How to Ablate Atrial Tachycardias in Patients with Congenital Heart Disease
John K. Triedman, MD
Patients who have undergone palliative surgery for congenital heart disease (CHD) have a high prevalence of both sinus node dysfunction and atrial tachycardias (AT) as they age into adult life, due to fibrosis, surgical scarring, and abnormal anatomy. Within 10 to 20 years of initial surgery, half or more of these patients will have such findings, causing significant morbidity in the young adult population with CHD.1 Unfortunately, these ATs tend to be quite refractory to medical management. For this reason, many centers have adopted an interventional approach to treatment and prophylaxis of ATs in CHD patients, either by catheter ablation or, in some cases, by adaptation of surgical maze procedures. Because these arrhythmias are most commonly macroreentrant and often highly organized by anatomy and scarring, they are extremely well suited to targeted, catheter-based approaches to mapping and ablation. Although the recurrence rate after ablation of these arrhythmias is high, advances in mapping and lesion generation, in combination with effective approaches to vascular access and improved understanding of the electroanatomical relationships that underlie these arrhythmia circuits, have been improving long-term outcomes.
Anatomical and Surgical Review
Review of the underlying anatomy of the patient’s congenital heart defect and the surgical modifications to this anatomy that the patient has experienced in the course of treatment are of critical importance to the operator. Although the variety of congenital heart defects is daunting, from the perspective of the ablating electrophysiologist, most patients can be characterized in one of three general types. The first group consists of patients with simple valvar and/or septal defects who have undergone a biventricular repair, such as tetralogy of Fallot, atrial and ventricular septal defects, and repair of anomalous pulmonary veins. These patients have cardiac anatomy that approximates normal and have atrial arrhythmias that principally reflect cardiac fibrosis and scarring. A second group includes patients who have undergone extensive intra-atrial baffling procedures, most notably those who have had a Mustard or Senning procedure for transposition of the great vessels. Although these patients may also have a physiologically biventricular repair, the postoperative anatomy of their atria presents special challenges for ablative intervention. The final group includes patients who have had a spectrum of palliative surgeries for functionally single ventricular physiology, typically varieties of the Fontan procedure. These patients often have highly complex atrial substrates that are highly arrhythmogenic (Figure 5.1).
In patients with ATs status postrepair of congenital heart defect, it is commonly the case that a variety of organized tachycardias can be induced by programmed stimulation, with varying cycle lengths (CLs) and P-wave morphologies (Figure 5.2). Ideally, after ablation, the tachycardia will be rendered noninducible. However, given the complexity of some atrial substrates in these patients, it is helpful to be able to identify and target clinically occurring ATs when possible based on review of prior electrocardiograms. The prior occurrence of atrial fibrillation (AF) should also be noted, as this identifies a subgroup of patients who may be more likely to have recurrences after successful targeted ablation of organized ATs and who might be considered as candidates for catheter-based or surgical maze procedures if recurrence occurs.
It is also important to note whether the patient has sinus node dysfunction, a concomitant finding in many CHD patients with AT. Both antibradycardia and antitachycardia pacing may be useful adjunct therapies for arrhythmia control in such patients. Those patients with a pacemaker in situ, either epicardial or transvenous, must be ablated with attention to the locations of the implanted leads to avoid causing exit block by ablation of underlying myocardium.
Prior to ablation, thorough evaluation should be performed with a focus on the identification of any hemodynamically significant lesions that should be investigated or addressed at the time of catheterization. In patients managed at an adult electrophysiology (EP) center, this may involve consultation with a pediatric or adult congenital cardiologist. A standard 2-dimensional (2D) echocardiogram should be performed, and consideration may be given in many patients to imaging using computerized tomography (CT) or magnetic resonance imaging (MRI). In addition to often providing superior views of intracardiac anatomical and functional properties, these data sets may be utilized in conjunction with electroanatomical mapping systems to guide catheter navigation and ablation planning.2
During ablation, standard electroanatomical mapping is often supplemented by incorporation of 3-dimensional (3D) data sets obtained during preoperative evaluation (Figures 5.3 and 5.4). In the case of the former, it is of significant value for the operating physician to participate directly in image segmentation and model construction of the CT or MRI datasets ( Videos 5.1 and 5.2). This is because the postoperative anatomy observed in these patients is frequently quite abnormal, and the imaging is sometimes difficult to interpret without specific anatomical knowledge of the patient. Additionally, it is the observation of the authors that time spent in meticulous anatomical review of the data is usually of significant benefit to the operator in procedure planning. Specific anatomical features that may be of value include the specific anatomy of atrial baffles in relation to the native atrial septum, the ostium of the coronary sinus (CS) and the atrioventricular (AV) valves, the presence of unusual venous connections, the anatomy of the atrial septum or baffle with respect to possible need for transseptal access, the presence of intracardiac thrombus and the relationships of the great vessels and semilunar valves with regard to possible need for retrograde catheterization.
