Indian Journal of Cardiology ISSN-0972-1622 2012 by the Indian Society of Cardiology Vol. 15, (3-4), 27-37 [ 27 Review Article Shomu Bohora Assistant Professor, Deptt. of Cardiology, U.N. Mehta Institute of Cardiac Research Centre, Vadodara, Ahmedabad. Introduction Invasive cardiac electrophysiology is a useful clinical technique for the investigation and treatment of cardiac rhythm disorders. Electrophysiology (EP) study can accomplish the following goals. Make a completely accurate diagnosis of an arrhythmia Establish the etiology for syncope/palpitations Evaluate prognosis and stratify risk of sudden cardiac death in patients at risk Guide antiarrhythmic drug therapy Evaluate the requirement, feasibility or outcome of non pharmacologic therapy (e.g. Pacemaker, radiofrequency ablation, antiarrhythmic surgery, or implantable cardiac defibrillator). Electrode catheter placement In typical studies of patients with supraventricular tachycardia (SVT) multipolar (Quadripolar/Decapolar) electrode catheters are positioned in the high right atrium (HRA), right ventricle (near apex (RVA)), across the tricuspid annulus to record a potential from the bundle of His (His) and coronary sinus (CS) under fluoroscopy guidance (Figure 1). When mapping and ablation are performed, left heart access may be obtained via either a trans-septal or retrograde aortic approach. Occasional epicardial mapping needs to be done. In pediatric patients, due to limited venous access less number of catheters may be used and occasional esophageal catheter for atrial signal recording may be used. Fig. 1 : Fluoroscopy depicting catheter positions in RAO and LAO views respectively. (CS proximal (CSP) and CS distal (CSD)) Address for Correspondence : Dr. Shomu Bohora, Assitt. Prof., U.N. Mehta Institute of Cardiac Research Centre, Vadodara, Ahmedabad.
Indian J Cardiol 2012 28 ] Vol. 15, (3-4), 27-37 Bohora S Baseline recordings Baseline recordings obtained during a typical electrophysiological study include surf ace electrocardiograms to time events from the body's surface, and intra-cardiac electrograms, all of which are recorded simultaneously. Electrical activation of the atrium is followed by that of the ventricle with a delay which occurs in the Atrio-ventricular (AV) node. The intracardiac recordings obtained from the electrodes are sharper unlike the waveforms of the surface ECG. These electrograms fall within the P wave or the QRS complex depending on the spread of activation within the chamber in which they are placed. Typical electrograms of the high right atrium electrode will show a sharp deflection in the early part of the P wave. Recording from the His electrode shows an A (Atrial) signal, a His signal and a V (Ventricular) signal. As the coronary sinus is in the left AV groove, a coronary sinus catheter records both the left atrial and left ventricular electrograms. RV catheter records ventricular signals from the right ventricle. Activation of the left atrium at the CS ostium (CSP) electrode is normally earlier than at the CS distal (CSD) electrode. Catheters record the signals from all the four chambers directly or indirectly (CS catheter) and the His bundle. Normal activation pattern obtained in sinus rhythm is shown in Figure 2a. In a normal person an electrical impulse can travel from the atrium to the ventricle or from the ventricle to the atrium via the AV node only. During sinus rhythm normal AH interval is 50-120 msec, which represents the conduction time through the AV node and proximal His Bundle. Normal HV interval is 35-55 msec and represents the conduction time through the distal His-Purkinje tissue. Any deviation from normal helps to diagnose cause of arrhythmia (Figure 2b). Fig 2 : Activation during sinus rhythm Fig. 2a: Simultaneous recording of surface and intracardiac electrograms during sinus rhythm showing the normal activation sequence. Pacing protocols and programmed electrical stimulation After baseline measurements are recorded, pacing is performed via intracardiac electrode catheters. Different chambers can be paced and the activation patterns noted. Burst (continuous) pacing in atrium and/or v entricle at v arious f ixed cycle lengths (mentioned in msec more often than as heart rate) as well as programmed electrical stimulation (PES) is
