Pediatric Cardiac Catheterization and Electrophysiology

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Chapter 69

Pediatric Cardiac Catheterization and Electrophysiology

Pediatric Cardiac Catheterization

Procedural Techniques

Catheterizations are performed with use of the standard Seldinger technique in the femoral artery and/or vein. Alternative venous access includes the internal jugular, subclavian, or transhepatic approaches. Arterial access also may be obtained via the arteries of the upper extremities or the carotid artery, but these approaches typically are restricted to larger patients or emergent conditions. Anticoagulation is managed with heparin, 80 to 100 U/kg or 5000 U in patients who weigh more than 50 kg, for an activated clotting time goal of 200 to 250 seconds, depending on the procedure to be performed. Antibiotics are administered to patients who receive an implanted device. Baseline hemodynamics (including pressures and oxygen saturations) and angiography are obtained in room air, whenever possible. Subsequently, relative blood flows and vascular resistances are calculated. The ratio of pulmonary blood flow (Qp) to systemic blood flow (Qs) is calculated according to the following formula: Qp/Qs = [(Ao2 − Mvo2)/(Pvo2 − Pao2)], where Ao2 is the aortic oxygen saturation, Mvo2 is the mixed venous oxygen saturation, Pao2 is the pulmonary arterial saturation, and Pvo2 is the pulmonary venous saturation. These data then determine whether additional information or interventions are necessary.

Semilunar Valve Stenosis and Balloon Valvuloplasty

Since transcatheter balloon pulmonary valvuloplasty for valvar pulmonary stenosis in infants was first reported in the early 1980s, it has become the first line of therapy. The recommended balloon/annulus diameter ratio is 120%.2 The reported success rate is greater than 90%, with major adverse events occurring in fewer than 1% of the procedures.3 Hemodynamic measurements include the right ventricular pressure compared with systemic arterial pressure and the peak-to-peak systolic pressure gradient across the pulmonary valve. The indication for balloon pulmonary valvuloplasty is the presence of at least moderate pulmonary valve stenosis. We use as a guideline the “rule of 50,” which is defined as a peak right ventricular systolic pressure of more than 50 mm Hg, a peak right ventricular systolic pressure more than 50% of the systemic systolic pressure, or a peak-to-peak systolic gradient across the pulmonary valve of more than 50 mm Hg.

Valvar aortic stenosis can be classified into two groups: disease that is severe enough that it presents at birth or within 1 year of age (10% to 15%), and disease that is not diagnosed until after age 2 years and will progress much more slowly, if at all.4,5 Mortality and the need for intervention are significantly skewed toward the infantile group. As with pulmonary stenosis, noninvasive imaging techniques have advanced to the point that nearly all anatomic and functional information about the valve may be obtained without catheterization. Catheterization is performed for valves that clearly merit intervention or when symptoms and imaging findings are incomplete or confounding.

Aortic valve stenosis is classified into the following categories: trivial, mild, moderate, severe, and critical. Critical aortic stenosis is not defined by a specific pressure gradient or valve orifice size but on the basis of physiologic manifestations. If the stenosis is such that the patient is unable to produce and maintain an adequate cardiac output, the stenosis is critical. Patients in this group may have a low valve gradient, as measured by echocardiography, because of decreased cardiac function and low cardiac output. Although some controversy still exists with regard to the most beneficial treatment method for this population (i.e., surgical valvotomy vs. percutaneous balloon valvuloplasty), most centers have adopted balloon valvuloplasty as the initial treatment of choice. Patients in this category do not tolerate the stress of any procedure well, and catheterization has immediate results comparable with those of surgery (i.e., a reduction in gradient and resultant valve regurgitation) with a shorter course in the intensive care unit after the procedure and a shorter overall hospital stay.