Acute intracardiac thrombosis has been anecdotally observed in atrial ablation in CHD patients, but concern for cerebrovascular events is considerably lower than for patients undergoing catheter-based procedures for AF. This is because the majority of intervention is typically performed on the systemic venous atrium, and the arrhythmias themselves appear to have somewhat lower thrombotic potential. Nevertheless, many patients are maintained chronically on warfarin, and while it is rare for bleeding to be an issue, it is generally considered prudent to interrupt chronic anticoagulation therapy for a period of days and ensure that the INR is not excessive prior to the procedure. During the procedure, heparinization is appropriate, with most laboratories targeting an activated clotting time (ACT) of 250 to 300 seconds as an indicator of adequate anticoagulation. Our laboratory protocol is an initial bolus dose of heparin of 100 U/kg, with assessment of the ACT hourly or more frequently, with intermittent follow-up dosing as necessary.
Because AT ablation in these patients is often a long procedure, patients in our lab are placed under general anesthesia. A urinary catheter is also placed in all patients both to monitor renal function and to balance intake and output over the course of the procedure, as the frequent use of irrigated-tip catheter may result in administration of 1 to 3 liters of excess fluid.
In general, AT ablations in congenital heart patients can be performed using a limited number of catheters—one catheter for mapping and ablation (usually requiring an 8-Fr sheath to allow for use of an irrigated-tip catheter) and a second for placement of a reference catheter in the atrium or coronary sinus (CS). At times, it is useful to place an esophageal lead as a reference electrogram (EGM), but the atrial signal obtained from these catheters is sometimes of such low amplitude and frequency content as to be of limited use. If it is desired to use intracardiac echocardiography (ICE), an adjunct imaging modality that we have increasingly found to be of value in these procedures, an additional 8- or 10-Fr sheath, is necessary.
Given these requirements, vascular capacitance is not often a limiting issue, although we typically divide sheaths between right and left femoral veins for organization of the field and ease of catheter manipulation. A much more important issue in congenital heart patients who have undergone many prior catheterizations, surgeries, and intensive care stays is whether a given femoral vein is open. Examination of prior catheterization notes may reveal that a femoral vein is occluded, or in the case of heterotaxy patients, that the inferior vena cava (IVC) is interrupted and drains via an azygos connection to the superior vena cava (SVC). Observation of the patient’s groin may reveal prior cutdown scars, especially if they were catheterized in the 1960s or 1970s or were ever placed on peripheral cardiopulmonary bypass. If it is possible to access a vessel percutaneously but not to easily pass a guidewire to the IVC and atrium, it is useful to perform an angiogram of the pelvis by hand injection and document the patency of the femoral and iliac vessels for future operators. If the femoral veins are obstructed, it is possible to catheterize these patients via the internal jugular approach. In rare cases, transhepatic puncture may be indicated.
Long Vascular Sheaths and Transseptal Puncture
In patients with extremely dilated right atria, the use of long vascular sheaths may be critical to augmenting the reach of the mapping and ablation catheter and ensuring that adequate endocardial contact is being made ( Videos 5.3 and 5.4). This is especially true in patients with older-style atriopulmonary Fontan anastomoses and in patients with severe Ebstein’s anomaly. The use of these sheaths is often particularly important to reach the anterior wall of the right atrium (RA).
