Effects of flecainide on defibrillation thresholds in the anesthetized dog

The effects of flecainide on defibrillation thresholds in 21 open chest, anesthetized dogs were studied. Defibrillation was accomplished using nontruncated exponential pulses delivered through two epicardial patches. Multiple shocks of varying energy were administered after 10 s of ventricular fibrillation in random order. The percent success was plotted against the energy delivered for each dog. A sigmoidal curve was fit to the data and the energy associated with 50% success (E50) calculated. Flecainide (n = 16) or saline solution (n = 5) was then infused and E50 again determined. Flecainide infusion produced mean (+/- standard error of the mean) plasma levels of 610 +/- 111 ng/ml. Defibrillation thresholds were obtainable in 10 of 16 dogs that received flecainide infusion. Flecainide infusion increased E50 by 75% (from 6.5 +/- 1.9 to 11.4 +/- 2.6 J) (P less than 0.05). Infusion of saline solution did not significantly affect defibrillation energy. Of 16 dogs that received flecainide infusion, 12 had one or more complications: 6 had ventricular fibrillation resistant to defibrillation, 6 developed severe hypotension after successful defibrillation and 5 had spontaneous ventricular fibrillation after successful defibrillation. These effects were not seen in any control dogs. Flecainide infusion significantly increases defibrillation threshold and has important adverse arrhythmic and hemodynamic effects in this experimental preparation.     

A prospective, randomized evaluation of effect of ventricular fibrillation duration on defibrillation thresholds in humans

The effect of ventricular fibrillation duration in humans on defibrillation efficacy as it pertains to the time of intervention of an automatic implantable defibrillator is unknown. If a difference in defibrillation efficacy exists in the early period after ventricular fibrillation onset, it may affect algorithms used by antiarrhythmic devices for arrhythmia detection and therapy. Therefore, a prospective, randomized evaluation was performed of the effect of ventricular fibrillation durations of 10 s and 20 s on defibrillation thresholds in 10 survivors of sudden cardiac arrest undergoing implantation of an automatic cardioverter defibrillator. The initial duration of ventricular fibrillation was chosen randomly. Subsequently, each patient served as his or her own control for the alternate duration of ventricular fibrillation to that chosen initially. The mean leading edge defibrillation threshold voltage was 411 +/- 114 V when ventricular fibrillation persisted for 10 s and 419 +/- 125 V when it persisted for 20 s (p = 0.73). The mean defibrillation threshold current was 11.4 +/- 2.8 A when ventricular fibrillation persisted for 10 s and 11.4 +/- 3.2 A when it persisted for 20 s (p = 0.97). The delivered energy defibrillation threshold was 11.5 +/- 5.9 J when ventricular fibrillation persisted for 10 s and 12.0 +/- 6.9 J when it persisted for 20 s (p = 0.67). These findings show that the defibrillation threshold does not change between 10 and 20 s of ventricular fibrillation in out-of-hospital survivors of cardiac arrest at the time of surgical implantation of an automatic defibrillator. The data may have influence on the programming of defibrillator detection algorithms.     

Optimization of transthoracic ventricular defibrillation-biphasic and triphasic shocks,waveform rounding,and synchronized shock delivery

The aim of this study is to optimize the truncated exponential waveform for transthoracic ventricular defibrillation. Discharge of a capacitor gives a fast-rising waveform with a spike; rounding of the waveform slows the rate of rise and removes the spike. Defibrillation thresholds for electrically induced VF were determined for rounded and conventional biphasic and triphasic waveforms (apex-anterior paddles; 130 microF capacitor; 3-10 ms phase duration), and for the Lown waveform in 29 anesthetized pigs. Rounding of the leading edge of the biphasic waveform reduced the threshold voltage and current for defibrillation at 3 + 3 ms and 6 + 6 ms phase duration, relative to the conventional unrounded biphasic or the Lown waveforms. The threshold delivered energy was lower for rounded truncated exponential biphasic shocks at 3 + 3 ms (55.3 +/- 2.5 J) than at 6 + 6 ms (67.6 +/- 2.9 J; reduction 15.9 +/- 3.8%; P <.001; n = 29) phase duration. Triphasic shocks (total duration 6-12 ms) showed no advantages over biphasic shocks in this model. The rounded waveform (6 + 6 ms phase duration) had a reduced delivered energy at threshold (9%) with transthoracic shock delivery synchronized to peak (71.1 +/- 4.2 J) or trough (71.5 +/- 4.9 J) of the high amplitude body surface electrocardiogram signal in ventricular fibrillation, compared with unsynchronized shocks (78.7 +/- 4.7 J; P <.05). In this study a biphasic, rounded waveform of total duration 6 or 12 ms, was optimal for the correction of electrically-induced ventricular fibrillation. Synchronization to the peak or trough of the high amplitude electrocardiogram signal gave a further reduction in the energy to defibrillate.     

