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Transseptal Catheterization and Interventions
By Ranjan Thakur, Andrea Natale Cardiotext Publishing, LLC
Copyright © 2010 Ranjan Thakur and Andrea Natale
All rights reserved.
ISBN: 978-1-935395-20-1
CHAPTER 1
History of Transseptal Catheterization
Gregory K. Feld, John Ross Jr.
Transseptal left heart catheterization, originally developed in experimental animal studies and then later in humans, was reported in 1959 by John Ross Jr, MD, at the National Institutes of Health (NIH). It was first used to measure left heart pressures as a diagnostic method and later as a research tool to study left heart dynamics in human diseases such as heart failure, heart valve dysfunction, and hypertrophic obstructive cardiomyopathy. In 1960, the needle used for transseptal catheterization (Figure 1.1) was slightly modified with a smaller needle tip by Edwin Brockenbrough, MD, a trainee in the NIH cardiovascular research program, for use with a larger catheter passed percutaneously by the Seldinger method. Subsequently the procedure became widely used as a diagnostic method to assess left atrial and left ventricular pressures in both adult and pediatric patients with valvular and congenital heart disease who were being considered as candidates for surgical repair procedures. With the developments of right heart, balloon, and thermodilution (Swan-Ganz) catheterization to estimate left atrial pressures and retrograde left ventricular catheterization, transseptal catheterization became less widely practiced in the 1970s and early 1980s, except in those centers performing procedures, such as mitral balloon valvulotomy, which require transseptal puncture.
During the 1980s, however, at a time when clinical cardiac electrophysiology was primarily a diagnostic specialty, the use of electrical fulguration for atrioventricular (AV) node ablation, septal accessory pathway ablation, and ventricular tachycardia ablation was described, rapidly advancing cardiac electrophysiology into an interventional specialty. Fortunately, considering the limitations of high-energy shocks for ablation, including the high failure rates and risks of complications from this technique, radiofrequency energy for catheter ablation was described. This led to an explosion in the use of catheter ablation to cure a variety of cardiac arrhythmias, including AV reentry, AV nodal reentry, atrial flutter, atrial tachycardia, and ventricular tachycardia.
In many cases, however, the successful application of radiofrequency energy via catheter for curative ablation required access to the left atrium and left ventricle. This was particularly true for left-sided accessory pathways, left ventricular tachycardia, atrial fibrillation and atypical atrial flutter. While often this could be accomplished by retrograde arterial catheterization across the aortic and mitral valves, usually from the femoral artery or rarely from the radial artery, in many cases easier access could be achieved by a transseptal catheterization approach. Thus, for the treatment of a variety of supraventricular and ventricular tachyarrhythmias there was a gradual resurgence in the use of transseptal catheterization in the field of interventional cardiac electrophysiology, beginning in the 1980s; this use increased dramatically in the 2000s as ablation of atrial fibrillation became widely practiced.
Thus, in the cardiac electrophysiology laboratory, transseptal catheterization has become particularly useful for obtaining access to the left atrium for radiofrequency catheter ablation of left-sided accessory pathways in patients with Wolff-Parkinson-White syndrome and refractory supraventricular tachycardia, in the occasional patient with failed right-sided slow or fast pathway ablation for AV nodal reentrant tachycardia, in those with failed right-sided AV node ablation for refractory atrial fibrillation, and in those with left ventricular tachycardia in whom access to the left ventricle is required for both mapping and ablation. Large numbers of transseptal catheterization procedures have been performed at numerous institutions worldwide over the last several decades, including many in electrophysiology laboratories, with very high success and very low complication rates, even if performed on an outpatient basis. The most serious potential risks from transseptal catheterization include cardiac or aortic perforation with pericardial effusion and tamponade, which may require percutaneous pericardial catheter drainage and, occasionally, surgical intervention.
