Electrical System of the Heart Of significance to the conduction system is the existence and location of the sinoatrial (SA) node. This is a small, cigar-shaped structure, located in the high right atrium near the junction of the high right atrium and the superior vena cava, at the top of the crista terminalis. The crista terminalis is a ridge of tissue formed by the boundary of the smooth venous muscle of the atrium and the trabeculated pectinate muscles which form the atrial appendage. The SA node has spontaneous automaticity and is the primary pacemaker of the heart. The fibrous skeleton is a fibrous tissue structure supporting the muscles of the heart and providing electrical insulation between the atria and the ventricles. Atrioventricular node (AV node) serves primarily to conduct impulses generated in the sinus node to the ventricles. It has spontaneous automaticity slower than the sinus node s and is therefore a secondary pacemaker of the heart. The distal portion of the AV node, which gives rise to the His bundle, is a compact, well-defined structure. This structure is located at the apex of Koch s triangle and is an anterior structure situated at the anterior aspect of the atrial septum. Koch s triangle is an important anatomic landmark whose borders are defined by the tricuspid annulus (the portion of the annulus next to the septal leaflet of the tricuspid valve), the tendon of Todaro, and the os of the coronary sinus. The os of the coronary sinus is a posterior structure. As atrial tissue approaches the compact AV node, the cells transition from having purely atrial behavior and characteristics to cells having typical AV nodal behavior. These transitional tissues appear to function as bands or tracts, the anterior tract which functions as the fast AV nodal pathway, and the posterior tract which functions as the slower AV nodal pathway. The electrical characteristics of the two pathways cannot be demonstrated in every EP study. The His-Purkinje system is that portion of the conduction system which consists of a band of parallel Purkinje fibers continuous with the distal AV node (His bundle), traveling through the interventricular septum to the right and left bundle branches and the Purkinje fibers, which are specialized cells of conducting tissue located throughout the ventricles. The Purkinje fibers are the tertiary pacemaker cells of the heart. The Electrophysiology Study Patient selection. AHA/ACC Guidelines exist providing guidance for identifying patient candidates for EP Studies. These guidelines should be followed both from the standpoint of providing appropriate care and for appropriate reimbursement opportunity. For example, exclusion criteria for a ventricular arrhythmia study would be: 1) Ventricular arrhythmias due to electrolyte imbalances, correctable drug effects, and 2) Arrhythmias due to reversible ischemia. Catheter placement and electrogram definitions. Three or four multipolar catheters (multiple electrodes) are commonly introduced into the right and/or left femoral veins. Other venous access can be used such as the jugular, brachial, or subclavian veins. The electrodes at the distal end are exposed, allowing for monitoring of electrical activity from nearby tissue. The proximal end of each electrode is connected through a small wire to a shielded pin that connects to a system for recording intracardiac electrograms (e-grams). Each set of two electrodes provides bipolar electrical signals. The most common EP catheter is a quadripolar (4 electrodes) catheter with a fixed or deflectable curve. An octapolar or decapolar catheter is commonly placed in the coronary sinus. Spacing of the exposed electrodes on the catheter ranges from 2-10 mm. Any two poles on a catheter may be used for pacing purposes. Most commonly, the two distal electrodes on the high right atrial (HRA) catheter and the right ventricular (RV) catheter are used for pacing purposes in the usual EP study. The remaining electrodes are used for recording purposes. The catheters are commonly placed in the following locations: High right atrium (HRA) electrogram: High lateral wall of right atrium, SA node region, superior portion of crista terminalis. His bundle (HBE) electrogram: Posterior aspect of the tricuspid valve recording electrical activity from the low septal right atrium, AV node, His bundle, and right ventricle. Coronary sinus (CS) electrogram: Catheter enters coronary sinus os posterior and slightly inferior to the tricuspid valve and follows the coronary sinus which lies posteriorly in the AV groove. Right ventricular apex (RVA) electrogram: Apex of right ventricle, distal portion of right bundle branch. May also be positioned in right ventricular outflow tract (RVOT), at the interventricular septum inferior to pulmonary artery outflow tract. If the patient has a pre-existing LBBB, when placing the RV catheter, preparations should be in place for immediate pacing if needed. The right bundle branch lies very superficial in the endocardium and conduction through it may be interrupted by the catheter, temporarily causing A-V block. Should this be the case, the patient may have asystole and would require pacing in the ventricle until the right bundle branch block resolves. Basic measurements (Figure 1). Measurements are made in milliseconds and seconds, often termed as intervals. Electrogram/ECG display speed is usually 100 mm/sec allowing for visualization and precise measurement of very short intervals. Each channel is labeled on the monitor screen and may be arranged in different order in individual labs. Basic cycle length (BCL) is measured from A to A on the HRA electrogram. PR, QRS, and QT are measured on surface ECG. It is important to place all electrodes in the correct locations on the patient to obtain 12-lead ECG recordings during the EP study. Three or four surface leads are continuously displayed on the monitor during the procedure (commonly Standard lead I, AVF, V1 and V6 or I, II and V1). IACT (Intraatrial