Bohora S Indian J Cardiol 2012 Vol. 15, (3-4), 27-37 [ 29 Fig. 2b : In a patient with accessory pathway (W PW Syndrome) during sinus rhythm, ventricular signals are seen earlier in the CSD electrode as compared to the His (HBED) electrode (arrow) suggesting activation of the left ventricle earlier than the ventricle near the His and with a short HV interval suggesting antegrade conduction through the left accessory pathway. administered. With PES, a number of stimuli at a fixed cycle length are delivered (e.g. ten beats at a fixed rate of 600 msec (100 beats/min)), followed by a premature beat. The premature beat is moved more and more premature, until the refractory period tissue is reached. Multiple premature stimuli can be given (generally 3-5). The sequence and intensity of protocols depends not only on the pathology for which the EP study is being planned but also on the operator's preference. Further is a brief overview of basic electrophysiology protocols during a study. Assessment of Sinus Nodal disease: Sinus nodal (SN) function is assessed by sinoatrial conduction time and sinus nodal recovery time. Measuring Sino-atrial conduction time (SACT) involves placement of a catheter near the sinus node from which progressively premature atrial extra stimuli are introduced after every eighth to tenth beat of either a stable sinus rhythm (Strauss method), or atrial pacing for 8 beats is done at a rate just lower than the sinus rate (Narula method). The difference of time of the recovery beat post pacing versus that of the sinus cycle length gives us 2 times the SACT value. Interval of upto 250 msec is considered normal. The sinus node recovery time (SNRT) is another method of assessing SN function and is performed by placing a catheter near the sinus node and pacing (overdrive suppression) for at least 30 seconds at a fixed cycle length. This is repeated at progressively shorter cycle lengths. Pacing rates up to 200 beats/ minute may be employed. The maximum SNRT is the longest pause from the last pacing stimulus to the first spontaneously occurring sinus beat at any paced cycle length. As the sinus cycle length (SCL) affects the SNRT, it is often normalized or corrected. The corrected sinus node recovery time (CSNRT) is determined by subtracting the SCL from the SNRT. Normal values considered are SNRT/SCL <150 percent and CSNRT < 550 milliseconds. A normal study however does not rule out clinical sinus nodal disease. Assessment of AV nodal disease: During sinus rhythm higher than normal AH and HV intervals (Figure 4) may represent abnormality in the conduction system. His bundle electrogram
Indian J Cardiol 2012 30 ] Vol. 15, (3-4), 27-37 Bohora S Fig. 3 : Sinus node recovery time as assessed by pacing the right atrium. Here the SNRT is 1816 msec and SCL is 1104 msec giving a CSNRT of 712 msec. (> 550 msec suggests sinus nodal dysfunction) duration reflects conduction through the short length of compact His bundle that penetrates the fibrous septum. This interval is normally short, 15 to 25 msec. Fractionation and prolongation, or even splitting of the His bundle potential, is seen with disturbances of His bundle conduction. The atrium then is paced at progressive faster rates and W enchebach point is noted. Programmed Fig. 4: Block after the His while pacing at cycle length of 500 msec in the atrium suggests His Purkinje disease and requires pacemaker implantation. Note that during conducted ventricular beat HV interval is prolonged (74 msec).
