(BQ) Part 2 book Practical clinical electrophysiology presents the following contents: Wolff-ParkinsonWhite syndrome and variants, ventricular tachycardia, bradycardias, syncope, sudden death syndromes, implantable cardioverter defibrillator indications, permanent pacemakers, clinical managementof patients with implantable cardioverter defibrillators, noninvasive diagnostic testing, antiarrhythmic drugs.
CHAPTER Wolff-ParkinsonWhite Syndrome and Variants Ventricular preexcitation occurs in 0.1 to 3.1 out of 1,000 people, and is defined as activation of the ventricular myocardium by an atrial impulse earlier than would be expected with normal atrioventricular (AV) conduction A delta wave is often seen on the surface electrocardiogram (ECG), which represents activation of the ventricle by an ‘‘accessory’’ pathway (AP) before activation by the conducting system (see Fig 9-1) Wolff-Parkinson-White (WPW) syndrome is defined as an AP-mediated tachycardia occurring in patients with ventricular preexcitation on a 12-lead ECG APs occur when there is an incomplete segmentation of the embryologic cardiac tube and formation of the fibrotic AV ring during fetal cardiac development The most common type of pathway is AV, formed by myocardial tissue connecting the atrium and ventricle, and most pathways are epicardial AV pathways may be ‘‘manifest,’’ which means that they conduct antegradely from the atrium to the ventricle and result in preexcitation which can be seen on the surface ECG, or ‘‘inapparent,’’ which means that preexcitation is not seen on the surface ECG, or concealed because normal AV conduction activates the ventricle faster than the AP or because the AP does not conduct in an antegrade manner These latter APs conduct only ‘‘retrograde’’ from the ventricle to the atrium, and are clinically relevant only when they participate in a tachycardia In fact a minority of APs only conduct in the antegrade manner (preexcitation) whereas the majority conduct in a retrograde direction Pathways exhibiting antegrade conduction so in an ‘‘all or none’’ manner 119 120 • Practical Clinical Electrophysiology FIGURE 9-1 Diagram of antegrade conduction over both the normal atrioventricular (AV) conducting system and a left-sided accessory pathway The amount of conduction over the accessory pathway corresponds to the degree of ventricular preexcitation or delta wave (See color insert.) 99% of the time Approximately 1% of antegradely conducting AV pathways exhibit decremental conduction, the vast majority of which are right sided APs can be located anywhere around the A-V ring except at the portion of the aortomitral continuity where there is no ventricular myocardium below the atrium They are often slanted, with the ventricular insertion point located closer to the septum and the atrial insertion more lateral in inferior APs and the ventricular insertion site lateral and atrial insertion site septal in anterior and posterior APs Less common variants of typical AV APs are atriofascicular, nodofascicular, nodoventricular, and fasciculoventricular pathways, representing AP conduction between combinations of the atrium, AV node, conducting system, and ventricle These variants are quite rare, but all except fasciculoventricular pathways may participate in tachycardias CLINICAL EVALUATION The first step in evaluating a patient who presents with preexcitation on an ECG is to take a thorough clinical history The presence of symptoms associated with preexcitation often determines the course of the clinical evaluation Symptoms may include sustained palpitations or syncope A history of syncope must be taken carefully to differentiate neurocardiogenic or vasovagal syncope from Wolff-Parkinson-White Syndrome and Variants V1 V4 V1 V4 V2 V5 V2 V5 V3 V6 V3 V6 A • 121 B FIGURE 9-2 Precordial leads V1 -V6 and a rhythm strip of lead are shown for a patient with preexcitation through a left-sided accessory pathway A: During atrial fibrillation (AF), overt preexcitation is seen with R-R intervals as short as 200 msec are seen, which may provoke hemodynamic instability and cardiac arrest B: After restoration of sinus rhythm, the same preexcitation pattern seen during AF can be seen on the electrocardiogram (ECG) syncope related to an AP-mediated tachycardia (see Chapter 12) Syncope due to an AP will often be preceded by palpitations and may even require urgent cardioversion or defibrillation if rapidly conducted atrial fibrillation (AF) is present (see Fig 9-2) Many patients will never have symptoms related to an AP, and the management of these patients is controversial (see discussion in the subsequent text) A family history of preexcitation or sudden cardiac death is important, as a familial association has been described In addition, the presence of congenital heart disease should be ascertained Ebstein anomaly is associated with right-sided APs, and when present the APs are often multiple and slowly conducting Ebstein anomaly may be seen in ‘‘corrected’’ or L-type