Decoding Cardiac Electrophysiology: Understanding the Techniques and Defining the Jargon

This book provides a concise overview of cardiac electrophysiology for cardiologists who are not electrophysiologists and for allied cardiovascular professionals, cardiology registrars and fellows who are new to the field. It familiarises them with the main procedures performed in the electrophysiology laboratory. Emphasis is placed on helping the reader develop a core understanding of how data is collected and interpreted in the electrophysiology laboratory, and how this is used to guide ablation for the commonest arrhythmias including AV nodal re-entry tachycardia, accessory pathways, atrial fibrillation and ventricular arrhythmias.

Decoding Cardiac Electrophysiology: Understanding the Techniques and Defining the Jargon will translate some of the technical terminology and data frequently used by electrophysiologists into terms and concepts familiar to the wider cardiovascular community. This includes the interpretation of electrograms and 3D electro-anatomicalmaps of common arrhythmias.  Accordingly, it offers a valuable resource for all non-electrophysiologists seeking a guide to the topic and for electrophysiology trainees establishing their core knowledge and skills in the field.  The aim is that this should be the first book anyone new to the field should choose to read.

               

• Language: English
• Publisher: Springer
• Release date: Nov 9, 2019
• ISBN: 9783030286729

Book preview

Part IThe Tools for Understanding Cardiac Electrophysiology

© Springer Nature Switzerland AG 2020

A. Sohaib (ed.)Decoding Cardiac Electrophysiologyhttps://doi.org/10.1007/978-3-030-28672-9_1

1. The Basic Language of Cardiac Electrophysiology—An Introduction to Intracardiac Electrograms and Electrophysiology Studies

Sandeep Prabhu¹, ², ³   and Afzal Sohaib¹  

(1)

Department of Cardiology, Barts Heart Centre, St Bartholomew’s Hospital, London, UK

(2)

The Heart Centre, Alfred Hospital, Melbourne, VIC, Australia

(3)

The Baker Heart and Diabetes Institute, Melbourne, VIC, Australia

Sandeep Prabhu

Email: [e-mail]

Afzal SohaibConsultant Cardiac Electrophysiologist (Corresponding author)

Email: [e-mail]

Abstract

This chapter aims to introduce the reader to some of the core principles which underpin clinical cardiac electrophysiology and how it is applied in cardiac catheter laboratories, where ablations take place. Key language and terminology are introduced before these concepts are explored further in subsequent chapters.

Keywords

ElectrogramsElectrophysiology studyAblationArrhythmia mechanismsRefractory periodsDecrementationRe-entryAutomaticity

1.1 Cardiac Electrophysiology—Why the Mystique?

To the uninitiated, there is a perceived ‘mystique’ about cardiac electrophysiology (EP) that often acts as a barrier to an even rudimentary understanding of EP procedures. Unhelpfully, this view of EP is not infrequently perpetuated by electrophysiologists themselves, with most apparently communicating in a language indecipherable, even to the seasoned general cardiologist. The purpose of this book is to simplify this jargon, and decode it into terms to allow newcomers to the field to find their way among the world of EP.

Unlike other realms of cardiology such as imaging, angiography and structural intervention, which have an obvious visual element (a regurgitant valve, a narrowed artery, etc.), electrophysiology instead relies upon the interpretation of electrical signals and the manner in which they propagate throughout the cardiac tissue. Since electricity itself cannot be physically visualised in real time, its presence and course through various cardiac structures instead needs to be inferred by the detection of electrical signals at various intra-cardiac sites. The direction and pattern of electrical wavefronts through tissue is determined by the timing of electrical signals in relation to each other, the surface ECG and/or a stable reference signal. For this reason, the ‘language’ of electrograms needs to be comprehended before electrophysiological procedures can make sense. Fortunately, like any language, there are key fundamental principles and concepts which will aid the interested student in understanding and eventually becoming proficient in this language. This chapter focuses on describing these key concepts. Subsequent chapters will then apply these principles to the treatment of various commonly treated conditions.

Decoding the Jargon and Beyond the Basics

Electrophysiologists have a tendency to very quickly slip into using jargon. For this reason throughout the book, we have highlighted commonly used terms and described them in decoding the jargon boxes. Each chapter is structured so that the core concepts are the main focus and some of the more complex concepts are discussed in sections entitled beyond the basics towards the end of each chapter.

