Catheter ablation using radiofrequency energy

Recently, transvenous catheter ablation using radiofrequency impulses has been applied in experimental animals as well as in humans. This review describes the biophysics, mechanisms, and tissue effects of radiofrequency alternating current. Ablation of the AV-junction has been performed in experimental studies as well as in a growing number of patients. Complete AV-block or modification of the AV conduction by RF-current application can be achieved in about 45-65% of patients. Presently about 50% of patients with rightsided accessory pathways can be successfully managed using radiofrequency impulses. Certain limitations exist using a biopolar "epi-endocardial" electrode configuration for leftsided accessory pathways. Only few studies are available concerning the use of RF ablation of ventricular tachycardia foci. As most experimental investigations were carried out in structurally nondiseased myocardium, the use of this new technique in fibrotic or scarred tissue warrants further studies. Radiofrequency catheter ablation seems to be feasible and relatively safe for the treatment of otherwise drug-refractory supraventricular arrhythmias; further studies are needed to assess the safety and efficacy in patients with drug-refractory ventricular tachyarrhythmias.

Radiofrequency coagulation of ventricular myocardium: improved prediction of lesion size by monitoring catheter tip temperature

To assess the importance of voltage, current, impedance and catheter tip temperature for the prediction of the size of tissue injury induced by transcatheter radiofrequency application, radiofrequency pulses (500 kHz) were delivered both in vitro and in vivo to isolated ventricular preparations and the intact canine heart, respectively. Radiofrequency coagulations were performed using unipolar electrode configuration. Besides measurements of current and voltage which were used to calculate the delivered power and tissue impedance, the catheter tip temperature was monitored during each application using specially designed 6F USCI catheters with a built-in nickel/chromium-nickel thermoelement. Lesion dimensions were measured and the correlation between lesion volume and delivered radiofrequency energy, maximum changes in catheter tip temperature and the integral of the temperature curve were calculated. First, in a pilot in vitro investigation, 50 radiofrequency coagulations (3.2 W-22.4 W, pulse duration 10 s) were performed in ventricular preparations from freshly excised dog hearts. The correlation between applied radiofrequency energy and lesion volume was 0.87; the correlation between maximal catheter tip temperature and lesion volume was 0.82; the correlation between temperature integral and lesion volume was 0.9. In the intact dog heart, 44 radiofrequency pulses were delivered to the left and right ventricular endocardium in 12 anaesthetized dogs (exposure time: 10 s). Delivered power ranged between 5.6 W and 24.6 W; tissue impedance varied between 92 omega and 364 omega; lesion volume measured 0-273 mm3; developed peak temperatures ranged from 16.25 degrees C to 196 degrees C. The calculated integral beneath temperature curves measured 126-1971 degrees C.s. The correlation between applied radiofrequency energy and lesion volume was 0.32; the correlation between maximal catheter tip temperature and lesion volume was 0.61. Temperature integral correlated best with the assessed volume of myocardial necrosis (r = 0.7). No significant arrhythmogenic or haemodynamic side-effects were observed. Macroscopic examination showed a central depression surrounded by a zone of homogeneous coagulation. Vaporization and crater formation up to a depth of 4 mm were observed following three radiofrequency discharges. In two of these cases, rapid changes and oscillation of catheter tip temperature were observed. Thus, monitoring of catheter tip temperature during radiofrequency energy application improves the prediction of lesion size. In addition, temperature monitoring might improve the safety of the procedure with respect to the risk of perforation.

