Atrial Fibrillation



Atrial Fibrillation










ATRIAL FIBRILLATION ABLATION

Prior to catheter ablation, surgical treatment of atrial fibrillation included the Cox-Maze operation, which was based on
the multiple wavelet hypothesis and decreased surface area by compartmentalizing the atrium through a series of strategically placed surgical incisions.16,17,18 Attempt to replicate the surgery using a transvenous ablation catheter was only modestly successful, time-consuming, and associated with a high complication rate.19 Attention turned to the PVs when it was demonstrated that ablation of focal PV triggers could eliminate atrial fibrillation.2,3,20 Longitudinal- and spiral-oriented muscle sleeves spreading from the funnel-shaped antrum into the PVs facilitate anisotropic conduction and reentry. The occurrence of PV stenosis as a complication of this approach shifted the ablation target to the PV ostium (or antrum) with the idea that atrial fibrillation could not occur despite rapid PV firing if the PVs were electrically disconnected from the atrium.21 Catheterbased strategies for atrial fibrillation include 1) PV isolation (PVI) and ablation of non-PV triggers, 2) substrate modification, and 3) vagal denervation. Pre-procedural (CT/MRI) and intraprocedural (intracardiac echocardiography [ICE], pulmonary venography) imaging facilitate an understanding of PV anatomy during ablation (Figs. 15-3 and 15-4).






FIGURE 15-1 Spontaneous LSPV firing initiating atrial fibrillation. Early atrial activity (asterisk) recorded from the ablation catheter in the LSPV initiates atrial fibrillation.


FOCAL TRIGGERS

The cornerstone of atrial fibrillation ablation is PVI—either by radiofrequency (RF) or cryoablation.


Radiofrequency Ablation

Electrophysiologically guided PVI involves recording PV potentials from a circular mapping catheter (e.g., Lasso) situated at the ostium of the targeted vein and therefore generally requires two transeptal punctures unless a double or retained guide wire approach is used. To help avoid entrapment of the catheter in the mitral valve apparatus, the circular mapping catheter should be torqued clockwise (posteriorly) when exiting the transeptal sheath and positioned in the PV away from the more anterior mitral valve. The circular mapping catheter records fused electrograms consisting of a blunt LA (far-field) and high-frequency PV (near-field) potential. During left PV ablation, these electrograms overlap during sinus rhythm because of synchronous activation and best separated by distal CS pacing (Fig. 15-5). Two approaches to PVI include segmental ostial and wide area circumferential ablation (WACA)—the latter having a higher success rate.22,23,24,25,26,27 Segmental ostial ablation successively targets the earliest breakthrough site of PV potentials around the circular mapping catheter until PV muscle conduction is eliminated or dissociated from the LA (Fig. 15-6).22 WACA targets the antrum of the PVs (≥5 mm outside the PV ostium), debulking the atrium and causing PVI with contiguous focal lesions around the PVs individually, ipsilaterally, or entirely en bloc (Fig. 15-7).23 Voltage abatement (≤0.05 mV) within encircled areas validate line continuity. Clues that the ablation catheter





is in the PV where RF delivery should be avoided include 1) fluoroscopic demonstration of the catheter tip beyond the cardiac silhouette, 2) ICE imaging of catheter tip beyond the PV ostium, 3) loss of electrical signals, and 4) increase in catheter impedance (>140-150 ohms). The endpoint of PVI is entrance block (elimination or dissociation of PV potentials from the LA) and exit block (PV stimulation causes PV sleeve capture without conduction to the atrium) (Figs. 15-8, 15-9, 15-10, 15-11 and 15-12).






FIGURE 15-2 Degeneration of AVNRT (top) and ORT (bottom) to atrial fibrillation. ORT uses a left free wall accessory pathway (AP).






FIGURE 15-3 PV anatomy (three-dimensional rotational atriography [ATG]). The ablation catheter is in the LSPV. The circular mapping catheter (Lasso) is positioned in each of the four PVs. Note that the left-sided PV ostia are seen “en face” in the RAO view and “longitudinally” in the LAO view. The opposite is true for the right-sided PV ostia. The longitudinal views allow visualization of the catheters exiting the cardiac silhouette.






FIGURE 15-4 PV anatomy ICE. A PentaRay catheter is positioned in each of the four PVs. The red tags denote ablation lesions around the PV antra.






FIGURE 15-5 Separation of LSPV potentials from LA electrograms by CS pacing. CS pacing separates the highfrequency (near-field) LSPV potentials (arrows) from LA (far-field) electrograms, which otherwise overlap during sinus rhythm. Barium paste highlights the esophagus and its proximity to the ablation site.






FIGURE 15-6 Segmental ostial ablation. Ablation circumferentially targeting the conducting fascicles around the RSPV incrementally changing activation on the circular mapping catheter until isolation is achieved.