Bohora S Indian J Cardiol 2012 Vol. 15, (3-4), 27-37 [ 31 atrial extra-stimuli can be used to determine the effective refractory period of the AV node (The atrial premature beat interval at which the conduction to the ventricle is blocked), which should be <450 milliseconds. If the block occurs in the AH segment at > 450 msec, in the absence of other abnormalities, there still is no clearly demonstrated risk of progression to AV block. However having a block in the HV segment suggests His Purkinje disease (Figure 4) and pacemaker implantation is advised. Atropine and procainamide can be given if required to stress the conduction system. Pacing protocols for tachycardia management. Atrial and ventricular pacing at increasing faster rates or PES at decreasing intervals with one or more than one extra-stimulus, with or without drugs like isoprenaline and atropine, can induce tachycardia (especially reentrant tachycardia) in most patients who have had documented tachycardia or symptoms. During tachycardia, based on different activation patterns and maneuvers an accurate diagnosis can be reached and localization can be done appropriately. Supraventricular and ventricular arrhythmias can also be terminated by burst pacing. Baseline intervals are recorded before pacing protocols are started. Ventricular pacing Normally during ventricular pacing the impulse travels retrograde into the atrium through the AV node. VA dissociation is referred when while pacing the ventricle, no impulse travels back to the atrium. Ventricular pacing is then done at decreasing cycle length till VA conduction is blocked. Thereafter ventricular extra stimuli (single or multiple) are given at decreasing interval till a VA block is obtained (Figure 5 b). During ventricular pacing atrial activation pattern whether central (His A is earliest) or eccentric (His A is later that A in either RA or LA recording catheter) is seen. Lengthening of VA interval with decrement in the stimulus interv al suggests decremental conduction. Central and decremental conduction generally suggests conduction through the nodal tissue (Figure 5a). Eccentric and non-decremental conduction suggests presence of a retrograde conducting accessory pathway (Figure 5c). Multiple (3 to 5) ventricular extrastimuli may be needed to induce ventricular arrhythmia sometimes. VA interval during RV apical and RV base pacing and VA interval of broad and narrow QRS during para-hisian pacing is useful to detect presence of a septal located bypass tract. Fig. 5 : Ventricular pacing. Figure 5 a: - Ventricular pacing showing central conduction with atrial signal at the His being the earliest as compared to the RA and CS atrial signals. (arrow)
Indian J Cardiol 2012 32 ] Vol. 15, (3-4), 27-27 Bohora S Fig. 5 b : Programmed ventricular extra stimuli showing central conduction (Arrow). An more premature timed ventricular extra stimuli causes no atrial activation (VA block) Fig. 5 c: Eccentric conduction during ventricular pacing as evidence by earliest atrial signals in CSD electrogram (arrow) and earlier than the A at the His suggesting presence of a retrograde conducting pathway in the left side.
Bohora S Indian J Cardiol 2012 Vol. 15, (3-4), 27-37 [ 33 Atrial Pacing Atrial pacing from either the right atrium or the left atrium as clinically indicated is done to see for antegrade conduction characteristics through the AV node. As discussed prior, Wenchebach point is noted. Most of the time a SVT is induced at Wenchebach point. In presence of WPW syndrome, there can be eccentric activation with increasing preexcitation which can be easily appreciated on a 12 lead electrocardiogram during the study. Atrial extra-stimuli are then given (single and if required multiple). Atrial extra stimuli may show an AH jump (wherein there is AH Fig. 6 : Atrial extra stimuli from the right atrium showing AH jump with initiation of tachycardia. prolongation of more than 50 msec with a decrement of 10 msec on atrial extra-stimuli interval), induce a tachycardia (Figure 6) or bring out preexcitation with or without decremental antegrade conduction with a preexcited QRS complex. Fig. 7a:- 1:1 AV relationship with short VA interval (<70 msec) and earliest A at His suggests AVNRT.
Indian J Cardiol 2012 34 ] Vol. 15, (3-4), 27-37 Bohora S Tachycardia Study Once tachycardia is induced relationship of the atrium and ventricular signals are noted. 1:1 AV activation generally suggests AVNRT (Figure 7a), AVRT Fig. 7b : 1:1 AV relationship with long VA interval (> 70 msec) and earliest A at CS D suggests AVRT Fig. 7c : V > A suggests Ventricular tachycardia (Figure 7b) and atrial tachycardia but rarely ventricular tachycardia (VT). A>V suggests atrial tachycardia and rarely AVNRT but never an AVRT or VT. V>A suggests ventricular tachycardia (Figure 7c). During SVT atrial activation patterns helps to diagnose the nature of tachycardia (Figure 7a and b).
