7 Left-lateral accessory pathways (AP) ablation is a common procedure that becomes necessitated often as the unforeseen result of a comprehensive electrophysiological (EP) evaluation of a supraventricular tachycardia (SVT). At other times, it is a procedure that can be suspected long in advance, based on a preexcited surface ECG pattern in patients with Wolff-Parkinson-White syndrome (WPW). In either situation, the target of ablation is a tract of myocardial tissue that crosses the atrioventricular groove, allowing the propagation of arrhythmias in a circuitous motion between the atria and ventricles. Patients with predetermined evidence of APs are routinely referred for transthoracic 2D echocardiography, to exclude conditions often associated with WPW, including Ebstein’s anomaly, tetralogy of Fallot, mitral valve prolapse, and atrial-septal defect. The presence of underlying structural heart disease may influence the selection of equipment to be used and may altogether preclude a transseptal approach to ablation. In asymptomatic patients referred to our attention based on a preexcited surface ECG, extended mobile cardiac outpatient telemetry is occasionally arranged to determine (1) the presence of asymptomatic SVT, and (2) the presence of an intermittent preexcitation pattern, suggesting a pathway associated with a low risk of sudden death. In those with minimal preexcitation (so-called “inapparent” preexcitation) or in those in whom the diagnosis is altogether questioned, an adenosine challenge is performed to search for AV conduction during complete blockade through the AV node. Patients referred for ablation are universally referred for routine labs prior to the procedure, including complete blood count, chemistries, and coagulation parameters, including partial thromboplastin time (PTT) and PT/INR. Those patients on warfarin for thrombophilia are usually not instructed to discontinue this medication, as our experience suggests that it is not associated with an increased risk of periprocedural bleeding complications. Furthermore, avoiding interruption of warfarin therapy allows for an earlier discharge and obviates the need for bridging with anticoagulant agents that may increase bleeding risk. Based on the need for ablating concealed left-lateral APs that become apparent following comprehensive EP evaluation of SVTs, our lab is equipped with a St. Jude Viewmate ZTM Intracardiac Ultrasound Console (St. Jude Medical, St. Paul, MN) and St. Jude Medical Viewflex Plus 9-Fr intracardiac echo (ICE) catheters (St. Jude Medical) to guide puncture across the interatrial septum. After informed consent is obtained and all requisite lab work drawn, patients are brought to the EP lab in a fasting state. As with a standard EP study, R2 pads are applied to the chest and back, as are standard 12-lead ECG electrodes. Conscious sedation is administered by the nursing staff through the duration of the procedure. Three femoral sheaths (9-Fr, 7-Fr, and 6-Fr) are introduced into the left femoral vein to accommodate quadripolar catheters advanced to the high right atrium (HRA), the bundle of His, and the right ventricular apex (RVA). One of these venous sheaths is primed with isuprel to facilitate induction of SVT. From the contralateral groin, a 7-Fr and an 8-Fr sheath are each introduced into the right femoral vein; the former is used to accommodate a deflectable decapolar catheter advanced into the CS, and the other permits exchange for a transseptal sheath. Prior to proceeding with transseptal puncture, a baseline ACT is measured. For ablation of left-lateral APs, our preferred guiding sheaths include the Medtronic Mullins (Medtronic, Minneapolis, MN) and Swartz SL1 (St. Jude, St. Paul, MN). The selected sheath is first advanced to the superior vena cava over a wire and then engaged with a blunted, RF-powered needle (Baylis Medical, Montreal, QC, Canada) all the way to the tip of the dilator. The selection of sheath usually influences the choice of needle: for sheaths with more curvature at the tip, a less curved needle such as the Baylis C0 61-cm transseptal needle is used, whereas a more curved needle is selected for use with those sheaths bearing less curve. The quadripolar catheter in the HRA is usually exchanged for the ICE probe to facilitate visualization of the intra-atrial septum (IAS) and guide transseptal crossing. Transseptal puncture is performed under biplane fluoroscopic and ICE guidance. Once adequate visualization of the IAS under ICE is obtained, the sheath-needle complex is slowly pulled down from the SVC until it engages the fossa