While the technique of transseptal catheterization remains largely unchanged since it was originally described and modified, several new technologies have made the approach considerably easier and potentially less risky, and are routinely employed today in many clinical electrophysiology laboratories. These new technologies include transesophageal echocardiography, now largely replaced by intracardiac echocardiography (ICE), to guide transseptal puncture, transseptal puncture performed using radio-frequency energy applied to a modified transseptal needle (NRG RF Transseptal Needle, Baylis Medical Company, Inc, Montreal QC, Canada), or use of a needle-tipped guidewire (SafeSept Transseptal Guidewire, Pressure Products, Inc, San Pedro, CA) passed through a standard Brockenbrough transseptal needle into the left atrium through a variety of pre-shaped transseptal sheaths (Fast-Cath, St. Jude Medical, Inc, St. Paul, MN). With the use of ICE, typically performed with a steerable 8 F or 10 F intracardiac ultrasound catheter (AcuNav, Siemens Medical Solutions USA, Inc, Malvern, PA), contrast injection to stain the septum is no longer required, since micro-bubbles in the saline flush provide adequate echo-contrast to ensure successful left atrial access, reducing the risk of cardiac perforation with the transseptal needle (Figure 1.2). The radiofrequency transseptal needle and the needle-tipped guidewire further reduce the risk of inadvertent needle perforation of the left atrial lateral wall or roof during transseptal catheterization. For ablation of left-sided accessory pathways, a numbered series of transseptal sheaths (eg, SL1, SL2 [Daig Corp, Minnetonka, MN]) with different lengths of the distal-shaped segment was developed that, when extended just beyond the sheath, position the ablation catheter at specific locations around the mitral valve annulus (Fast-Cath, St. Jude Medical, Inc, St. Paul, MN).
With the recognition that electrical isolation of the pulmonary veins with radio-frequency catheter ablation may cure atrial fibrillation, an entirely new procedure in interventional cardiac electrophysiology was launched in the late 1980s, namely the ablation of atrial fibrillation. This approach required transseptal catheterization of the left atrium and, in most cases, double transseptal catheterization (Figure 1.3) because both a circular mapping catheter and an ablation catheter need to be introduced into the left atrium to ensure successful isolation of the pulmonary veins. This can be accomplished either by performing two separate transseptal punctures or a single transseptal puncture through which an ablation catheter is guided either by ICE or fluoroscopy after withdrawing the sheath (through which the initial puncture was made) back into the right atrium while retaining a guidewire in the left atrium. Once the ablation catheter is passed through the transseptal puncture into the left atrium, the retained sheath can be passed over the guidewire back into the left atrium, through which the circular mapping catheter can then be deployed in the left atrium for mapping the pulmonary veins. Both approaches may be guided by ICE, and recent advances in 3-D echo may further enhance success and reduce the risks of the procedure. Studies suggest that long-term complications are similar with either approach, and while the single transseptal puncture approach may result in patency of the puncture site for a longer period of time than the double transseptal puncture approach, closure eventually occurs with either approach.
Another aspect of transseptal catheterization that has evolved with the advent of atrial fibrillation ablation has been the use of intensified anticoagulation regimens for prevention of thromboembolic events, including maintaining an activated clotting time (ACT) >350 seconds during ablation and front-loading with heparin before transseptal puncture, as well as consideration of transseptal catheterization and ablation while fully anticoagulated with warfarin with an INR of 2-3. Detection of left atrial thrombus before or during radiofrequency ablation of atrial fibrillation has also been enhanced by the use of ICE, which may be an important tool to reduce the risk of thromboembolic events.
