Chapter 88 Management of Lead Systems for Implantable Devices
Techniques and Interventions
Pacemaker and implantable cardioverter-defibrillator (ICD) systems have undergone enormous changes and innovation since the initial invention of the permanent implanted pacemaker.1 Leads have had to keep pace with the constant demand for improvement, and novel products and patients are surviving longer. It is hardly surprising that this rapid innovation and expanded usage increases the risk of dysfunction. This chapter describes the normal and abnormal functions of implanted leads. The incidence, mode of presentation, and treatment of the common lead-related complications, including lead fracture, insulation failure, perforation, dislodgment, and lead infection, are discussed. Finally, the indications, methods, and complications associated with lead extraction are reviewed.
Lead Design
Pacemaker and ICD leads are designed to transmit current from the generator to the myocardium as well as from the myocardium to the generator. The connection is made through a terminal pin electrically continuous with one or more conductor coils, which, in turn, are continuous with the electrodes making contact with the myocardium (Figure 88-1). The lead tip is stabilized at the myocardial interface by an active fixation mechanism such as a helix or a passive mechanism with tines to reduce the risk of lead dislodgment. The number of conductor coils within each lead varies with the function and complexity of the lead. For example, simple unipolar and bipolar pacemaker leads have one and two conductor coils, respectively.
Implant and Related Complications
Perforation
The incidence of symptomatic perforation is 0.1% to 0.8% for pacemakers and 0.6% to 5.2% for ICDs.2,3 However, the incidence of asymptomatic perforation rates found incidentally by computed tomography (CT) scan can be much higher: up to 15% for atrial leads and 6% for ventricular leads.4 Most perforations occur within the first month after implantation; however, they can occur up to 10 months after implantation.5 This risk appears to be higher when leads are inserted into the right ventricular apex or atrial free wall and with smaller diameter leads.2,3,5
Patients with lead perforation may present with symptoms of pleuritic chest pain, diaphragmatic stimulation, and, less frequently, hypotension caused by cardiac tamponade. Interrogation often reveals poor sensing and a high capture threshold with a variable effect on impedance.6 Imaging often reveals a change in the lead tip position compared with the initial chest radiograph. Lead perforation requires emergent lead revision. The authors of this chapter prefer to perform lead revision in the operating room, where access to urgent pericardiocentesis or thoracotomy, if necessary, in the event of tamponade is available. This risk is low, however, and many centers perform lead revision in an implant room without on-site surgical backup.
Dislodgment
Lead dislodgment can occur in up to 2% of ventricular permanent pacemaker leads, 5% of atrial leads and 5% of ICD leads.3,7–9 Dislodgment should be suspected if a significant change in sensed amplitude, lead impedance, or capture threshold occurs, particularly within the first month after lead insertion. Subsequent imaging may show that the lead tip has changed compared with the postprocedure radiograph.10 Most cases of lead dislodgment require surgical revision. In rare cases, the risk of lead revision is prohibitive, and the risk of the lead causing mechanical injury is low. In these cases, programming may satisfactorily resolve abnormal interrogation parameters.
High Defibrillation Threshold
Defibrillation threshold (DFT) testing is performed to assess the minimum energy needed for the device to successfully terminate ventricular fibrillation. The need for DFT testing is still being debated. The authors of this chapter perform DFT testing routinely in patients who have had a cardiac arrest and in cases where circumstances that may impair defibrillation efficacy (e.g., the lead tip is in a nonapical lead position, the generator is on the right side, or the patient is on medications known to increase defibrillation threshold) are present. In the remaining cases, the decision to test is operator dependent and performed in approximately 50% of cases. During the era when DFT testing was routinely performed, DFT was typically less than 10 J.11 An increase in DFT can be caused by lead factors (Table 88-1) or other factors such as patient factors, medications, and recreational drugs.12–24 Lead factors that may increase DFT are lead fracture, reversal of the coil terminal pins in a dual-coil system, and all causes that increase the high-voltage lead impedance.
| LEAD CAUSES |
Lead Performance Monitoring
Lead performance monitoring is essential to ensuring appropriate device function and helps be prepared for lead failure. This begins at device implantation by the documentation of specific lead, patient, and implantation factors (Table 88-2). These factors provide a baseline to compare subsequent interrogations.
