Abstract:
The speciality of electrophysiology has evoloved dramatically encompassing both detection, diagnosis, and treatment of complex cardiac arrthymias. This chapter provides the fundamentals to appreciate EP studies and apply new techniques in the EP laboratory.
Keywords:
electrophysiology, pacemaker, ICD, CRT, ablation, arrhythmia
During the past two decades, the dedicated electrophysiology laboratory has evolved into a highly specialized procedure room where a variety of procedures are offered, ranging from diagnostic electrophysiologic (EP) studies, curative catheter ablation procedures, implantation of loop recorders, and pacemakers, defibrillators, and resynchronization therapy devices to extraction of chronic in-dwelling leads.
The electrophysiologic study (EPS) is an invasive procedure that involves the placement of multipolar catheter electrodes at various intracardiac sites. Electrode catheters are routinely placed in the right atrium (RA), across the tricuspid valve annulus in the area of the atrioventricular (AV) node and His bundle (a special part of the conduction system), in the right ventricle (RV), in the coronary sinus, and sometimes in the left ventricle (LV; Fig. 7.1 ). The general purposes of EPS are to characterize the EP properties of the conduction system, to induce and to analyze the mechanism of arrhythmias, and to evaluate the effects of therapeutic interventions. Invasive EP techniques and procedures are routinely used in the clinical management of patients who have supraventricular and ventricular arrhythmias ( Box 7.1 ). In today’s laboratory, computer-generated electroanatomical maps are very much a part of the jargon, and even a novice must be able to recognize the color-coded activation patterns of common arrhythmias shown later in this chapter. Individuals seeking a more in-depth discussion of the procedures and concepts described should refer to the Suggested Readings section later in this chapter.
Diagnostic
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Diagnose SND
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Determine site of AV nodal block
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Define cause of syncope of unclear origin
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Differentiate VT from SVT in cases of wide-complex tachycardia
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Define mechanism of SVT or VT and map site of origin of tachycardia
Therapeutic
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Guide drug therapy for sustained VT, aborted sudden death, or SVT
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Select appropriate candidates for cardioverter-defibrillator and antitachycardia pacing therapy
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Test efficacy of device therapy for ventricular tachyarrhythmias
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Select appropriate candidates for catheter ablative and surgical therapy
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Test efficacy of ablative and surgical therapies
Interventional
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AV nodal ablation or modification for AF
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Ablation for atrial tachycardia and atrial flutter
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AV nodal modification (slow-pathway or fast-pathway ablation)
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Accessory pathway ablation in WPW syndrome
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Ablation of VT
Prognostic
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Risk stratification in asymptomatic WPW syndrome
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Risk stratification in patients after myocardial infarction
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Risk stratification in patients with nonsustained VT
AF, Atrial fibrillation; AV, atrioventricular; SVT, supraventricular tachycardia; SND, sinus node dysfunction; VT, ventricular tachycardia; WPW, Wolff-Parkinson-White.
Equipment
An EP laboratory is equipped with radiographic imaging systems, a recording and monitoring system, a stimulator, and all drugs and equipment required for advanced cardiovascular life support (ACLS) ( Fig. 7.2 ). Although a dedicated laboratory would be preferable, in many institutions, these procedures are performed in the cardiac hemodynamic-angiographic catheterization laboratory. If used for pacemaker and defibrillator implantation procedures, the room should have air filtering equivalent to that in a surgical operating room.
Although expensive and elaborate equipment cannot substitute an experienced and careful operator, the use of inadequate equipment may prevent adequate amounts of data from being collected and can make all the difference between success and failure. The arrhythmia targeted determines which equipment is required. A complete evaluation of most arrhythmias that may require activation mapping necessarily involves the use of multiple catheters, several recording channels, a programmable stimulator, and sophisticated and computerized three-dimensional (3D) mapping systems. Thus, an appropriately equipped laboratory should provide all of the equipment necessary for the most detailed study.
Electrode catheters
Diagnostic catheters
The hallmark feature of an EP catheter is the presence of at least two ring electrodes that can be used for bipolar and unipolar pacing and recording of local myocardial electrical activity. The material used to construct these catheters may be of the woven Dacron variety or synthetic materials, such as plastic or polyurethane. The number of electrodes in these catheters can vary between 2 and 20, interelectrode spacing between 2 and 20 mm, and thickness between 4 F and 7 F. The shape of these catheters can vary on the basis of the structures that they are designed to map: the crista terminalis, His bundle, coronary sinus, or pulmonary vein ostium. Typical EP catheters are shown in Figure 7.3 .
