Pacemaker Terminology

A letter code, initially established in 1974 and revised as technology advances, standardizes nomenclature for pacemakers.¹¹ Table 80-1 includes an explanation of the five-letter code scheme and the standard abbreviations in each category. The first three code letters are used most commonly. Using this table, one should be able to understand the features of any pacing mode. For example, a VDD pacemaker is capable of pacing only the ventricle, sensing both atrial and ventricular intrinsic depolarization, and responding by dual inhibition of both atrial and ventricular pacing if intrinsic ventricular depolarization occurs; a paced ventricular beat is triggered in response to a sensed intrinsic atrial depolarization. The codes of a permanent pacemaker that are used most frequently and the indications, advantages, and disadvantages of each are listed in Table 80-2. Detailed algorithms for matching a patient with a pacemaker exist.¹⁰ The majority of permanent pacemakers are dual chamber and often rate adaptive.¹²

Pacemaker Components

All pacemaker systems have three basic components: the pulse generator, which houses the power source (battery); the electronic circuitry; and the lead system, which connects the pulse generator to the endocardium.

Nearly all implanted pacemakers are lithium powered. Lithium-powered pulse generators function normally for 4 to 10 or more years, depending on the pacemaker features, such as single versus dual chamber, pacing threshold, and rate adaptiveness. This long “battery life” and the fact that the output voltage of the lithium-iodine cell decreases gradually rather than abruptly, as occurred with the early mercury-zinc cell, make sudden pulse generator failure an unlikely cause of pacemaker malfunction.

Permanent pacemakers have endocardial leads that are positioned in contact with the endocardium of the right ventricle and, in the case of a dual-chamber device, the right atrium, with a subclavian or cephalic vein approach used for insertion. Occasionally, an epicardial lead may be implanted during open-heart surgery performed for another indication, such as prosthetic valve insertion or correction of a congenital cardiac defect. Pacemaker leads, like power sources, continue to undergo major technical improvements.¹ Innovations include resilient plastic insulation surrounding the electrodes that reduces the chance of complete lead disruption or breakage (resulting in failure to pace or sense) and the chance of partial fracture (resulting in a “make or break” contact with intermittent failure to sense or pace). Despite these advances, problems with the electrical circuitry remain the most common cause of pacemaker malfunction.¹³ A lead capable of active fixation is more commonly used in patients with cardiomyopathies and right ventricular dilation complicated by tricuspid regurgitation.

Pacemaker leads may be either bipolar or unipolar in configuration. A bipolar endocardial lead has both the negative (distal) and the positive (proximal) electrodes, separated by approximately 1 cm, within the heart. A unipolar lead has the negative electrode in contact with the endocardial surface, and the positive pole is the metallic casing of the pulse generator. Each lead system has potential advantages and disadvantages.¹ The unipolar configuration is not compatible with ICD systems and is prone to oversensing of myopotentials and electromagnetic interference but is of smaller diameter and less susceptible to fracture. The bipolar configuration is compatible with ICD systems but is larger and more prone to lead fractures. Oversensing, however, is rarely a problem. The selection of lead configuration usually depends on the experience and preference of the operator.

The Standard Electrocardiogram during Normal Cardiac Pacing

The modern pacemaker has two basic functions: to stimulate the heart electrically and to sense intrinsic cardiac electrical activity. Additional functions are available and are noted in the pacemaker code system (see Table 80-1, letters 4 and 5). The pacemaker delivers an electrical stimulus to either the atrium or the ventricle if it does not recognize (sense) any intrinsic electrical activity from that chamber after a selected time interval. This interval is usually programmed at the time of implantation and can be changed noninvasively at a later time, if necessary, with use of a programming and “interrogating” device provided by the pacemaker manufacturer. If the pacemaker recognizes or senses an intrinsic atrial depolarization (P wave) or ventricular depolarization (QRS complex), it inhibits or resets its output to prevent competition with the underlying intrinsic rhythm. The stimulus intensity and sensing threshold (amplitude of electrical activity that is detected as being intrinsic) are typically set at the time of implantation but can also be reprogrammed later.

