Fred H.M. Wittkampf

Control of radiofrequency lesion size by power regulation.

The influence of power and exposure duration on lesion size in radiofrequency catheter ablation was investigated in 15 closed-chest dogs. Radiofrequency energy was delivered to the right ventricular endocardium between the tip of a standard 6F electrode catheter and a large W and durations of 5, 10, 20, 30, and 60 seconds. At necropsy 1 week later, well-demarcated homogeneous lesions were found when power had exceeded a threshold level that decreased from 1.8 W at 5 seconds to 0.7 W at 60 seconds. Lesion size ranged from 0 to 7.5 mm in depth and 0 to 9 mm in diameter. For the 5, 10, and 20 second ablations, lesion size was determined by exposure duration and power level. However, after a 20 second exposure, lesion size had reached maturity and was related to delivered power only. Therefore, a gradual, controlled growth of the lesion can be obtained by a stepwise increase of the radiofrequency power level with ample exposure duration at each level to allow for stabilization. At levels exceeding 7 W, the formation of a thin insulating layer of blood coagulum on the electrode surface caused an abrupt increase of impedance within approximately 30 seconds. Therefore, lesion size is limited to 8.5 mm in radiofrequency ablation with a standard 6F endocardial electrode catheter.

LocaLisa New Technique for Real-Time 3-Dimensional Localization of Regular Intracardiac Electrodes

BACKGROUND Estimation of the 3-dimensional (3D) position of ablation electrodes from fluoroscopic images is inadequate if a systematic lesion pattern is required in the treatment of complex arrhythmogenic substrates. METHODS AND RESULTS We developed a new technique for online 3D localization of intracardiac electrodes. Regular catheter electrodes are used as sensors for a high-frequency transthoracic electrical field, which is applied via standard skin electrodes. We investigated localization accuracy within the right atrium, right ventricle, and left ventricle by comparing measured and true interelectrode distances of a decapolar catheter. Long-term stability was analyzed by localization of the most proximal His bundle before and after slow pathway ablation. Electrogram recordings were unaffected by the applied electrical field. Localization data from 3 catheter positions, widely distributed within the right atrium, right ventricle, or left ventricle, were analyzed in 10 patients per group. The relationship between measured and true electrode positions was highly linear, with an average correlation coefficient of 0.996, 0.997, and 0.999 for the right atrium, right ventricle, and left ventricle, respectively. Localization accuracy was better than 2 mm, with an additional scaling error of 8% to 14%. After 2 hours, localization of the proximal His bundle was reproducible within 1.4+/-1.1 mm. CONCLUSIONS This new technique enables accurate and reproducible real-time localization of electrode positions in cardiac mapping and ablation procedures. Its application does not distort the quality of electrograms and can be applied to any electrode catheter.

Inverse Relationship Between Electrode Size and Lesion Size During Radiofrequency Ablation With Active Electrode Cooling

BACKGROUND Clinical efficacy has driven the use of larger electrodes (7F, length > or =4 mm) for radiofrequency ablation, which reduces electrogram resolution and causes variability in tissue contact depending on electrode orientation. With active cooling, ablation electrode size may be reduced. The purpose of this study was to examine the effect of electrode length on tissue temperature and lesion size with saline irrigation used for active cooling. METHODS AND RESULTS In 11 anesthetized dogs, the thigh muscle was exposed and bathed with heparinized canine blood. A 7F ablation catheter with a 2- or 5-mm irrigated tip electrode was positioned perpendicular or parallel to the thigh muscle. Radiofrequency current was delivered at constant voltage (50 V) for 30 seconds during saline irrigation (20 mL/min) to 148 sites. Tissue temperature at depths of 3.5 and 7 mm and lesion size were measured. In the perpendicular electrode-tissue orientation, radiofrequency applications at 50 V with the 2-mm electrode compared with the 5-mm electrode resulted in lower power at 50 V (26 versus 36 W) but higher tissue temperatures, larger lesion depth (8.0 versus 5.4 mm), and greater diameter (12.4 mm versus 8.4 mm). Also, in the parallel orientation, overall power was lower with the 2-mm electrode (25 versus 33 W), but tissue temperatures were higher and lesions were deeper (7.3 versus 6.9 mm). Lesion diameter was similar (11.1 versus 11.3 mm) for both electrodes. CONCLUSIONS The smaller electrode resulted in transmission of a greater fraction of the radiofrequency power to the tissue and resulted in higher tissue temperature, larger lesions, and lower dependency of lesion size on the electrode orientation.