Ablation catheters
Ablation catheters of various designs allow the operator to map and to deliver energy in a very precise manner. These catheters vary with respect to the length of the ablation/tip electrode, which can range from 3.5 to 10 mm in length. Figure 7.4 shows commonly used ablation catheters. Notice that the tip of the catheter can be deflected to allow the arrhythmogenic myocardium to be reached. Conventionally, the tip of the ablation catheter is longer than the electrode of a diagnostic catheter to prevent overheating of the ablation electrode with consequent coagulum formation. Prevention of overheating of the ablation electrode can also be achieved by actively cooling with saline irrigation.
Electroanatomical mapping catheters
In the mid 1990s, a novel technology termed nonfluoroscopic electroanatomical mapping revolutionized the practice of interventional EP. Electroanatomical mapping systems integrate three functionalities: (1) nonfluoroscopic catheter localization in 3D space, (2) 3D display of activation sequences and electrogram voltage, and (3) integration of this electroanatomical information with noninvasive images of the heart (i.e., computed tomography, magnetic resonance images, or ultrasound images [image fusion]). Two leading mapping systems are available and most laboratories use one or both. They are (1) the CARTO 3 system, manufactured by Biosense Webster, Inc., and (2) the NavX system, manufactured by St. Jude Medical.
CARTO 3.
In the mid 1990s, Biosense Webster, Inc. created a catheter that has the appearance of a standard ablation catheter with a magnetic sensor within the shaft near the tip. Together with a reference sensor, it can be used to map precisely the 3D spatial location of the catheter ( Fig. 7.5 ). The electroanatomical mapping system is called the CARTO 3 system and consists of the reference and catheter sensor, an external ultra-low magnetic emitter ( Figs. 7.6 and 7.7 ), and a processing unit. The amplitude, frequency, and phase of the sensed magnetic fields contain information to solve the algebraic equations, yielding the precise locations (see Fig. 7.6 ) in three dimensions (x, y, and z axes) and orientation of the catheter tip sensor (roll, pitch, and yaw). An electrogram can also be recorded simultaneously in space, and an electroanatomical map can be generated. The catheter can also be moved without fluoroscopy, thus decreasing radiation exposure. An example of atrial tachycardia arising from a focal point that was mapped and ablated successfully with the use of the CARTO 3 system is shown in Fig. 7.7 for focal tachycardia. A left anterior oblique (LAO) view of an electroanatomical map of the right and left atria is shown as well as the coronary sinus. The activation data seen with the color scale show the arrhythmia to arise from the ostium of the coronary sinus.
NavX.
The NavX system uses three low-amplitude high-frequency current fields that are generated in three axes over the patient’s thorax to compute the position of an electrode in the thorax relative to a reference electrode that can be placed in the heart or on the patient’s thorax. On the basis of these measurements, the system then displays the position of any EP catheter. The advantage of this system is that multiple catheters can be displayed, and unlike the CARTO 3 system, they are not limited to the products of a single manufacturer. Figure 7.8 shows an example of an LAO cranial view of an electroanatomical map of the left atrium (LA, purple shell) along with the four pulmonary veins and the left atrial appendage (LAA, green) constructed with NavX. Also seen are the coronary sinus catheter, tip of the ablation catheter, and a circular mapping catheter. This map was performed in a patient undergoing catheter ablation of atrial fibrillation (AF).
