Ian D. McRury

Linear Atrial Ablations in a Canine Model of Chronic Atrial Fibrillation Morphological and Electrophysiological Observations

BACKGROUND To test the hypothesis that susceptibility to sustained atrial fibrillation may be decreased by creation of linear atrial ablations, we established a canine model of chronic atrial fibrillation and used a novel catheter design to create atrial ablations. METHODS AND RESULTS Chronic atrial fibrillation was induced in 16 dogs by creation of mitral regurgitation and rapid pacing of the atria. Temperature-controlled radiofrequency ablations were attempted along empirically derived, preselected atrial target sites in 11 dogs (ablation group), and a sham procedure was performed in 5 dogs (control group). Follow-up electrophysiology study and pathological examination were conducted 13+/-5 days after the initial procedure. Immediately after ablation, sustained atrial fibrillation could be initiated in 1 of 9 surviving ablation dogs and 5 of 5 controls (P=.004). Four dogs died within 24 hours of the procedure. Permanent pacing was required in 4 dogs. At follow-up, 0 of 7 ablation dogs and 5 of 5 controls had atrial fibrillation (P=.001). Furthermore, 2 of 7 ablation dogs had sustained atrial tachycardias, one of which was successfully ablated. Pathological examination demonstrated frequent incomplete lesion sets and discontinuous lesions. CONCLUSIONS In this model, a reduction in the susceptibility to sustained atrial fibrillation can be achieved by long linear atrial ablations created with specially designed coil electrode catheters. Complete lesion continuity was not required to achieve a therapeutic effect.

Morphological and Physiological Characteristics of Discontinuous Linear Atrial Ablations During Atrial Pacing and Atrial Fibrillation

Discontinuous Atrial Ablations. Introduction: Linear atrial ablations are thought to he necessary to accomplish successful catheter ablation of atrial fibrillation. In order to investigate the conduction characteristics of atrial myocardium in regions of linear lesion discontinuity gaps), we performed activation sequence mapping in gap regions during atrial pacing and atrial fibrillation.

Temperature measurement as a determinant of tissue heating during radiofrequency catheter ablation: an examination of electrode thermistor positioning for measurement accuracy.

Thermometry and Radiofrequency Catheter Ablation. Introduction: Temperature monitoring has been proposed as a control for lesion occurrence and dimension during radiofrequency transcatheter ablation. Effective temperature measurement depends on thermistor positioning relative to the heated cardiac tissue and the convective cooling effects of the circulation. But the accuracy of a single tip thermistor as a measure of peak electrode‐tissue interface temperature is unknown.

Effects of dispersive electrode position and surface area on electrical parameters and temperature during radiofrequency catheter ablation

Positioning of the dispersive electrode has no significant effect during radiofrequency ablation. Doubling the surface are of the dispersive electrode results in a lower impedance, higher current delivery, and increased tip temperatures, particularly if the baseline impedance is >100 ohms. These findings may have important implications for optimizing radiofrequency energy delivery using currently available radiofrequency generators.

Radiofrequency multielectrode catheter ablation in the atrium.

We developed a temperature-controlled radiofrequency (RF) system which can ablate by delivering energy to up to six 12.5 mm long coil electrodes simultaneously. Temperature feedback was obtained from temperature sensors placed at each end of coil electrodes, in diametrically opposite positions. The coil electrodes were connected in parallel, via a set of electronic switches, to a 150 W 500 kHz temperature-controlled RF generator. Temperatures measured at all user-selected coil electrodes were processed by a microcontroller which sent the maximum value to the temperature input of the generator. The generator adjusted the delivered power to regulate the temperature at its input within a 5 degrees C interval about a user-defined set point. The microcontroller also activated the corresponding electronic switches so that temperatures at all selected electrodes were controlled within a 5 degrees C interval with respect to each other. Physical aspects of tissue heating were first analysed using finite element models and current density measurements. Results from these analyses also constituted design input. The performance of this system was studied in vitro and in vivo. In vitro, at set temperatures of 70 degrees C, 85% of the lesions were contiguous. All lesions created at set temperatures of 80 and 90 degrees C were contiguous. The lesion length increased almost linearly with the number of electrodes. Power requirements to reach a set temperature were larger as more electrodes were driven by the generator. The system impedance decreased as more electrodes were connected in the ablation circuit and reached a low of 45.5 ohms with five coil electrodes in the circuit. In vivo, right atrial lesions were created in eight mongrel canines. The power needed to reach 70 degrees C set temperature varied between 15 and 114 W. The system impedance was 105+/-16 ohms, with one coil electrode in the circuit, and dropped to 75+/-12 ohms when two coil electrodes were simultaneously powered. The length and the width of the lesion set varied between 17.6+/-6.1 and 59.2+/-11.7 mm and 5.9+/-0.7 and 7.1+/-1.2 mm respectively. No sudden impedance rises occurred and 75% of the lesions were contiguous. From the set of contiguous lesions, 90% were potentially therapeutic as they were transmural and extended over the entire target region. The average total procedure and fluoroscopy times were 83.4 and 5.9 min respectively. We concluded that the system can safely perform long and contiguous lesions in canine right atria.