1.2 What Happens in the EP Lab

Before exploring some of these fundamental concepts it is useful to have an overview of the range of procedures which happen in the electrophysiology laboratory and the rest of the book will deal with each of these areas in detail. Procedures can be divided into standard EP and complex EP. This is an oversimplification and often what is classified as standard can be very complex and vice versa, but it is a useful way to orientate oneself, and these categories are frequently used by various bodies involved in overseeing electrophysiology [1].

Standard EP usually refers to procedures which can deal with arrhythmias frequently from the right sided circulation and includes AV node ablation, AV nodal re-entry tachycardia (AVNRT), typical atrial flutter, and ablation of accessory pathways. These often can be done with a relatively simple arrangement of catheters and fluoroscopy. The risks are usually at the lower end, and procedure duration is usually at the lower end of the spectrum for electrophysiology procedures.

Complex EP usually refers to procedures with deal with arrhythmias from the left sided circulation and includes ablation for atrial fibrillation, left sided atrial tachycardias, and ventricular tachycardia (VT). Procedures tend to be slightly longer. Heparinisation is required as the left sided circulation is being instrumented. Consequently, the risks are at the higher or more serious end of the spectrum. Key elements of these procedures are described in Table 1.1.

Table 1.1

Procedures performed in the EP laboratories. Access and equipment required is listed

A standard EP procedure may become complex depending on the nature of the arrhythmia. For example, a narrow complex tachycardia may initially look like a form of accessory pathway mediated tachycardia (AVRT), however, upon further testing, may reveal itself (after appropriate diagnostic manoeuvres), to be a left sided focal atrial tachycardia. Similarly, what may appear like a complex VT or ventricular ectopic ablation, may arise from the right ventricular outflow tract, which is generally readily accessible from the right femoral vein and may require just a single catheter (ablation catheter) to treat. Therefore, the distinction between standard and complex EP is somewhat artificial.

1.3 Important Cardiac Anatomy

A thorough understanding of cardiac anatomy is of particular importance in electrophysiology as literally any part of the heart tissue may be involved in the mechanism of an arrhythmia and/or be a target for ablation. A review of some key anatomical features specifically relevant to electrophysiology procedures are described here. More specific details for some anatomical structures may be found in the relevant chapters discussing specific procedures [2].

1.3.1 Accessing the Heart Chambers

The chambers of the heart are shown in Fig. 1.1. The right atrium (RA) is most readily accessible chamber of the heart either inferiorly (via the right femoral vein and inferior vena cava (IVC)) or superiorly (via the internal jugular and superior vena cava (SVC)). Several key structures are located in or accessible via the RA. The right ventricle (RV) is readily accessible from the RA by advancing across the tricuspid valve. The right ventricular outflow tract (not shown in this figure), the region of the RV inferior to the pulmonary valve, is often a source of arrhythmia and a frequent target for ablation in the RV [3]. Its anatomy will be described in detail in the relevant section. The left atrium (LA) can be accessed in 3 main ways: (1) via trans-septal puncture across the inter-atrial septum, (2) epicardially from the coronary sinus (see below) or (3) retrogradely through the aortic valve and mitral valve (uncommonly). Similarly, the left ventricle (LV) can be accessed in the same 3 ways as the LA, however, limited parts of the epicardial surface may be accessible via branches from the coronary sinus (such as the great cardiac or middle cardiac veins). In the case of trans-septal access, the LV is reached by advancing across the mitral valve. Modern techniques such as epicardial access via the pericardium can also allow access to most cardiac structures epicardially. This approach is most often utilised to access the epicardium of the LV or RV in complex ventricular tachycardia ablation procedures. Currently, this is usually limited to being performed in highly specialised tertiary or quaternary centres with experienced operators.

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Fig. 1.1

Accessing the chambers of the heart. The access to each chamber is described by arrows and colour coded by chamber. IVC access is the most common for right heart structures, although the Ra can be accessed via the SVC from the internal jugular vein if required. Access to the left structures is via the interatrial septum (via trans-septal puncture), through the coronary sinus (down LV branches to access LV epicardium) or retrogradely from the aorta. Any chamber can be accessed via an epicardial approach however this is most commonly used for ventricular chambers

1.3.2 Cardiac Structures with Electrophysiological Significance

Figure 1.2 outlines some key anatomical structures that have specific relevance for electrophysiological procedures.