Balloon valvuloplasty has been associated with an increased rate of reintervention compared with surgical valvotomy as a result of recurrent stenosis or worsening regurgitation. Given that residual aortic valve disease, especially regurgitation, may progress over time, the recommendation for the valvuloplasty technique is more conservative, with a smaller maximal balloon diameter (80% to 100% of the annulus) than that recommended for the pulmonary valve (100% to 120%). The valve may be approached retrograde from the aorta, using a soft-tipped J-wire to cross the narrowed valve orifice and obtaining arterial access in the femoral artery (more commonly) or the carotid artery. The valve also may be approached prograde by crossing an existing atrial communication or by performing a transseptal puncture to access the left side of the heart. Once in the left ventricle, angiography is performed to measure the annulus of the valve and obtain landmarks for valvuloplasty. The diameter of the balloon should not exceed 80% to 90% of the valve annulus. The smaller balloon diameter, compared with a similar sized pulmonary valve annulus, is recommended to decrease the amount of valve tearing and resultant acute regurgitation.

Many centers have adopted rapid right ventricular pacing at the time of balloon inflation. This rapid pacing transiently reduces cardiac output and the shearing force transmitted to the balloon as it is inflated across the valve annulus. The goal is to reduce the motion on the fragile valve leaflets and prevent excessive damage and regurgitation. Repeat angiography and echocardiography after inflation are essential to evaluate the success of the valvuloplasty and monitor for regurgitation or other complications.

The differentiation between noncritical stenosis categories is made by noninvasive echocardiographic measurements of valve area and Doppler gradient. A normal valve area is 2 cm2/m2. Mild obstruction is consistent with valve areas less than 2 cm2/m2 but greater than 0.7 cm2/m2, and severe obstruction is consistent with valve areas less than 0.5 cm2/m2. Mean echocardiographic Doppler gradients are good predictors of the peak-to-peak pressure gradient measured at catheterization. Gradients less than 25 mm Hg are considered trivial, those 25 to 50 mm Hg are mild, those 50 to 75 mm Hg are moderate, and those >75 mm Hg are severe. These measurements are made with the understanding that the cardiac function and cardiac output are normal.

Catheterization is not recommended for trivial or mild stenosis. Moderate and severe stenoses are approached with primary balloon valvuloplasty using the techniques previously described.

Balloon or Stent Angioplasty

Since the late 1980s, when balloon angioplasty was performed with low-pressure balloons (<5 atmospheres), balloons capable of dilations at higher pressure have been introduced. Since the introduction of high-pressure balloons, standard balloon angioplasty has resulted in a successful result of more than 70% for pulmonary artery angioplasty. As for coarctation or recoarctation, balloon angioplasty also has been used with some success. Patients who have severely hypoplastic pulmonary arteries with multiple stenoses that are refractory to high-pressure angioplasty may not be treatable using currently available medical, surgical, or catheter-based tools. In such cases, a “waist” persists during angioplasty at the maximum recommended inflation pressure, or even at pressures exceeding the recommended maximum, because of inadequate stretching or tearing of the vessel wall. For this patient population, cutting balloon angioplasty has become a therapeutic option with use of balloons up to 8 mm in diameter. Cutting balloon angioplasty often is followed by standard angioplasty for the best final result. Balloon-expandable stents have improved results in proximal vessels, eliminating the need for surgery in most patients, but they are of limited value in distal pulmonary vessels. Furthermore, stents are contraindicated in noncompliant vessels that cannot be expanded using high-pressure balloons. Stent angioplasty also has been utilized for coarctation of the aorta in postoperative and native lesions.

Pulmonary artery stenosis is a form of congenital heart disease that can occur in isolation or as part of more complex malformations such as tetralogy of Fallot. Although obstructions confined to the main or proximal branches can be repaired surgically, many patients can receive adequate palliation with transcatheter balloon angioplasty techniques. In addition, more distal obstructions that cannot be repaired operatively require angioplasty with balloons delivered to the distal stenotic segments as previously described. The inflation of a balloon whose maximum diameter is two to three times the diameter of the lesion will tear the vessel wall within the stenotic segment, resulting in an increase in lumen diameter. Although balloon catheters have not specifically been approved by the Food and Drug Administration as a treatment for pulmonary artery stenoses, transcatheter techniques have become the mainstay of treatment for distal vessel obstruction.

