Radiofrequency Catheter Ablation: The Effect of Electrode Size on Lesion Volume In Vivo

Radiofrequency current is a promising alternative to high voltage direct current defibrillator discharges for catheter ablation of arrhythmias. However, lesions produced with radiofrequency current are relatively small and use of high power is limited by the impedance rise that occurs with desiccation of tissue and coagulum formation. The effect of electrode size on radiofrequency ablation was assessed by comparing results of radiofrequency application using a standard 6 French electrode catheter (distal electrode 2 mm in length) to those using catheters modified with longer distal electrodes (3, 4, 6, 8, and 10 mm in length). Radiofrequency ablation was performed at 47 left ventricular endocardial sites in 20 anesthetized dogs. A constant power of 13.3 +/- 1.3 watts at 550 kHz was applied between the distal catheter electrode and a skin electrode until a total of 500 joules had been delivered or a rise in impedance occurred. Increasing electrode length from 2 to 4 mm more than doubled lesion volume from a mean of 143 to 326 mm3 (P = 0.025). Increasing electrode length beyond 4 mm produced progressively smaller lesions (157 mm3, 155 mm3, and 67 mm3 for 6-, 8-, and 10-mm electrode lengths, respectively). Impedance rise was significantly less likely with larger electrodes and took longer to occur. Increasing the size of electrodes used for radiofrequency ablation allows application of higher power without an impedance rise. Optimizing electrode size (3 or 4 mm in this study) results in larger lesions and may improve the effectiveness of radiofrequency ablation of arrhythmias.     

Simultaneous Multipolar Radiofrequency Ablation in the Monopolar Mode Increases Lesion Size

Delivery of radiofrequency (RF) energy from the distal tip of electrophysiology catheters produces lesions that may be too small to ablate arrhythmogenic sites during a single application of RF energy. To produce larger lesions, we delivered RF energy via a quadripolar catheter in which all four electrodes were connected in unipolar fashion. The catheter (Webster Labs) had a 4-mm tip, 2-mm ring electrodes, and 2-mm intereiectrode distance. Lesion size was compared using RF energy delivered in a multipolar configuration with that delivered only to the distal tip using fresh bovine ventricular tissue. In vivo, RF lesions were made in dogs using the distal tip as well as all four poles of the same catheter inserted percutaneously. RF energy was delivered using a constant voltage at a frequency of 400 kHz. Preliminary experiments were conducted to determine the maximum power deliverable without coagulation using each electrode configuration. The use of simultaneous muitipolar RF ablation produced significantly larger lesions both in vitro and in vivo. The length of the lesion was increased by a factor of approximately 2 in both the in vitro and in vivo experiments. There was a trend toward an increasing depth of the lesion by simultaneously applying RF energy to all four electrodes. Lesion width was significantly increased in the in vivo studies. We concluded that simultaneous muitipolar delivery of RF energy produces larger lesions than can be obtained with delivery of RF energy to the distal tip alone. This technique may offer a means of increasing lesion size, leading to a decrease in the number of applications of RF energy necessary for ablation of arrhythmias.     

Comparison of Gold Versus Platinum Electrodes on Myocardial Lesion Size Using Radiofrequency Energy

During radiofrequency (RF) catheter ablation of arrhythmias, temperatures that approach 100°C cause a coagulum to form on the ablation electrode that results in an increase in electrical impedance and prevents further energy delivery. Since gold has nearly four times the thermal conductivity as platinum, the metal commonly used, it was postulated that gold tip electrodes could deliver more power and produce deeper lesions because of its greater heat dissipation from the electrodetissue interface to the circulating blood. To test this hypothesis, RF energy was applied to fresh bovine ventricular myocardium using 6 French catheters with 2-mm long distal electrodes made from gold or platinum. Similar studies were also conducted using 7 French catheters with 4-mm long distal electrodes. Maximum lesion depth was defined as that produced with the level of energy just below that causing an impedance rise. A maximum lesion depth of 6.2 ± 0.7 mm (mean ± SD) was obtained with the gold 2-mm electrode and 4.7 ± 0.5 mm with the platinum electrode (P = 0.003). The 4-mm gold electrode produced a maximum lesion depth of 7.2 ± 1.4 mm, while a catheter with a 4-mm platinum electrode caused a maximum lesion depth of 5.8 ± 0,7 mm (P = 0.05). We conclude that deeper lesions should be able to be made when RF energy is delivered to a gold rather than platinum tip electrode.     

