An electrophysiology (EP) test (study) is a minimally invasive procedure where electrode-tipped catheters are placed into the heart to record electrical activity in various areas. The EP study may be used as an isolated diagnostic procedure or in combination with an ablation procedure. As a diagnostic procedure, the EP study may clarify a diagnosis that is not clear from surface electrocardiograms (ECGs), allowing for informed decision-making regarding further treatment options. In combination with ablation procedures, the EP study clarifies the mechanism of an arrhythmia to guide an ablation treatment strategy.¹

Venous access from the femoral veins and/or the internal jugular veins is required to perform an EP study. The presence of deep venous thrombosis or venous occlusion is a contraindication to performing an EP study.

Two main fluoroscopic views are utilized to localize which structures a catheter is in during an EP study: right anterior oblique (RAO), which shows anterior and posterior positions in the heart, and left anterior oblique (LAO), which delineates
right and left orientation in the heart (Figure 53.1). Using a combination of these two views, recordings from the catheter, and an understanding of cardiac anatomy, the location of catheters can be determined.

Because most catheters are placed into the central veins, they will first enter the right atrium. The sinus node is the primary pacemaker site in the heart and lies in the high lateral right atrium near the junction with the superior vena cava. A catheter placed in this area should record the earliest atrial depolarization during sinus rhythm.

A catheter can be placed into the coronary sinus by advancing it to the inferior tricuspid annulus and then rotating it posteriorly. The coronary sinus lies anterior to the fossa ovalis (ie, interatrial septum), which is itself anterior to the plane of the superior and inferior vena cava junction with the right atrium. The coronary sinus courses within the left atrioventricular (AV) groove between the left atrium and left ventricle, and is closer to the atrium than to the left circumflex artery. Hence, recordings from catheters within the coronary sinus generally have near-field atrial signals and far-field ventricular signals (discussed in more detail subsequently). Catheters in the coronary sinus can sometimes be advanced further toward the great cardiac vein (along the anterolateral AV groove) and the anterior interventricular vein (which lies along the interventricular septum, and generally rightward of the left anterior descending artery). Ventricular branches of the coronary sinus course anteriorly from the body of the coronary sinus. It is common for the tip of a catheter within the coronary sinus to be wedged into a ventricular branch, leading to a more near-field ventricular signal.

The bundle of His is the only normal AV connection across the central fibrous body of the heart. It connects the compact AV node with the His-Purkinje system (the right and left bundles). It can be located along the superior aspect of the membranous septum. A catheter wedged between the septal and anterior leaflet of the tricuspid valve can record electrical activity in the His bundle. The His-bundle EGM can also be recorded from the left side of the membranous septum (left ventricular aspect). The aortic valve, specifically the junction
of the noncoronary cusp and right coronary cusp, lies just superior and leftward of the His bundle.

The right bundle extends down the right side of the interventricular septum and then across to the free wall of the right ventricle via the moderator band. There are many exits for the right bundle into the right ventricular myocardium near the apex of the right ventricle and from branches off the moderator band to the free wall. For an electrical impulse from the right ventricle to propagate retrograde via the AV node to the atrium in the absence of an accessory pathway (ie, in the majority of patients), the impulse must enter the right bundle near the apex (near the standard position of a right ventricular catheter) or moderator band branches.

In cases where catheters need to be placed in the left atrium, access can be obtained through a patent foramen ovale, atrial septal defect, or by performing transseptal puncture through the interatrial septum at the fossa ovalis. The fossa ovalis lies posterior to the coronary sinus and aortic root. Crossing the interatrial septum requires significant caution and guidance with fluoroscopy and/or intracardiac echocardiography to avoid crossing anteriorly into the aorta or posteriorly into the pericardial space. Access to the left ventricle can be performed via transseptal (by crossing the mitral valve) or retrograde aortic (by crossing the aortic valve) approaches. Anticoagulation with heparin should be performed before placing catheters and sheaths in the left atrium or ventricle to prevent formation of blood clots and potential stroke or systemic embolism. Significant care should also be taken to prevent air within sheaths and catheters in the left side of the heart or arterial system to avoid air embolisms.

The EP study is performed by inserting electrode-tipped catheters into the body through central venous sheaths (generally from the right/left femoral veins and/or right internal jugular vein) and positioning them within the heart (Figure 53.1). Catheters are placed within the specific chambers/vessels using fluoroscopy and/or three-dimensional electroanatomic mapping (EAM). There are many shapes and sizes of electrode-tipped catheters, based on the purpose of the catheter. The most standard catheters are quadripolar (four recording electrodes) and have a preformed curve, and are manipulated only by torqueing and advancing/retracting the catheter. Many catheters designed for placement in structures such as the coronary sinus, or for mapping cardiac structures have more electrodes (10+) and are deflectable. Most catheters are between 4 and 8 French in diameter. The specific catheters used are tailored toward the purpose of the EP study. At the end of the EP study procedure, all catheters and sheaths are removed and hemostasis is obtained by manual compression and/or vascular closure device.

Intracardiac EGMs are recordings from these electrodes. In contrast to surface ECG that records the summation of cardiac electrical activity, EGMs record localized cardiac activity only between two electrodes on the catheter, called a bipolar electrode. Hence, this provides electrical information only within the vicinity of the recording electrodes. EGMs contain several rapid deflections, representing depolarization of myocardial tissue between the two recording electrodes. Remote (far-field) cardiac electrical activity can also be seen, characterized by less rapid deflections (Figure 53.2). Less commonly used are unipolar lead configurations that measure cardiac electrical activity directly beneath the electrode. High-pass and low-pass filters are applied to EGMs for display to reduce noise and interference. The usual filter range for bipolar EGMs is 30 to 500 Hz. A notch filter is also generally applied to remove electrical noise (50 or 60 Hz).

EGMs are displayed on the recording screen by the position of the catheter (eg, HRA for high right atrium, HIS for His bundle, CS for coronary sinus, RV for right ventricle) or by the name of the catheter (eg, DD for duodecapolar or Abl for ablation catheter) and the pair of electrodes from which the recording is originating. Basic catheters inserted in the HRA, HIS, and RV are quadripolar, with two or three EGMs displayed for each catheter. Other catheters are multipolar catheters with multiple pairs of recording electrodes. Pairs of electrodes can be distinguished by a subscript p for proximal, m for middle, and d for distal (eg, HIS_p, HIS_m, HIS_d) or by number pair, where by convention, electrode 1 is the most distal (eg, CS 1,2 representing the most distal bipolar recording pair of electrodes, CS 3,4 representing the second most distal pair) (Figure 53.2).

The amplitude of EGMs depends on multiple factors: electrode contact to the myocardial tissue, the proximity of an electrode to the tissue from where the EGM originates, and the health of the myocardial tissue. Contact against a myocardial surface can be determined by tactile sensation, visualization of catheter bend by fluoroscopy or EAM, presence of sharp near-field EGMs, or springs within the catheter shaft that can measure contact force (generally available with ablation catheters only). Catheters placed on the annulus will record both atrial and ventricular EGMs with approximately the same amplitude. As the catheter moves away from the annulus and toward the atrium, the atrial EGM will progressively become higher amplitude than does the ventricular EGM, and vice versa for a catheter moving away from the annulus from the ventricular side. Areas of myocardium with a significant amount of fibrosis will have relatively lower amplitudes than will the normal myocardial tissue. On the display screen, the gain can be adjusted to improve visualization of low-amplitude signals. The standard sweep speed for visualization during an EP study is also much faster than that of surface ECG to enable better resolution of temporal relationships (100-200 mm/second during EP study vs 25 mm/second on surface ECG).