Electrophysiologic, haemodynamic and antiarrhythmic effects of the new class Ic agent 1-(2'-biphenyloxy)-2-tert.-butylamino-propanol-2-hydrochloride

The electrophysiological, antiarrhythmic and haemodynamic profile of the new compound GK 23-G (1-(2'-biphenyloxy)-2-tert.-butylamino-propanol-2-hydrochloride, proposed INN: bipranol) was examined using dogs models relevant to conditions in humans. In the first part of the study, dose-related effects of cumulatively increasing doses of GK 23-G (0.2-12.8 mg/kg) on intracardiac conduction, ventricular refractoriness and on haemodynamic parameters of the non-ischemic heart were determined in six anesthetized mongrel dogs. In the second part of the study, antiarrhythmic actions of bipranol on "delayed reperfusion ventricular arrhythmias" following release of coronary artery occlusion after 2 h of obstruction were investigated in another six dogs. The results show: GK 23-G causes a significant prolongation of HV-time, QRS-duration and ventricular refractory period at mid-range and high doses (greater than or equal to 3.2 mg/kg). QT-time does not change. Atrial refractory period is significantly lengthened at the maximum dose of 12.8 mg/kg. There are no significant changes in heart rate, systolic and diastolic aortic pressure and cardiac output. Up to 12.8 mg/kg, GK 23-G does not influence left ventricular contractility (dp/dtmax). In acute myocardial necrosis "delayed reperfusion arrhythmias" are almost completely abolished at a dose of 1.6 mg/kg + 50 micrograms/kg x min. Thus, because of its antiarrhythmic potency, further experimental and clinical testing of the new compound seems promising.

[Ion regulation disorders and cardiac arrhythmia. The relevance of sodium, potassium, calcium, and magnesium]

1. Among the dysionemias hyper- and hypokalemia are particularly important for clinical arrhythmogenesis. Disorders in sodium- and calcium concentrations, however, are relevant only in single cases. The impact of magnesium concentration disorders on cardiac rhythm is not yet totally elucidated. 2. In hypokalemia tachycardic arrhythmias are most important, while bradycardic and tachycardic arrhythmia can be caused by hyperkalemia. An important factor in arrhythmogenesis is the rate of development of hypo- or hyperkalemia. Hypokalemically-induced arrhythmia can be suppressed by potassium substitution. 3. Although the importance of a magnesium dysionemia for arrhythmogenesis has not been confirmed, magnesium can be used for the treatment of arrhythmias with good results. Besides an antiarrhythmic efficacy, an antifibrillatory activity is suggested in acute myocardial ischemia.

Coagulation of ventricular myocardium using radiofrequency alternating current: bio-physical aspects and experimental findings

Within the last years, a variety of different energy sources has been investigated to test their feasibility for catheter ablation of myocardial tissue. This report summarizes our experience of the use of radiofrequency alternating current (500 kHz, unipolar mode) for coagulation of ventricular myocardium in canine experiments. Under standardized in vitro conditions, we found a significant correlation between actually delivered radiofrequency energy and assessed myocardial necrosis (r = 0.87). However, this did not hold for percutaneous application of radiofrequency alternating current to the beating dog heart (r = 0.32). In the intact dog heart, the size of induced lesions paralleled catheter contact pressure which was varied until ST-egment elevation was either 0-2 mV or 5-7 mV. However, no statistical significant differences in either calculated energy or tissue impedance were observed. Under in vivo conditions, a significant improvement in the predictability of the resulting size of lesions was observed when catheter tip temperature, measured via a built-in Ni/CrNi thermoelement, was monitored (r = 0.07). Changes in tip temperature during coagulation also indicated the quality of catheter contact, catheter damage and the appearance of carbonization at the tip of the ablation catheter. Total perforation of the myocardial wall and proarrhythmogenic effects were only rarely observed. In conclusion, catheter coagulation of myocardial tissue using radiofrequency energy can be considered as safe and effective. Since changes in catheter tip temperature occurring during coagulation were found to predict the extent of induced tissue necrosis, the development of temperature controlled radiofrequency devices seems promising and necessary.