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Fig. 1.2

Key anatomical structures for electrophysiological procedures. The listed structures have key significance in electrophysiological procedures. Details are described further in the text

1.3.2.1 AV Node Complex

This will be discussed in more detail in the section on AVNRT. In the absence of an accessory pathway, the AV node is the only electrical communication between the atria and the ventricles, and the pathway used for normal impulse conduction. The compact AV node is located in the centre of the heart. It is electrically insulated and so does not have any detectible electrical signal, however the His bundle located just distal to the AV node, does record a small electrical signal (called a His deflection), allowing a catheter spanning across it to identify the approximate the location of the AV node. The AV node often receives inputs from extensions with differing degrees of decremental properties (often termed slow and fast pathways), which are involved in the mechanism of AVNRT (see chapter on AVNRT). The AV node complex is often described with a triangular border known as the Triangle of Koch, which delineates the slow and fast inputs into the complex. Decremental conduction is a key property of the AV node and is discussed below.

1.3.2.2 The Coronary Sinus

The coronary sinus is the collection point of the venous drainage of the coronary circulation. It drains the anterior interventricular vein, the posterior interventricular vein and middle cardiac vein (along with other lateral venous branches). Crucially it runs on the epicardial surface along the atrio-ventricular groove (predominately along the atrial aspect) along the posterior/inferior surface of the left atrium, before draining into the right atrium at the inferior interatrial septum. The coronary sinus is of crucial importance in electrophysiology procedures for several reasons:

1.

By positioning catheters in the coronary sinus, electrical information is obtained regarding the left atrium (LA), without the catheter having to be in the systemic circulation (unlike, for example if it were sitting inside the LA), avoiding the need for anticoagulation and minimising the risk of embolic complications.

2.

Unlike catheters inside the LA, the coronary sinus provides access to the epicardial surface of the LA.

3.

Branches of the coronary sinus, may allow limited access to certain epicardial aspects of the left ventricle.

1.3.2.3 The Cavo-Tricuspid Isthmus

The cavo-tricuspid isthmus (CTI) is the region on the floor of the right atrium from the inferior aspect of the tricuspid valve to the anterior portion of the IVC. This area contains complex anatomy making the surface far from smooth, particularly at the septal aspect. The Eustachian ridge and the often trabeculated myocardial tissue in this area lead to slowed conduction—a kind of electrical ‘bottleneck’. This slowed conduction can allow electrical circuits to sustain by allowing an opportunity for an excitable gap to form in the circuit. See Sect. 1.4.4. Typical atrial flutter utilises the cavo-tricuspid isthmus as part of its re-entry circuit around the tricuspid annulus—often termed CTI-dependent flutter. This electrical bottleneck is the target for the ablation of this tachycardia with an ablation line from the electrically inert annulus to the inferior vena cava (blue dotted line in Fig. 1.2) effectively interrupting this circuit.

1.3.2.4 The Cristae Terminalis

The cristae terminalis (CT) is a curved ridge delineating the border between the smooth and trabeculated endocardial surfaces of the RA. It runs superior-inferiorly from the IVC to the SVC along the postero-lateral aspect of the RA. It is also an area of slowed conduction and its specific fibre orientation promotes electrical conduction superiorly and inferiorly along its edges rather than transversely across it. The superior aspect of the CT gives rise to the sinus node complex. The body of the cristae acts as an electrical channel, promoting the conduction around the tricuspid annulus in the case of atrial flutter. The atrial tissue in the cristae is a common source of focal atrial tachycardia.

1.3.2.5 The Inter-atrial Septum

Understanding the anatomy of the inter-atrial septum is crucial as trans-septal puncture has become the primary route of access to the LA in contemporary electrophysiology. Like the heart itself, the septum does not sit in the true antero-posterior plane of the heart but is angled to the left. This explains the reason during trans-septal puncture to direct the catheter tip posteriorly about 45° (4:30 position on a clock face) to ensure the septum is engaged perpendicularly to its plane (Fig. 1.3). The fossa ovalis is the thinnest part of the septum and the only true shared membrane between the RA and the LA and is therefore the target for trans-septal puncture. Puncturing outside this area runs the risk of a catheter running an extra-cardiac course prior to entering the LA resulting in cardiac tamponade. Detailed anatomy of the inter-atrial septum is described in the section explaining trans-septal puncture technique.

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Fig. 1.3

The orientation of the interatrial septum. The interatrial septum sits in an oblique plane in the body as shown in the transverse cartoon as viewed from below (like a CT scan). The fossa ovalis is the true single membranous connection between the RA and LA, the thinnest part of the IAS and the target location for trans-septal puncture. To puncture outside this region runs the risk of