Assessment of Implantable Devices

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Chapter 13

Assessment of Implantable Devices

Patients with implanted pacemakers or automatic implantable cardioverter-defibrillators (AICDs) are commonly seen in the emergency department (ED). Fortunately, the increased reliability of these devices has prevented a marked increase in patients with true emergencies related to device malfunction, but such patients clearly have serious underlying medical problems that must be considered. Pacemaker complications are not uncommon, with rates ranging from 2.7% to 5%.1 Many pacemakers fail within the first year.2 AICD complication rates, including inadvertent shocks, occur in up to 34% of patients with the device.3 The basic evaluation and treatment of patients with cardiac complaints are not substantially different in patients with pacemakers and AICDs than in those without. However, a general knowledge of the range of problems, complications, and techniques for evaluating or inactivating pacemakers or AICDs is important for emergency clinicians. These devices are complicated, so appropriate consultation may be necessary, depending on the clinical situation.

Pacemaker Characteristics

In essence, a pacemaker consists of an electrical pulse–generating device and a lead system that senses intrinsic cardiac signals and then delivers a pulse. The pulse generator is hermetically sealed with a lithium-based battery device that weighs about 30 g and has an anticipated lifetime of 7 to 12 years. A semiconductor chip serves as the device’s central processing unit. The generator is connected to sensing and pacing electrodes that are inserted into various locations in the heart, depending on the configuration of the pacemaker. Newer models are programmable for rate, output, sensitivity, refractory period, and modes of response.4 They can be reprogrammed radiotelemetrically after implantation.

Pacemakers are classified according to a standard five-letter code developed by the North American Society of Pacing and Electrophysiology/British Pacing and Electrophysiology Group (Table 13-1). Known as the NBG code, it consists of five positions or digits. The first letter designates the chamber that receives the pacing current; the second, the sensing chamber; and the third, the pacemaker’s response to sensing. The fourth letter refers to the pacemaker’s rate modulation and programmability, and the fifth describes the pacemaker’s ability to provide an antitachycardia function. Whereas standard pacemakers generally do not have an antitachycardia function, AICDs do have this capability and overdrive pacing is the device’s first response to tachycardia. In normal practice, only the first three letters are used to describe the pacemaker (e.g., VVI or DDD).5

Pacemaker wires are embedded in plastic catheters. The terminal electrodes, which may be unipolar or bipolar, travel from the generator unit to the heart via the venous system. In a unipolar system, the lead electrode functions as the negatively charged cathode, and the pulse generator case acts as the positively charged anode into which electrons flow to complete the circuit. The pulse generator casing must remain in contact with tissue and be uninsulated for pacing to occur. In the case of bipolar systems, both electrodes are located within the heart. The cathode is at the tip of the lead, and the anode is a ring electrode roughly 2 cm proximal to the tip. Bipolar leads are thicker, draw more current than unipolar leads, and are commonly preferred because of several advantages, including a decreased likelihood of pacer inhibition as a result of extraneous signals and decreased susceptibility to interference by electromagnetic fields.6

The typical entry point for inserting the leads is the central venous system, which is generally accessed via the subclavian or cephalic vein. The terminal electrodes are placed either in the right ventricle or in both the right ventricle and the atrium under fluoroscopic guidance. Proper lead placement and sensing and pacing thresholds are assessed with electrocardiograms (ECGs).7 The typical radiographic appearance of an implanted pacemaker is shown in Figure 13-1.



Figure 13-1 A, Various radiographs of an implanted pacemaker and automatic implantable cardioverter-defibrillator showing battery and lead wires. Posteroanterior (PA; A1) and lateral (A2) chest radiographs demonstrate a biventricular pacing system. There are three leads—the first is positioned in the right atrium, the second is in the right ventricular apex, and the third courses posteriorly in the coronary sinus and into the posterolateral cardiac vein. B, PA chest radiographs of a dual-chamber pacemaker. B1, The ventricular lead is passing through an atrial septal defect into the left ventricle. B2, The lead is repositioned in the right ventricular apex. C, A dual-chamber implantable cardioverter-defibrillator with active fixation leads has been implanted via a transvenous approach to place the atrial lead in the systemic venous atrium and the ventricular lead across the baffle into the morphologic left ventricle. D, PA chest radiograph of a patient with a dual-chamber pacemaker (D1). The atrial lead, originally positioned in the right atrial appendage, is clearly no longer positioned in the right atrial appendage. A lateral view (D2) also shows definite dislodgment of the atrial lead. E, Close-up view of a portion of the PA chest radiograph of a patient with a single-chamber pacemaker. The lead has fractured (a subtle finding) at the point where it passes below the clavicle (arrow). The device showed intermittent ventricular failure to capture and intermittent failure to output on the ventricular lead. Impedance was intermittently measured at more than 9999 Ω. Inset, Diagram of the fracture site. (E inset, Courtesy of Telectronics Pacing Systems, Englewood, CO.)

