Impedance decrement indexes for avoiding steam-pop during bipolar radiofrequency ablation: An experimental study using a dual-bath preparation.

INTRODUCTION: This experimental study was conducted to explore impedance monitoring for safely performing bipolar (BIP) radiofrequency (RF) ablation targeted to arrhythmia focus. METHODS AND RESULTS: Using a newly designed dual-bath experimental model, contact-force-controlled (20-g) BIP ablation (50 W, 60 s) was attempted for porcine left ventricle (17.0 ± 2.7 mm thickness). BIP ablation was successfully accomplished for 60 s in 75 of the 89 RF applications (84.3%), whereas audible steam-pop occurred in the other 14 RF applications (15.7%). Receiver operating characteristic analysis demonstrated the optimal predictive values regarding the occurrence of steam-pop as follows; thinner myocardial wall (≤14.8 mm), low minimum impedance (≤89 ohm), greater total impedance decrement (TID) (≤ -25 ohm) and %TID (≤ -22.5%). Greater impedance decrement was not observed immediately preceding the occurrence of steam-pop but appeared around 15 s before. Four steam-pops happened before reaching the optimal predictive values of minimum impedance, whereas all 14 steam-pops developed 11.5 ± 9.2 and 8.1 ± 8.1 s after reaching the optimal predictive values of TID and %TID, respectively. Total lesion depth (endocardial plus epicardial) was 10.7 ± 1.2 mm on average, and was well correlated with TID and %TID. Transmural lesion through the myocardial wall was created in 22 RF applications. CONCLUSION: Relatively thinner areas of the myocardium are likely to be at greater risk for steam-pop during BIP RF ablation. Lowering the RF application energy to reduce the impedance decrement may help to lessen this risk.

Bepridil Inhibits Premature Ventricular Complexes Induced by Cardio-Sympathetic Nerve Stimulation in a Canine Experimental Model.

Sympathetic nerve activity has arrhythmogenic potential for ventricular arrhythmias associated with structural heart diseases. However, a sufficient amount of beta-blockers occasionally cannot be prescribed in some patients.An experimental study was performed to clarify the therapeutic effects of bepridil, a multiple ionic current inhibitor that does not affect beta-adrenergic receptors, for premature beats occurring during enhanced sympathetic nerve activity. Cardio-sympathetic nerve activity was augmented via stellate-ganglion (SG) stimulation in a canine model (n = 8), and the arrhythmogenic potential and anti-arrhythmic effects of bepridil (2 and 4 mg/kg intravenously) were assessed. For safe use, vagal-stimulation-induced slow HR and programmed electrical stimulation were applied to evaluate possible pro-arrhythmic effects of the drug. Heart rate variability (HRV) indexes were used to estimate cardio-autonomic nerve activity.Either side of the SG-stimulation increased BP and HR. Premature beats were induced in 10/16 SG-stimulations and it was more frequent in left (8/8) rather than right stimulation (2/8). Following 2 mg/kg drug administration, premature beats were still inducible in 8/16 stimulations (7/8 in left and 1/8 in right), but burden of the premature beats decreased from 87.1 ± 46.8 to 62.1 ± 42.6 beats. After 4 mg/kg administration, premature beats were inducible in one SG-stimulation. Proarrhythmic effects were not observed in all experiments. Steady-state HRV indexes and percent increases in SG-stimulation-induced BP-elevation and HR-acceleration were similar among the 3 periods (before, 2 and 4 mg/kg of the drug).Bepridil may be an option for ventricular arrhythmias developed during enhanced cardio-sympathetic nerve activity with minimal effect on autonomic nerve responses.

Therapy-Resistant Ventricular Arrhythmias Developed More Often in Advanced Than in Therapeutic Mild Hypothermic Condition.

