Israel A. Byrd

Contact Sensing Provides a Highly Accurate Means to Titrate Radiofrequency Ablation Lesion Depth

RF Ablation Lesion Depth Estimation Using Contact Sensing. Background: Transmural lesions are essential for efficacious ablation. There are, however, no accurate means to estimate lesion depth.

Contact geometry affects lesion formation in radio-frequency cardiac catheter ablation.

One factor which may be important for determining proper lesion creation during atrial ablation is catheter-endocardial contact. Little information is available that relates geometric contact, depth and angle, to ablation lesion formation. We present an electrothermal computer model of ablation that calculated lesion volume and temperature development over time. The Pennes bioheat equation was coupled to a quasistatic electrical problem to investigate the effect of catheter penetration depth, as well as incident catheter angle as may occur in practice. Biological experiments were performed to verify the modelling of electrical phenomena. Results show that for deeply penetrating tips, acute catheter angles reduced the rate of temperature buildup, allowing larger lesions to form before temperatures elevated excessively. It was also found that greater penetration did not lead to greater transmurality of lesions. We conclude that catheter contact angle plays a significant role in lesion formation, and the time course must be considered. This is clinically relevant because proper identification and prediction of geometric contact variables could improve ablation efficacy.

Voltage-based device tracking in a 1.5 tesla MRI during imaging: initial validation in swine models

Voltage‐based device‐tracking (VDT) systems are commonly used for tracking invasive devices in electrophysiological cardiac‐arrhythmia therapy. During electrophysiological procedures, electro‐anatomic mapping workstations provide guidance by integrating VDT location and intracardiac electrocardiogram information with X‐ray, computerized tomography, ultrasound, and MR images. MR assists navigation, mapping, and radiofrequency ablation. Multimodality interventions require multiple patient transfers between an MRI and the X‐ray/ultrasound electrophysiological suite, increasing the likelihood of patient‐motion and image misregistration. An MRI‐compatible VDT system may increase efficiency, as there is currently no single method to track devices both inside and outside the MRI scanner.

MRI Guided Electrophysiological Intervention with a Voltage- Based Electro-Anatomic Mapping System

Background MRI visualizes luminal & vessel-wall anatomy, and identifies edema & scar tissue, contributing to improved electrophysiological (EP) ablative procedures for treatment of Ventricular Tachycardia & Atrial Fibrillation. MRI-guided EP interventions will be performed for the foreseeable future partially in & outside MRI, due to the need for X-ray/Ultrasound-compliant devices. Electromagnetically tracked catheter procedures, today’ sn orm for most EP procedure phases; vascular navigation, Electro-Anatomic-Mapping (EAM, the diagnostic and therapeutic phases), can only be performed outside MRI. Separate MRI tracking is required in MRI, complicating EP procedures which require moving in & out of the bore [1,2]. Continuous catheter tracking using a single system would allow registration-free EAM in & outside MRI. The goal was developing an MR-compatible St. Jude Medical (SJM) EnSite NavX (ESN) voltage-based tracking [3]. ESN applies 5.8/8.0 kHz voltage bursts between 3 pairs of electrodes on the chest, detecting a catheter’s position [4], so a challenge for intra-MRI use is MR gradient ramps which interfere with ESN operation. Minimal MR Image Quality (IQ) reduction also needs to be insured, as well as <2oC patient-skin heating due to components in MRI.