Determinants of Impedance Rise During Catheter Ablation of Bovine Myocardium with Radiofrequency Energy

Recently, radiofrequency (RF) energy has been used as an alternative energy source to direct-current (DC) electricity for catheter ablation of recurrent tachyarrhythmias. Since delivered energy is inversely related to impedance, factors that cause impedance rise during catheter ablation impede the ability to ablate tissue. To elucidate some of the factors responsible for impedance rise during RF (750 kHz) catheter ablation using a constant voltage RF generator, the effects of the following variables on impedance were studied in an in vitro bovine heart model: power setting (10-70 W), pulse duration (10-60 sec), catheter contact pressure (5-120 gm), repeated applications (2-4), and immersion media (saline vs citrated blood). Baseline impedance in blood was twice that of saline (190 vs 80 ohm) and rises in impedances occurred more rapidly in blood for the same energy settings. Increased power settings (greater than or equal to 30 W) and pulse duration (greater than or equal to 30 sec at 20 W) were associated with impedance rises in blood medium. Typically, impedance rises in blood were associated with blood coagulum on the catheter electrodes. Impedance rises in both saline and blood media were also associated with tissue charring and endocardial surface disruption. Once a rise in impedance occurred at the ablation site, repeated applications to the same site resulted in a more rapid rise in impedance. Catheter contact pressure of 80 gm or more also resulted in rapid impedance rise. These data suggest that factors other than set power and duration may also contribute to impedance rises during RF ablation. These findings may have important clinical implications in performing catheter ablation with RF energy.     

Myocardial Temperature Response During Radiofrequency Catheter Ablation

During radiofrequency catheter ablation, steady-state electrode-tissue interface temperatures are reached within 5 seconds. Within the myocardium, however, a much slower temperature rise has been observed in vitro with stabilization after approximately 2 minutes. This discrepancy suggests that tissue temperature rise time depends on distance from the ablation electrode and, thus, that temperature rise measured at the electrode-tissue interface does not correspond with temperature rise within the myocardium. In five beagles, closed-chest radiofrequency catheter ablation was performed in the vicinity of intramural thermocouples. Sequences of 60 seconds, 10- and 25-watt pulses were delivered in the unipolar mode via the 4-mm distal electrode of a 7 French steerable catheter. At all distances > 3 mm from the ablation electrode, the rate of myocardial temperature rise was low: relative rise after 5, 10, 20, and 30 seconds was 22%, 32%, 48%, and 63% of that achieved at 60 seconds, and even then steady-state temperatures had not yet been reached. Temperature rise was faster at sites closer to the ablation electrode. There was no difference in rate of rise between first and second pulses at the same site. A 6% higher myocardial temperature was reached with a second identical pulse at the same site. Tissue temperatures achieved with 25 watts were 2.4 times higher than with a preceding 10-watt pulse at the same ablation site.     

Thermal Catheter Disruption During Closed-Chest Radiofrequency Ablation of the Atrioventricular Conduction System

Radiofrequency ablation of the atrioventricular conduction system was attempted in a 63-year-old man with drug refractory atrial fibrillation. A total of 5 radiofrequency pulses (750 kHz, power setting: 25-50 W, pulse duration: 9-20 sec) were delivered in a unipolar fashion via the distal electrode of a 7 Fr bipolar electrode catheter without induction of permanent AV block. No direct measurements of current (I) and voltage (U) were made. During the fifth pulse catheter disruption occurred at the interface of the shaft and the proximal electrode. Inspection of the catheter shaft revealed carbonized insulation material indicating overheating of the catheter tip. Overheating was presumably due to an impedance rise with unrecognized clot formation on the distal electrode. This led to progressive melting of insulation material during repeated radiofrequency applications and short circuiting of current flow to the proximal ring electrode that resulted in catheter disruption. This case report is the first to describe a serious complication of radiofrequency ablation. The complication might have been prevented by measurements of U and I, reflecting changes in impedance or by measurements of catheter tip temperature (T). It is concluded that measurements of U, I, and/or T are necessary to control the coagulation process thereby reducing the risk of serious complications during transcatheter radiofrequency ablation.     