ovalis. The trajectory of the needle is checked under RAO and LAO projections and adjusted to ensure that the needle does not course too anteriorly in an RAO projection. In the LAO projection, the sheath and needle should appear to point leftward. The sheath and needle are then advanced together and cinched up against the IAS until adequate tenting of the latter is visualized. The needle is then advanced out beyond the dilator tip and a 3-second RF burst is delivered through the tip of the Baylis needle. This approach is usually successful in crossing the septum. Occasionally, the sheath and needle tend to “ride up” the septum when efforts are made to better engage it, in which case the needle is removed from the sheath and more curve is introduced into it. The exchange-length wire sometimes must be reintroduced through the dilator and the sheath readvanced into the SVC before being re-engaged with the needle. Once the tip of the needle has crossed, LA position must be confirmed before advancing the dilator and sheath. First, an O2 saturation is measured on a sample of blood aspirated from the tip of the needle. Next, pressure is transduced through the needle to ensure that the LA and not the aorta has been penetrated. Finally, a small injection of agitated, heparinized saline is injected through the needle while the ICE image is inspected for echo contrast in the LA. After these steps are taken, the sheath, dilator and needle are advanced slightly further into the LA to avoid losing transseptal position before the dilator and then the sheath are gradually “telescoped” into the LA. Clockwise or posterior torque is sometimes applied to the sheath and dilator while advancing over the septum to avoid coursing too laterally in the LA and perforating the appendage. Once the sheath has entered the LA, the needle and dilator are withdrawn while aspirating through the sidearm of the sheath to avoid introducing air. Weight-based heparin (usually 100 U/kg of body weight) is then immediately injected and flushed through and a heparinized saline infusion of 50 U/hr is run through the sheath. ACT is then checked, and heparin titrated to maintain ACT above 350 seconds. Finally, the mapping/ablation catheter is introduced through the sheath. With manifest preexcitation of the ventricles, as in WPW, confirmation of a left-lateral AP is usually suspected based on surface ECG delta-wave morphology and confirmed at the time of invasive EP evaluation. Ablation of a left-lateral AP can be achieved by targeting either its atrial or ventricular insertion along the mitral valve annulus. During sinus rhythm or atrial pacing with maximal ventricular preexcitation, the AP’s approximate location may be estimated by examination of the earliest ventricular EGM in the CS in relation to the delta wave. This approach is of particular importance, as our lab has recently come across patients with exclusively antegrade conduction over the AP, precluding the ability to map during retrograde activation. It is important to note, however, that APs often traverse the atrioventricular groove in an oblique fashion1 so that the atrial and ventricular insertions may be separated by some distance along the annulus. Further confounding matters is that CS recordings give only an approximation of transannular activation sequence, as the CS is an epicardial structure and is itself superiorly displaced from the AV groove. Our lab’s practice has been to ablate the LA insertion of the AP, usually identified during orthodromic AVRT (Figure 7.1) or via pacing from the RVA. Assuming the effective refractory period (ERP) of the AV node is longer than that of the APs, mapping of its atrial insertion is achieved during pacing from the RVA at a cycle length shorter than the AV node’s ERP so that retrograde atrial activation while mapping is exclusively via the AP. In rare instances, the ERP of the AP is longer than or equal to that of the AV node, which can make mapping extremely difficult unless tachycardia may be readily induced and sustained. Figure 7.1 Induction of orthodromic AVRT with sensed double atrial extrastimuli in a patient with a concealed left lateral accessory pathway. Note the “bracketing” of earliest atrial activation during tachycardia in CS 5–6.
Ablation of Left-Lateral Accessory Pathways
Abram Mozes, MD; Mohamed Aljaabari, MBBS;
Mark S. Link, MD; Ann C. Garlitski, MD; Jonathan Weinstock, MD;
Munther Homoud, MD; N. A. Mark Estes III, MD
Introduction
Preprocedural Planning
Procedure
Patient Preparation
Transseptal Puncture
Mapping
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