Voltage-based electroanatomic mapping system for MR-guided cardiac electrophysiology: preliminary swine validations

Background MRI produces images that serve as luminal, edema, & scar maps to assist in the Electrophysiological (EP) treatment of ventricular and atrial arrhythmias [1]. Until MR-compatible EP devices are widely available, there will be an eed to perform EP partially in the MRI for imaging, and partially outside the MRI for ablation, puncture & navigation. An MR-conditional voltage-based Electroanatomic Mapping (EAM) system would allow MR-guided EP in MRI & registration-free EP to be performed outside the MRI during X-ray, Intra-Cardiac-Echo (ICE) or EAM guidance. Previously a 1.5T MR-conditional St. Jude Medical EnSite Velocity (Velocity) voltage-based EAM system was presented [2]. The study objective was to conduct a multicatheter registration free EAM (localization & intracardiac Electrogram (EGM) measurement) both in & outside of the MRI. Methods

Human & swine studies of concurrent 12-lead ECG & MRI

Background 12-lead Electrocardiogram (ECG) is a clinical standard for patient physiological monitoring. An MRI-conditional 12-lead ECG should permit detection of acute myocardial ischemia during MR imaging or MRI-guided therapy, which may improve the handling of patients with ischemic histories. MRI visualization of ischemic episodes can also enhance the understanding of ischemic progression. Previously an MR-conditional 12-lead ECG system was presented. The system was equipped with GradientRampR (2) detect S-wave to T-wave (ST) ECG elevation & perform MR imaging of a Left Anterior Descending (LAD) balloon occlusion from the onset of ischemia to death in a swine model.

Cathether contact geometry affects lesion formation in radio-frequency cardiac catheter ablation

One factor which may be important for determining proper lesion creation in an atrial ablation procedure is catheter-endocardial contact. Little information is available that relates geometric contact, depth and angle, to ablation lesion formation. We present an electrothermal computer model of ablation that calculates lesion volume and temperature development over time. The Pennes bioheat equation was coupled to a quasistatic electrical problem. This method simulates importantly, not just catheter penetration depth, but also several different incident catheter angles as may occur in practise. Results show that for deeply penetrating tips, greater catheter angles reduce the rate of temperature buildup, allowing for larger lesions to form before temperatures become dangerous. It was also found that greater penetration may not lead to greater transmurality in lesion formation. We conclude that catheter contact angle plays a significant role in lesion formation, and the time course must be considered. This is clinically relevant because it makes proper identification and prediction of geometric contact variables a necessity in order to improve ablation efficacy and safety.

Phase angle shift is a better determinant for catheter electrode contact with tissue compared to a catheter sensed electrogram

Convention holds that the magnitude of an electrogram (EGM) recorded from an ablation catheter indicates proximity to the tissue and may be used to guide tip placement. The shift in capacitance (phase angle) as the electrode touches the tissue may be a better guide. We compared these two methods over a range of distances in close proximity to heart tissue. This study suggests that EGM is not a reliable predictor of proximity to tissue within a few millimeters of the surface. Thus, EGM alone should not be used to guide electrode placement for ablation, as a millimeter off the surface will shift a greater percentage of delivered energy to the blood pool rather than the target tissue. EGM should also not be used to gauge force of the catheter into tissue. Phase angle is a better predictor of both variables, but an optimal combination of predictors remains to be found.

Delayed Termination Following Pacing Induced Shifts from Monomorphic to Polymorphic Ventricular Tachycardia: Implications for Antitachycardia Pacing

Interactions between paced wavefronts and monomorphic ventricular tachycardia (VT) dictate antitachycardia pacing outcomes. Monomorphic VTs were initiated in isolated rabbit hearts (n=6) that were endocardially cryoablated to limit viable tissue to the visible epicardium and the ablated apex served as an anatomic anchor. Preparations were optically mapped during single and dual site pacing at 50% to 90% of VT cycle length with 8 pulses per trial. Of these trials, responses to the 48 single site pulses and to the 172 dual site pulses that captured tissue were analyzed. Overall, we found most pulses reset the VT, and a small number of pulses that abruptly terminated the VT. Of particular interest, we found 12 pulses that shifted the anatomically anchored VT to functionally reentrant wavefronts, and thereby induced polymorphic VT. Delayed termination was observed following 6 of these instances, and the underlying non-sustained polymorphic VTs presented temporal characteristics similar to those presented by delayed termination after antitachycardia pacing in ICD patients