The two basic functions of a pacemaker can be easily recognized and confirmed on a standard 12-lead electrocardiogram (ECG) or rhythm strip. The normal function of a single-chamber VVI pacemaker is most easily recognized (Fig. 80-1). After a programmed interval is surpassed during which intrinsic ventricular activity does not occur, a pacer “spike” or stimulus artifact appears. The pacer spike is a narrow deflection that is usually less than 5 mm in amplitude with a bipolar lead configuration and usually 20 mm or more in amplitude with a unipolar lead. A wide QRS complex appears immediately after the stimulus artifact. Depolarization begins in the right ventricular apex, and the spread of excitation does not follow normal conduction pathways. Characteristically, a left bundle branch block conduction pattern is seen. A right bundle branch pattern is abnormal and suggests lead displacement. In VVI pacing the paced QRS complexes are independent of intrinsic atrial depolarization if present (AV dissociation).

The recognition of normal dual-chamber pacing is more complex owing to the interactive sensing and pacing of the right atrium and ventricle (Fig. 80-2).¹⁴ Pacing intervals are preprogrammed, may be changed noninvasively at a later time, and are generally specific to the patient’s needs. Pacing rates and delay intervals typically vary from patient to patient. Dual-chamber devices are typically used in patients with nonfibrillating atria coupled with intact AV conduction. A normal-appearing QRS complex may follow an intrinsic “P” wave as a result of normal sinoatrial node discharge if the intrinsic atrial depolarization is conducted to the ventricles. The intrinsic p wave and QRS complex inhibit the atrial and ventricular circuitry. A normal QRS complex follows a paced p wave if the paced atrial beat is conducted through the AV node and the programmed AV delay period is not exceeded. If it is not conducted to the ventricles (AV delay period exceeded), the pacemaker stimulates the ventricle, resulting in a paced p wave and a wide, paced QRS complex with left bundle branch block configuration.

Recognition of the interactivity of the paced chambers is important. A paced p wave may be mistaken for failure to sense or pace, and malfunction may be diagnosed when it is not present (pseudomalfunction). In addition, if the programmed rate of the pacemaker approximates the patient’s intrinsic heart rate, fusion of paced and native beats may occur and represents another common type of pseudomalfunction (Fig. 80-3).

Complications of Implantation

Pacemaker Malfunction

The term pacemaker malfunction refers specifically to problems with the circuitry or power source of the pulse generator, the pacemaker lead (most commonly displacement or fracture), or the interface between the pacing electrode and the myocardium (pacing or sensing threshold). In addition, environmental factors, such as extracardiac or extracorporeal electrical signals, may interfere with normal pacemaker function.20,21 With use of the standard ECG, pacemaker malfunction can be separated into three broad categories: (1) failure to capture (no pacemaker spikes or spikes not followed by an atrial or ventricular complex), (2) inappropriate sensing (oversensing or undersensing spikes occur prematurely or do not occur even though the programmed interval is exceeded), or (3) inappropriate pacemaker rate. Symptomatic pacemaker malfunction after implantation occurs in less than 5% of patients and is rarely immediately life-threatening. Malfunction is most commonly a result of inappropriate sensing, followed by failure to capture.¹⁷ Typical presentations and causes of pacemaker malfunction are listed in Box 80-3.

In the context of suspected pacemaker malfunction, knowledge of the pacing modalities (see Table 80-1) and what is normal for a given pacing modality are critical when the ECG is reviewed. Fortunately, patients are provided with important identifying information, usually in the form of a wallet card, after pacemaker implantation. The most important information is provided in the five-letter code. If this information is not available, a standard posteroanterior and a lateral chest radiograph can provide critical information. A single lead in the apex of the right ventricle indicates a VVI pacemaker. With VVI pacing, only one stimulus artifact or spike is seen with each stimulated ventricular depolarization (see Fig. 80-1). If sinus node activity is present, the paced QRS complex is dissociated from the intrinsic P waves. If separate leads are identified in the right atrium and right ventricle, the pacing modality is most often DDD or DVI, and paced p waves and QRS complexes (two spikes for each QRS complex) are seen (see Fig. 80-2). Although DDD and DVI units are capable of pacing both the right atrium and the right ventricle, only one spike may be seen (Fig. 80-4). Failure to identify two spikes with a DDD or DVI unit can represent normal pacemaker function.

A magnet placed externally over the pulse generator is frequently used in the assessment of pacemaker function. Magnet application causes closure of a reed switch within the pacemaker circuitry, converting the pacemaker to an asynchronous or fixed-rate pacing mode, and the pacemaker is no longer inhibited by the patient’s intrinsic electrical activity. The technique is most commonly used when the patient’s intrinsic heart rate exceeds the pacemaker’s set rate and pacemaker function is inhibited. Magnet application then allows pacing to occur, despite the patient’s native cardiac activity, and pacing rate and the presence of capture can be determined. Magnets are made by each manufacturer, but any cardiac pacemaker magnet will typically activate the reed switch within any of the devices.