Clinical Follow-up
Long-term monitoring of lead performance occurs simultaneously with ICD and permanent pacemaker generator follow-up. The recommended frequency of follow-up varies between ICDs and permanent pacemakers (Table 88-3).25–27 At the authors’ center, patients are seen in the clinic 1 week and 3 months after implantation, regardless of device type, and then every year for pacemakers and every 6 months for ICDs. In addition to in-clinic visits, trans-telephonic monitoring and remote monitoring systems facilitate lead follow-up by transmitting interrogation parameters to the device clinic at scheduled times. This allows the clinician to review the trends in lead performance and to be notified should lead performance deviate from predetermined normal values. Furthermore, should the interrogation parameters deviate from their expected values, the remote monitoring system can automatically transmit this information to the device follow-up team. The authors use remote monitoring systems for patients with leads or generators on advisory, in patients living remote from ready access to a follow-up clinic, and increasingly for all patients as capacity permits.
| PERMANENT PACEMAKER FOLLOW-UP | ICD FOLLOW-UP |
|---|---|
| At implantation | At implantation |
| Within 72 hours of implantation | Within 72 hours of implantation |
| 2–12 weeks following implantation | 2–12 weeks following implantation |
| Every 3–12 months | Every 3–6 months |
ICD, Implantable cardioverter-defibrillator.
Many manufacturers have warning systems that alert the patient to system malfunction in the form of an auditory or vibratory stimulus emitted from the device. These warning systems alert the patient to aberrations in the system function of the device. The primary lead parameter that these alerts monitor is lead impedance. If lead impedance deviates outside the programmable range, the alert is triggered, prompting the patient to seek medical attention.28 At each in-person visit, the pacing or ICD system (generator and lead) is reviewed to ensure normal function. This includes a focused patient history, physical examination, and device interrogation.
Measured Data
In their current iterations, many devices routinely assess lead performance by performing impedance, sensing, and capture threshold tests (Figures 88-2 through 88-4). These tests are typically performed daily and allow interrogation parameters to be tracked over time. At each clinic follow-up, lead performance is confirmed manually to ensure that the values measured daily by the device are valid and not discrepant from those measured automatically. Specifically, evaluation of lead function includes a pacing capture threshold test, impedance measurement, and P-wave and R-wave sensing.
Cardiac Resynchronization Therapy–Specific Follow-up
Cardiac resynchronization therapy (CRT) follow-up is similar to pacemaker and ICD follow-ups. In addition, left ventricular lead threshold should be assessed, anodal stimulation should be excluded, and right/left ventricular timing optimization should be evaluated. In newer devices, one can perform a capture threshold on both right and left ventricular leads individually. The authors find that it is helpful, and often necessary, to perform the capture threshold test with simultaneous 12-lead rhythm strips to determine when each lead fails to capture. Unipolar left ventricular leads use the lead electrode tip as the cathode and the right ventricular proximal ring or right ventricular coil electrode as the anode, depending on whether the lead is a true bipolar lead or an integrated bipolar lead. Occasionally, the anode will also capture the myocardium, as is more frequently seen at higher pacing outputs or with right ventricular leads that are true bipolar or pace/sense leads. This can result in unwanted right ventricular capture in the case of programmed left ventricular only capture or triple-site capture (left ventricular cathodal and right ventricular cathodal and anodal captures) with biventricular pacing. Performing a left ventricular lead capture threshold test with a 12-lead rhythm strip will aid in distinguishing anodal stimulation, pure left ventricular capture, and loss of capture.29
Lead Failure and Related Lead Problems