Comparison of Electrode Cooling Between Internal and Open Irrigation in Radiofrequency Ablation Lesion Depth and Incidence of Thrombus and Steam Pop

Background— Electrode cooling by circulating fluid within the electrode (closed loop) or open irrigation facilitates radiofrequency (RF) ablation. This study compared lesion parameters between closed loop and open irrigation with the use of a canine model. Methods and Results— In 8 anesthetized dogs, the skin over the thigh muscle was incised and raised, forming a cradle superfused with heparinized blood (activated clotting time >350 seconds) at 37°C. A 7F 4-mm closed loop electrode (irrigation 36 mL/min) and 7.5F 3.5-mm open irrigation electrode (irrigation 17 mL/min) were positioned perpendicular to the thigh muscle at 10 g contact weight. RF was applied (n=121) at 20 or 30 W for 60 seconds in low (0.1 m/s) or high (0.5 m/s) pulsatile blood flow. Temperatures were measured in the electrode, electrode-tissue interface, and within the tissue at 3- and 7-mm depths. After each RF, the cradle was emptied to examine the electrode and interface for thrombus. There was no difference between closed loop and open irrigation in impedance, lesion depth, or tissue temperature at 20 or 30 W. Interface temperature and electrode temperature were greater in the closed loop application. Thrombus occurred in 32 of 63 closed loop versus 0 of 58 open irrigation RF applications (P<0.05) with interface temperature ≥80°C in all 32 (electrode temperature <40°C in 1, 40°C to 50°C in 26, and >50°C in 5). With closed loop, interface temperature and thrombus incidence were greater at 30 W and low blood flow. With open irrigation, interface temperature remained low (≤71°C) with no difference between 20 and 30 W or between low and high blood flow. Steam pop occurred at 20 W in 4 of 35 closed loop and 0 of 30 open irrigation and at 30 W in 15 of 28 closed loop and 4 of 28 open irrigation applications (P<0.05). Conclusions— Lower interface temperature, thrombus, and steam pop, especially in low blood flow, indicate greater interface cooling with open irrigation.

RF Catheter Ablation: Lessons on Lesions

The present treatment of atrial fibrillation by radiofrequency catheter ablation requires long continuous lesions in the thin walled left atrium where side effects may lead to serious complications. Better understanding of the physical processes that take place during ablation may help to improve the quality, safety, and outcome of these procedures. These processes include the distribution of power between blood, tissue, and patient; the mechanisms of tissue heating and coagulum formation; the relation between tissue and electrode temperatures; and the effects of increased electrode size and internal and external electrode cooling.

Pulmonary Vein Ostium Geometry Analysis by Magnetic Resonance Angiography

Background—During a catheter ablation procedure for selective electrical isolation of pulmonary vein (PV) ostia, the size of these ostia is usually estimated using fluoroscopic angiography. This measurement may be misleading, however, because only the projected supero/inferior ostium diameters can be measured. In this study, we analyzed 3-dimensional magnetic resonance angiographic (MRA) images to measure the minimal and maximal cross-sectional diameter of PV ostia in relation to the diameter that would have been projected on fluoroscopic angiograms during a catheter ablation procedure. Methods and Results—In 42 patients with idiopathic atrial fibrillation who were scheduled for selective electrical isolation of PV ostia, the minimal and maximal diameters of these ostia were measured from 3-dimensional MRA images. Thereafter, these images were oriented in a 45° right or left anterior oblique direction and the projected diameter of the PV ostia were measured again. The average ratio between maximal and minimal diameter was 1.5±0.4 for the left and 1.2±0.1 for the right pulmonary vein ostia. Because of the orientation and oval shape of especially the left pulmonary vein ostia, their minimal diameters were significantly smaller than the projected diameters. Conclusion—Pulmonary vein ostia, especially those at the left, are oval with the short axis oriented approximately in the antero/posterior direction. Consequently, PV ostia may sometimes be very narrow despite a rather normal appearance on angiographic images obtained during a catheter ablation procedure.