Prospective comparison of sequential pulse and single pulse defibrillation with use of two different clinically available systems

Sixteen out-of-hospital survivors of ventricular fibrillation underwent a prospective, randomized, intraoperative comparison of sequential pulse and single pulse defibrillation with use of two distinct electrode systems and waveform shapes currently available for clinical use. Defibrillation was tested alternately with either the single pulse or the sequential pulse system 10 s into an episode of ventricular fibrillation. Sequential pulse defibrillation was performed with two 4 ms truncated exponential pulses of constant duration delivered to three equally spaced oval epicardial patch electrodes composed of concentric coils. The posterior left ventricular electrode served as the common cathode. The first anode was over the anterior right ventricle and the second anode was over the anterior left ventricle. Single pulse defibrillation was performed with the standard intracardiac defibrillation system with use of a single truncated exponential pulse with a fixed 65% tilt delivered across two rectangular, wire mesh epicardial patch electrodes positioned over the anterior right ventricle and posterolateral left ventricle. During defibrillation threshold determination, voltage and current waveforms were recorded and used to determine pulsing resistance and delivered and stored energy. Average defibrillation threshold leading edge voltage for the single pulse technique was 273 +/- 101 V compared with 246 +/- 67 V (11% less) for the sequential pulse technique (p = 0.136). Defibrillation threshold leading edge current for the single pulse technique was 6.7 +/- 2.5 A compared with 5.2 +/- 1.7 A (29% less) for the sequential pulse method (p = 0.005). The defibrillation threshold delivered energy was 5.6 +/- 4.0 J for the single pulse technique and 3.5 +/- 1.8 J (38% less) for the sequential pulse technique (p = 0.021).(ABSTRACT TRUNCATED AT 250 WORDS)     

Increasing fibrillation duration enhances relative asymmetrical biphasic versus monophasic defibrillator waveform efficacy

Biphasic waveforms reduce defibrillation threshold compared with corresponding monophasic waveforms. However, effects of fibrillation duration on relative efficacy of monophasic and biphasic waveforms are unknown. This study used a newly developed defibrillation model, the isolated right- and left-sided working rabbit heart, with epicardial defibrillation electrodes, to compare threshold for a monophasic waveform (5 msec rectangular) and an asymmetrical biphasic waveform (5 msec each pulse, V2 = 50% V1). Mean voltage defibrillation threshold (V50) was determined from sigmoidal probability of successful defibrillation versus shock intensity curves after 5, 15, and 30 seconds of fibrillation in a paired study with 10 hearts. Results showed that biphasic waveforms had significantly lower voltage and energy thresholds at all fibrillation durations and that their relative efficacy improved with increasing fibrillation duration. Biphasic voltage threshold was 38.2 +/- 2.2, 44.7 +/- 4.8, and 46.6 +/- 3.2 V after 5, 15, and 30 seconds of fibrillation compared with monophasic thresholds of 51.7 +/- 4.4 (p less than 0.002), 63.0 +/- 7.6 (p less than 0.05), and 72.1 +/- 3.9 V (p less than 0.005). Biphasic waveform energy threshold was 0.67 that for the monophasic waveform after 5 seconds of fibrillation (0.12 +/- 0.01 versus 0.18 +/- 0.03 J, p less than 0.05). The ratio between biphasic waveform threshold and monophasic waveform threshold (B/M) decreased to 0.62 at 15 seconds. At 30 seconds, B/M was 0.52 (0.17 +/- 0.02 versus 0.33 +/- 0.04 J, p less than 0.02). This study also showed that biphasic waveform threshold was a nonlinear function of monophasic waveform threshold so that improved biphasic defibrillator waveform efficacy was greatest for hearts having higher monophasic thresholds.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in pacing threshold and R wave amplitude after transvenous catheter countershock