conduction time), PA, or P-LSRA (P to Low Septal Right Atrium) interval are measured from the beginning of the P wave on surface ECG to A wave deflection on His bundle electrogram, and when combined with AH and HV intervals = PR interval. The AH interval (AV node conduction time) is measured on His bundle electrogram from the first rapid deflection of the atrial electrogram to the onset of the His deflection. Normal value is 50-120 msec. AV node disease often produces prolongation of the AH interval. HV interval reflects conduction through the His-Purkinje system and is measured on the His bundle electrogram from beginning of the His deflection to the earliest identified ventricular activity on the surface ECG. Normal value is 35-55 msec. Distal conducting system disease often produces prolongation of the HV interval. Antegrade or A-V activation sequence (Figure 2). The following represents normal conduction through the heart and normally occurs in the time frame as cited above. It begins in the SA node to/through atria; then the AV node (slowly); and then to the His-Purkinje system. Next there is the ventricular activation sequence: 1) intraventricular septum, 2) apices of right and left ventricles, 3) ventricular free walls, to 4) ventricular bases (latest activation in left ventricular posterobasilar area). Pacing protocols. Characteristics of various tissues are demonstrated by using various pacing techniques. Extrastimulus pacing technique. Eight paced beats (traditional drive train labeled S1) followed by a premature impulse (extrastimulus S2) at a programmed coupling interval is decremented with each drive train. Additional extrastimuli may be added at programmed coupling intervals (labeled S3, S4, etc.) Demonstration of relative refractory period (RRP); extrastimulus pacing, which demonstrates the longest coupling interval at which a premature impulse results in slowed conduction through that tissue. Demonstration of effective refractory period (ERP) and functional refractory period (FRP): 1) ERP: Extrastimulus pacing which demonstrates the longest coupling interval for which a premature impulse is blocked or fails to propagate through that tissue (Figures 3 and 4); and 2) FRP: The shortest interval between two impulses that are conducted through a tissue. Induction of supraventricular or ventricular arrhythmias. The extrastimulus pacing technique may be used in any chamber or area of the heart. Incremental pacing technique: long trains of paced beats, decrementing in cycle length with successive train: 1) AV block rate: atrial pacing cycle length at which intermittent conduction through the AV node occurs. The AV node typically displays decremental conduction properties leading to a Wenckebach periodicity, particularly with antegrade activation (Figure 6). 2) V-A block rate: ventricular pacing cycle length at which intermittent retrograde conduction through the AV node occurs. Retrograde or V-A activation sequence (Figure 5): Of those which occur over normal AV conduction system, the first is the ventricular myocardium (Purkinje fibers), the second is the His bundle, and the third is the low septal right atrium (the AV node has no manifest depolarization). The normal retrograde atrial activation sequence consists of the atrial septum (A on His electrogram), the right atrial (A on HRA electrogram), and the left atrial (A on CS electrogram). If VA conduction is demonstrated, you understand that 1) it is present in most individuals, and 2) retrograde conduction, if present, is a useful method of demonstrating AV bypass tracts (atrial activation sequence will be abnormal). SA Node Assessment To determine sinus node recovery time (SNRT), there is an assessment of automaticity of the SA node by a 30-second pacing run, which then is stopped abruptly. The pause induced by overdrive suppression of the SA node (interval following the cessation of pacing) is measured. The second sinus interval may be longer than the first (secondary pause); thus, it is measured as the sinus node recovery time. A value of > 1,500 msec is abnormal. Since the basic cycle length (BCL) affects the SNRT, the corrected sinus node recovery time (CSNRT) is obtained by subtracting the BCL from the SNRT: CSNRT = SNRT - BCL (Normal = 100 msec = significant disease. Usually treated with permanent pacemaker; 4) Splitting of His deflection = significant disease (arguable, according to Josephson); and 5)To identify AV node and His-Purkinje ERP, note the following: 1) Block in His-Purkinje system at H1-H2 interval of > 400 msec = significant distal conduction system disease, and is generally indication for permanent pacing; 2) Wenckebach cycle length > 450 msec = significant disease or AV nodal blocking drugs; 3) HV lengthening or distal block with pacing at 400 msec or greater = significant His-Purkinje block; 4) If AV nodal ERP is reached before His-Purkinje ERP is reached, and infranodal disease is suspected, autonomic maneuvers should be undertaken to reduce the refractoriness of the AV node (atropine). Arrhythmia Induction Extrastimulus technique may be used either in the atrium or the ventricle to induce tachycardia. A drive train of eight (usually) stimuli is delivered followed by a stimulus (S2) at a shorter cycle length. This procedure is repeated with the S2 interval getting progressively shorter until refractoriness is reached or tachycardia is induced. Additional stimuli may be added (S3, S4, etc.) (Figure 7). If tachycardia is induced, it is recorded for analysis and the patient s clinical response is observed. If tachycardia termination is not spontaneous, burst overdrive pacing may be attempted (antitachycardia pacing). If syncope occurs, the patient is cardioverted immediately. Short runs of burst pacing at various cycle lengths may also be used as programmed electrical stimulation. Conclusion. Electrophysiology is a fast-growing field; hopefully, this has been a thorough review for anyone thinking of transferring into the EP lab.