The incidence of pacemaker lead failure is highly variable. At one end of the spectrum, optimally performing leads fail at a rate of 0.2% to 1.0% per year after 5 to 10 years. In contrast, some leads fail at an unexpectedly high rate of 15% to 35% at 5 years, as high as 5.7% per year.30–32 However, it is reasonable to expect the annual failure rate of modern pacing leads to be less than 0.5%.30,33–35 ICD leads are much more complex, are larger, and thus have an attendant higher “expected” failure rate. In the current generation of ICD leads, the reported average annual failure rate ranges from 0.58% to 3.75%.36–38 The failure rate increases as leads age and can be as high as 20% per year in leads more than 10 years old.38 The estimated overall survival of ICD leads at 2, 5, and 8 years is approximately 91% to 99%, 81% to 95%, and 60% to 95%, respectively (Table 88-4).36–43 Lead design directly affects lead performance. For example, the type of insulation material correlates well with the risk of insulation failure. Although the median time to insulation failure is 7.2 years, this is significantly shorter with polyurethane insulation than with silicone insulation (5.7 years vs. 9.4 years).44
In addition, certain alterations in lead design have adversely affected lead safety and performance. The Accufix (Telectronics Pacing Systems, Engelwood, CO) atrial lead incorporated a preformed J-curved wire into the distal portion of the lead to allow for ease of positioning of the lead tip within the right atrial appendage. Over time, this wire protruded from the insulation resulting in laceration of the right atrium and, rarely, fatalities. In other cases, the J-wire fractured and embolized into the pulmonary circulation.45 Ironically, more patients died during interventions to remove the leads than from consequences of lead malfunction.45
More recently, an apparent improvement in reducing the size of an ICD lead resulted in suboptimal performance with the Sprint Fidelis ICD lead. This lead is a 6.6-F bipolar high-voltage lead, whose distribution was suspended in 2007 because of a higher-than-normal fracture rate.46 Although implantation technique and patient factors (age and ejection fraction) may contribute to lead fracture, the lead’s reduced diameter and construction are presumed to be the primary reasons for the increased risk of fracture.46
The rates and causes of lead failure are estimated on the basis of clinical variables and interrogation parameters. For example, lead fracture is often diagnosed with significant increases in lead impedance in the absence of other causes. Confirmation of the precise cause of lead failure is often not possible, as many leads that fail are either abandoned or damaged during extraction. These inferential observations, however, are often inaccurate in predicting the cause of lead failure and likely overestimate the incidence. When the leads with suspected failure are returned to the manufacturer for analysis, a substantial portion are found not to have the proposed malfunction.36
Mode of Presentation
Most (68%) of the pacemaker lead malfunctions are detected at routine follow-up or during scheduled remote follow-up monitoring. The remaining lead malfunctions are detected at unscheduled clinic visits (21%) or at the time of pulse generator replacement (9.1%).44 The indicators of lead failure include failure to capture on ECG (33%), high pacing threshold (14% to 30%), undersensing (13%), oversensing (12%), high impedance (5%), low impedance (12%), and a combination of capture and sensing abnormalities (13% to 15%).31,44
Patients with ICD lead malfunction most commonly present with inappropriate shocks (33% to 76%) caused by oversensing of intracardiac signals, particularly T-wave oversensing or sensing of artifact.38 The remainder of the patients are found to have an abnormally high-voltage lead impedance (56%), an increased capture threshold (22%), or noise on the lead (11%). These findings are typically discovered at routine testing (24% to 61% of lead problems) or at the time of elective generator replacement (2%).36,38,40,42
Lead Impedance Abnormalities
Lead impedance is an estimate of the combined effects of capacitance, resistance, and inductance in opposition to current flow. In simpler terms, it is a measure of stimulation resistance. Lead impedance is affected by lead electrode design (size, configuration, and materials). For example, smaller-diameter leads and bipolar leads tend to have higher impedance. The normal trend in pacemaker or pace/sense lead impedance is to slowly decrease over lead lifespan.47,48 However, lead impedance should not change significantly from visit to visit. A marked change from the previous measurement (i.e., over 200 ohms) is abnormal and warrants careful assessment.