Transvenous electrode catheter countershock in patients with recurrent ventricular tachyarrhythmias may be followed by transient bradycardia and require temporary pacing with a catheter. The serial changes in R wave amplitude and stimulation threshold after catheter countershock in 11 halothane-anesthetized open chest dogs ranging in weight from 11.8 to 24 kg were studied. Ventricular fibrillation was electrically induced and followed by catheter defibrillation using nonsynchronized trapezoidal waveform (65% tilt) current discharge in incremental doses (5 to 50 J). Significant decreases in bipolar R wave amplitude (8.3 +/- 1 versus 2 +/- 0.2 mV, p less than 0.001) and increases in stimulation threshold (1 +/- 0.1 versus 2.3 +/- 0.4 V, p less than 0.001) were observed using the countershock catheter 15 seconds after countershock; these changes persisted for up to 10 minutes. To determine whether these changes were localized to the defibrillating catheter and whether they were species-specific, a second electrode catheter was positioned in the right ventricle distant from the countershock catheter in five pigs. Increases in stimulation threshold were observed only at the countershock catheter, suggesting that changes were secondary to local changes at the catheter-myocardium interface. No significant change in R wave amplitude or stimulation threshold was observed at the countershock catheter in three pigs given transthoracic shocks (60 to 250 J). It is concluded that current discharge through the countershock catheter results in a significant temporary reduction in R wave amplitude and an increase in pacing threshold. This may make pacing through the countershock catheter unreliable after shock delivery.     

Atrial defibrillation threshold in humans minutes after atrial fibrillation induction; "A stitch in time saves nine".

AIMS: To assess the effects of atrial fibrillation duration on the defibrillation threshold in atrial fibrillation patients seconds or minutes after initiation of the arrhythmia. METHODS AND RESULTS: Nineteen patients with recurrent symptomatic atrial fibrillation were evaluated. After programmed induction of atrial fibrillation, the defibrillation threshold was assessed after two sequential periods of arrhythmia in the same patient: an "ultrashort" period of 30 s duration and a "short" period, which lasted 10 min. After the specified period, internal cardioversion was attempted using a balloon-guided catheter that allows the delivery of biphasic shocks between one electrode array placed in the left pulmonary artery and a proximal electrode array on the lateral right atrial wall. The defibrillation threshold was assessed with energy steps of 0.5 J with a starting level of 0.5 J. Mean time from induction to successful defibrillation was 92+/-30 s after the "ultrashort" period of atrial fibrillation and 910+/-86 s after the short period. The defibrillation threshold was significantly greater after 10 min of atrial fibrillation than after 30 s of arrhythmia (2.32+/-0.61 J vs 1.31+/-0.66 J, P<0.001). Clinical data were not found to affect the defibrillation threshold. CONCLUSIONS: Prolongation of atrial fibrillation over minutes in patients with paroxysmal arrhythmia increases the energy requirements for successful defibrillation.     

Ischemic ventricular fibrillation: The importance of being spontaneous

Although the energy level required to defibrillate normal myocardium is low and constant, as determined from studies of induced ventricular fibrillation, little is known of the specific energy requirements in regionally ischemic hearts for spontaneous or induced ventricular fibrillation. In this study the lowest energy threshold for defibrillation was determined in 10 open chest dogs with reversible 10 minute coronary occlusions at various sites for each of 44 events of ventricular fibrillation, using apical and superior vena caval electrodes with a generator providing variable output of 1 to 30 watt seconds. The ischemic mass, quantitated from postmortem angiographic and planimetric data, was 52 ± 9 percent (mean ± standard deviation) of the left ventricle in dogs with induced ventricular fibrillation (Group I), 52 ± 12 percent in dogs with spontaneous ventricular fibrillation after occlusion (Group II) and 54 ± 9 percent in dogs with spontaneous ventricular fibrillation after reperfusion (Group III). Defibrillation thresholds in watt seconds were 9 ± 7 in Group I (n = 12), 19 ± 10 in Group II (n = 13) and 18 ± 10 in Group III (n = 19). (Group I versus Groups II and III, probability [p]<0.025). In nonischemic hearts, the defibrillation threshold was 3 ± 2 (n = 32) (p <0.001 compared with values in Group I, II or III). Thus, despite similar masses of ischemia, twice as much energy was required for defibrillation of spontaneous ventricular fibrillation (whether after occlusion or reperfusion) as for induced ventricular fibrillation, suggesting that these conditions are caused by different metabolic or pathologic derangements. Such differences should be considered in assessing interventions such as drug therapy designed to inhibit the repetitive ventricular response and in design of implantable defibrillators.     