Electrophysiologic and Histologic Observations of Chronic Atrioventricular Block Induced by Closed-Chest Catheter Desiccation with Radiofrequency Energy

Direct-current or laser energy has been used to induce atrioventricular (AV) block, but certain complications associated with this type of energy have been reported. We have previously documented that radiofrequency (RF) energy can effectively and safely induce acute AV block in ciosed-chest dogs during the 4–7 days of follow-up. This study was undertaken Io determine if the ablation was permanent and to define the chronic pafhoiogy and site of AV block. Gomplete AV block was successfully achieved in four dogs immediately after ablation with a bipolar "standard" RF output (750 kHz) delivered between the tip electrode of a standard 7F USCI catheter and an external patch electrode on the left iateral chest wall. During 2 months of follow-up, three dogs had persistent complete AV block with a stable escape rhythm; the other had persistent 2:1 AV block. Repeat His bundle recordings were performed at 2 months prior to sacrifice of the dogs. Supra-His AV block was noted in two dogs; His bundle potential could not be recorded in another two. Histologically, the damaged area was well delineated. In all animals, the AV node and, in some dogs, part of the His bundle were completely replaced by granulation tissue and/or cartilage. There was fatty infiltration and also chronic inflammatory cells around the lesions. Neither perforation, hemorrhage nor vacuolation was seen in the adjacent area. Thrombus was not present. It is concluded that RF energy can effectively achieve chronic AV block and produce well-circumscribod pathological lesions.     

Impedance Monitoring During Radiofrequency Catheter Ablation in Humans

Radiofrequency catheter ablation of accessory pathways and the atrioventricular junction often requires multiple applications of energy. The inability to determine the effects of any given application on the underlying tissue may contribute to this problem. In the present study, impedance was monitored in 20 patients undergoing radiofrequency catheter ablation, and the relationship between an initial decrease in impedance and subsequent effects were examined. An initial fall in impedance of more than 10 omega was 78% sensitive and 88% specific for predicting subsequent evidence of tissue heating (interruption of conduction or an abrupt rise in impedance due to coagulum formation). In contrast, initial values of voltage, current, or impedance did not distinguish between effective and ineffective applications of radiofrequency energy. Continuous monitoring of impedance may facilitate radiofrequency catheter ablation.     

Cellular Electrophysiological Changes Induced In Vitro by Radiofrequency Current: Comparison with Electrical Ablation

The purpose of this study was to examine the cellular electrophysiological effects of radiofrequency energy delivery in an in vitro canine epicardial preparation and compare the effects to those of high energy electrical ablation in a similar preparation. Ten joules of direct current energy or 40 volts of radiofrequency energy were delivered by a 6 French 2-mm tip catheter to the epicardial surface of 2 × 3 cm epicardial strips superfused with Tyrode's solution. Direct current energy delivery produced a crater and central zone of necrosis surrounded by a border zone of viable but damaged tissue that extended up to 10–12 mm from the site of energy delivery. Cellular electrophysiological abnormalities that included a less negative resting membrane potential, decreased peak dV/dT, decreased action potential amplitude, and decreased action potential duration (APD) were approximately linearly related to the distance from the crater edge. In addition, viable and inexcitable cells were frequently interspersed. Between 2 and 5 mm from the crater edge, 36.4% of the cells were inexcitable whereas others displayed normal action potential characteristics. In contrast, radiofrequency current produced a central zone of necrosis surrounded by a smaller border zone. Cellular damage that was qualitatively similar to that produced by direct current energy extended only up to 6–8 mm from the edge of the crater. In addition, severe abnormalities were noted in intracellular potentials recorded within 2 mm of the ablation site, and only minor abnormalities further away. Lesions were relatively homogeneous. Between 2 and 5 mm from the ablation site only 2.6% of the cells were inexcitable (P < 0.05 vs direct current). In conclusion, radiofrequency current produces lesions that are smaller and more homogeneous than those produced by direct current ablation. Although the border zone is small, a region of partially depolarized but viable myocardium is present after radiofrequency current energy delivery. These findings provide a cellular basis for several clinical observations that have been made following radiofrequency current energy delivery.     

Is a Cooled Tip Catheter the Solution for the Ablation of the Cavotricuspid Isthmus?

