-voltage lead pin configurations.

Top, Combined pacing/shocking, high-voltage defibrillation (DF-4) header configuration. Bottom, Low-voltage standard (IS-4) header configuration. The specifications for the pins and the header are extremely precise, permitting the interchange of leads between models and manufacturers. Although both configurations are quadripolar, the low-voltage IS-4 lead tip will not fit into a high-voltage DF-4 receptacle cavity, enhancing safety. Although both lead heads are 3.2 mm in diameter, the diameter of the distal pin is larger for the low-voltage leads.

(From Proposed IS-4 Standard; American Association of Medical Instrumentation presentation at Cardiostim, Nice, France, 2004; and ISO Standard: Active implantable medical devices: four-pole connector system for implantable cardiac rhythm management devices—dimensional and test requirements. Geneva, 2009, International Standards Organization—E:\ISO FDIS 27186 (E) 25June2009.doc STD Version 2.1c2.)

For procedures involving early, nonstandard ICD connector blocks, however, the operator must be familiar with the existing system of leads and generator in the patient before surgery, and technical support from the manufacturer may be required at operation. A full range of adapters, or various pulse generator header designs, to mate a replacement generator to the existing leads, must be available. Ensuring tight and proper connections between the generator and the lead, and with any adapters and lead extenders, avoids malfunction and current leak. Although early adapters used an uncured medical adhesive to seal set screws in the connector block of the device, some newer adapters use set-screw seals similar to those found in pacemaker pulse generators.

Special Indications for Replacement of ICD Generators

As outlined in Box 24-1, the ICD generator may need to be replaced for other reasons, as discussed here.

Identification of Pulse Generator Make and Model

The most straightforward means of identifying a pulse generator is to obtain information directly from the patient (Box 24-5). An identification card specifies the type of device, model and serial number, implantation date, name of implanting or monitoring physician, and lead model and serial numbers. This information may also be obtained from records from the manufacturer, implanting or monitoring physician, transtelephonic or remote service that monitors the patient, or institution where device was placed. If none of these is helpful, alternative methods must be used to identify the pulse generator. Identification of the make and model of the existing pulse generator is crucial to determining its true functional status and, with earlier leads, to have the necessary information to select a compatible replacement or upgraded device. In the rare case of a pulse generator that cannot be identified before surgery, the implanting physician must have a full array of leads, generators, and adapters available at reoperation.