Often, aortic obstruction after an end-to-end surgical repair at the isthmus is a result of aortic narrowing within the transverse arch. These obstructive lesions are further defined as either the proximal transverse arch between the innominate artery and left carotid artery or the distal transverse arch, defined as the region between the left carotid artery and the left subclavian artery. Although few data exist regarding stent angioplasty within the distal transverse aortic arch, general experience has been that this procedure is safe and effective. An arterial monitoring catheter is placed in the right upper extremity and a 4 Fr sheath is used to advance a 4 Fr pigtail to the aorta from the right radial artery. This catheter is used to monitor pressures during stent angioplasty of the distal transverse arch and to perform cine angiograms to determine appropriate stent placement distal to the takeoff of the left carotid artery.

Transcatheter stent angioplasty for postoperative recoarctation of the aorta at the isthmus has been demonstrated to be safe and effective.5 The technique usually involves a femoral arterial approach. An exchange length wire is placed in the ascending aorta or right subclavian artery. An angioplasty balloon is used with a maximum diameter that is equal to or less than the diameter of the normal aortic segments adjacent to the stenotic region and/or the diameter of the descending thoracic aorta at the diaphragm. The stent is mounted on the angioplasty balloon and passed through a sheath at least 1 to 2 Fr larger than that required by the balloon. The stent length is dependent on the lesion length but usually is at least 36 mm in adults. The stent is fully dilated in most cases, but at times it is deemed safer to serially dilate the lesion over two procedures.

Septal and Vascular Occlusion Devices

Although diagnosing the presence of an atrial septal defect (ASD) rarely requires cardiac catheterization, today many patients are undergoing cardiac catheterization for therapeutic device closure.6 These patients require assessment of associated anomalies such as abnormalities of pulmonary venous connections. A step-up in oxygen saturations in the right atrium and pulmonary arteries is characteristic for an ASD, and the degree of left-to-right shunting or the pulmonary to systemic blood flow ratio (Qp : Qs) can be determined. The ideal age or timing for elective ASD closure is 2 to 5 years of age or within 6 to 12 months of diagnosis. Rarely, a child with an ASD presents with severe congestive heart failure and requires intervention in the first year of life.

Percutaneous ASD closure has been established as a safe and effective alternative to operative repair. The technique involves transcatheter delivery of a device in its retracted state via the femoral vein under fluoroscopic and echocardiographic guidance. Echocardiographic guidance can be transthoracic in young children and transesophageal, intracardiac, or even three-dimensional (3D) in older children. The most commonly available devices today consist of a double disc design made of nickel and titanium (Nitinol) with deployment of the first disc on the left atrial aspect of the septum followed by deployment of the second opposing disc on the right atrial wall. The expanded discs are tightly approximated, thus closing the defect. The device becomes endothelialized during the next 3 to 12 months while the patient is treated with antiplatelet medications. Contraindications include some very large defects, the absence of adequate septal tissue margins, close proximity to vital cardiac structures, and very small children. ASD devices require an adequate tissue margin (minimum 7 to 8 mm) for deployment. The ASD must be small enough that the device can be deployed and held in the atrial septum (an adequate margin must be present on all sides other than the anterior superior rim by the aorta, where some splay of the device around the aorta will compensate for deficient rim) without impingement on adjacent structures or significant pressure on the walls of the atrium. Venous inflow to both the right and left atria must not be impeded, and the tricuspid and mitral valves should not come into contact with the device.

The world experience of transcatheter ASD closure using the Food and Drug Administration–approved Amplatzer Septal Occluder (AGA Medical Corporation, Golden Valley, MN) has been reported with a greater than 97% immediate success rate.7 The occlusion rate reached 100% by 3 years. An overall 2.8% adverse event rate was found, and no procedural deaths occurred. Several studies regarding comparisons of device occlusion versus surgical closure have been performed prospectively. These reports include the Amplatzer and Helex (W.L. Gore & Associates, Flagstaff, AZ) devices for ASD closure. Findings include shorter hospital stays, less discomfort, and shorter durations for convalescence in patients undergoing successful closure with use of a device. Hospital costs are similar. Regression of right ventricular dilatation was similar for both groups of patients; however, it was dependent on the patient’s age at the time of closure, with greater regression following earlier intervention.

Procedural adverse events are uncommon but include embolization into the right or left atrium, pulmonary artery, left ventricle, and aorta; stroke as a result of a clot or air embolization; and bleeding complications.8 Both acute and late embolization have been reported, and thus it is critical for the radiologist to be familiar with the location of the interatrial septum on radiography to ascertain correct device position in the anteroposterior and lateral chest radiograph. Figure 69-1, A, demonstrates a septal occluder device in the appropriate position within the interatrial septum, and Figure 69-1, B, demonstrates the device positioned incorrectly in the left pulmonary artery.