[The incidence of ventricular arrhythmia following direct current ablation, high-frequency current ablation and laser photo-ablation]

Incidence and severity of ventricular arrhythmias (VA) following transvenous catheter ablation have so far not been fully elucidated. In the present study we evaluated the comparative incidence of postablation ventricular arrhythmias following high voltage-direct current electrical ablation (DCA), radiofrequency-ablation (RFA), and laser-photoablation (LPA). Experiments were performed on a total of 26 anesthetized mongrel dogs (BW: 20-30 kg). DCA (n = 14; 150-200 J) and RFA (n = 7; 38.5-72.5 J) were performed unipolarly via a 6F USCI catheter, LPA (n = 5; 40-80 J) was delivered through a quarz core fiber (diameter 0.4 mm) housed within a special designed catheter. Energies were delivered to various sites of free wall and apical endocardium of the left ventricle. Immediately after DCA fast runs of ventricular tachycardia (VT) developed in 13 out of 14 dogs degenerating into ventricular fibrillation in two animals. Mean cycle length of induced VT was 298 +/- 86 ms. Persistent VA, morphologically mainly characterized by an accelerated idioventricular rhythm interrupted by runs of ventricular salvoes, occurred in 12 animals (mean rate: 78 +/- 13 VPB/min 3 h after ablation). During VT early endocardial activations were recorded from the ablation site. No significant correlation between total applied energy (150-550 J) and incidence of arrhythmogenic effects was observed. RFA and LPA induced ventricular salvoes and runs of non-sustained ventricular tachycardia, in one animal ventricular fibrillation occurred during RFA; however, no persistent arrhythmic activity developed after RFA and LPA, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

[Experimental and initial clinical experiences with the new class Ib anti-arrhythmia agent barucainide]

Barucainide is a new class Ib-antiarrhythmic compound which causes a reduction of the upstroke velocity and shortens the duration of the action potential in a dose-dependent fashion. The electrophysiological, hemodynamic and antiarrhythmic effects of the new drug were analyzed in experimental and clinical investigations. The results show that barucainide, in a dose range varying between 0.125-4.0 mg/kg i.v. fails to exert any significant effect on cardiac conduction and refractory times, heart rate, arterial blood pressure, cardiac output and left ventricular contractility. In a standardized animal model of ventricular arrhythmia, barucainide proved to be superior to lidocaine with respect to its antiarrhythmic potency. In a group of patients displaying chronic, otherwise therapy-resistant ventricular arrhythmias, the drug was effective in the majority of cases. Tolerance was excellent over a maximum observation period of 18 months. According to our first experimental and clinical observations, further testing of barucainide appears promising. The drug may be of particular value for treatment of ventricular arrhythmias in cases with left ventricular dysfunction, heart failure and severe conduction disturbances.

Calcium antagonists and acute myocardial ischemia: comparative effects of gallopamil and nifedipine on ischemia-induced and reperfusion-induced ventricular arrhythmias, epicardial conduction times, and ventricular fibrillation thresholds