FIGURE 15-7 WACA. The red and white tags denote ablation lesions.






FIGURE 15-8 Entrance and exit block. LIPV pacing causes sleeve capture (arrows) without conduction to the atrium (exit block). CS pacing captures the atrium but fails to conduct into the LIPV (entrance block). White arrowheads denote circular mapping catheter (Lasso) on ICE.



Pulmonary Vein Potential Pitfalls

After PVI, adenosine or isoproterenol can unmask dormant PV conduction and/or non-PV triggers and, therefore, need for further ablation.29,30,31,32 Far-field electrograms from neighboring electrically active structures (LA appendage [LAA] for LSPV and SVC for RSPV) can be misinterpreted as PV potentials resulting in unnecessary RF applications, particularly with widely spaced electrode pairs having a larger “antenna” (Figs. 15-14 and 15-15). Far-field electrograms can be differentiated from true PV potentials by 1) morphology, 2) distribution, and 3) pacing techniques. PV potentials demonstrate near-field characteristics (high frequency, sharp, narrow width) and are circumferentially distributed around the PV, while far-field electrograms (low frequency, dull, wider width) are segmentally distributed to sites overlying the electrically active neighboring structure. Pacing the neighboring structure (e.g., LAA) and capturing the electrogram (potential anticipation) confirms a far-field signal (Fig. 15-14).33,34 (Conversely, when demonstrating exit block by pacing within the PV, it is also important not to far-field capture the electrically active neighboring structure and therefore atrium, which could be falsely interpreted as persistent conduction.)


Cryoablation

Rather than point-by-point focal RF ablation, cryoablation is an alternative PVI technique using a balloon catheter to occlude the PV and liquid nitrogen to freeze its antrum.35,36,37 A circular mapping catheter inserted through the lumen of the cryoballoon allows monitoring of PV potentials while providing distal support, and therefore, only a single transeptal puncture is required. The transeptal puncture site should be low and anterior on the fossa ovalis allowing for a large turning radius of the cryocatheter to reach the posterior and superior PVs (particularly, the more difficult right inferior pulmonary vein [RIPV]). Keeping the cryoballoon coaxial to the vein with maintenance of forward pressure, advancing the sheath to the back hemisphere of the balloon for proximal support, and using the circular mapping catheter as a distal luminal supporting wire (“rail”) in different vein branches (inferior for inferior veins/superior for superior veins) facilitate balloon occlusion of the targeted vein. Because the balloon heads superiorly toward the PV antrum, inferior leaks during initial attempts at vein occlusion are common (worst site of balloon-to-PV contact) and can be sealed by a 1) “pull down” technique (pulling the entire assembly down to prevent the balloon from “riding” upward) or 2) curving the cryosheath and pushing the assembly inferiorly downward toward the antrum (“hockey stick” sign—especially for RIPV). A large common or oval ostium can make complete occlusion difficult. Signs of PV occlusion are 1) difficult contrast injection with retention in the vein (“hang up”), 2) absence of leaks on color Doppler sweep, and 3) pulmonary venous pressure waveforms (transition from LA [venous A and V wave] to pulmonary


artery pressure [only higher pressure V wave]). Because of the different viscosities between contrast and saline, it is important to adequately flush any dye out of the lumen of the cryoballoon in order to accurately record pulmonary venous pressures (Figs. 15-16, 15-17, 15-18, 15-19 and 15-20).38 Optimal cryoablation endpoints include 1) nadir temperatures (<−50°C), 2) time to isolation (TTI) <60 sec, 3) freezing time (−30°C by 30 sec, −40°C by 60 sec) (rapid freezing with shorter freezing times correlates with effective cryoablation), and 4) rewarming time (interval thaw time to 0°C by ≥10 sec) (longer rewarming times indicate more therapeutic ice crystallization with tissue-ice bonding at 0°C).39,40,41,42 In contrast to the abrupt loss of PV potentials that signify entrance block during RF ablation, cryoablation causes LA-PV delay prior to disappearance of PV potentials (Fig. 15-21).43 (A distally positioned circular mapping catheter can be pulled back to the nose of the cryoballoon within 10 sec of the cryofreeze to record PV potential before the central lumen freezes.) Pacing from the circular mapping catheter can also show PV-LA delay prior to exit block. The dosing and duration of cryofreeze applications depends on how successful the cryoablation endpoints were achieved (e.g., 180 sec [TTI <90 sec] or 150 sec [TTI <30 sec]). After the freeze, the cryoballoon should not be manipulated until the temperature reaches 35°C during the thaw in order to avoid tissue tearing from cryoadherence.

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Oct 13, 2019 | Posted by in CARDIOLOGY | Comments Off on Atrial Fibrillation
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