The first transcatheter interventional procedure was closure of a persistently patent ductus arteriosus (PDA), which was performed by Portsmann in 1967. Since that time a number of different PDA closure devices have been studied, some of which are no longer available. The goal of the procedure has always been to achieve 100% closure of the PDA without obstruction of either adjoining blood vessel (i.e., the aorta and the left pulmonary artery) with minimal risk of complications. The difficulty is that the PDA exhibits extreme variability in size and shape. Krichenko et al.9 classified the PDA into five anatomic types based on the lateral aortic angiogram. The most common (“type A”) ductus is conical with a narrowed pulmonary arterial end and large aortic ampulla. Other types include those with a narrowed aortic end, narrowing at both ends, and a tubular configuration. Early device closure procedures were complicated by a lack of choices for vessels of different sizes and shapes. The early devices had unsatisfactory rates of complications and residual leaks and were abandoned. In 1992, the first report was published using the Gianturco embolization coil (Cook Medical, Bloomington, IN) for closure of small PDAs. The Gianturco coil, which is available in multiple lengths and diameters, has now been in use for more than 20 years for blood vessel occlusion, including unwanted collateral vessels, fistulae, and arteriovenous malformations. The successful use of the coil in PDA closure, combined with sharing of ingenious techniques to deploy multiple coils and secure the coils before deployment by individual operators, provided the needed variety of approaches for successful PDA closure. At present, tens of thousands of patients around the world have had PDAs closed with embolization coils.

As expected, the recommended technique for coil embolization is variable, depending on the size and shape of the PDA. The vessel can be approached from the venous or arterial side, and the coils may be deployed “free hand” or secured with a bioptome or modified catheter. Smaller coils, such as the Flipper coil (Cook Medical), also are available; these coils are attached to a delivery system and are released once they are verified to be in the proper position.

A more recent addition to the interventional cardiologist’s armamentarium for PDA closure is the Amplatzer Duct Occluder.10 The PDA occluder is a self-expanding wire mesh device that is attached to a delivery cable and deployed through a long sheath from the venous system. Figure 69-2, A, demonstrates a lateral projection angiogram of the descending thoracic aorta and a left-to-right shunting PDA. Figure 69-2, B, demonstrates device occlusion of the PDA. The device has a retention skirt to occupy the aortic ampulla and tapers slightly to the pulmonary artery end. The device is filled with a polyester mesh that stimulates thrombus formation within the lumen of the PDA. The shape of the device and self-expanding properties exert radial force on the walls of the PDA, holding the device in place until endothelialization occurs. The device has excellent closure rates approaching 100% at 1 month after the procedure. The main limitation is the size and bulky nature of the device. The PDA must have an aortic ampulla adequate to accommodate the retention skirt without creating aortic obstruction. Furthermore, the device can create left pulmonary artery stenosis by compressing adjoining structures once it is released.

Accessory blood vessels are a common area of concern for the interventional radiologist and interventional cardiologist. These accessory blood vessels may include aortopulmonary or venovenous collaterals, arteriovenous fistulae and malformations, surgically placed shunts, and transhepatic access tracts. Techniques for closure are similar to those described for the PDA, predominately involving Gianturco and similar coils, the Amplatzer Duct Occluder, or the Amplatzer Vascular Plug. The vascular plug is similar to the duct occluder in that it is a self-expanding wire mesh design. It differs in that it is cylindrical in shape with no retention skirt, no tapering through its length, and no polyester mesh interior fabric. The device has excellent occlusion results but has been unsuccessful in short arterial vessels, such as aortopulmonary collaterals and the PDA. It is thought that the lack of occlusive material through the center of the device does not provide enough restriction to arterial blood flow to stimulate thrombosis and occlude the vessel.