The pacemaker is typically programmed to pace at a rate of 60 to 80 beats/min. A significantly different rate usually indicates malfunction. When the battery is low, the rate generally begins to drop and gets slower as the battery fades. Sensing of intracardiac electrical activity is a combination of recognizing the characteristic waveforms of P waves or QRS complexes while discriminating them from T waves or external interfering signals, such as muscle activity or movement. The pacing electrical stimulus is a triphasic wave consisting of an intrinsic deflection, far-field potential, and an injury current, which typically delivers a current of 0.1 to 20.0 mA for 2 msec at 15 V.8

Pacemakers have a reed switch that may be closed by placing a magnet over the generator externally on the chest wall; this inactivates the sensing mechanism of the pacemaker, which then reverts to an asynchronous rate termed the magnet rate. Essentially, the magnet turns the demand pacemaker into a fixed-rate pacemaker. The magnet rate is usually, but not always the same as the programmed rate.

Several innovations in rate regulation have been incorporated into some pacemakers. When present, the hysteresis feature causes pacing to be triggered at a rate greater than the intrinsic heart rate. When the hysteresis feature is used in a single-chamber ventricular pacemaker, it is designed to maintain atrioventricular (AV) synchrony at rates that are lower than what would be normal for a ventricular-paced rhythm alone. To illustrate, were the hysteresis feature of the pacemaker set at 50 beats/min, an intrinsic rate lower than 50 beats/min would trigger ventricular pacing. Unlike a standard ventricular pacemaker, the hysteresis feature might be set to offer a ventricular pacing rate at 70 beats/min or greater once the pacer is triggered.9

Rate modulation by sensor-mediated methods is an additional feature triggered and mediated by a sensed response to various physiologic stimuli.9 The primary application for this rate modulation feature is in patients with pacemakers who continue to engage in vigorous physical activity. When present, the rate regulation feature is engaged and modulated through motion sensors installed within a pulse generator device, with a corresponding increase or decrease in the pacing rate depending on the degree of motion sensed by the pacemaker device. Other physiologic sensors that may be installed as part of the pacemaker system include those designed to sense minute ventilation, the QT interval, temperature, venous oxygen saturation, and right ventricular contractions. The latter sensors generally require that additional leads be placed.

Characteristics of AICDs

The basic components of an AICD, including sensing electrodes, defibrillation electrodes, and a pulse generator (Fig. 13-2), can be seen on a chest radiograph. Transvenous electrodes have obviated the previous need for surgical placement. They are inserted into the pectoralis muscle. Many transvenous systems consist of a single lead containing a distal sensing electrode and one or more defibrillation electrodes in the right atrium and ventricle.10 Leads are inserted through the subclavian, axillary, or cephalic vein into the right ventricular apex. The left side is preferred because of a smoother venous route to the heart and a more favorable shocking vector.11 In an effort to improve the efficiency of defibrillation, an additional defibrillation coil may be used.11 Various placements of AICDs are demonstrated in Figure 13-3.

The pulse generator is a sealed titanium casing that encloses a lithium–silver–vanadium oxide battery. It has voltage converters and resistors, capacitors to store charge, microprocessors and integrated circuits to control analysis of the rhythm and delivery of therapy, memory chips to store electrographic data, and a telemetry module.12 Whereas a pacemaker can draw the voltage required for function from its component battery, the energy needed for defibrillation requires a battery that is prohibitively large.6 To circumvent this problem, an AICD contains a capacitor that maximizes the voltage required by transferring energy from the battery before discharge. To achieve the energy required, AICDs use capacitors that are charged over a period of 3 to 10 seconds by the battery and then release this energy rapidly for defibrillation.10 The maximal output is 30 J in most units and 45 J in higher-energy units.6 This energy is high enough that a discharge is very obvious and often distressing to the patient.

