Cooling dynamics: a new predictor of long-term efficacy of atrioventricular nodal reentrant tachycardia cryoablation

New Perspectives on Risk Stratification and Treatment in Patients with Atrial Fibrillation: An Analysis of Recent Contributions on the Journal of Cardiovascular Disease and Development

The medical approach to atrial fibrillation (AF) underwent a paradigm shift over time, evolving from considering AF as a simple arrhythmic phenomenon to a complex nosological entity [...].

Cryoablation of atypical atrioventricular nodal reentry tachycardia

AIM

Data on ablation for atypical recurrent atrioventricular nodal reentry tachycardia (AVNRT) and long-term follow-up are generally sparse. Furthermore, the rate of recurrence and safety of cryoablation for atypical AVNRT has not been established. We compared patients cryoablated for atypical AVNRT and typical AVNRT during long-term follow-up.

METHODS

All patients (n = 2612) who underwent catheter ablation for AVNRT at the Karolinska University Hospital between January 2009 and August 2019 were analyzed. A total of 91 patients undergoing first-time cryoablation for atypical AVNRT were included. A control group with first-time cryoablation for typical AVNRT was matched in a 1:1 ratio. Patients were followed-up for recurrences for a median of 5.0 years (interquartile range: 3.1-7.5 years).

RESULTS

After 5 years, AVNRT recurrence occurred in 10 patients (11.0%) in the atypical AVNRT group and in 8 patients (8.8%) in the typical AVNRT group (hazard ratio: 1.31 [95% confidence interval: 0.52-3.32]; p = 0.568). The duration of the index procedure was significantly longer for atypical compared to typical AVNRT ablation (132.1 ± 49.2 min vs. 110.1 ± 38.8 min; p = 0.001). Transient AV blocks occurred in a similar fashion in the atypical compared to typical group (11 [12.1%] vs. 4 [4.9%]; p = 0.103). However, no ablation induced persistent AV block developed in either group.

CONCLUSION

Cryoablation for atypical AVNRT showed similar rate of recurrences and safety compared to typical AVNRT during long-term follow-up.

Slow pathway elimination using antegrade conduction improvement with fast atrial pacing during AVNRT radiofrequency ablation: a proof-of-concept study

BACKGROUND

Radiofrequency (RF) ablation of slow pathway (SP) is usually performed in sinus rhythm while monitoring the occurrence of a slow junctional rhythm (JR). JR although sensitive, is not specific for elimination of SP conduction. Our objective was to prospectively evaluate feasibility and safety of SP elimination using fast atrial rate pacing (FAP) during RF delivery.

METHODS

Consecutive patients admitted for atrioventricular nodal re-rentrant tachycardia (AVNRT) ablation were included. The rate of proximal coronary sinus (CS) pacing was set to a value constantly yielding antegrade SP conduction, while carefully monitoring the AH interval. RF delivery (at the lower part of Koch's triangle) was considered successful if the AH shortened ≥ 14 ms or if transition from Wenckebach (WK) periods to a 1:1 conduction occurred.

RESULTS

24 patients were included (54 ± 20 y). Typical AVNRT was induced in all (cycle length 349 ± 83 ms). RF delivery during CS pacing (335 ± 73 ms) led to AH shortening by 51 ± 25 ms in 13 patients. In 10 patients, a transition from 3:2 or 4:3 WK periods to 1:1 conduction occurred during the successful pulse. In one patient, atrial fibrillation was systematically induced during FAP, requiring conventional ablation. Non-inducibility, and SP conduction disappearance was obtained in all patients. No patient developed AV block. After a follow-up of 12 ± 3 months, no recurrences were observed.

CONCLUSION

SP ablation using FAP during RF delivery allows direct visualisation of its disappearance. In our cohort of patients, this technique was feasible without safety compromise.

Calorimetric analysis of ice onset temperature during cryoablation: a model approach to identify early predictors of effective applications

Aim of the present study is to analyze thermal events occurring during cryoablation. Different bovine liver samples underwent freezing cycles at different cooling rate (from 0.0075 to 25 K/min). Ice onset temperature and specific latent heat capacity of the ice formation process were measured according to differential scanning calorimetry signals. A computational model of the thermal events occurring during cryoablation was compiled using Neumann’s analytical solution. Latent heat (#1 = 139.8 ± 7.4 J/g, #2 = 147.8 ± 7.9 J/g, #3 = 159.0 ± 4.1 J/g) of all liver samples was independent of the ice onset temperature, but linearly dependent on the water content. Ice onset temperature was proportional to the logarithm of the cooling rate in the range 5 ÷ 25 K/min (#3a = − 12.2 °C, #3b = − 16.2 °C, #3c = − 6.6 °C at 5K/min; #3a = − 16.5 °C, #3b = − 19.3 °C, #3c = − 11.6 °C at 25 K/min). Ice onset temperature was associated with both the way in which the heat involved into the phase transition was delivered and with the thermal gradient inside the tissue. Ice onset temperature should be evaluated in the early phase of the ablation to tailor cryoenergy delivery. In order to obtain low ice trigger temperatures and consequent low ablation temperatures a high cooling rate is necessary.