transposition of the great arteries in which the tricuspid valve (TV) is the left AV valve Asymptomatic Patients The evaluation of patients presenting without identifiable symptoms or history of syncope and preexcitation on an ECG is controversial The two risks to such patients are the development of an AP-mediated supraventricular tachycardia (SVT) and the occurrence of AF with rapid conduction over the AP leading to ventricular fibrillation and/or cardiovascular collapse The incidence of the latter is extremely low (90% accurate and all have limitations When interpreting a preexcited ECG, the duration of the PR interval and the vector of the delta wave are examined In general, preexcitation caused by right-sided APs result in a shorter PR interval due to proximity to the • Practical Clinical Electrophysiology Vagal maneuvers Preexcited tachycardia (initial forces identical to delta wave) ina tio n Vagal maneuvers rm IV b-blocker, IV diltiazem, or verapamil Typical BBB pattern inconsistent with preexcitation or VT Te IV adenosine 12-lead ECG Termination Run 12-lead rhythm strip during administration Narrow QRS (120 msec) Preexcited atrial fibrillation b-blockers Tachycardia in a patient with known or suspected preexcitation Avoid calcium channel blockers, digoxin, adenosine, and 124 VT or unknown Emergent therapy for VT (see Chapter 10) IV procainamide, IV ibutilide IV amiodarone, or DCCV FIGURE 9-3 An algorithm for the management of tachycardia in a patient with known or suspected preexcitation DCCV, direct current cardioversion; ECG, electrocardiogram; BBB, bundle branch block; VT, ventricular tachycardia; IV, intravenous; AV, atrioventricular sinus node, with the terminal portion of the P wave often interrupted by the onset of the delta wave (see Fig 9-4) APs excite the ventricular myocardium from the site of insertion at the base of the ventricle and activation spreads from this point Therefore, the vector of the delta wave is determined by the site of ventricular insertion As an example, a posterior (previously described as left lateral, see subsequent text) AP activates the posterior and lateral portion of the ventricle first and activation spreads anteriorly and to the right, resulting in a rightward axis of the delta wave and positive delta wave in the precordial leads (see Fig 9-5) The traditional nomenclature describing APs was developed in the pathologic and surgical literature, and did not accurately locate the pathways as the heart sits in the chest cavity Revised nomenclature has been developed which more accurately reflects the anatomic location of APs around the AV ring Figure 9-6 depicts the location of APs using the revised nomenclature along with traditional designations A synthesis of algorithms for localization of APs is presented in this figure, which can only be used as a general guide for localization Factors which may affect ECG interpretation are multiple bypass tracts, rapid AV nodal conduction, intra-atrial conduction defects, hypertrophy, congenital heart disease, and prior myocardial infarction More • Wolff-Parkinson-White Syndrome and Variants I aVR V1 V4 II aVL V2 V5 III aVF V3 V6 125 FIGURE 9-4 A 12-lead electrocardiogram (ECG) demonstrating an anterior (right free wall) accessory pathway is shown Note the left inferior axis of the delta wave and the short PR interval The delta wave interrupts the P wave (arrow), characteristic of right-sided accessory pathways accurate localization and characterization of APs requires an electrophysiologic study Fully preexcited tachycardias often present a diagnostic dilemma, as it may be difficult to differentiate the ECG from ventricular tachycardia (VT) Algorithms have been developed in an attempt to differentiate a preexcited tachycardia from VT Absence of initial RS complexes across the precordium and other factors have been cited as morphologic criteria for VT (see Chapter 10) This criterion is useful, as negative initial forces suggest I aVR V1 V4 II aVL V2 V5 III aVF V3 V6 FIGURE 9-5 A 12-lead electrocardiogram (ECG) demonstrating a posterior (left free wall) accessory pathway is shown Note the longer PR interval and vector of the delta wave consistent with the posterior location of the accessory pathway There is a lesser degree of preexcitation due to the relative distance from the sinus node, which allows a greater degree of activation through the native conduction system • 126 I II Practical Clinical Electrophysiology Septal "paraseptal" Variable pattern common Superoparaseptal "anteroseptal" + aVR − V1 − + aVL + V2 − III +/− aVF I II III + V3-6 + Anterior "right free wall" I + aVR +/− V1 + II + aVL +/− V2 + III − aVF +/− V3-6 +/− + aVR +/− V1 +/− + aVL + V2 + − aVF − V3-6 + Posterior "left lateral" H TV I II MV III Inferoparaseptal "posteroseptal" I II III + aVR +/− V1 + _ aVL + V2 − − aVF − V3-6 ? − aVR +/− V1 + + aVL − V2 + + aVF + V3-6 + Infero-posterior "left posterior free wall" I II III + aVR +/− V1 + _ aVL +/− V2 + − aVF − V3-6 + R/S