Relationship of left ventricular mass to defibrillation threshold for the implantable defibrillator: a combined clinical and animal study

Defibrillation results when a critical mass of myocardium is depolarized. The relationship between echocardiographic determinations of left ventricular mass, volume, and cavity radius to wall thickness ratio and defibrillation threshold for the implantable defibrillator was examined. Ten patients with two large patch defibrillating lead systems were studied. Defibrillation threshold was determined intraoperatively as the lowest energy terminating ventricular fibrillation. Left ventricular mass, volume, and radius/posterior wall thickness ratio were calculated from two-dimensional echocardiograms. A significant correlation was found between left ventricular mass and defibrillation threshold (r = 0.78, p less than 0.01). The correlations between defibrillation threshold and left ventricular volume (r = 0.59) and radius/wall thickness ratio (r = 0.55) were not significant. Subsequently, 11 dogs undergoing defibrillation trials with a transvenous catheter and a chest wall patch were studied. Defibrillation threshold was defined as the lowest energy-terminating ventricular fibrillation (four separate attempts). Subsequently, the heart was dissected, and the left ventricle (including the septum) was weighed. The correlation between left ventricular weight and defibrillation threshold (r = 0.76) was significant (p less than 0.01). We conclude that noninvasive assessment of left ventricular mass and direct measurement of left ventricular weight are significantly correlated with defibrillation threshold and consistent with the critical mass hypothesis.     

Intraoperative comparison of sequential-pulse and single-pulse defibrillation in candidates for automatic implantable defibrillators

Sixteen survivors of cardiac arrest underwent intraoperative comparison of the effectiveness of sequential-pulse and single-pulse defibrillation. Defibrillation was tested alternately with the single-pulse or sequential-pulse technique 10 seconds into an episode of ventricular fibrillation that was induced with alternating current. The sequential-pulse defibrillation technique using truncated exponential pulses was performed with a right ventricular endocardial catheter and a left ventricular epicardial patch electrode. The first pulse was delivered between the right ventricular apical and the superior vena caval electrode on the right ventricular endocardial catheter. The second pulse was delivered between the right ventricular apical electrode and the left ventricular patch electrode 0.2 ms after termination of the first pulse. Single-pulse defibrillation was performed with a standard intracardiac defibrillation system in which a single truncated exponential pulse was delivered across 2 epicardial patch electrodes positioned over the anterolateral right ventricle and the posterolateral left ventricle. During defibrillation threshold determination, voltage and current waveforms were recorded and integrated to determine delivered energy. Average defibrillation threshold leading-edge voltage for the sequential pulse technique was 496 +/- 140 V, compared with 365 +/- 157 V for the single-pulse technique (p less than 0.005). Defibrillation threshold leading-edge current for the sequential-pulse technique was 6.0 +/- 2.3 A, compared with 10.6 +/- 5.1 A for the single-pulse method (p less than 0.0005).(ABSTRACT TRUNCATED AT 250 WORDS)     

Prospective Evaluation of the Effect of Biphasic Waveform Defibrillation on Ventricular Pacing Thresholds