Since the methods for ablation of atrial fibrillation vary significantly, from segmental antral pulmonary vein isolation (Figure 1.3) to circumferential pulmonary vein ablation and isolation with or without additional linear ablation (Figure 1.4), different transseptal sheaths have been employed to achieve the goals of these ablation procedures, including fixed-curve sheaths, such as the Fast-Cath SL1 sheath (St. Jude Medical, Inc, St. Paul, MN), and steerable sheaths, such as the Agilis sheath (St. Jude Medical, Inc, St. Paul, MN). In addition, robotic systems, such as the Sensei X (Hansen Medical, Inc, Mountain View, CA), have recently been introduced, which control large, steerable sheaths, such as the Artisan sheath (14 F) (Hansen Medical, Inc, Mountain View, CA), through which any radiofrequency ablation catheter can be positioned for ablation of atrial fibrillation. The type of catheters used for ablation of atrial fibrillation vary widely as well, but most commonly include either a standard (4-5 mm tipped) or large-tipped (8-10 mm tipped) ablation catheter, an internally perfused ablation catheter (Chilli II, Boston Scientific, Inc, Natick, MA) or an externally perfused ablation catheter (ThermoCool, Biosense Webster, Inc, Diamond Bar, CA). These ablation catheters generally are 8 F or smaller in diameter and fit through the standard fixed-curve or steerable transseptal sheaths commonly used today. In addition, a circular mapping catheter (Lasso, Biosense Webster, Inc, Diamond Bar, CA) is usually passed through a second transseptal sheath to map the pulmonary veins, or occasionally a balloon mapping catheter (EnSite Array, St. Jude Medical, Inc, St. Paul, MN) passed into the left atrium for 3-D mapping, in the event of evidence that a non-pulmonary vein focus is triggering atrial fibrillation. Due to the tendency for atrial fibrillation to recur despite apparently initially successful ablation, repeat procedures may be difficult due to septal scarring, requiring radiofrequency or guidewire-needle assistance for transseptal catheterization. In addition, unusual vascular anatomy has resulted in the development of alternative approaches for transseptal catheterization, such as a superior vena caval approach, in the event of congenital or surgical interruption of the inferior vena cava. A future intervention requiring transseptal catheterization, likely to be employed by electrophysiologists (as well as interventional cardiologists), is the deployment of the left atrial appendage occluder device (WATCHMAN, Atritech, Inc, Plymouth, MN) to prevent systemic thromboembolic events, such as stroke, in patients with persistent or recurrent atrial fibrillation despite attempted ablation or antiarrhythmic drug therapy who wish to discontinue use of warfarin or who have relative or absolute contraindications to the use of warfarin.
In summary, the basic approach to transseptal catheterization has changed little since its first description in 1959. However, its clinical application has dramatically expanded, particularly in the field of cardiac electrophysiology, as recently reviewed by Babaliaros et al. Furthermore, advances in imaging technology and adjunctive devices have increased the ease with which transseptal catheterization can be performed, and arguably its safety as well.
CHAPTER 2
Embryology and Anatomy of the Atrial Septum
Siew Yen Ho
Increasingly, transcatheter interventional procedures require access to the left heart chambers via the atrial septum. To ensure that these procedures are successful and to reduce the risk of complications, a detailed knowledge and understanding of the atrial septum, which separates the left and right atrial chambers, is crucial. While morphologists can view the heart from all angles and display the atrial septum to best advantage, their views sometimes differ from those commonly used by interventional electrophysiologists. Adopting McAlpine's attitudinal approach for describing the locations of the heart's structures can improve our understanding of its anatomy. Nevertheless, there remains the issue of speaking a common language. The terminology used when describing cardiac embryogenesis is confusing because, historically, the same structures have been given different names, and eponyms are rather common. Furthermore, the conversion of Latin terms into English, while laudable for readers who are fluent in English, can be confusing for others.