Most AICDs use a system in which the pulse generator is part of the shocking circuit, often described as a “can” technology, and most of them have a dual-coil lead with a proximal coil in the superior vena cava and a distal coil in the right ventricle.13 Current flows in a three-dimensional configuration from the distal coil to both the proximal coil and the generator.14 This dispersion of the electrical field increases the likelihood of depolarizing the entire myocardium at once, thereby leading to successful defibrillation.14

AICDs may have the same programming capabilities as pacemakers and can be single chambered, dual chambered, or used with cardiac resynchronization therapy (CRT) .15 Single-chamber devices have only a right ventricular lead. They often have difficulty identifying atrial arrhythmias, which can result in inappropriate defibrillation of atrial tachycardias. Dual-chamber AICDs have right atrial and right ventricular leads and improved ability to discriminate rhythms. In most studies, dual devices have been found to offer improved discrimination between ventricular and supraventricular arrhythmias, thus decreasing inappropriate shocks as a result of rapid supraventricular rhythms or physiologic sinus tachycardia.16 Approximately 50% of AICDs implanted in the United States are dual-chamber devices.17 CRT devices add an additional left ventricular lead that is placed in the coronary sinus or epicardium. In patients requiring both AICD and pacemaker functions, both these devices are placed together. The advent of technology has allowed placement of a single device that can perform both pacemaker and defibrillator functions.

AICDs use a combination of antitachycardia pacing, low-energy cardioversion, defibrillation, and bradycardiac pacing in a combination also known as tiered therapy. They are programmed with specific algorithms that identify and treat specific rhythms. Ventricular arrhythmias may initially be converted (or undergo attempts at conversion) with antitachycardiac pacing as opposed to immediate defibrillation. This overdrive pacing may terminate the rhythm without the need for electrical defibrillation in up to 90% of events. It is most successful for terminating monomorphic ventricular tachycardia with a rate of less than 200 beats/min.1 Overdrive pacing is better tolerated by patients than cardioversion and reduces the risk for inducing atrial fibrillation.18 These events may be silent, not felt by the patient, and discovered only by interrogating the device.

If unsuccessful, the next intervention may be low-energy cardioversion (<5 J). The device may be programmed to very low levels of electricity that, again, are better tolerated by the patient. This works best for ventricular rates higher than 150 and lower than 240 beats/min.14 This may be followed by a high-energy defibrillation. Traditionally, the energy level of the first shock is set at least 10 J above the threshold of the last defibrillation measured.12 If the first shock fails, a backup shock may be required, but this may induce or aggravate ventricular arrhythmias (see the later section “Pacemaker-Mediated Tachycardia”). Unlike the proarrhythmic effects of medication, these arrhythmias are almost never fatal, although they may be associated with increased morbidity.11 Currently used biphasic waveforms have improved defibrillation thresholds.12

This tiered approach obviates the need for unnecessary energy requirements. The devices also have antibradycardiac pacing that allows these patients to have one device instead of separate units. Additional complications associated with AICDs that have antibradycardiac pacing algorithms include a tendency toward oversensing, increased current drain, potential detection problems, and an increased incidence of hardware and software design problems.1 At the time of insertion the amount of energy required for various AICD functions, such as the defibrillation threshold, is determined for any given patient, and output and sensing functions can be adjusted by reprogramming as needed.

Indications for Placement of Implantable Pacemakers and Aicds

The most common indication for placement of a cardiac pacemaker is for the treatment of symptomatic bradyarrhythmias.19 Roughly 50% of pacemakers are placed in such patients for the treatment of sinus node dysfunction (sick sinus syndrome). Other diagnoses include symptomatic sinus bradycardia, atrial fibrillation with a slow ventricular response, high-grade AV block (including Mobitz type II and third-degree AV block), tachycardia-bradycardia syndrome, chronotropic incompetence, and selected prolonged QT syndromes. Though not classified as absolute indications, pacemakers are sometimes placed for the treatment of severe refractory neurocardiogenic syncope, paroxysmal atrial fibrillation, and hypertrophic or dilated cardiomyopathy.