Bohora S Indian J Cardiol 2012 Vol. 15, (3-4), 27-37 [ 35 Fig. 8 : Pacing maneuvers during tachycardia Fig. 8 a : Continuous ventricular pacing during narrow QRS tachycardia shows VA during pacing 168 ms ec and VA during tachycardia 64 msec. A difference of > 100 msec suggests AVNRT. Fig. 8b : Programmed single ventricular extra stimuli during narrow QRS tachycardia when the His is refractory, does not advance the next atrial signal, suggests AVNRT.
Indian J Cardiol 2012 36 ] Vol. 15, (3-4), 27-37 Bohora S Pacing maneuvers during tachycardia Continuous pacing at a slightly faster than that of the tachycardia (Figure 8a) or programmed single extra-stimuli when the His is refractory (Figure 8b), can be used to differentiate the mechanism of arrhythmia. Mapping for localizing site of ablation In many cases, catheter ablation immediately follows the diagnostic electrophysiologic study. Mapping refers to careful movement of a mapping or ablation catheter in the area of interest, probing for the site at which radiofrequency ablation will be successful at curing the arrhythmia. pattern to that of tachycardia, with difference in return cycle length of < 20 msec from that of the cycle length of the arrhythmia, suggests the site of slow conduction and possible site for ablation. It is also called concealed entrainment (Figure 9c). Presence of abnormal diastolic potentials, fractionated potentials or areas of continuous activity may suggest areas of slow conduction and possible sites of ablation. Post ablation protocols To confirm the success of ablation all pacing Fig. 9a: - Activation mapping. Signals in the RFD channel (mapping catheter) during ventricular tachycardia are earlier than the earliest QRS complex suggesting that the site is in the area of earliest activation and potentially near the origin of the arrhythmia Activation mapping: - Where the activation signal in the mapping catheter shows earlier signals as compared to the surface P/QRS (Figure 9a) suggests the area to be near the origin of the arrhythmia and suggests an appropriate site for treatment. Pace-mapping: - In a patient especially with ventricular tachycardia, pacing at the same cycle length as the tachycardia from the mapping catheter and comparing the 12 lead ECG obtained during pacing with that of tachycardia is called pace-mapping. If there is a 12/12 match it suggests the origin of the tachycardia to be nearby (Figure 9b) and is especially helpful for treatment of idiopathic ventricular outflow tachycardias. Entrainment : Pacing during tachycardia from the site of interest, when results in similar activation protocols need to be repeated (especially protocol on which the tachycardia was induced earlier) after successful treatment of the arrhythmia with and without isoprenaline, atropine and adenosine whenever required. Conclusion Electrophysiology Studies requires rigorous training to recognize the deviations in intra-cardiac electrograms and interpret the results of the pacing maneuvers in real time to reach to a successful procedure in the least duration of time. With advances in technology and presence of 3 D anatomical mapping systems anatomical localization and ablation of a complex tachycardia is much simpler than before; however in the absence of a solid foundation of the
Bohora S Indian J Cardiol 2012 Vol. 15, (3-4), 27-37 [ 37 Fig. 9b : Pace-mapping. 12 lead ECG of tachycardia is taken. The tachycardia is terminated and then pacing from given site at the same rate is done and the two compared. A 12/12 match of the QRS complexes as shown in figure suggests mapping catheter site to be near the tachycardia origin. Figure 8 e: - Concealed entrainment. basic electrophysiology procedure and protocols, is sometimes becomes difficult to do even the simplest of procedure. References 1. Arora R, Kadish A. Fundamentals of Intracardiac Mapping. In: Huang S, Wood M, eds. Catheter Ablation of Cardiac Arrhythmias. Elsevier Saunders; 2011: 103-126. 2. Josephson M. Electrophysiologic Investigations: General Concepts. Josephson M, eds. Clinical Cardiac electrophysiology: Techniques and Interpretations. Lea & Febiger;1993: 22-70. 3. Josephson M. Supraventricular tachycardias. Josephson M, eds. Clinical Cardiac electrophysiology: Techniques and Interpretations. Lea & Febiger;1993: 181-274.