Effect of Defibrillation on Pacing Thresholds. Introduction : Significant increases in ventricular pacing threshold have been observed following monophasic waveform ventricular defibrillation shocks. High-output pacing is recommended to ensure consistent capture, particularly in pacemaker-dependent patients who are likely to be defibrillated. Whether biphasic waveform defibrillation compounds this problem is not known. The purpose of this prospective study was to examine serial changes in ventricular pacing thresholds following single, multiple, low- and high-energy biphasic defibrillation sbocks from an implanted defibrillator.
Methods and Results : Bipolar pacing thresholds before and after defibrillation, and the adequacy of pacing capture at three times preshock threshold in the immediate aftermath of ventricular defibrillation, were prospectively evaluated in 67 consecutively tested recipients of a biphasic implanted cardioverter defibrillator. Overall, serial pacing thresholds following successful defibrillation were completely unchanged after 141 of 177 (80%) ventricular fibrillation inductions. In no case did the threshold pulse width increment > 0.06 msec from its baseline value after shock, nor did pacing at a pulse width of three times preshock threshold from dedicated bipolar pacing electrodes fail to result in successful ventricular capture. Changes in threshold were not related to when measured from the time of shock, defibrillation energy, number of shocks, electrode system, chronicity of leads, shock orientation, or to clinical factors.
Conclusions : No clinically important changes in pacing threshold were observed after biphasic waveform defibrillation. Bradycardia pacing at conventional pacemaker outputs of three times baseline pulse width threshold from bipolar electrodes dedicated exclusively to pacing or sensing (but not defibrillation) consistently allowed for an adequate safety margin following defibrillation.     

Transesophageal low-energy synchronous cardioversion of atrial flutter/fibrillation in the dog.

The purpose of this study was to determine the feasibility and efficacy of terminating atrial flutter/fibrillation using low-energy synchronous shocks delivered through a transesophageal catheter in dogs with talc-induced pericarditis. Atrial flutter/fibrillation was induced by employing the pulse train method. The minimum effective cardioversion energy level was compared for three different methods--method A, delivery between a distal esophageal electrode and a proximal esophageal electrode; method B, delivery of shocks through a distal esophageal electrode and a plate placed on the chest; method C, transthoracic cardioversion. The minimum effective cardioversion energy level did not differ significantly between methods A and B (1.30 +/- 0.46 joules versus 1.29 +/- 0.35 joules). Transesophageal cardioversion decreased the defibrillation threshold three- to fourfold from that of conventional transthoracic cardioversion. There were no complications of heart block, ventricular fibrillation, or any pathologic evidence of esophageal injury. Thus transesophageal low-energy synchronous cardioversion is considered a feasible and effective method for the treatment of atrial flutter/fibrillation.     

Effects of pacing rate and timing of defibrillation shock on the relation between the defibrillation threshold and the upper limit of vulnerability in open chest dogs

To test the relation between the defibrillation threshold and the upper limit of vulnerability, the shock strength associated with 50% probability of successful defibrillation (DFT50) and that associated with 50% probability of reaching the upper limit of vulnerability (ULV50) were determined in 20 open chest dogs with use of the delayed up-down method, with pacing drive cycle lengths of 150 to 500 ms and either single 6-ms shocks (10 dogs) or 12-ms biphasic shocks (10 dogs) given at the mid-upslope, peak and mid-downslope of the T wave of electrocardiographic lead II. The shocks were given by means of a patch-patch configuration on the anterior and posterior surfaces of the heart, which was paced from a stimulating electrode attached to the left ventricular apex. Analysis of variance showed no statistically significant differences in ULV50 as determined with different pacing cycle lengths. For monophasic shocks, DFT50 (331 +/- 66 V or 5.8 +/- 2.7 J) was not significantly different from ULV50 determined at the mid-upslope of the T wave (318 +/- 64 V or 5 +/- 2 J). The correlation coefficients between the two values were 0.74 (p = 0.014) for voltage and 0.67 (p = 0.034) for energy. In contrast, DFT50 was significantly higher than ULV50 as determined at the peak of the T wave (219 +/- 43 V or 2.3 +/- 1 J) and mid-downslope of the T wave (200 +/- 38 V or 1.9 +/- 0.9 J). In three dogs, ventricular fibrillation could not be induced at the mid-downslope of the T wave with any baseline pacing (Si) cycle length.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reentry site during fibrillation induction in relation to defibrillation efficacy for different shock waveforms