The Effects of Reverse Atrial Electrical Remodeling on Atrial Defibrillation Thresholds

EVERETT, T.H. et al.: The Effects of Atrial Electrical Remodeling on Atrial Defibrillation Thresholds. Electrical remodeling of atrial fibrillation may account for the increase in atrial defibrillation thresholds over time. The aim of this study was to examine the time course of electrical remodeling and the benefit of early defibrillation on the defibrillation threshold. Twenty‐six mongrel dogs weighing 27.6 ± 3.3 kg were induced into AF by repeated high output burst atrial pacing. Eight dogs were paced for multiple time periods of 5, 20, 40, and 60 minutes. Five dogs each had burst pacing for 4 hours and 8 hours, and eight dogs were paced at a high rate (640 beats/min) for 48 hours. Biphasic atrial defibrillation shocks with a pulse width of 3/3 ms synchronized to the left apical electrogram were delivered to coil electrode catheters positioned in the lateral left and right atria. Defibrillation voltage was increased from 50 V in 20‐ to 30‐V steps until defibrillation was successful. As the pacing period increased, a decrease in atrial fibrillation cycle lengths and atrial effective refractory period was not observed before 8 hours. Similarly, the defibrillation threshold did not change significantly until the 8‐hour pacing period was reached. The defibrillation thresholds were 69 ± 28 V for 5 minutes, 64 ± 20 V for 20 minutes, 99 ± 85 V for 40 minutes, 78 ± 51 V for 60 minutes, 78 ± 38 V for 4 hours, 124 ± 33 V for 8 hours, and 133 ± 32 V for 48 hours (mean ± SD) (P < 0.05). Atrial electrical remodeling in a rapid atrial pacing canine model is not observed until after 4 hours of burst atrail pacing. The atrial defibrillation threshold increases with increasing duration of burst atrial pacing, and follows a similar time course to other parameters of electrical remodeling.

Efficacy of multiple ring and coil electrode radiofrequency ablation catheters for the creation of long linear lesions in the atria

Atrial fibrillation is an arrhythmia that may potentially be treated by creating long linear lesions in the atria to create lines of electrical conduction block. While this has been performed with success with open-heart surgery, it has been proposed that a less invasive catheter-based approach could achieve similar success. Radiofrequency energy catheter ablation was performed in vivo with two novel electrode catheters. Each was an expanding loop design: one with 3 mm ring electrodes; and one with 12.5 mm coil electrodes. Power delivery was controlled automatically with temperature (70 degrees C target) feedback from thermistors embedded in each electrode. A total of 39 lines of ablation were created in the atria of 11 normal dogs. The coil electrodes were more effective in creating lesions than the ring electrodes with a similar prevalence of transmurality (89% vs. 85%) but a higher prevalence of continuous transmurality (35% vs. 5%). Sequential electrode energy delivery was better than simultaneous multipolar delivery due to varying efficiencies of tissue heating. Inadequate heating was observed in 47% of simultaneous versus 1% of sequential multipolar deliveries, and excessive heating in 6% versus 1% of cases, respectively. It is feasible to create linear atrial lesions with an expanding loop electrode catheter. Catheters with coil electrodes are more effective than those with ring electrodes. In order to avoid coagulum formation and inefficient heating, sequential electrode energy delivery is preferable to multipolar delivery.

In vivo atrial ablations with a multielectrode RF system

We tested the performance of a multielectrode RF ablation system during in vivo procedures in the right atrium. A total of 75% of the RF applications resulted in complete lesions at the gaps between electrodes and 90% of them were considered potentially therapeutic, as they were transmural and expended along the intended contour. The average total procedure and fluoroscopy times were 83.4 and 5.9 min, respectively. Average power varied between 15 and 114 W. No sudden impedance rise occurred and no char was found by post-ablation inspection. The average impedance was 105/spl plusmn/16 /spl Omega/, with one electrode on, and 75/spl plusmn/12 /spl Omega/ when two electrodes were simultaneously on. The average maximal lesion length and width were 59.2/spl plusmn/11.7 and 7.1/spl plusmn/1.2-mm.