Pediatric Electrophysiology

Catheter Ablation in Children

History and Indications

The surgical elimination of an accessory atrioventricular pathway in 1968 heralded the era of curative therapy of tachycardia substrates.11 Transvascular catheter delivery of direct current to create atrioventricular block in adults with troublesome atrial fibrillation was first reported in 1983.12 However, the modern era of catheter ablation began in 1987 with the use of alternating current in the radiofrequency range (about 550 kHz) to ablate an accessory pathway.13 Unlike direct current, radiofrequency current causes resistive heating, thus creating a fairly well-circumscribed lesion and minimizing collateral damage. This technology rapidly expanded to include children14 and patients with congenital heart disease.15 Almost no tachyarrhythmia substrate is now considered exempt from catheter ablation therapy thanks to newer catheter designs, new electroanatomic mapping technologies, and additional energy sources, especially cryoenergy. Indications to perform catheter ablation in children remain somewhat limited by the benign natural history of some tachyarrhythmia substrates,16 continued concern of damage to nearby structures, and concern for scar expansion with somatic growth.17 The arrhythmia substrates that commonly undergo ablation in children appear in e-Table 69-1, and published indications for performing ablation appear in e-Box 69-1.18

e-Table 69-1

Tachyarrhythmia Types and Anatomic Targets for Catheter Ablation

Tachyarrhythmia Type Substrate Target
Atrial flutter Zone of slow conduction between conduction barriers
Atrial ectopic tachycardia Focus of earliest activation
Atrioventricular reciprocating tachycardia Accessory pathway
Atrioventricular nodal reentry tachycardia “Slow inputs” to the atrioventricular node
Ventricular tachycardia in the normal heart Focus of earliest activation
Ventricular tachycardia after congenital heart surgery Zone of slow conduction between conduction barriers

e-Box 69-1   Indications for Radiofrequency Catheter Ablation in Pediatric Patients

Class II B

Clear divergence of opinion exists regarding the need for the procedure in persons with the following conditions:

1. Asymptomatic preexcitation (WPW pattern on an electrocardiograph), age >5 years, with no recognized tachycardia, when the risks and benefits of the procedure and arrhythmia have been clearly explained

2. SVT, age >5 years, as an alternative to chronic antiarrhythmic therapy that has been effective in control of the arrhythmia

3. SVT, age <5 years (including infants), when antiarrhythmic medications, including sotalol and amiodarone, are not effective or are associated with intolerable adverse effects

4. Intraatrial reentrant tachycardia, one to three episodes per year, requiring medical intervention

5. Atrioventricular node ablation and pacemaker insertion as an alternative therapy for recurrent or intractable intraatrial reentrant tachycardia

6. One episode of VT associated with hemodynamic compromise and that is amenable to catheter ablation

Technical Considerations

The guiding principle of electrophysiologic testing and catheter ablation involves coupling of anatomic structures to electrical phenomena. Most cases can be accomplished with standard fluoroscopy and multiple multielectrode catheters positioned in the right ventricle, right atrium, coronary sinus, and His bundle region of the tricuspid valve annulus, with one for mapping and ablation (Fig. 69-3). In a normal heart, these anatomic sites can be easily accessed from a combination of femoral, internal jugular, and subclavian venous approaches. However, in some patients with complex congenital cardiovascular anomalies (e.g., interrupted inferior vena cava in some patients with heterotaxy) and in others having undergone certain congenital heart operations (e.g., cavopulmonary connection or extracardiac conduit in patients having single ventricle physiology), some or all of these venous sites will not allow access to the heart. Moreover, patients who have undergone multiple prior procedures may have permanent venous occlusion of some of these access pathways. In these instances, alternate approaches may include transhepatic venous,19 arterial/retroaortic, and transthoracic access.20 Furthermore, occasional arrhythmia substrates are primarily epicardial, requiring transpericardial access. Transthoracic or intracardiac echocardiography21 may aid in understanding anatomic details and in the positioning of catheters.

Even high-resolution fluoroscopy in multiple projections is insufficient to display the internal cardiac topography after complex congenital heart surgery. Therefore technologies using mathematically derived reconstructions have been developed for real-time creation of 3D cardiac chambers and associated structures.

The CARTO system (Biosense Webster, Diamond Bar, CA) allows an endocardial map to be created using a low-energy, triple-source transmitter located on a position pad mounted beneath the patient, a receiver in the tip of the specialized mapping/ablation catheter, and global positioning system technology. A second electrode catheter in a fixed intracardiac position serves as a temporal reference, thus permitting electroanatomic coupling by the mapping catheter as it is manipulated to create an anatomic rendition of the chamber(s) of interest. Isochronal activation, isopotential, and animated activation maps may be produced (e-Fig. 69-4).