Up until recently, high-voltage ICD lead impedance could only be tested following a shock or at device replacement or implantation. Presently, the high-voltage lead impedance can be measured during routine follow-up interrogation in all ICDs at regular intervals and through remote monitoring systems. This has provided insight into the normal trend and fluctuation in lead impedance of high-voltage leads. Normal high-voltage lead impedance falls slightly in the first few months after lead implantation, after which it can be expected to rise to baseline values within a year of implantation. From visit to visit, fluctuations in ICD lead impedance up to 6 ohms are often seen. However, a change in impedance of over 12 ohms should prompt an investigation, as this is unusual in normally functioning leads.49 Large fluctuations in impedance or aberrations in lead impedance, which are induced by provocative maneuvers at the time of device interrogation, may be caused by problems that manifest intermittently, such as conductor fracture, insulation break, or a loose set screw.
A marked increase in lead impedance in the short term after lead implantation is most likely caused by lead dislodgment, perforation, or an incomplete circuit caused by a header-connector problem, such as a loose connection between the lead and the pulse generator or when the lead is not fully inserted into the header. A rise in lead impedance beyond the early postimplant period may be caused by lead conductor fracture. A marked reduction in lead impedance after the early implant phase is most often caused by an insulation break (see below).41
Output Failure
Failed output occurs when an expected current is not generated at the conductor tip. This can be the result of generator or lead malfunction or more often erroneously suspected in normally functioning devices (Table 88-5). Lead-associated problems that result in failure to output include conductor fracture or a header connector problem, such as incomplete insertion of the lead pin into the connector block, incompatible lead and pulse generator, or a loose set screw.7
| NORMAL DEVICE FUNCTION (PSEUDOFAILURE) |
PVARP, Postventricular atrial refractory period; PVC, premature ventricular contraction; ECG, electrocardiogram.
Abnormalities in Capture Threshold
Capture threshold is the minimum amount of current necessary to elicit an evoked potential. The long-term trend in capture threshold varies from lead to lead. Historically, capture threshold would often rise after implantation because of the inflammatory response at the distal electrode. Current leads elute steroid from the distal electrode, typically eliminating this problem. As such, capture threshold should not deviate significantly from baseline over the lead’s lifetime.47,48
Failure to capture may or may not be lead related (Table 88-6). Unrelated causes include intrinsic cardiac disease, metabolic or electrolyte derangement, or medications.7,21,50,51 The most common causes of early lead-related failure to capture are lead dislodgment and lead perforation.52,53 Lead insulation break and conductor fracture are the most common causes of failure to capture later in the follow-up period.7 Local exit block is an uncommon cause of failure to capture and can occur at any time after lead insertion. In the acute setting, this is caused by inflammation at the lead-electrode interface. In the long term, fibrosis develops at the lead tip–myocardium interface, resulting in an associated increase in capture threshold.
Abnormalities in Sensing
Sensing is the ability of the device to detect intrinsic myocardial depolarization. The long-term R-wave amplitude is predicted by the R wave at first follow-up.47,48,54 Sensing abnormalities can be in the form of undersensing when intrinsic myocardial depolarization is not detected or oversensing when signals that do not represent myocardial depolarization are sensed.
Undersensing can result in inappropriate pacing in both pacemakers and ICDs and in the inappropriate withholding of therapy in ICDs (anti-tachycardia [ATP] and shocks). Undersensing can be caused by patient factors, generator factors, or lead factors or can be erroneously suspected in a normally functioning device (Table 88-7).7,51 Lead causes of undersensing include lead dislodgment, fibrosis at the tip electrode, poor initial lead position, lead insulation defect, and lead conductor coil fracture.7
| LOW-AMPLITUDE ELECTROGRAM |