Management

Disposition of the Emergency Department Patient with a Pacemaker

As a result of the current design of modern pacemakers and the frequent follow-up evaluation of patients with pacemakers, life-threatening emergencies resulting from pacemaker malfunction requiring immediate ED intervention are rare. Most instances of malfunction are subtle and difficult to recognize without interrogation of the pacemaker with manufacturer-specific devices by someone skilled in the technique. In all instances of suspected pacemaker malfunction, the patient’s cardiologist should be consulted.

Advanced Cardiac Life Support Interventions

Electrical defibrillation at recommended shock strengths (200, 300, and 360 J) can be safely performed in the patient with a pacemaker.¹⁷ If the sternal paddle is placed adjacent to the sternum, it is at a safe distance (>10 cm) from the pulse generator. Alternatively, defibrillation electrodes can be placed in an anteroposterior configuration. A cardiologist or technician should ensure that the pacing parameters of the unit are not altered if the resuscitation is successful. A chest radiograph should also be obtained after resuscitation to ensure that the pacing catheter was not displaced during chest compression, although this is an extremely uncommon occurrence.

Immediate return of pacing (capture) may not occur after defibrillation; this is commonly the result of global myocardial ischemia and increased pacing threshold and is not an indication of pacemaker malfunction. Temporary transcutaneous pacing may be needed if the pacemaker cannot be reprogrammed or normal pacing does not resume spontaneously. Transcutaneous pacing can also be safely used because the anterior and posterior pacing electrodes, if properly positioned, are distant from the pulse generator. Attempting temporary transvenous pacing is usually not necessary and is unlikely to be successful, especially if undertaken without fluoroscopic guidance. Chronic venous thrombosis, which is common and most often asymptomatic after pacemaker insertion, may preclude temporary catheter insertion through the neck veins. Insertion through the femoral vein is also difficult because the permanently implanted catheter may prevent entry into the right ventricle. Blind insertion may also dislodge the permanent catheter.

Terminology and Components

The majority of ICDs are now placed percutaneously in a manner similar to that of the standard pacemaker. A transvenous electrode system has largely replaced epicardial lead placement, which required thoracotomy. An epicardial defibrillation lead may occasionally be placed during coronary artery bypass surgery or in a few patients who cannot be defibrillated with use of existing transvenous electrode systems.

The typical modern ICD consists of components similar to those in the standard permanent pacemaker namely, a power source, electronic circuitry, and lead system. In addition, the standard ICD has a high-voltage capacitor and complex microprocessor memory.²³ The power source is lithium chemistry based with a battery life of 5 to 10 years. The longevity is largely determined by the frequency of shocks. All ICDs are also ventricular pacemakers, providing pacing for bradyarrhythmias.

The right ventricular lead is used for sensing and pacing, and shocks are typically delivered between a coil in the right ventricular lead and the pulse generator. If dual-chamber pacing is required, a second lead is placed in contact with the endocardium of the right atrium. A biphasic waveform is currently the preferred waveform for internal defibrillation. The shape and characteristics of the shock waveform vary among manufacturers. The biphasic waveform is more effective at lower energies than earlier monophasic waveforms and allows a smaller capacitor to be used, thereby reducing the size and increasing the comfort of the ICD unit.

The diagnostic and treatment functions of the ICD are determined at the time of implantation. In most instances, the cardioversion and defibrillation thresholds are determined at the time of ICD insertion by inducing ventricular tachycardia (VT) and ventricular fibrillation (VF) and adjusting the shock strength at a level above the minimum required to terminate the induced rhythm. Optimally, the required shock strength for defibrillation is less than half the maximum output (approximately 30 J) of the device. VT is typically managed with use of either low-energy shocks or programmed pacing that interrupts the VT reentrant circuit. Programmed pacing is less likely to have proarrhythmic effects and requires less energy, thereby extending battery life. In the setting of VF, ICDs are capable of delivering up to five additional discharges if the first shock fails.