Junction box and recording apparatus
The junction box and recording apparatus consists of pairs of numbered multiple pole switches matched to each recording and stimulation channel and permits the ready selection of any pair of electrodes for stimulation or recording. Current computer junction boxes come in banks of 8 or 16. Nowadays, the signal processor (filters and amplifier), visualization screen, and recording apparatus are incorporated as a single unit in the form of a computerized system. GE Healthcare, EP Med Systems (St. Jude Medical), and Bard Electrophysiology manufacture popular systems. Eight to 14 amplifiers should be available to process surface electrocardiogram (ECG) leads simultaneously with multiple intracardiac electrograms. The number of amplifiers can be as many as 128 in some systems. Intracardiac recordings must be displayed simultaneously, with at least three surface ECG leads. Most computers allow several pages to be stored, with one page displaying a 12-lead ECG. Thus, an operator can always have a 12-lead ECG recorded simultaneously while observing intracardiac electrogram data. The amplifiers used for recording intracardiac electrograms must have the ability to have gain modification and to alter both high and low band pass filters to permit appropriate attenuation of the incoming signals. For example, the His bundle electrogram is most clearly visualized when the signal is filtered between 30 and 40 Hz (high pass) and 400 and 500 Hz (low pass; Fig. 7.9 ). In addition, assessing unipolar electrograms also requires acquiring open filters (0.05 to 500 Hz).
Stimulation apparatus
Most EPSs require a complex programmable stimulator that has (1) a constant current source, (2) minimal current leakage, (3) the ability to pace at a wide range of cycle lengths (100 to 2000 ms) from at least two simultaneous sites, (4) the ability to introduce multiple extrastimuli, and (5) the ability to synchronize the stimulator to appropriate electrograms during spontaneous and paced rhythms. The stimulator is equipped with dials or switches by which the pacing intervals and coupling intervals of the extrastimuli may be adjusted ( Fig. 7.10 ). A junction box that interfaces with the recording system and stimulator facilitates changes in the pacing site without the need to disconnect catheters. The stimulator should be able to deliver variable currents that can be accurately controlled, with a range from 0.1 to 10 mA. The ability to change pulse widths is also useful. The results of programmed stimulation can be influenced by the delivered current, and for consistency and safety, stimulation is generally performed at two and a half times diastolic threshold.
It is preferable that the stimulator, computerized data recorder, and other devices used in EP are permanently installed. Most laboratories use a stimulator and computer system that modifies all input signals and stores them in an optical disc. All equipment must be grounded, and other aspects of electrical safety must be ensured because even small amounts of leakage current can pass to the patient and potentially induce arrhythmias. A technical engineer must check the equipment so that leakage current remains <10 mA. Figure 7.2 shows an illustration of the organization of the relevant equipment required during an EPS.
Defibrillator
A functioning defibrillator should be available at the patient’s side throughout all EPSs. A backup defibrillator is optimal in case of a rare but potentially disastrous failure of one defibrillator. Defibrillators should be tested before each study and equipped with an emergency power source. Many laboratories use commercially available R2 pads, which are placed on the patient before the EPS procedure begins. One pad is placed under the right scapula and the other on the anterior chest over the LV apex and connected to the defibrillator with an adapter. In rare instances in which transthoracic defibrillation fails to convert induced ventricular fibrillation (VF), emergency defibrillation through an intracardiac electrode catheter may be effective in terminating the arrhythmia ( Fig. 7.11 ). It is our practice to have biphasic defibrillators in our laboratories.
Because the physician’s attention is often focused on the stimulator and electrograms, he or she relies heavily on the nurse to monitor the patient’s condition and to communicate significant changes. The nurse usually sits between the patient and the cardioverter-defibrillator and crash cart. The nurse monitors the patient’s blood pressure, heart rate (HR), rhythm, and oxygen saturation via a pulse oximeter; administers drugs for diagnostic and therapeutic interventions during EPS; and performs cardioversion or defibrillation when an induced hemodynamically unstable arrhythmia appears. Optimally, a second nurse is available during procedures to administer medications or assist in technical aspects of the procedure.
Implantation of pacemakers, defibrillators, and resynchronization therapy devices
Pacemakers
Pacemakers, the original devices implanted for cardiac rhythm management, have the ability to provide electrical stimulation (pace) to the heart. They are indicated in patients who no longer have the intrinsic ability to provide adequate electrical stimulation to maintain a functional HR or complete conduction from the atria to the ventricles. They can be either single or dual chamber devices with a lead in the atrium, ventricle, or both, depending on the patient, pathology, and clinical scenario. Recently, a leadless pacemaker has been approved. This device is different from standard pacemakers in that the entire device is contained within the heart, which eliminates the need for a generator and a pocket, therefore reducing the risk of infection. These devices are single-chamber ventricular pacemakers implanted via the femoral vein and are an alternative for transvenous pacemakers for patients who only require ventricular pacing. ( Fig 7.12 )
A technique that has been showing benefit in recent research is that of His bundle pacing.