The Ensite system (St. Jude Medical, St. Paul, MN) uses a noncontact multielectrode array that is mounted on a balloon catheter and centrally positioned in the chamber of interest (intracavitary). Ring electrodes on this catheter, proximal and distal to the array, serve as receivers from a low-current “locator” signal delivered from any second standard electrode catheter. As this second catheter is rapidly swept along all endocardial surfaces of the chamber of interest, a 3D computer model of the endocardium is generated. Far-field electrical activity recorded from each electrode on the array is enhanced and resolved based on an inverse solution to Laplace’s equation. The inverse solution considers how a signal detected at a remote point (the array) will appear at its source (endocardial surface), thus superimposing a real-time isopotential map on the geometry matrix, even from a single heartbeat (e-Fig. 69-5).22

Both of these systems now have the capacity to merge previously obtained digital imaging and communication in medicine–formatted computerized tomographic or magnetic resonance images (MRI) of the patient’s heart chambers with the real-time anatomic renderings previously described (e-Fig. 69-6).

This technology has been of greatest value in the mapping and ablation of atrial or ventricular muscle tachycardias whose substrates are within the wall of a chamber and are defined by areas of slow or absent conduction and not only by structural conduction boundaries, such as venous ostia or valve annuli. Each system has its own idiosyncrasies and limitations and requires extra equipment with attendant costs.

Radiation Exposure

Fluoroscopy remains the workhorse for cardiac catheter ablation procedures in most institutions. Under the direction of John Kugler, the Pediatric Electrophysiology Society began a pediatric radiofrequency registry in 1990, which has demonstrated a progressive reduction in procedural fluoroscopy duration from 61.5 minutes in 199414 to 38.3 minutes in 2004,23 at least among patients with supraventricular tachycardia. Most operators limit x-ray exposure by reducing frame rates to 15 and even 7.5 frames/sec and by minimizing the use of magnification. That said, clever use of electroanatomic mapping systems may markedly reduce the duration of fluoroscopy.22,24 St. Jude Medical’s Ensite NavX system permits continuous, real-time, 3D rendering of all electrode catheters, anatomic features, and tagged structures (including locations of previously delivered ablation lesions) with use of a global positioning system–like technology, which is facilitated by three pairs of skin surface patches, serving as x-y-z axis low-energy transmitters. Using continuous impedance measurements, the electrode catheters are the receivers. Virtual elimination of ionizing radiation for simple catheter ablation procedures has been reported with use of this system.25

Data on actual x-ray exposure during catheter ablation procedures comes largely from adult series, in which thermoluminescent dosimeters and/or anthropomorphic radiologic phantoms were used.26–30 Considering fluoroscopy times ranging from 41 to 60 minutes, a single procedure has been estimated to carry a risk of fatal malignancy in 0.03% to 0.13% of individuals and to result in birth defects in 0.00012%.27,28,30 These figures are approximately equivalent to 1% and 0.1% of the spontaneous incidences, respectively. Geise and colleagues31 reported that radiation doses to exposed skin were 6.2 to 49 mGy/min in nine children, which calculated to total doses of 0.09 to 2.35 Gy. In our laboratory, our median fluoroscopy time is 32 minutes per case, and we limit fluoroscopy times to 120 minutes, even for complicated cases.