The patient with an ICD should have close follow-up monitoring by a cardiologist familiar with ICD programming. This allows the cardiologist to determine the frequency of ICD activation (programmed antitachycardia pacing or shocks) and to confirm the programmed functions of the device. The majority of patients with ICDs have underlying heart disease, most commonly extensive atherosclerotic coronary artery disease, which is complicated by a low ejection fraction and congestive heart failure. The patient’s medications and metabolic status, such as electrolyte disorders that accompany diuretic usage, may also affect ICD function.

Complications of Implantation

Complications of ICD implantation are nearly identical in type and frequency to those of permanent pacemaker implantation. They include infection of the wound, the subcutaneous pouch fashioned for the device, and the lead system as well as acute thrombophlebitis and chronic thrombosis of the veins traversed for lead insertion. Management of these complications is similar to that for patients with permanent pacemakers.

Malfunction

Patients with ICD malfunction usually come to the ED with a limited number of specific symptoms (Box 80-4).

In contrast to patients with a permanent pacemaker, ICD patients are aware of when the ICD discharges to terminate VT or VF. The most common complaint of ICD patients is the occurrence of frequent shocks (i.e., occurring at a rate greater than they are accustomed to).²⁴ An increasing shock rate may be appropriate and not indicative of ICD malfunction if the patient is experiencing an increase in the frequency of VT or VF episodes. An increase in the frequency of episodes may occur in the setting of hypokalemia, hypomagnesemia, ischemia (with or without infarction) related to underlying coronary artery disease, or the proarrhythmic effect of drugs administered to decrease the frequency of ventricular tachyarrhythmias. Many ICD patients, particularly those to whom the technology is new, report that their device discharged, but subsequent device interrogation reveals that no discharge occurred.

An increase in the shock frequency is a manifestation of ICD sensing malfunction if (1) a supraventricular tachyarrhythmia is inappropriately sensed as VT, (2) shocks are delivered for nonsustained VT, or (3) intracardiac T waves detected by the ICD system are sensed as QRS complexes and the ICD interprets this as an increased heart rate. Temporary ICD deactivation with magnet application may be necessary if oversensing is the problem. Syncope, near-syncope, dizziness, or lightheadedness in the patient with an ICD may indicate undersensing of sustained VT or inappropriately low shock strength to terminate the rhythm. An approach to the evaluation of ICD malfunction is shown in Figure 80-8.

Advanced Cardiac Life Support Interventions

An ICD does not prevent sudden death in all patients at risk, and a patient with an ICD may arrive in cardiac arrest (2% annual incidence in patients with implanted devices). Cardiac arrest is not necessarily an indication of ICD malfunction. Appropriate repeated shocks may have been delivered but were ineffective. Alternatively, the ICD may not have sensed VF or the ventricular ectopic activity that typically precedes VF. Resuscitation efforts in the patient with an ICD should be undertaken in accordance with current recommendations. Transthoracic defibrillation can be performed in the standard manner with a monophasic or biphasic defibrillator if VF is the arrest rhythm. The sternal electrode or paddle should be placed in a parasternal location approximately 10 cm from the ICD subcutaneous pouch if the device has been implanted in the right deltopectoral area. If it has been implanted in the left deltopectoral region, this recommended safety distance is usually exceeded.

ICD discharge during manual chest compressions poses no risk to providers, although the rescuer may feel a weak shock. Although this generally is not indicated, the device can be deactivated with magnet application during resuscitation efforts. Deactivation is probably more important in the immediate postresuscitation period because recurrent ventricular dysrhythmias are common at this time after prolonged global myocardial ischemia during the arrest period, reperfusion, and the hyperadrenergic state, which is worsened by the use of intravenous epinephrine during resuscitation. ICD malfunction should be assumed, and these postresuscitation rhythms treated with standard pharmacologic agents (lidocaine and amiodarone). Although class 1 antidysrhythmic agents may raise the defibrillation threshold of the ICD, their impact on the defibrillation threshold during transthoracic countershock is clinically inconsequential owing to the high shock strengths that are used.