RV apical pacing causes interventricular dysynchrony, which can lead to adverse hemodynamics and progression to pacing induced cardiomyopathy (PCIM) in some patients. While biventricular pacing may be an option, it introduces a nonphysiologic ventricular activation sequence. Permanent His bundle pacing is an alternative option as it directly engages the His-Purkinje system, utilizing normal physiology to maintain synchronized ventricular activation.
Implantable cardiac defibrillators
Where pacemakers are used for bradycardic indications, defibrillators are designed to treat tachyarrhythmias, specifically ventricular tachycardia (VT) and VF. These devices have all of the functions of a pacemaker, but in addition, they have the ability to defibrillate (shock) the heart. The primary difference in these devices in comparison with a pacemaker is the use of a high-voltage lead in the RV with an active “can” (pulse generator [PG]) to complete the defibrillation circuit. The lead has at least one shock coil and acts with the PG to provide a defibrillation wave front across the heart if the need arises. In addition, all current defibrillators have the ability to pace the ventricle rapidly (anti-tachycardia pacing [ATP]) to attempt to terminate ventricular tachyarrhythmias and to prevent the need for a shock.
In addition to transvenous defibrillators, there are also subcutaneous defibrillators (SICD) that have become available in recent years. With transvenous ICDs, insertion of electrodes into the central venous circulation and inside cardiac chambers can cause vascular obstruction, thrombosis, infection, and cardiac perforation. In addition, lead failure has been estimated to be up 20% in 10 years. The SICD consists of a 3-mm tripolar parasternal lead (12 F, 45 cm) connected to an electrically active pulse generator. The lead is vertically positioned in the subcutaneous tissue of the chest, parallel and 1 to 2 cm to the left sternal mid line. The pulse generator is positioned in the subcutaneous tissue of the left lateral chest. ( Fig. 7.13 ). These devices are currently only capable of defibrillation and only have postshock (subcutaneous) pacing. They are not capable of performing any other pacing function, including ATP pacing to terminate arrhythmias or bradycardia pacing.
When a patient with an implantable cardiac defibrillator (ICD) undergoes a change in his or her antiarrhythmic drug regimen, the device may need to be tested, because changes in rate of tachycardia may necessitate reprogramming the device’s rate detection criteria, and changes in the defibrillation threshold may be caused by certain medications.
Primary prevention of sudden death
The survival rate after out-of-hospital cardiac arrest is extremely low, and attention has been directed at identifying high-risk patients who may benefit from prophylactic treatment as a means of primary prevention of sudden cardiac death. Two primary prevention trials indicated that patients with coronary disease, significant LV dysfunction (left ventricular ejection fraction [LVEF] 35% to 40%), spontaneous nonsustained VT, and inducible sustained ventricular arrhythmia by EPS experienced a survival benefit from prophylactic ICD implantation. There is no evidence that EPS-guided antiarrhythmic drug therapy is effective as preventive therapy for sudden cardiac death in high-risk individuals. A primary prevention trial, which did not require spontaneous or induced ventricular arrhythmias as entry criteria, concluded that prophylactic ICD implantation benefited patients with coronary artery disease and found an LVEF of less than 30%. This study suggests that poor LV function alone is a strong predictor of subsequent sudden cardiac death. Indications for implantation of pacemakers and ICDs are listed in Table 7.1 .
| Pacemaker | |
| SND |
|
| Acquired AV block in adults |
|
| |
| |
| Chronic bifascicular block |
|
| After acute phase of MI |
|
| Hypersensitive carotid sinus syndrome and neurocardiogenic syncope |
|
| After cardiac transplantation |
|
| Recommendations for permanent pacemakers that automatically detect and pace to terminate tachycardias |
|
| Pacing to prevent tachycardia |
|
| Pacing to prevent AF |
|
| Recommendations for pacing in patients with HCM |
|
| Recommendations for permanent pacing in children, adolescents, and patients with congenital heart disease |
|
| |
| |
| Implantable Cardiac Defibrillator | |
| |
| |
Cardiac resynchronization therapy
In the mid 1990s, a new tool was developed to assist in the management of patients with systolic heart failure (HF): biventricular pacing to improve systolic function. Since that time, the use of cardiac resynchronization devices has become a mainstay of an EP practice. These devices have undergone numerous refinements with time, and as clinical trials have been published, the patient population that can benefit from such therapy has greatly expanded ( Fig. 7.14 ). This section provides a broad overview of cardiac resynchronization therapy (CRT) as a common procedure performed in the EP laboratory.