The comparative effects of the calcium-antagonists gallopamil and nifedipine on ischemia-induced and reperfusion-induced ventricular arrhythmias, particularly ventricular fibrillation (VF), were assessed in a total of 40 mongrel dogs in two experimental preparations. In part I of the study, changes in the time course of spontaneous ventricular arrhythmias and VF parallel to changes in epicardial conduction following acute coronary artery occlusion lasting 20 minutes and followed by subsequent reperfusion were determined. In part II, repeated coronary artery occlusions (20 min) followed by reperfusion (60 min) were performed, and changes in ventricular fibrillation threshold (VFT) were assessed. Gallopamil proved to be highly effective in preventing ventricular arrhythmias and VF following coronary delay was reduced. The ischemia-induced fall in conduction delay was reduced. The ischemia-induced fall in VFT occurring during the first few minutes after occlusion (phase Ia) was significantly reduced. In contrast, nifedipine failed to influence the incidence of ventricular arrhythmias and VF. Following reperfusion, neither drug reduced the incidence of VF nor the associated fall in VFT at the onset of reperfusion. The time course of recovery of epicardial conduction was not affected by either drug. However, the increase in the VFT during the early postreperfusion period was significantly enhanced by both agents. The effects of gallopamil were more pronounced than those of nifedipine. Delayed reperfusion ventricular arrhythmias arising 5 to 10 minutes after release of coronary artery obstruction were significantly reduced by gallopamil whereas nifedipine proved ineffective. The results show that calcium antagonists display direct antiarrhythmic and cardioprotective actions in acute transient myocardial ischemia. The different effectiveness of gallopamil compared to nifedipine can be explained by differences in electrophysiological properties of the drugs. Enhanced ventricular vulnerability following acute transient coronary artery occlusion and subsequent release of coronary artery obstruction, first described by Tennant and Wiggers, has been extensively investigated over the past decade in a variety of experimental and clinical settings. However, the basic mechanisms underlying ischemia- and reperfusion-induced ventricular arrhythmias and ventricular fibrillation (VF) have not yet been fully elucidated. Furthermore, the results of pharmacological approaches to prevent ventricular arrhythmic activity are conflicting. The present study aimed to evaluate the antiarrhythmic efficacy of calcium antagonists in acute myocardial ischemia and reperfusion. We have examined the effects of gallopamil and nifedipine on the time course of ventricular arrhythmias during the first 20 minutes after acute coronary artery occlusion and subsequent reperfusion. We have studied the underlying mechanisms by mapping epicardial conduction and by assessing the electrically induced ventricular fibrillation threshold (VFT) both within and outside ischemic areas.

Comparative investigations on the antiarrhythmic and electrophysiologic effects of class I-IV antiarrhythmic agents following acute coronary artery occlusion

The effects of class I to IV antiarrhythmic drugs on ventricular arrhythmias, particularly ventricular fibrillation, in relation to epicardial conduction delay during acute myocardial ischaemia were investigated in 40 open-chest anaesthetized dogs. In these animals transient coronary artery occlusion lasting 20 minutes was performed. Sixteen dogs served as controls; four groups of 6 dogs each received lidocaine (2 mg kg-1 + 50 micrograms kg-1 over 20 minutes), metoprolol (2 mg kg-1 i.v.), d-sotalol (5 mg kg-1 i.v.) and verapamil (0.25 mg kg-1 i.v.), respectively 5 minutes prior to coronary occlusion. Epicardial conduction delay was assessed by means of an epicardial mapping electrode array consisting of 42 bipolar electrodes. In the control group conduction delay displayed a bimodal time course in the ischaemic area with a maximum of 38 +/- 10 ms 6 minutes after coronary occlusion followed by a partial improvement after 12 to 15 minutes. After pretreatment with lidocaine the peak conduction delay in the ischaemic area was significantly increased (43.2 +/- 12), whereas metropolol (21.5 +/- 1.9), d-sotalol (16.6 +/- 3.7) and verapamil (16.3 +/- 3.1) significantly reduced conduction delay. Correspondingly, ventricular arrhythmias and fibrillation were almost completely prevented by metoprolol, d-sotalol or verapamil, but increased by lidocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of diprafenone, a new potent propafenone-analogue, in acute experimental myocardial ischaemia and infarction

Diprafenone (D) is a new class I c antiarrhythmic agent, structurally similar to propafenone. We assessed its antiarrhythmic and anti-fibrillatory effects during acute coronary occlusion and reperfusion and the underlying mechanisms of action by epicardial mapping of the conduction delay; also the effects of D on stimulus-induced ventricular tachycardia 18-24 h after permanent coronary occlusion were assessed. Experiments were performed on 32 mongrel dogs with temporary coronary occlusion lasting 20 min and subsequent reperfusion. Control ligations in 16 animals were compared to ligations after pretreatment with D (2 mg kg-1) in 6 dogs. In another 10 dogs a permanent coronary occlusion was performed and the inducibility of ventricular tachycardia was assessed by programmed stimulation before and after D (2.4 mg kg-1). Following D the incidence of ventricular arrhythmias including rapid ventricular tachycardias was not reduced during acute coronary occlusion, but even enhanced in some animals, whereas the frequency of ventricular fibrillation was diminished. No significant difference was observed following reperfusion. Conduction delay in the ischaemic area increased significantly during both phase Ia and Ib following pretreatment with D. During reperfusion conduction delay was significantly prolonged in the D group. At 18-24 h after permanent coronary occlusion the new compound proved to be highly effective in suppressing stimulus-induced ventricular tachycardia. During acute coronary occlusion D diminished the incidence of ventricular fibrillation. D is similar to other class Ic compounds; however, there are some important differences with respect to its additional beta-sympatholytic activity.