Why a large tip electrode makes a deeper radiofrequency lesion: effects of increase in electrode cooling and electrode-tissue interface area.

Increase in RF Lesion Depth with Larger Electrode. Introduction: Increasing electrode size allows an increase in radiofrequency lesion depth. The purpose of this study was to examine the roles of added electrode cooling and electrode‐tissue interface area in producing deeper lesions.

Effect of right ventricular pacing on ventricular rhythm during atrial fibrillation

In 13 patients with atrial fibrillation, the effect of right ventricular pacing at various rates on spontaneous RR intervals was studied. Five hundred consecutive RR intervals were recorded and measured before and during varying right ventricular pacing rates. As anticipated, all RR intervals longer than the right ventricular pacing intervals were abolished. However, RR intervals shorter than the right ventricular pacing intervals were also eliminated. It is difficult to explain the elimination of RR intervals shorter than the pacing intervals with the accepted concepts concerning the mechanisms governing the rate and rhythm of the ventricular response to atrial fibrillation. An alternative explanation may be that during atrial fibrillation the atrioventricular node behaves as a nonprotected pacemaker that is electrotonically modulated by the chaotic atrial electrical activity. The result is a random ventricular rhythm. With right ventricular pacing, the automatic focus is depolarized by the retrogradely concealed conducted ventricular impulses, the short RR intervals are not generated as a consequence and the rhythm becomes pacemaker dependent.

Identification of the Substrate of Atrial Vulnerability in Patients With Idiopathic Atrial Fibrillation

BACKGROUND Experimental studies have shown that atrial fibrillation (AF) causes remodeling, which facilitates AF perpetuation. AF may also, however, occur in patients without remodeling and underlying structural cardiac disease. The substrate for enhanced vulnerability in these patients is unknown. METHODS AND RESULTS We studied 43 patients without structural heart disease: 18 patients with documented sporadic paroxysmal AF and 25 control patients without AF. In each patient, a decapolar catheter was positioned against the right atrial free wall, and a quadripolar catheter was positioned in the right atrial appendage. Unipolar electrograms were recorded. Atrial vulnerability was assessed according to an increasingly aggressive stimulation protocol. Mean local fibrillatory interval (FI) was used as an index of local refractoriness. Spatial dispersion of refractoriness was assessed through the calculation of the coefficient of dispersion (CD), which was defined as the SD of mean local FI expressed as a percentage of the mean FI. In the AF group, AF was induced with a single extrastimulus in 16 of 18 patients; the CD was 5.4+/-2.6, and the mean FI was 164+/-29 ms. In the control group, AF could be induced only with more aggressive pacing in 23 of the 25 patients; the CD was 1.4+/-0.7 (P<0.0001), and the mean FI was 175+/-26 ms (NS). CONCLUSIONS Patients with idiopathic AF showed increased dispersion of refractoriness, which may be the substrate for the observed enhanced inducibility and spontaneous occurrence of AF.

Bradycardia Dependent QT Prolongation and Ventricular Fibrillation Following Catheter Ablation of the Atrioventricular Junction witb Radiofrequency Energy

Recurrent ventricular fibrillation was observed in a 67‐year‐old woman following catheter ablation of the AV junction using radiofrequency energy. This serious complication has been reported following direct current energy ablation of the AV junction, but not after using radiofrequency energy. This life‐threatening arrhythmia seemed pause and bradycardia dependent. It was followed by QTc prolongation of the QRS escape rhythm 1 day after the procedure. Ventricular arrhythmias were suppressed by rapid ventricular pacing.