SCAVÉE, C., et al. : Is a Cooled Tip Catheter the Solution for the Ablation of the Cavotricuspid Isthmus? To test the theoretical superiority of irrigated tip catheters to achieve complete cavotricuspid isthmus block, a 4-mm cooled tip catheter was compared to a conventional 8-mm tip catheter with a double temperature sensor in the cavotricuspid isthmus (CTI) ablation. The study prospectively enrolled 60 patients (47 men, mean   65 ± 10   years) with common flutter divided in group 1   (n = 30)   assigned to an 8-mm tip catheter versus group 2   (n = 30)   , assigned to an internal circuit, irrigated tip catheter. Linear radiofrequency applications were performed in a point-by-point protocol to achieve complete CTI block. Complete CTI block was achieved in 29 patients in each group. Mean durations of procedure and fluoroscopy were 91 versus 90 and 40 versus 33 minutes in group 1 versus 2, respectively, (NS). The mean number/patient of RF pulses to interrupt atrial flutter was four in group 1 and eight in group 2   (P = 0.034)   , and 11 and 13, respectively, to interrupt CTI conduction (NS). The total energy delivered was similar in both groups (29,237 vs 23,236 W/s, NS). CTI ablation with a conventional 8-mm tip catheter versus an irrigated tip catheter was associated with similar success rates, procedure duration, and fluoroscopic exposure. The technical complexity of the cooled tip catheter renders it less competitive. (PACE 2003; 26[Pt. II]:328–331)     

Microwave, Irrigated, Pulsed, or Conventional Radiofrequency Energy Source: Which Energy Source for Which Catheter Ablation?

ERDOGAN, A., et al .: Microwave, Irrigated, Pulsed, or Conventional Radiofrequency Energy Source: Which Energy Source for Which Catheter Ablation? The aim of the study was to compare the diameter of endomyocardial lesions induced with the delivery of microwave, cooled, or pulsed energy versus conventional RF energy. In vitro tests were performed in fresh endomyocardial preparations of pig hearts in a 10-L bath of NaCl 0.9% solution at 37°C and constant 1.5 L/min flow. Ablation 7 Fr catheters with 4-mm tip electrodes were used, except for the delivery of microwave energy. Energy delivery time was set to 60 s/50 W in all experiments. Cooled energy delivery was performed with a closed irrigation catheter. Pulsed energy delivery was performed using a special controller with a duty-cycle of 5 ms. Microwave energy was delivered with a 2.5-GHz generator and 10-mm antenna. Electrode temperature and impedance were measured simultaneously. After ablation, lesion length, width, and depth were measured with microcalipers, and volume calculated by a formula for ellipsoid bodies. Each energy delivery mode was tested in ten experiments. The deepest lesions were created with cooled energy delivery, and the largest volume by microwave energy delivery. Pulsed RF produced significantly deeper lesions than conventional RF energy delivery. Cooled or pulsed RF energy delivery created deeper transmural lesions than conventional RF. To create linear lesions at anatomically complex sites (isthmus), microwave energy seemed superior by rapidly creating deep and long lesions. (PACE 2003; 26:[Pt. II]:504–506)     

Prospective Comparison of Temperature Guided Microwave and Radiofrequency Catheter Ablation in the Swine Heart

Microwave energy has been proposed as an alternative to radiofrequency energy for use during catheter ablation procedures. The purpose of this study was to prospectively compare, in an animal model, the lesion size associated with temperature guided catheter ablation using either microwave or radiofrequency energy. Eleven swine underwent catheter ablation with either radiofrequency (N = 4) or microwave energy (N = 7). In each animal catheter ablation was performed at 7–15 sites. At each site energy was delivered for 60 seconds using closed loop feedback temperature control to achieve a target temperature of 70°C. Cardiac catheterization was performed before and after ablation. Animals were sacrificed approximately one month following the ablation procedure. Analysis of lesion size demonstrated that overall lesions created using radiofrequency energy were larger than those created using microwave energy. In the ventricle, lesions created using microwave energy were longer, but had a similar width and depth as those created using radiofrequency energy. An important relation was observed between tbe depth of lesions created using microwave energy and catheter stability, as evidenced by the temperature profile. Overall, lesions created using microwave energy are smaller than those created using radiofrequency energy. Catheter stability has an important impact on lesion size.     