Device Therapy in Children

Fundamentals of Pacing Hardware

Bradycardia devices (i.e., pacemakers) and antitachycardia devices (i.e., implantable cardioverter defibrillators [ICDs]) require two basic hardware components, the pulse generator and conductors (primarily, “leads”). The pulse generator consists of an energy source (battery), microcircuitry, titanium alloy housing, and a plastic connector block for conductor attachment. In addition, the ICD contains capacitors to store deliverable energy. The lead consists of one (unipolar) or two (bipolar) wires, silicon or polyurethane insulation, a connector pin (or pins) that insert(s) into the pulse generator connector block, and a fixation end that attaches to myocardium (via a tiny screw, fish hook, plaque, or other device). Transvenous bipolar leads generally have a radiodense distal electrode and a slightly more proximal “ring” electrode, whereas the unipolar lead has only a distal electrode. Epicardial leads are mostly unipolar, but bifurcated plaque electrodes and in-line bipolar leads also exist. In addition, the ventricular lead for an ICD may have one or two additional insulated conductors that are exposed on the outer surface of the lead (so-called coils) and participate in the shock field. Arrays and patches may be necessary to optimize cardioversion or defibrillation and are inserted in subcutaneous or intrapericardial sites. Figures 69-7 and 69-8 illustrate the radiographic appearance of this hardware. An entirely subcutaneous cardioverter-defibrillator (which can sense and shock only) is currently in clinical trials.

Radiography of Pacing Systems

The radiologist will be called upon to interpret radiographs from children with devices immediately after device implantation, during routine follow-up, and when component failure is suspected.

A systematic approach will enable the radiologist to interpret the appearance of the hardware. The first step is to identify the location of the pulse generator. When the device is infraclavicular, the conductors usually are transvenous. Epicardial leads are generally tunneled to a subcutaneous abdominal device, but subcostal and flank locations (especially in premature infants) also may be used. Hybrid systems imply use of a combination of transvenous, epicardial, and/or subcutaneous conductors, configured to accommodate restricted venous access, and/or a preexisting lead that is considered valuable, and/or an optimized shock vector in the case of ICDs. The pulse generator will be positioned in a location optimal to the complex configuration of the conductors. The second step is to describe each conductor, including the type (e.g., lead, lead with coils, array, or patch), its course from the pulse generator to the heart or other thoracic site, its form of attachment to the heart in the case of leads, and whether the lead is unipolar or bipolar. Magnification of the lead tip may be required. In young children with epicardial lead(s), redundant lead body is coiled anterior to the heart. Lead may be partially looped within the right atrium in the case of transvenous leads (the so-called growth loop). The third step is to correlate the congenital and surgical anatomy with lead locations and courses. Understanding the appropriateness of the course of each lead often requires some knowledge of the surgical anatomy. Figures 69-9 and 69-10, and e-Figure 69-11 illustrate the radiographic appearance of patients who have complex cardiac device therapy. Finally, leads attached to both right and left ventricles suggest an attempt at ventricular resynchronization as a result of ventricular dysfunction. The left ventricular lead may be transvenous to the coronary sinus or epicardial surface of the heart.

In children undergoing chronic device therapy, symptoms suggestive of device malfunction may develop, such as syncope, skeletal muscle twitching, hiccoughs, new onset of fatigue, palpitations, and, in the case of ICDs, inappropriate shocks. Radiographic abnormalities that may suggest the etiology include lead conduction fracture, lead dislodgement (especially if symptoms occur soon after implantation), lead stretch as a result of somatic growth, and connector pin separation from pulse generator. Transvenous leads usually fracture beneath the clavicle, and epicardial leads tend to fracture at the level of the diaphragm or within the active fixation component (especially when it is a screw). A caveat: The Medtronic model 4968 (Medtronic, Minneapolis, MN) bifurcated, epicardial, double-plaque lead always has the appearance of near-fracture at the union of the two conductors (Fig. 69-12). Finally, the radiologist may be called upon to identify the type of implanted device (i.e., the manufacturer and model number) that is in a patient. Each pulse generator has a radiodense alphanumeric identifier (often its model number) that can be referenced in any of the major companies’ device encyclopedias (see Fig. 69-7). Unfortunately, if the face of the device is facing posteriorly or if sufficient obliquity is present, it may not be readable.

Caring for Children with Devices While in the Radiology Department

Cardiac devices became interactive with the first inclusion of demand circuitry, which was developed in 1965 to allow sensing of intrinsic electrical activity. We now communicate with these devices for purposes of reprogramming, functional testing, and telemetry using a computerized programmer and radiofrequency signals. Hence, despite various forms of protective shielding and electronic filters, all devices may be affected by certain sources of electromagnetic interference (EMI) that may be present in the radiology department. It should be emphasized that ionizing radiation used for diagnostic procedures usually is not a source of EMI. High-dose x-rays during computerized tomography, only when applied directly to the device, can rarely result in oversensing.32 This effect theoretically can inhibit a device, resulting in loss of output. However, the effect is transient and reverses as the beam moves away from the device.