Therapy-resistant ventricular arrhythmias can occur during accidental advanced hypothermic conditions. On the other hand, hypothermic therapy using mild cooling has been successfully accomplished with infrequent ventricular arrhythmia events.To further clarify the therapeutic-resistant arrhythmogenic substrate which develops in hypothermic conditions, an experimental study was performed using a perfusion wedge preparation model of porcine ventricle, and electrophysiological characteristics, inducibility of ventricular arrhythmias, and effects of therapeutic interventions were assessed at 3 target temperatures (37, 32 and 28°C).As the myocardial temperature decreased, myocardial contractions and the number of spontaneous beats deceased. Depolarization (QRS width, stimulus-QRS interval) and repolarization (QT interval, ERP) parameters progressively increased, and dispersion of the ventricular repolarization increased. At 28°C, VF tended to be inducible more frequently (1/11 at 37°C, 1/11 at 32°C, and 5/11 hearts at 28°C), and some VFs at 28°C required greater defibrillation energy to resume basic rhythm.An advanced but not a mild hypothermic condition had an enhanced arrhythmogenic potential in our model. In the advanced hypothermic condition, VF with relatively prolonged F-F intervals and a greater defibrillation energy were occasionally inducible based on the arrhythmogenic substrate characterized as slowed conduction and prolonged repolarization of the ventricle.

Electrode Contact Force-Controlled Bipolar Radiofrequency Ablation: Different Effects on Lesion Size between Dual- and Single-Bath Preparations.

BACKGROUND: During bipolar (BIP) radiofrequency (RF) ablation using two catheters in humans, each catheter is placed in separate cardiac chambers or spaces. We developed a contact force-controlled experimental preparation, and compared measurements made with two catheters placed in a single bath (SB), versus each catheter placed in separate baths, in order to assess the preparation-dependent differences in the results of BIP-RF ablation. METHODS: In the SB experiments, a porcine heart was placed in the center of the bath, while in the dual-bath (DB) experiments, it was placed between two half baths communicating through windows. RESULTS: The initial impedance was greatest (110.5 ± 7.2 Ω) with the BIP-DB, followed by the BIP-SB (92.0 ± 5.6 Ω) and the unipolar (UNIP) DB (84.9 ± 4.7 Ω) configurations. During 50-W ablation for 60 seconds at a 20-g contact force, the root mean square voltage was 75.7 ± 2.5 V in the BIP-DB, 68.0 ± 2.1 V in the BIP-SB, and 66.8 ± 2.0 V in the UNIP-DB. The mean surface lesion diameters were similar among the three configurations. However, the endocardial lesion depth was 5.60 ± 0.56 mm with the BIP-DB, 4.71 ± 0.64 mm with the BIP-SB, and 4.24 ± 0.58 mm with the UNIP-DB configuration. On average, the endocardial lesions were significantly deeper than the epicardial ones. CONCLUSIONS: BIP ablation created much deeper lesions as compared to UNIP ablation. Lesion depth could be different depending on experimental preparation, and contact force-controlled DB preparation may be a much more appropriate model for studying the effects of BIP ablation.

Enhanced arrhythmogenic potential induced by renal autonomic nerve stimulation: Role of renal artery catheter ablation.

BACKGROUND: Renal artery catheter ablation has been reported as a possible therapeutic option for drug-refractory ventricular arrhythmias (VAs) associated with structural heart diseases. OBJECTIVE: To further clarify its therapeutic background, we examined the relationship between electrical nerve stimulation (ENS)-induced blood pressure (BP) elevation and occurrence of VAs by using an acute canine model of renal artery ablation. METHODS: Using a decapolar electrode catheter, ENS was successively applied from the distal, mid, and proximal segments of the renal artery in 8 beagles. The same ENS was repeated after accomplishment of radiofrequency ablation at the ostial site of the renal artery by using an irrigation catheter. RESULTS: Before ablation, ENS increased BP from 140 ± 11/77 ± 11 to 167 ± 20/98 ± 16 mm Hg and heart rate from 100 ± 21 to 131±33 beats/min as well as induced VAs in 20 of the 45 ENS applications. Occurrence of VAs was associated with a greater magnitude of sympathetic nerve augmentation, and VAs were more frequently observed by ENS at the distal (67%) rather than mid/proximal segments of the renal artery (33%). Renal artery ablation was accomplished without any angiographic stenosis, and ENS-induced BP elevation, heart rate acceleration, and VAs occurrence were attenuated not only at the close segment (proximal) but also at the remote segments (mid/distal) of the renal artery. CONCLUSION: The renal autonomic nerves are considered as one of the therapeutic targets for suppression of frequent VAs because its activation has arrhythmogenic potential and induces premature ventricular beats.