Disposition of the Emergency Department Patient with an Implantable Cardioverter-Defibrillator

As a result of the difficulty in documenting or excluding ICD function or malfunction in the patient with transient symptoms, the device should be interrogated to guide further evaluation and therapy. In cases in which the patient reports a single ICD shock, an assessment for acute cardiac ischemia, worsening of chronic congestive heart failure, symptoms of new-onset heart failure, and electrolyte abnormalities should be performed. In the absence of a change in clinical status, such patients can be discharged in consultation with the managing or consulting cardiologist after timely follow-up is ensured. For patients reporting multiple shocks, interrogation is essential, because in many of these cases the defibrillator has not discharged and the patient is experiencing hiccoughs, diaphragmatic twitching, or other nonelectrical phenomena. In such cases, discharge home is the rule. When multiple defibrillator discharges are confirmed by interrogation, immediate consultation is required along with admission to a monitored setting for extended telemetric observation. If frequent ventricular ectopy is noted, intravenous amiodarone should be given.²⁴ ICD interrogation allows assessment of ICD function and preceding dysrhythmia episodes.²⁵ Reprogramming may be necessary. If a lead problem is detected, reimplantation is required. A magnet can be placed over the ICD to inactivate the defibrillator. This should be done only if the emergency physician is confident that the ICD is delivering inappropriate shocks, such as a supraventricular tachycardia.

Biventricular Pacing

Biventricular pacing, also known as cardiac resynchronization therapy, is a therapy for patients with left-sided heart failure and ventricular dyssynchrony. It is beneficial for patients with New York Heart Association (NYHA) class III or IV heart failure despite optimal medical therapy, a left ventricular ejection fraction of 35% or less, and sinus rhythm with QRS duration of 120 msec or greater.26,27 Left bundle branch block causes an altered sequence of depolarization of the left ventricle such that the interventricular septum contracts before the left ventricular free wall, leading to inefficient mechanical pumping. Biventricular pacing “resynchronizes” the ventricles by simultaneously pacing the left and right ventricles, eliminating the delay in left ventricular free wall contraction and improving systolic function. Right atrial and right ventricular leads are positioned as for conventional atrial and univentricular pacing. The left ventricular lead is positioned in a left ventricular epicardial location via the coronary sinus and veins, preferably in a posterolateral or lateral location. The QRS duration of paced ventricular complexes is often but not always less than the QRS duration measured before resynchronization therapy.²⁸

Biventricular pacing can usually be recognized on the standard ECG (Fig. 80-9). Two stimulus artifacts or “spikes” may be seen preceding a paced QRS complex. With biventricular pacing, a predominantly negative QRS complex is seen in lead I, in contrast to the typical upright complex seen with right ventricular pacing (see Fig. 80-2). A predominantly positive QRS complex is seen in lead V₁ with biventricular pacing.²⁸

The complications and malfunctions inherent with conventional cardiac pacing are also observed with biventricular pacing. In addition, biventricular pacing has unique complications related to placement of the left ventricular pacing lead through the coronary sinus. In large clinical trials, coronary sinus dissection occurred in 0.3 to 4.0% of patients and coronary vein or coronary sinus perforation in 0.8 to 2.0% of patients. Cardiac tamponade caused by perforation of the coronary venous system is seen in less than 1% of patients. Dislodgement of the left ventricular electrode with resultant loss of pacing occurs as an early complication in approximately 10% of patients. Patients with malfunction of a biventricular pacing system frequently report palpations or acute decompensation of chronic heart failure.

Left Ventricular Assist Devices

Mechanical ventricular assistance devices have been used as a “bridge” to transplantation since the 1960s.²⁹ These implanted devices replace or support the pump function of the left ventricle and were originally bulky and mechanically complex owing to the pulsatile nature of the pump mechanisms. This usually resulted in extended in-hospital care while the patient awaited cardiac transplantation. Newer devices, such as the Jarvik 2000 and HeartMate II, are continuous flow pumps that are portable and powered with a comparatively smaller battery pack. Implantation is associated with fewer infectious complications and lower immediate postoperative mortality, and survival for more than 1 year is increasingly common. The greatest mortality is noted within the first 30 days after implantation and during hospitalization. Left ventricular assistance devices (LVADs) are commonly used as “destination” therapy in patients who do not qualify for cardiac transplantation.^29,30

Patients with an LVAD typically require care at cardiac transplant centers. As technology progresses, emergency physicians not based at a transplant center may encounter a patient with an LVAD. In such cases, telephone consultation with an expert at a transplant center may help avoid missteps. The more common complications with this device include infection and embolic stroke. Device failure usually results in severe heart failure. If necessary, dopamine, dobutamine, or a combination of these drugs may be given to treat congestive heart failure symptoms while the patient awaits transfer. If cardiac arrest occurs, chest compressions may dislodge the device. The hand pump should be used to provide circulation if this backup method is available and early transition to cardiopulmonary bypass should be considered (Table 80-3).

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