Transcatheter Radiofrequency Ablation of Atrial Tissue Using a Suction Catheter

Closed chest ablative technique that avoid barotrauma would be attractive for ablation at thin walled cardiac structures, such as the atrial free wall or coronary sinus. Transcatheter radiofrequency (RF) currents produce tissue necrosis the size of which is dependent on the contact between the tissue and the electrode. In order to assess the effects of transvenous RF ablation of atrial free wall using a suction electrode catheter, we delivered in ten dogs, one single unmodulated RF pulse 1.2 MHz, in a unipolar mode, through the distal electrode of a lumen catheter (USCI 8F) (USCI, Billerica, MA, USA) located in the right appendage. During the pulse an 80 KPa vacuum depression was applied to the lumen of the catheter. Each pulse had a 10 seconds duration and the mean delivered power was 4.3 ± 1.4 W. Aortic pressure and electrocardiogram were monitored during the procedure. A right airial electrophysiological study was performed at the ablated site, at control, after suction application and after RF pulse delivery. The animals were sacrificed after 14 or 21 days. Atrial pacing threshold values decreased after suction application in comparison to control values after the pulse (0.42 ± 0.06 vs 0.60 ± 0.23 mA, P < 0.05) but increased after the pulse delivery (2.60 ± 1.85 mA, P < 0.01). In contrast, the atrial effective refractory period did not significantly change after suction, nor after RF pulse delivery. Aortic pressure remained unchanged throughout the procedure. Complex arrhythmias were not observed during or after RF pulse delivery. One dog died suddenly at the first day after ablation, but this death was most probably unrelated to RF ablation. Anatomic lesions had a length of 8.8 ± 3.3 mm, a width of 4.6 ± 2.5 mm and a depth of 3.6 ± 1.1 mm. They were transmural in nine of the ten dogs but without atrial wall perforation in any case. Lesions suggesting tissue volatilization were present in four dogs. These results demonstrate that low energy RF currents delivered with a suction electrode catheter can produce transmural necrosis of free wall, without risk of perforation. Such ablative technique would be of interest for ablation of right sided accessory pathways or atrial ectopic foci. Further experimental data are required in order to define the optimal energy level required to avoid tissue volatilization.     

Radiofrequency Catheter Ablation of Sinus Node Reentrant Tachycardia

Sinus node reentrant tachycardia is a relatively uncommon (5%-5%) form of recurrent paroxysmal supraventricular tachycardia (SVT). We describe a case of symptomatic sinus node reentrant tachycardia in a 67-year-old male with ischemic heart disease, congestive heart failure, and depressed ventricular function. Adenosine administered during an electrophysiology study caused prolongation of the tachycardia cycle length due to atrial cycle length prolongation (without atrio-His prolongation) prior to tachycardia termination. Right atrial mapping revealed the earliest site of atrial activation in the high lateral right atrium just below the superior vena cava. Low energy (10 and 20 W) radiofrequency lesions were applied ai this site with termination of the tachycardia within 3 seconds of radiofrequency energy delivery. Tachycardia could not be reinduced after delivery of the radiofrequency lesions. The sinus node function immediately and 6 weeks after radiofrequency catheter ablation remained normal and the patient was without clinical recurrence of SVT. Mapping of sinus node reentrant tachycardia and elimination of the reentrant circuit with radiofrequency catheter ablation is possible without causing sinus node dysfunction. Adenosine causes prolongation of the atrial cycle length followed by termination of sinus node reentrant tachycardia.     

Catheter Ablation of the Atrioventricular Junction Using a Helical Microwave Antenna: A Novel Means of Coupling Energy to the Endocardium

Catheter ablation with either direct current defibrillator discharges or radiofrequency energy produces tissue injury via current flow from an electrode into the adjacent myocardium. In order to affect tissue at a distance, excessive power density may be produced at the electrode-tissue interface with the possibility of explosive gas formation or coagulum formation. A novel microwave catheter was developed with a helical antenna distally. This coil, although not in direct contact with the endocardium, radiates an electromagnetic field into the tissue that, in turn, causes thermal injury. The utility of this system for ablation was assessed in six dogs. The antenna catheter was introduced percutaneously and positioned so as to record the largest His electrogram. Microwave power (50 watts at 2,450 MHz) was applied for 114 +/- 118 seconds. Complete AV block was produced in all six animals with 1.8 +/- 1.2 applications. There was no ventricular ectopy or change in blood pressure during microwave ablation. One dog died 6 days after ablation. The remaining five dogs had persistent, complete AV block during 6 weeks of follow-up. Pathological analysis at 6 weeks revealed a large (mean 2.8 x 4.7 mm) fibrovascular scar in the region of the AV junction. Percutaneous microwave ablation of the endocardium appears feasible. By radiating an electromagnetic field without direct contact, this system can produce large lesions without being limited by desiccation of tissue and impedance rise.     