Theory
HF remains an extensive and expensive problem in the United States, and the majority of HF patients have systolic dysfunction. As systolic dysfunction becomes progressively worse, both mechanical and electrical remodeling occurs. The electrical component manifests in the QRS duration. As QRS duration increases, morbidity and mortality levels from systolic dysfunction significantly increase. The electrical delay often leads to delayed activation of the LV. Because of this delay in activation, the septal wall is not held stable by simultaneous contraction of the LVs and RVs, leading to a less efficient contraction. This has led to the advent of cardiac resynchronization devices to allow for pacing of both chambers to resynchronize the contraction and stabilize the septum for a more effective LV contraction.
Clinical response to CRT is variable and 30% of patients are considered “nonresponders.” One of the most important correctable causes for lack of response to CRT is suboptimal coronary sinus lead position. As a result, multisite pacing (MSP) has been developed, which may be one way to improve the number of nonresponders in an appropriately selected patient population.
Implantation procedure
The primary difference in implanting a CRT device compared with standard pacemakers and defibrillators is in the placement of an LV lead in a branch of the coronary sinus. This adds an additional level of complexity to the standard implant procedure, and thus, knowledge of the coronary sinus anatomy is essential. Given that cardiac electrophysiologists routinely place catheters in the coronary sinus for EPSs, they are very experienced in the intricacies of working in this vessel and are natural implanters for these devices. The coronary sinus is often accessed via the axillary vein and a long sheath is passed into the coronary sinus to deliver the lead. LV leads are designed to be wedged into a branch vessel of the coronary sinus, and the lead tip usually has a cant or tab to help maintain the location of the lead ( Figs. 7.15 , 7.16 , and 7.17 ). Once a lead is advanced and located in a branch of the coronary sinus, the sheath is split and removed. The lead is sutured in place and can interface with a CRT defibrillator (CRT-D) or CRT pacemaker (CRT-P) PG.
Not only is it necessary to find a branch vein of the coronary sinus to accept the lead, location of the lead is also very important. Subgroup analyses of many major trials have investigated the importance of lead location and found that patients obtain the most benefit from a basilar location on the posterior or lateral LV. Anterior placement of the lead has no benefit because of the lack of distance between LV and RV pacing (apical RV) locations, so true resynchronization does not occur. In addition, an apical placement of the lead has been shown to be harmful because patients tend to benefit less on subgroup analyses.
With the higher complexity of implanting a LV lead comes a higher level of complications from the procedure. Procedure times tend to be longer for CRT device implantation when compared with dual chamber devices. In addition, a third lead increases the risk of short-term and long-term mechanical problems. Lead dislodgement occurs at a higher rate because of the passive mechanisms used to retain the lead in place. Often, the number of possible branches available is limited in any given patient who can accept a lead, and pacing thresholds can frequently be elevated compared with acceptable thresholds for RA and RV leads. Diaphragmatic stimulation can also be a problem, because branches from the coronary sinus can traverse very near the phrenic nerve, which can allow stimulation from the LV lead. However, even with the higher level of complications, the benefit from such resynchronization therapy can be substantial and in most patients these additional risks are easily justified.
Implantable cardiac monitors
The use of implantable cardiac monitors has recently grown in popularity as the size of devices has been reduced and the implantation procedure simplified. Implantable cardiac monitors, also known as loop recorders, can record and store arrhythmias. They are implanted subcutaneously and generally have a battery life of approximately 3 years. Such devices can be useful for patients with rare symptoms, in whom traditional monitoring is unlikely to provide a diagnosis, or with patients unwilling or unable to wear traditional noninvasive monitors. A relatively new indication for these monitors has been in the area of cryptogenic stroke. A significant portion of cryptogenic strokes is caused by asymptomatic paroxysmal AF. The implantable cardiac monitor provides a method of monitoring these patients for AF. A diagnosis of this arrhythmia would change therapy for a stroke with the initiation of anticoagulation ( Figs. 7.18 and 7.19 ).
