Electrophysiology Study: Interpretation of Intracardiac Electrocardiograms

Abstract

Although electrocardiography is more than 100 years old, it still holds the key to diagnose many disorders and is one of the most commonly used diagnostic tools not only in cardiology but throughout medicine. Most often a surface electrocardiogram (ECG) is made, which represents a summarized electrical activity of the heart. However, by inserting catheters into the heart, it is possible to make an ECG from different localized areas. This chapter focuses on introducing the readers to the world of cardiac electrophysiology providing an overview of the basic principles of the electrophysiology study.

1. Introduction

Cardiac electrophysiology study (EPS) is helpful to assess the heart’s electrical system. This is an invasive percutaneous cardiac procedure used for the investigation and treatment of certain arrhythmias. During the examination, catheters are inserted to the appropriate position within the heart mainly via large veins to record the electrical signals of the heart and to pace from different localized areas. In this way, EPS can help evaluating the function of the conduction system, determining the mechanisms of brady- and tachyarrhythmias, and identifying areas which may be the targets of often curative catheter ablation. Many cardiac arrhythmias that previously required the use of potentially harmful antiarrhythmic drugs can now be routinely cured in the electrophysiology laboratory by means of transcatheter ablation techniques.

In this chapter, the basics of cardiac electrophysiological studies are presented.

2. Principles of the electrophysiology study

2.1 The setup of the electrophysiology study

Equipment necessary for EPS includes an operation table, fluoroscopy unit, recorders, programmable stimulator, a multichannel lead switching box, an oscilloscope, and emergency instruments. In addition, tools for vascular access and electrode catheters are also required [1]. A standard schematic set-up for typical EPS is shown in Figure 1.

Figure 1.

Standard schematic set-up for typical electrophysiology study. Abbreviation: RF—radiofrequency.

2.2 Signal recording, amplification, and filtering

Electrical signals from humans obtained by surface or intracardiac electrodes are <10 mV in amplitude. These electrocardiograms must be amplified and filtered before digitalization, displaying, and storage for interpretation and analysis [2]. Amplification means the increase of the signal’s amplitude. However, the signals are plagued with electrical noise, thus amplification results in increasing not only the original signals but the amplitude of noise also. For this reason, avoidance of extraneous signals is essential: all electrical tools used in EPS should be appropriately earthed and shielded. In addition, filtering is required to eliminate unnecessary components of the electrical signals. High-pass filters remove components below a given frequency, while low-pass filters eliminate high-frequency components of the electrical signal. Electrophysiological signals are often contaminated with power line noise (i.e., 60 Hz in North America and 50 Hz in Europe), thus notch filtering is often used to eliminate it [3].

Standard ECG devices run at 25 mm/s Increasing the paper output speed, subtle ECG findings hidden in the tracings become more evident. During an EPS, surface ECG leads and intracardiac electrocardiograms (IEGM) are generally displayed and interpreted at a sweep speed of 100 or 200 mm/s (Figure 2).

Figure 2.

Same normal sinus beat at a paper speed of 25, 50, 100, and 200 mm/s.

2.3 Standard catheter positions

For a standard EPS, a standard number of four catheters is necessary. Based on the operator’s decision, EPS is also feasible using only three diagnostic catheters. Diagnostic catheters have two or multiple electrodes, and for each pair of consecutive electrodes, a distinct intracardiac electrogram gets recorded. Traditionally, catheter placement is carried out under fluoroscopy guidance. In the EP lab, three main fluoroscopy projections are used: anteroposterior (AP), left anterior oblique (LAO), and right anterior oblique (RAO) views (Figure 3).

Figure 3.

Standard catheter positions in left anterior oblique (LAO), anteroposterior (AP), and right anterior oblique (RAO) projections. Abbreviations: CS—coronary sinus decapolar catheter; HRA—high right atrium; RVa—right ventricle apex.

2.3.1 High right atrium

A diagnostic catheter is positioned from the femoral vein and contacted with the lateral wall of the right atrium at right atrium—superior vena cava junction.

2.3.2 Coronary sinus

The coronary sinus runs transversely in the left atrioventricular groove on the posterior side of the heart. A multielectrode catheter is inserted into the coronary sinus from femoral, jugular internal, or subclavian vein. For femoral approach, steerable catheters are used. CS catheter allows to record IEGMs coming from the left atrium and ventricle. Moreover, this position is easily reproducible and serves as a reference point during the EPS. Thus, CS catheters play an important role in EP labs.

2.3.3 His bundle

For recording His bundle electrogram, a catheter is inserted via femoral vein to the high septal part of the right ventricle and pulled back slowly with clockwise torquing till characteristic His bundle electrogram appears.

2.3.4 Right ventricular apex

A diagnostic catheter is advanced from femoral vein to apical right ventricle, which allows to record local ventricular IEGMs.

2.4 Introduction to evaluate intracardiac electrocardiograms, basic intervals

Generally, IEGMs mean the electrical activity between two electrodes at the tip of the catheter (bipolar recording) [4]. The main difference between surface ECG and IEGMs is that the surface ECG records a summation of the electrical activity of the heart, while in contrast, IEGMs show only the electrical activity of a localized area, i.e., IEGMs are local intracardiac electrograms. Importantly, these are displayed together on the monitor system facilitating accurate interpretations of the electrical signals (Figure 4).

Figure 4.