Radiofrequency Current Ablation of Porcine Right Atrium: Increased Lesion Size with Bipolar Two Catheter Technique Compared to Unipolar Application In Vitro and In Vivo

Interruption of atrial flutter and fibrillation by RF catheter ablation may be favored by large, elongated lesions. We administered RF current in unipolar and bipolar mode in porcine right atrium. Bipolar ablation was performed between the tip electrodes of two serially coupled catheters. With 4-mm tip electrodes in vitro, lesion length increased from a mean (SD) of 7.9 (1.2) mm at 3 mm-interelectrode distance (IED) to 13.3 (3.3) mm at 9-mm IED, but decreased at 12-mm IED due to nonconfluent lesions (P < 0.0001), With 4 mm distal electrodes and 8 mm IED, bipolar lesions were 65% longer than corresponding unipolar ablations. Switching to bipolar mode increased the lesion length more than increasing electrode tip length to 6 mm in unipolar mode. Power and temperature controlled ablation created equally sized lesions. Twelve anesthetized pigs were randomized to unipolar or two catheter bipolar temperature controlled ablation of the right atrial free wall. Bipolar ablation created confluent lesions with endocardial length × width of 13.5 (5.8) × 7.3 (3.7) mm, unipolar ablation 6.4 (2.8) × 4.6 (1.4) mm (P < 0.001 when comparing length and P = 0.013 for lesion width). The atrial lesions in both groups were transmural and extended into hilar lung lesions with maximal depth of 3.0 (1.1) and 2.6 (1.0) mm, respectively (P = 0.44). Five bipolarly and four unipolarly ablated pigs developed right diaphragmal paresis. We conclude that bipolar ablation may be preferable in situations where large, elongated lesions are favorable. The two catheter technique is feasible in porcine right atrium. Both bipolar and unipolar ablation of the porcine right atrial free wall may frequently be complicated by injury to the phrenic nerve and adjacent lung tissue.     

Permanent AV Block or Modification of AV Nodal Function by Selective AV Nodal Artery Ethanol Infusion

The acute and chronic effects of selective AV nodal artery ethanol infusion on AV nodal function was studied in 9 closed chest anesthetized dogs. Using standard percutaneous techniques of arterial catheterization, a 2.2 French infusion catheter was positioned in the AV nodal artery. Ten minute infusions into the AV nodal artery of 25%, 50%, or 100% ethanol in normal saline at rates of 0.5 mL/min acutely resulted in complete AV nodal block (AVB) in 5 dogs, 2:1 AV nodal block in 1 dog, and prolongation of AV nodal effective refractory period and/or Wenckebach cycle length in the remaining 3 dogs. One dog died with persistent complete AV block 1 week after the ethanol infusion. When restudied 4 weeks later, 7 of the 8 surviving dogs had persistent modification of AV nodal function, including complete AV block in 5 dogs and lengthening of AV nodal effective refractory period and/or Wenckebach cycle length without AV block in 2 dogs. Pathologic examination of the animals exhibiting chronic modification or ablation of AV nodal function revealed healing infarction of the AV node or its approaches. Distant myocardial necrosis was not observed and left ventricular function was normal. Slow infusion of low concentrations of ethanol into the AV nodal artery results in AV nodal modification or ablation due to localized necrosis in or around the AV node. This technique may have a role in AV nodal modification or ablation, particularly in patients who have failed DC shock or radiofrequency ablation.     