Repeated high-dose radiation therapy may damage the silicone and silicone oxide insulation necessary for the complementary metal oxide semiconductor chip technology of cardiac devices. Device manufacturers have provided guidelines to minimize risk of damage to ICDs resulting from radiotherapy.33 EMI-device interactions may result from MRI, defibrillation, electrocautery, peripheral nerve stimulation, transcutaneous electrical nerve stimulation, diathermy, radiofrequency ablation, and lithotripsy. Untoward responses by the device may include oversensing, noise reversion, power-on reset, permanent circuit failure, and damage to the lead-tissue interface, causing a permanent rise in the stimulation threshold. A glossary of these terms appear in e-Box 69-2. Some of these potentially harmful medical procedures, specific responses to EMI, and ways to prevent these responses appear in e-Table 69-2.34 MRI theoretically may cause device malfunction as a result of static magnetic, gradient, and radiofrequency fields and may cause tissue heating at the conductor-cardiac interface. Nevertheless, the performance of a nonthoracic MRI (at 1.5 T) in a patient with a pacemaker35 or ICD36 has been upgraded to a relative, not absolute, contraindication. An MRI-compatible pacemaker (but not ICD) manufactured by Medtronic also recently has become available.

e-Box 69-2   Glossary of Terms Applicable to the Electromagnetic Interference-Cardiac Device Interactions

Activity sensor: A pacing device component that responds to a physiologic or nonphysiologic patient parameter (e.g., motion or minute ventilation) to provide a faster pacing rate.

Asynchronous mode: A mode of operation in which the pacemaker is insensitive to incoming signals from the chamber being paced.

Back-up mode: A pacing mode typically similar to that observed when the device battery has reached critical depletion; usually VVI (see below).

DDD mode: Dual-chamber mode of operation in which pacing and sensing occur in both the atrium and ventricle; atrioventricular synchrony is thus maintained from the lower programmed rate to the upper P wave tracking (sensing) rate.

Inhibition: A pacemaker response in which a stimulus is withheld in response to a sensed event.

Noise reversion mode: A pacing mode that is activated when electrical noise is sensed. The mode usually consists of fixed-rate pacing for one pacing cycle, but in some devices this mode may continue as long as the electrical noise is sensed, and normal pacemaker function resumes when the noise is no longer sensed.

OOO mode: A programmable mode in some devices in which no chambers are sensed and no chambers can be paced; in essence, the device is off.

Oversensing: When the sensing circuitry of a pacemaker senses noncardiac electrical activity (such as electromagnetic interference) and interprets it as cardiac activity; this may result in inappropriate inhibition (see above) or inappropriate pacing, depending on the underlying programmed pacing mode.

Pacing mode: A programmable feature of all cardiac devices that defines the cardiac chamber or chambers that can be paced and sensed from and how the device will respond to a sensed intrinsic electrical event; the mode is abbreviated using an internationally recognized three- to five-letter code.

Pacing threshold: The minimum programmable energy output required to result in a propagated response by the cardiac chamber of interest; this value is dependent on complex lead, tissue, and metabolic interactions.

Reed switch: A magnetically activated component of most pacemakers that “closes” when exposed to certain magnetic fields. This action typically results in asynchronous pacing (see above), thus ensuring pacemaker output in the presence of possible electromagnetic interference–induced oversensing (see above) and inappropriate inhibition.

Sensing threshold: The minimum cardiac chamber electrogram amplitude that can still be identified by the pacemaker circuitry as an intrinsic electrical event; this value depends on complex lead orientation, tissue, and metabolic interactions.

VDD mode: A dual-chamber mode of operation in which pacing can only occur in the ventricle but sensing can occur in both chambers.

VOO mode: A single chamber mode of operation that is asynchronous, and only pacing occurs in the ventricle at the programmed rate, irrespective of the intrinsic ventricular rate.

VVI mode: A single-chamber mode of operation in which both pacing and sensing occurs in the ventricle; intrinsic ventricular beats are sensed by the pacemaker, resetting the lower rate timing circuitry and thus avoiding competitive pacing.

References

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