Transseptal puncture is often not necessary in ablation of AT in congenital heart patients, as the substrate for ablation is often located in the systemic venous atrium. The most common scenario in which transseptal puncture is indicated is with patients who have portions of the native RA that contain the AT substrate baffled to the pulmonary venous atrial chamber.3 This includes patients who have undergone a Mustard or Senning procedure and some forms of the Fontan procedure, which involve RA baffling (“lateral tunnel” procedures and RA baffles designed to direct blood from an atrial septal defect to a right-sided AV valve), or placement of an extracardiac conduit. In these patients, the cavotricuspid isthmus (CTI) must usually be ablated from the pulmonary venous atrium, necessitating a transseptal puncture. In recent years, it has been demonstrated that this procedure, while sometimes difficult, can generally be safely performed.3
It is generally wise to consider and search for any native or postoperative interatrial communication, which might be used for transseptal access prior to performance of a puncture. When transseptal puncture must be performed, care must be taken, as they often involve passage through prosthetic and/or thickened septae, and the anatomy of the systemic and pulmonary venous atria is almost always unique. ICE imaging may be a valuable adjunct in crossing a septum safely (Figure 5.5), but in our laboratory, we use fluoroscopy and angiography as primary tools to understand the geometry of the procedure. Performance of a levophase angiogram of the pulmonary venous atrium is often useful in defining the transseptal course ( Videos 5.5 and 5.6). Although transseptal sheath and dilator will sometimes pass easily into the left atrium (LA) with mild pressure, it is common that considerable force may still not result in passage of the sheath. In these cases, the best strategy for safe puncture may be to search for a better spot to apply pressure, with moves of as little as a millimeter or two sometimes revealing a much easier spot to pass the sheath. Valuable adjunct techniques include use of the radiofrequency (RF) transseptal needle4 (Baylis transseptal needle, Baylis Medical, Montreal, Canada) and/or placement of a fine exchange wire through the transseptal needle lumen and use of a coronary balloon to dilate the hole.
Successful transseptal puncture can be confirmed with visualization of contrast ( Video 5.7). Attention must be paid to careful removal of the dilator and wire, lest air become entrained in the sheath and subsequently embolized when a catheter is introduced.
We routinely use RF for energy delivery for performing AT ablation and, most commonly, RF catheters enhanced by open irrigation. This allows for increased power delivery in atrial chambers, which are frequently large and have low local blood-flow velocities. Under these conditions, normal temperature-controlled catheters are frequently quite limited with respect to power. It has been our and others’ observation that irrigated ablation in these patients has not increased the incidence of adverse events, and the increased efficacy of ablation using this technology has been demonstrated in patient series specific to CHD.5,6 A modest disadvantage of irrigated ablation is that the open-system irrigation that we most commonly use (THERMOCOOL, Biosense Webster, Diamond Bar, CA) results in a significant volume load over longer procedures. This is managed by placement of a Foley catheter, monitoring of urinary output, and administration of IV diuresis as needed. Contact force-sensing catheters add another promising dimension to our ability to ensure formation of lesions large enough to block conduction corridors, but have not yet been shown to affect acute procedural outcomes.7
During ablation of APs, the AV annulus may be effectively located by analysis of the EGM itself, in combination with fluoroscopic support. In contrast, mapping of a macroreentrant circuit that may involve many areas of the atrial surface requires a technique for recording and integrated display of that entire surface. Although attempts to ablate AT in complex congenital heart patients were initiated prior to the availability of electroanatomical mapping tools such as CARTO™ (Biosense Webster, Diamond Bar, CA) and NavX (St. Jude Medical, Sunnyvale, CA), rapid and effective interpretation of activation sequences was exceedingly difficult and success rates relatively low.
3D mapping is an enabling technology for these ablation procedures.8 The process of mapping in ablation of ATs thus involves creation of an anatomical shell for mapping and superimposition of an electrical activation pattern onto this model. The original versions of 3D mapping tools collected anatomical and electrical information entirely from the recorded movements of the catheter tip; as the mapping electrode moved through the cardiac chamber and recorded electrical signals from the endocardium, a shell representing the endocardial surface was sequentially built. Successful efforts have been made to integrate anatomical imaging with electroanatomical mapping, either by import, segmentation, and registration of cardiac MRI or CT data sets2 or by real-time integration of navigated ICE (Figures 5.6 and 5.7).9 Each of these techniques has been demonstrated to be of great potential value in understanding the often complex anatomy of patients with congenital heart defects and complex atrial surgical palliation. They each ensure that important anatomical detail is not overlooked in the mapping and ablation process and allow reduction in fluoroscopy exposure in these cases.10