Echoing Whitmore's sentiments in "Terminologia Anatomica: New Terminology for the New Anatomist" that terminology must accommodate all users, I hope the terminology used in this chapter will suit cardiologists, morphologists, and scientists alike. Latinate terms will be offered in association with their commonly used English equivalents. The first part of this chapter is a review of the embryologic development of the atrial septum. As this section is meant to enhance the understanding of structures in the definitive heart, I will use attitudinal orientations as much as possible when describing those locations. The bending and growth of the heart tube during development continually shifts these structures, however, so their locations can be described only in general terms. Traditionally, embryologists use ventral, dorsal, cephalad, and caudad as their compass points because those terms can be applied to both bipeds and quadrupeds. I prefer the terms anterior, posterior, superior, and inferior, respectively, which are more compatible with descriptions of the human postnatal heart.
Developmental Anatomy
At the early stages of cardiac development, the embryonic heart tube is more or less straight with a venous pole at the inferior end and an arterial pole at the superior end. The tube is attached posteriorly along its length by mesocardium to the body of the embryo. This tube gives rise to the ventricles and, through the process of looping, detaches much of its length from the mesocardium. During the looping process, the inferior pole, which is also the inlet part of the tube, expands to form the atrial component, concomitant with the expansion of the superior pole, the outlet portion, through recruitment of extracardiac cells. Wide venous channels, which carry blood from both the right and left sides of the embryo back to the heart, connect to the developing atrial portion. The paired veins form the so-called horns of the sinus venosus. The common cardinal veins (ducts of Cuvier) drain from each side of the embryo into the sinus, as do the umbilical veins from the placenta and the vitelline veins from the yolk sac.
At the fourth week of development, there are no anatomical borders between the sinus venosus and the primitive atrium. The sinus venosus portion becomes asymmetrical as the left horn diminishes in size and is incorporated into the developing left atrioventricular junction, while the right horn grows rapidly. The left horn, which ultimately receives only the left duct of Cuvier, persists as the coronary sinus. Consequently, the entire systemic venous component opens to the right portion of the primitive atrium, which, in turn, continues to the developing atrioventricular junction at the atrioventricular canal (Figure 2.1a).
While the primitive atrium is developing, endocardial cushions form within the canal in preparation for the remodelling of the atrioventricular junction and cardiac septation (Figure 2.1b). Two cushions, the superior and inferior cushions, project into the lumen; they meet in the middle, dividing the common orifice of the canal into right and left orifices. The union of the endocardial cushions is thought to form the septum inter-medium, but the cushions are not the only contributors to septation of the atrioventricular canal (discussed below).
The remaining systemic veins remodel. The vitelline and umbilical veins become a single vessel, the inferior caval vein (inferior vena cava). Thus, the venous component, which includes the entrances of three veins, the inferior caval vein and the right and left ducts of Cuvier, opens into the posterior aspect of the primitive atrium. Valve-like structures develop to the right and left sides of the venous component to demarcate its junction with the atrium, allowing anatomical distinction between the sinus venosus and the primitive atrium. These valves, described as the right and left venous valves, fuse at their superior and inferior extremities (Figure 2.1a). The right venous valve continues into the posterosuperior wall of the atrium, forming the septum spurium, which, as the name suggests, is a false septum because it reaches its fullest development during the third month and then diminishes to become the sagittal bundle in the formed heart. The posteroinferior wall of the sinus venosus folds inward to form the so-called sinus septum (eustachian ridge); this fold divides the right venous valve into two portions (Figure 2.1c). The valvar remnants become the eustachian and Thebesian valves, which guard the orifices of the inferior caval vein and the coronary sinus, respectively. The tendinous commissure between the two venous valves extends through the sinus septum, which later becomes the antero-inferior rim of the foramen ovale and a border for the triangle of Koch (discussed below). As the sinus venosus becomes incorporated into the primitive atrium, the left and right sides of the primitive atrium expand to form the atrial appendages. In the definitive heart, the right border between the sinus venosus and the atrial appendage is marked internally by the crista terminalis (terminal crest).
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Excerpted from Transseptal Catheterization and Interventions by Ranjan Thakur, Andrea Natale. Copyright © 2010 Ranjan Thakur and Andrea Natale. Excerpted by permission of Cardiotext Publishing, LLC.
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