In recent years, CRT has emerged as a primary approach for patients with severe diastolic dysfunction and a low left ventricular ejection fraction (LVEF).19 Commonly, such patients manifest low-grade AV blocks and left bundle branch block.20 The resultant delay in left ventricular conduction often results in corresponding biomechanical delays in ventricular contraction, which in turn causes a further decrement in cardiac output and worsening congestive heart failure. Such prolongation may occur in as many as 33% of patients with advanced heart failure.20 This electromechanical “dyssynchrony” has been associated with increased risk for sudden cardiac death.21

CRT comprises atrial-synchronized, biventricular pacemaking, which overcomes the atrial and ventricular blocks while optimizing both preload and LVEF.22 Clinical trials and systematic reviews have confirmed the efficacy of CRT, with decrements in mortality of 22% to 30%, as well as improved LVEF and quality of life.23,24 It is therefore likely that emergency physicians will see the CRT configuration with increasing frequency in patients with implanted pacemakers and AICDs.

The 2008 American Heart Association guidelines for implantation of a cardiac pacemaker are summarized in Box 13-1.19 AICD technology is used principally for both primary and secondary prevention in patients at risk for sudden death. Primary prevention is an attempt to avoid a potentially malignant ventricular arrhythmia in patients identified as being at high risk.25 Secondary prevention is for patients who have already had a ventricular arrhythmia and are at risk for further events. In addition, AICDs are implanted for a number of other congenital or familial cardiac conditions. Box 13-2 is a summary of class I indications for the placement of AICDs.26

Box 13-1   2008 American Heart Association Class I Indications for Pacing

B Acquired Atrioventricular Block in Adults

1. Third-degree and advanced second-degree atrioventricular (AV) block at any anatomic level associated with bradycardia, with symptoms (including heart failure) or ventricular arrhythmias presumed to be due to AV block.

2. Third-degree and advanced second-degree AV block at any anatomic level associated with arrhythmias and other medical conditions that require drug therapy resulting in symptomatic bradycardia.

3. Third-degree and advanced second-degree AV block at any anatomic level in awake, symptom-free patients in sinus rhythm with documented periods of asystole of 3.0 seconds or longer or any escape rate less than 40 beats/min or with an escape rhythm that is below the AV node.

4. Third-degree and advanced second-degree AV block at any anatomic level in awake, symptom-free patients with atrial fibrillation and bradycardia with one or more pauses of at least 5 seconds or longer.

5. Third-degree and advanced second-degree AV block at any anatomic level after catheter ablation of the AV junction.

6. Third-degree and advanced second-degree AV block at any anatomic level associated with postoperative AV block that is not expected to resolve after cardiac surgery.

7. Third-degree and advanced second-degree AV block at any anatomic level associated with neuromuscular diseases with AV block, with or without symptoms.

8. Second-degree AV block with associated symptomatic bradycardia regardless of the type or site of block.

9. Third-degree AV block at any anatomic site with average awake ventricular rates of 40 beats/min or faster if cardiomegaly or left ventricular dysfunction is present or if the site of block is below the AV node. Second- or third-degree AV block during exercise in the absence of myocardial ischemia.

J Pacing in Children, Adolescents, and Patients with Congenital Heart Disease

1. Permanent pacemaker implantation is indicated for advanced second- or third-degree AV block associated with symptomatic bradycardia, ventricular dysfunction, or low cardiac output.

2. Permanent pacemaker implantation is indicated for SND with correlation of symptoms during age-inappropriate bradycardia. The definition of bradycardia varies with the patient’s age and expected heart rate.

3. Permanent pacemaker implantation is indicated for postoperative advanced second- or third-degree AV block that is not expected to resolve or that persists for at least 7 days after cardiac surgery.

4. Permanent pacemaker implantation is indicated for congenital third-degree AV block with a wide QRS escape rhythm, complex ventricular ectopy, or ventricular dysfunction.

5. Permanent pacemaker implantation is indicated for congenital third-degree AV block in infants with a ventricular rate lower than 55 beats/min or with congenital heart disease and a ventricular rate lower than 70 beats/min.