Ultrastructural injury to chick myocardial cells in vitro following "electric countershock"

Tissue heating during radiofrequency catheter ablation: a thermodynamic model and observations in isolated perfused and superfused canine right ventricular free wall

The characteristics of radiofrequency catheter ablation induced injury in the heart are not well characterized. Since the mechanism of injury by radiofrequency energy is thermal, this study was performed to determine the temperature gradient in myocardial tissue during radiofrequency (RF) catheter ablation, and to validate a thermodynamic model derived to describe these observations. Lesions were created by RF heating in an experimental model of isolated perfused and superfused canine right ventricular (RV) free wall. RF power output was adjusted to maintain electrode tip temperature at 80 degrees C for 120 seconds in 151 serial lesions and radial temperature gradients were measured. With increasing distance from the electrode, the temperature of the myocardium decreased in a hyperbolic form that was closely predicted by a derived thermodynamic model (P = 0.0001, r = 0.98). This gradient and resultant lesion sizes were unaffected by the rate of coronary perfusion. The utility of tip temperature monitoring as a predictor of lesion size was tested in 104 serial lesions with tip temperatures that were varied between 50 and 85 degrees C. The tip temperature correlated closely with lesion depth (P = 0.0001, r = 0.92) and width (P = 0.0001, r = 0.88), and was a better predictor of lesion size than measurements of power, current or energy. The temperature at the margin between viable and nonviable tissue was estimated to be 47.9 degrees C. These data demonstrate that during radiofrequency catheter ablation, the radial temperature gradient is predictably hyperbolic and appears to be independent of intramyocardial perfusion if constant electrode temperature is maintained. The use of tip temperature monitoring can accurately predict the ultimate size of radiofrequency-induced lesions.

Current-based versus energy-based ventricular defibrillation: a prospective study

Defibrillation is thought to be mediated by a depolarizing current; however, the present method of defibrillation is based on delivering an empiric dose of energy to all patients. The hypothesis of this study was that for equivalent efficacy rates, a current-based defibrillation method would result in delivering less energy and peak current than would the standard energy-based method. In a group of 86 consecutive patients with ventricular fibrillation, every other patient was prospectively assigned to receive shocks according to method 1 or method 2. Method 1 was current based and delivered successive shocks of 25, 25 and a maximum of 40 A; method 2 was energy based and delivered shocks of 200, 200 and 360 joules. Patients in both groups were similar with respect to age, gender, weight, cardiac diagnosis, ejection fraction, antiarrhythmic therapy, chest circumference, chest depth and transthoracic impedance. Each method had statistically equivalent first shock (79% current-based versus 81% energy-based) and cumulative shock success rates. The mean first shock energy was 120 +/- 30 joules for patients receiving the current-based method and 200 joules for patients receiving energy-based shocks (p = 0.0001). The mean peak current was 24 +/- 2.3 and 33 +/- 5.0 A, respectively (p = 0.0001). Therefore, for equivalent first shock success rates, the energy-based method delivered 67% more energy and 38% more current than the current-based method. High transthoracic impedance (greater than or equal to 90 omega) predicted first shock failure only in patients undergoing defibrillation by the energy-based method (p = 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of electrical defibrillation of the heart

This paper discusses the development of theoretical models of heart defibrillation by a bipolar impulse. These are based on theoretical mechanisms as well as our results obtained in a series of fibrillation/defibrillation experiments carried out in anesthetized mongrel dogs. The procedures for fibrillation and defibrillation have been previously described. During the experiments, blood pressure, ECG, and ventricular wall contraction strength are continuously recorded. The methods of recording have been previously described.

Transvenous catheter ablation of extranodal accessory pathways

Twelve patients with an accessory pathway and recurrent symptomatic reciprocating tachycardia or atrial fibrillation, or both, underwent attempted transvenous catheter ablation of the accessory pathway. In one patient with a small right coronary artery, the pathway was along the right free wall. In 11 patients, the pathway was located at or within 15 mm of the coronary sinus os. For these patients, a quadripolar electrode catheter was placed in the coronary sinus and positioned, if possible, so that the proximal pair of electrodes straddled the pathway. For those patients with a pathway greater than 5 mm within the coronary sinus, the most proximal electrode was placed at the os. This proximal pair of electrodes was connected to the cathodal output of a defibrillator with an anterior chest wall patch serving as the current sink. Two shocks were then delivered for a cumulative energy of 500 to 600 J (stored energy). Among the eight patients with a pathway at or within 5 mm of the coronary sinus os, conduction over the pathway was abolished in five and modified in one. Among the four patients with a pathway farther from the os (10 to 15 mm) and along the right free wall, pathway conduction was modified only in two. Rupture of the coronary sinus did not occur in any patient. There were no serious complications. Minor damage surrounding the area of ablation was seen at the time of surgical division of the accessory pathway in two of five patients with unsuccessful ablation who subsequently underwent surgery.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of catheter-delivered electrical discharges near the tricuspid anulus in dogs