Snapshot from an electrophysiology study. The upper four channels represent lead I, II, V1, and V6 of surface ECG. The paper speed is 200 mm/s. on the distal His (His d) channel, we can recognize three different wavefront characteristics of the His bundle: the first one is the A (atrial) wave (synchronous to P wave on surface ECG), the last is called V (ventricular) wave (synchronous to QRS complex on the surface ECG). In the middle, a sharp signal represents His bundle electrogram. AH interval could be measured from the beginning of A to the sharp His signal. HV interval is measured on the His bundle electrogram from the beginning of the His deflection to the earliest identified ventricular activity on the surface ECG. CS electrograms show atrial activation (synchronous to P wave on surface ECG again). Note that first activation occurs on CS 9,10 which is the proximal pair of electrodes. CS 9,10 is at the ostium of the coronary sinus, thus these electrodes are the closest to the sinus node. In the case of a correctly positioned CS catheter, CS 9,10 should be activated first during normal sinus rhythm. Finally, a local ventricular electrogram can be easily identified on the catheter at RVa position. Asterix represents PA interval.

After catheter placement, a routine EPS starts with the measurement of basic intervals [4]. Ideally basic intervals should be measured during sinus rhythm.

2.4.1 PA interval

The PA interval represents the interval between the earliest atrial activation (recording in any channel) in the region of the sinus node and at the region of the atrioventricular node. Usually, the earliest atrial activation is represented by the P wave onset on the surface ECG. Normal value is 25–55 milliseconds (ms).

2.4.2 AH interval

AH interval represents the conduction time from the low-right atrium at the interatrial septum through the atrioventricular (AV) node to the His bundle. It is measured between the atrial electrogram recorded by the His bundle catheter and the beginning of the His electrogram itself. The normal range is 55–150 ms [4]. The AH interval is sensitive to autonomic tone. A prolonged AH interval may indicate AV nodal disease or high vagal tone, whereas a shorter than normal AH can occur during sympathetic activation.

2.4.3 HV interval

HV interval reflects conduction through the His-Purkinje system and is measured on the His bundle electrogram from the beginning of the His deflection to the earliest identified ventricular activity on the surface ECG. An HV interval of 35–55 ms is considered normal. In the presence of anterograde conducting accessory pathway, the HV interval may be shorter. Prolonged HV interval represents infrahisian conduction disturbances.

2.5 Time measurement and pacing

2.5.1 Cycle length

Time measurements are reported in milliseconds in the case of EP procedures. To characterize the heart rate, the cycle length (CL) is used instead of the frequency. CL represents the length of time between each atrial or ventricular beats. For example, a tachycardia with a heart rate of 150 beats per minute has a CL of 400 ms (Figure 5). The faster the heart rate the shorter the CL.

Figure 5.

Atrial pacing at a cycle length of 400 ms, which means a rate of 150 beats per minute. Sharp pacing artifacts are present before P waves. P waves are negative in the inferior leads (pacing from coronary sinus ostium). Paper speed is 100 mm/s.

2.5.2 Pacing

Besides IEGM recordings, electrode catheters previously inserted in the heart are also used for pacing. An external stimulator is connected to the catheters. When pacing starts, electrical current is passed by catheters resulting in cardiac cells’ depolarization near the catheter’s electrode. The depolarization of these cells generates an electrical wavefront, spreading over the heart as the impulse originating from the sinus node. As a result, stimulator pacing generates cardiac impulse artificially. Carefully positioned catheters can impulse the heart from almost any position. During the EPS, pacing is used to introduce electrical impulses in predetermined patterns and at precise time intervals. Such pacing is called programmed stimulation [1]. Programmed stimulation consists of the main type of pacing technique: burst and extrastimulus pacing.

2.5.2.1 Burst pacing

Burst pacing consists of implementing a series of electrical impulses (so-called drive train) at a fixed cycle length. By definition, each impulse is called S1 and the difference between the impulses is the same (Figure 6).

Figure 6.

Ventricular burst pacing from RVa position. Note the pacing artifact on RVa channel and before the QRS complexes. Pacing cycle length is fixed (500 ms). Paper speed is 50 mm/s.

2.5.2.2 Extrastimulus testing

Extrastimulus testing means introduction of a drive train (usually 8 beats, S1) followed by one or more extrastimuli with shorter coupling interval than the cycle length of the drive. S2 means the first programmed extrastimulus, S3 is used for the second, and so on (Figure 7).

Figure 7.

Atrial extrastimulus testing from coronary sinus catheter (CS). The drive train consists of 8 beats at a cycle length of 400 ms is followed by an extrastimulus with a shorter coupling interval (300 ms). Ablation catheter (ABL) is in the His region showing His bundle electrogram. Paper speed is 50 mm/s.

2.6 Measurement of refractory periods

During EPS, refractory periods of cardiac tissues can be characterized by measuring effective, functional, and relative refractory periods [5].

2.6.1 Effective refractory period

Most commonly used as it is part of a routine EPS. It represents the longest coupling interval that fails to capture the tissue or be conducted over the structure (Figure 8).

Figure 8.

Programmed atrial stimulation from CS 9,10. Drive cycle length is 400 ms. Panel A shows ventricular contraction (i.e., QRS complex) after S2 extrastimulus at a coupling interval of 240 ms. However, Panel B represents atrioventricular block after S2 extrastimulus at a coupling interval of 230 ms. In this case, effective refractory period of the AV node (AVNERP) is 230 ms.

2.6.2 Functional and relative refractory period

A routine EPS does not include the measurements of the functional and relative refractory period. Functional refractory period means the lower limit shortest “output” coupling interval that can be produced by any “input” interval. Relative refractory periods represent the point at which latency begins to occur. RRP means “input” interval to a tissue at which the “output” interval just begins to differ from the “input” interval.

3. Summary

This chapter summarized the basics of electrophysiological studies of the heart. The author sincerely hopes that the chapter may have contributed to a deeper understanding of the world of electrocardiograms.

Conflict of interest

The author declares no conflict of interest.