Experimental Investigation on Thermal Conductivity and Thermal Diffusivity of Ex-Vivo Bovine Liver from Room Temperature down to -60 °C

Ex vivo animal tissues (e.g., bovine liver) as well as water-agar gel are commonly used to simulate both experimentally and numerically the response of human tissues to cryoablation treatments. Data on the low temperature thermal properties of bovine liver are difficult to find in the literature and very often are not provided for the whole temperature range of interest. This article presents the thermal conductivity and thermal diffusivity measurements performed on ex-vivo bovine liver samples using the transient plane source method. Regression coefficients are provided to determine these properties in different temperature ranges except for the phase transition during which no results were obtained, which suggests an ad hoc calorimetric analysis. A quick procedure is also suggested to determine the water mass fraction in the tissue. Moreover, an attempt to estimate the liver density in the frozen state using measurements performed solely at room temperature is also presented. The measured thermal conductivity and thermal diffusivity values are compared with data reported in literature highlighting a spread up to 40%. Moreover, it emerges that water-agar gel usually made with 2% by weight of agar does not show the same thermal properties as the bovine liver.

Efficacy of cryo-ablation during atrioventricular nodal reentrant tachycardia

Because of the low atrioventricular (AV) block risk during cryo-ablation, it has become possible to treat AV nodal reentrant tachycardia (AVNRT) during arrhythmia. This study aimed to investigate the clinical outcomes of performing cryo-ablation for AVNRT during arrhythmia. Twenty-three patients with AVNRT treated by cryo-ablation during arrhythmia were enrolled. Cryo-ablation was performed gradually from the bottom to above the paraseptal tricuspid annulus until AVNRT was terminated. If the slow pathway was not eliminated despite cryo-ablation terminating the AVNRT, additional cryo-ablation was performed at a higher site until the slow pathway elimination was achieved. AVNRT was terminated by cryo-ablation in all 23 patients. However, the slow pathway was only eliminated in 6 patients. Among the remaining 17 patients, the slow pathway could not be ablated because transient AV block occurred during cryo-ablation at a higher site in 8 patients; however, cryo-ablation at a higher site successfully eliminated the slow pathway in the other 9 patients. In these 9 patients, the distance from the bottom of tricuspid annulus to the site of slow pathway elimination was significantly longer than that from bottom of tricuspid annulus to the AVNRT termination site (20.1 ± 5.3 vs 14.7 ± 4.5 mm: p = 0.027). During follow-up, AVNRT recurrence was confirmed in 3 patients. In 1 of these 3 patients, even a slow pathway elimination was achieved by cryo-ablation at the AVNRT termination site. The AVNRT termination site may not be the ideal site for performing cryo-ablation.

Radiofrequency Catheter Ablation: How to Manage and Prevent Collateral Damage?

Radiofrequency catheter ablation has become the standard of care for the management of various arrhythmias and, in fact, the first-line therapy for many tachyarrhythmias. It entails creating scar tissue in the heart in regions where abnormal impulses form or propagate to restore normal cardiac conduction. As the heart is a complex organ and is surrounded by and related to many other anatomical structures, it is important to avoid the collateral damage that can happen from radiofrequency (RF) ablation on the endocardium as well as on the epicardium. This review explores methods for mitigating or limiting collateral damage during catheter ablation.

Thermodynamic properties of atrial fibrillation cryoablation: a model-based approach to improve knowledge on energy delivery

The objective of this study is to describe a suitable model of atrial fibrillation cryoablation thermodynamic properties. Three different thermal loads were applied to a cylindrical copper element simulating the cryoprobe, thermally coupled with a Peltier stack producing the freezing effect, and in contact with a bovine liver sample. Thermal events occurring inside the samples were measured using mirror image technique. Heat subtracted flux during ice formation and minimum temperature measured at probe-tissue interface were, respectively, 1.33 W cm^-2 and -27.8°C for Sample#0, 1.88 W cm^-2 and -35.6°C for Sample#1 and 1.82 W cm^-2 and 1.44 W cm^-2 before and after the ice trigger, respectively, and -29.3°C for Sample#2. Ice trigger temperature was around -8.5°C for Sample#0 and Sample#2, and -10.4°C for Sample#1. In all the investigated samples, ice front penetration was proportional to the square root of time and its velocity depended on the heat flux subtracted. The fraction of the useful energy spent for ice formation was less than 60% for Sample#0, and about 80% for Sample#1 and for Sample#2, before the reduction of the removed heat flux. Freezing time exceeding a cut-off, according to the heat subtracted flux, does not improve the procedure effectiveness and is detrimental to the surrounding tissues.

Catheter Ablation of Atrioventricular Nodal Re-entrant Tachycardia: Facts and Fiction

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