Radiofrequency catheter ablation with the split-tip electrode in the temperature-controlled mode

The 7 Fr "split-tip electrode" (2.5-mm tip electrode divided longitudinally into four electrodes with an adjacent 2-mm ring electrode) improves mapping resolution due to its small recording electrodes and narrow interelectrode distances (0.1 mm). The purpose of this study was to examine the temperature-controlled ablation properties of this electrode. In seven anesthetized dogs, the thigh muscles were exposed and superfused with canine blood. A split-tip catheter electrode (with a thermocouple in each of the five electrodes) and a conventional 4-mm catheter electrode were positioned at constant pressure perpendicular or parallel to the surface of the thigh muscle. Impedance measured between each split electrode and a skin patch correlated with the degree of contact with blood and tissue. In the parallel catheter to tissue orientation, split electrodes not in contact with tissue had a low impedance (mean 210-224 ohms), and the split electrode almost entirely in contact with tissue had the highest impedance (380 +/- 56 ohms). In the perpendicular catheter to tissue orientation all split electrodes had a similar impedance (mean 279-286 ohms). A total of 75 radiofrequency (RF) lesions were produced in the temperature-controlled mode with the 4-mm electrode (target 60 degrees C) or the split-tip electrode (power limited by the hottest electrode reaching 70 degrees C) with current delivered to all five electrodes simultaneously, or only to electrodes in contact with tissue. Lesion depth was not significantly different between electrodes in the parallel orientation (5.2 +/- 0.9 vs 5.1 +/- 1.4 vs 5.3 +/- 1.1 mm), but significantly deeper with the conventional 4-mm tip electrode in the perpendicular orientation (6.7 +/- 1.2 vs 5.3 +/- 1.3 vs 5.6 +/- 0.9 mm, P < 0.05). This was due to higher power delivered to the conventional 4-mm electrode (27 +/- 9 vs 17 +/- 7 vs 15 +/- 7 W, P < 0.05) because convective cooling by the blood flow was less effective for the split-tip electrode due to a reduced heat conduction across the interelectrode space from the hottest electrode to cooler areas of the group of five electrodes (mean temperature difference between the hottest split electrodes and the ring electrode: 24 degrees C). Electrode cooling or heat conduction was not effected by the elimination of current delivery to non-contact electrodes. Steam pops occurred in 36% of applications with the conventional 4-mm electrode in the perpendicular orientation but never with the split-tip electrode in spite of the higher target temperature. Measurement of impedance from the split electrodes allow the determination of electrode tissue contact and RF lesions produced with the split-tip electrode in the temperature-controlled mode using a target of 70 degrees C were of reasonable size and not associated with steam pops.     

In Vivo Ventricular Lesion Growth in Radiofrequency Catheter Ablation

While radiofrequency catheter ablation has proved highly effective in the treafment of various supravenfricular tQchyarrhythmias, resulls in the trentment of ventricular tachycardia invite improvement. Knowledge of lesion growth in vivo might improve understanding of this discrepancy. So far only information from in vitro and in vivo studies using a small 2 mm tip eiectrode is available. Growlh of ventricular radiofrequency lesions created with a 4 mm ahlalion electrode was studied in 11 closed-chest dogs. Endocardia] ablations were performed at 31 left and 35 right ventricuiar sites at a power setting of 25 Watts and 5, 10, 20, 30 or 60 seconds pulse duration. Macroscopic and histopathologic lesion examination were performed after one week survival. Mean lesion volume increased from 52 mm³ after 5 seconds pulse duration to a maximum 388 mm³ and approximately 7 mm depth after 30 seconds. Lesions were prolate spheroid in form, with a sparing of subendocardial myocardium and maximum lesion diameter at some millimeters depth. Results indicate that catheter positioning at no more tlian 7 mm from the target is required for successful ablation. Due to lesion geometry, subendocardial targets demand even more exact catheter positioning, while subepicardial substrates may not be ammenable to ablation if ventricular wall thickness exceeds 7 mm at the ablation site. Repeated pulses at adjacent sites may be required for ablation of extended arrhytbmogenic areas. Volume at 5 seconds was only approximately 15% of mature lesions. Therefore, the use of a short'test pulse after careful mapping may be useful to pinpoint the most appropriate site for ablation in discrete pathways.     

Catheter Ablation of Canine Myocardium with Radiofrequency Energy

High energy direct-current shocks delivered via an electrode catheter have been used to ablate the atrioventricular junction since 1981.¹ This technique has also been adapted for ablation of other cardiac tissues including the atrium, posterior interatrial septum and ventricular myocardium. The limitations of this technique include inadequate control of the energy source, poor understanding of the mechanisms of myocardial injury, and untoward complications possibly related to barotraumatic injury. Radiofrequency energy has been shown to create ablative injury when delivered lo the myocardium via standard electrode catheters. This report will review our experience with radiofrequency catheter ablation of the canine myocardium with specific emphasis on the biophysical aspects of lesion formation.