Comparative investigations on the antiarrhythmic and electrophysiological effects of various calcium antagonists (diltiazem, verapamil, gallopamil, nifedipine) following acute transient coronary artery occlusion and reperfusion

The effects of various calcium antagonists on ventricular arrhythmias, particularly fibrillation (VF), in relation to epicardial conduction delay during acute myocardial ischaemia and reperfusion were investigated in 40 open-chest anaesthetized dogs. Acute transient coronary artery occlusion lasting 20 min was performed in all animals. Sixteen dogs served as controls; diltiazem (D) (0.5 mg kg-1 iv), verapamil (V) (0.25 mg kg-1 iv), gallopamil (G) (0.13 mg kg-1 iv) and nifedipine (N) (0.04 mg kg-1 iv) were given in six animals each 5 min prior to coronary occlusion. Epicardial conduction delay was assessed by means of an epicardial mapping electrode array consisting of 42 bipolar electrodes. In the control group, conduction delay showed a bimodal time course in the ischaemic area with a maximum of 38 +/- 10 ms 6 min after coronary occlusion followed by partial improvement. After pretreatment with D, V or G the peak in conduction delay as well as the maximum dispersion of conduction times in the ischaemic area were significantly diminished, whereas N failed to improve conduction in the ischaemic area. Correspondingly, ventricular arrhythmias and VF were almost completely suppressed by D, V or G, but not affected by N. Following release of coronary artery occlusion none of the compounds proved to influence the rapid and heterogeneous improvement of conduction immediately after the onset of reperfusion. Correspondingly, none of the drugs diminished the incidence of VF immediately after release. Delayed ventricular reperfusion arrhythmias, arising parallel to complete restoration of conduction, were significantly reduced by D, V and G but not affected by N. Delayed and inhomogeneous activation of the ischaemic myocardium plays an important role in the genesis of ventricular arrhythmias in the early stage of acute myocardial ischaemia; thus a reduction in conduction delay and dispersion of conduction times seems to be a precondition for antiarrhythmic action. The different effects of calcium antagonists type V and type N on ischaemia-induced conduction delay and ventricular arrhythmias can be assumed to result from differences in the electropharmacological properties of the compounds.

Electrophysiologic effects of asocainol, a new antiarrhythmic agent with predominant class I action

In experiments on 6 mongrel dogs the electrophysiologic effects of the new antiarrhythmic compound (+/-)-6,7,8,9-tetrahydro-2,12-dimethoxy-7-methyl-6-phenethyl-5H- dibenz[d,f]azonin-1-ol (asocainol) were examined in the non-ischemic heart. The PR interval increased significantly following doses of 5 mg/kg caused by a conduction delay in the His-Purkinje system. At high doses (10 mg/kg) the QRS duration was also prolonged. Atrial and ventricular effective refractory periods were only slightly increased. Asocainol significantly increased the atrial fibrillation threshold, whereas the ventricular fibrillation threshold was not altered. Hemodynamics were not changed except for a slight increase in heart rate after 10 mg/kg asocainol. Thus asocainol predominantly exhibited class Ia antiarrhythmic properties, whereas calcium antagonistic actions could not be established in this intact dog heart model.