The possibility of using electrical discharges to ablate right free wall accessory pathways by delivering a series of catheter shocks near the tricuspid anulus was assessed in a canine model. Before the shock, the amplitudes of the atrial and ventricular electrograms recorded from the distal electrodes were compared (A/V ratio), and the atrial pacing threshold was determined. To assess effects on function and arrhythmogenicity, right heart pressures were measured and programmed ventricular stimulation was performed before the shock and prior to sacrifice 7 to 10 days after the shock. Nine dogs received a total of 24 discharges at varying energies (50 to 400 J). Nonsustained ventricular tachycardia occurred with 13 shocks (62%) and transient atrioventricular block with 9 shocks (43%). There was no worsening in cardiac or valvular function as determined by right heart pressure measurements or right ventriculography. Programmed ventricular stimulation performed before the shocks and repeated before sacrifice failed to induce ventricular arrhythmias. The endocardial lesion produced by the shock was roughly circular and its area correlated with both the magnitude of the shock as well as the atrial pacing threshold. Transmural necrosis always occurred at the anulus when the A/V ratio was between 1.00 and 1.50 and preshock atrial pacing threshold suggested adequate wall contact (less than 1.5 mA). There was mild inflammation of the adventitia of the right coronary artery near two discharge sites (both 200 J) and inflammation of the media near one discharge site (400 J); no intimal involvement was seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Histologic changes and arrhythmogenicity after discharge through transseptal catheter electrode

Ventricular tachycardia commonly arises within the intraventricular septum and successful catheter ablation of septal tachycardia might be enhanced by transseptal electrode placement. We have evaluated the safety of a transseptal ablation procedure. Arrhythmogenicity and histology were examined after high-energy capacitor discharges were delivered to an intracavitary cathode-anode pair placed on opposite sides of the interventricular septum in pentobarbital-anesthetized dogs. After two discharges of 200 or 100 J proved lethal, paired discharges of 30 or 50 J (10 dogs) or a single discharge of 100 J (four dogs) was used to induce 28 lesions. Acute rhythm changes and risk of induction of ventricular tachycardia by programmed stimulation were measures of arrhythmogenicity. Gross and histologic examination of the hearts after 20 min to 28 days allowed characterization of the evolution of lesions. The conduction system in nearby and remote locations was extensively examined in four dogs. Refractory ventricular fibrillation developed with paired shocks at 200 or 100 J. At lower energy levels, acute ventricular fibrillation occurred with 12 of 20 shocks (60%), but defibrillation was consistently achieved. After ablation, no dog had ventricular tachycardia or fibrillation induced with programmed stimulation. Matching anodal and cathodal lesions spanned the septum without perforation in 10 of 16 dogs, and the lesions were of similar histology. Each contained central areas of hemorrhage surrounded by a region of coagulation necrosis merging with normal myocytes peripherally. There was necrosis and edema without inflammation at 20 min, acute inflammatory cell infiltration at 1 to 2 days, and myocyte replacement by granulation tissue after 6 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Catheter ablation for patients with cardiac arrhythmias

Catheter ablative techniques have recently been introduced as therapy for patients with drug-resistant cardiac arrhythmias. These techniques include delivery of high-energy direct-current shocks in order to electrocoagulate the atrioventricular junction for those with supraventricular arrhythmias. This technique, if successful, results in arrhythmia control but induces chronic pacemaker dependency. The evidence, to date, suggests that 90% of these patients are improved after attempted atrioventricular junctional ablation. Catheter ablative procedures have been used for attempted ablation of accessory pathways and has been shown to be especially effective for those with posteroseptal accessory pathways. More recently, techniques for ablation of ventricular tachycardia foci have been introduced but experience with this procedure is too limited to allow for definitive recommendations.

Effects of varying electrode configuration with catheter-mediated defibrillator pulses at the coronary sinus orifice in dogs

We compared two methods of delivering single damped sine-wave defibrillator pulses to the coronary sinus orifice in 20 dogs. Ten dogs had "unipolar" (coronary sinus to precordial disc) and 10 had "bipolar" (coronary sinus proximal to coronary sinus distal electrode) discharges. Delivered voltage, current, and energy were recorded during each pulse. Electrophysiologic testing was done before and 4 weeks after the procedure. Histologic examination of the atrioventricular groove was done at 1 mm serial sections. For the unipolar configuration a 200 J defibrillator pulse resulted in a peak voltage of 3370 +/- 125 V, a peak current of 21 +/- 4 A, and a delivered energy of 253 +/- 29 J as compared with 3010 +/- 99 V, 70 +/- 4 A, and 144 +/- 18 J, respectively, for the bipolar configuration (p less than .001). Three dogs (two with bipolar, one with unipolar pulses) had gross coronary sinus rupture and died from acute pericardial tamponade. In addition, irrespective of electrode configuration, all dogs showed microscopic rupture of the coronary sinus internal elastic membrane. Transmural atrial scarring occurred in all 10 dogs that received a unipolar pulse but in only two dogs that received a bipolar pulse (p = .0004). Unlike the atrium, injury to the left ventricle was limited in both groups. Similarly, injury to the periannular myocardium was inconsistent and not transmural in either group. No significant electrophysiologic changes were observed. With the present technique, unipolar rather than bipolar catheter-mediated defibrillator pulses result in transmural atrial injury that might prevent accessory pathway conduction. Regardless of electrode configuration, high-energy defibrillator pulses consistently cause some degree of coronary sinus rupture, most likely related to a barotraumatic mechanism.

Investigation of the possibilities of cardiac defibrillation by ultrasound

In 15 experiments on dogs ultrasound with a frequency of 500 kHz and an intensity of 10 W/cm2, exerted a defibrillatory and antiarrhythmic effect and stopped ventricular fibrillation in 28% of the animals. Electrophysiological studies on isolated right ventricles of rabbits in oxygenated chambers demonstrated that the antiarrhythmic action of the ultrasound acted by prolonging the refractory period of the myocardial cells. Ultrasound with an intensity of 4 W/cm2 and 30 s duration increased the refractory period up to 37%. Any further increase of intensity and duration of the ultrasound action resulted in a shortening of the refractory period and inhibition of the electrical activity of the myocardial cells. Investigations of the harmful effects of ultrasound with a frequency of 500 kHz and an intensity of 10 W/cm2 showed that ultrasound for up to 2 min did not cause evident histological changes.