INTRODUCTION: Unsuccessful defibrillation shocks may reinitiate fibrillation by causing postshock reentry. METHODS AND RESULTS: To better understand why some waveforms are more efficacious for defibrillation, reentry was induced in six dogs with 1-, 2-, 4-, 8-, and 16-msec monophasic and 1/1- (both phases 1 msec) 2/2-, 4/4-, and 8/8-msec biphasic shocks. Reentry was initiated by 141+/-15 V shocks delivered from a defibrillator with a 150-microF capacitance during the vulnerable period of paced rhythm (183+/-12 msec after the last pacing stimulus). The shock potential gradient field was orthogonal to the dispersion of refractoriness. Activation was mapped with 121 electrodes covering 4 x 4 cm of the right ventricular epicardium, and potential gradient and degree of recovery of excitability were estimated at the sites of reentry. Defibrillation thresholds (DFTs) were estimated by an up-down protocol for the same nine waveforms in eight dogs internally and in nine other dogs externally. DFT voltages for the different waveforms were positively correlated with the magnitude of shock potential gradient and negatively correlated with the recovery interval at the site at which reentry was induced by the waveform during paced rhythm for both internal (DFT = 1719 + 64.5VV - 11.1RI; R2 = 0.93) and external defibrillation (DFT = 3445 + 150VV - 22RI; R2 = 0.93). CONCLUSION: The defibrillation waveforms with the lowest DFTs were those that induced reentry at sites of low shock potential gradient, indicating efficacious stimulation of myocardium. Additionally, the site of reentry induced by waveforms with the lowest DFTs was in myocardium that was more highly recovered just before the shock, perhaps because this high degree of recovery seldom occurs during defibrillation due to the rapid activation rate during fibrillation.     

A prospective randomized evaluation of biphasic versus monophasic waveform pulses on defibrillation efficacy in humans

Biphasic waveforms have been suggested as a superior waveform for ventricular defibrillation. To test this premise, a prospective randomized intraoperative evaluation of defibrillation efficacy of monophasic and biphasic waveform pulses was performed in 22 survivors of out of hospital ventricular fibrillation who were undergoing cardiac surgery for implantation of an automatic defibrillator. The initial waveform used in a patient for defibrillation testing, either monophasic or biphasic, was randomly selected. Subsequently, each patient served as his or her own control for defibrillation testing of the other waveform. The defibrillation threshold was defined as the lowest pulse amplitude that would effectively terminate ventricular fibrillation with a single discharge delivered 10 s after initiation of an episode of ventricular fibrillation induced with alternating current. Each defibrillation pulse was recorded oscilloscopically, and defibrillation pulse voltage, current, resistance and stored energy were measured. Fifteen (68%) of the 22 patients had a lower defibrillation threshold with the biphasic pulse, 3 (14%) had a lower threshold with the monophasic pulse and 4 (18%) had equal defibrillation thresholds (within 1.0 J) regardless of waveform. The mean leading edge defibrillation threshold voltage was 317 +/- 105 V when the monophasic pulse was used and 267 +/- 102 V (16% less) when the biphasic pulse was used (p = 0.008). Mean leading edge defibrillation threshold current was 7.9 +/- 3.7 A when the monophasic pulse was used and 6.8 +/- 3.8 A (14% less) when the biphasic pulse was used (p = 0.051).(ABSTRACT TRUNCATED AT 250 WORDS)     

Low energy synchronous transcatheter cardioversion of atrial flutter/fibrillation in the dog

The feasibility and effectiveness of low energy synchronous transcatheter cardioversion of atrial flutter and fibrillation were examined in dogs with talc-induced pericarditis. A conventional electrode catheter was positioned transvenously in the right atrial appendage. Atrial flutter/fibrillation was induced by using the train pulse method, and the tachyarrhythmia-inducing threshold was determined. The minimal effective cardioversion energy levels were compared in three different cardioversion methods: method A = delivery of shock between the proximal electrode (cathode) and the backplate (anode), method B = delivery between the proximal electrode (cathode) and the distal electrode (anode) and method C = conventional external cardioversion. In both methods A and B, all 149 cardioversion attempts were successful with shocks of less than or equal to 5 J. Shocks of less than or equal to 1 J resulted in successful cardioversion in 57 (70%) of 81 attempts, 50 (74%) of 68 attempts and 5 (12%) of 41 attempts with methods A, B and C, respectively. The mean minimal effective cardioversion energy levels were not significantly different between methods A and B (0.62 +/- 0.67 versus 0.58 +/- 0.71 J). Transcatheter cardioversion decreased the defibrillation threshold 3- to 75-fold (mean 6- to 7-fold) from that of transthoracic cardioversion. The defibrillation threshold was not influenced by the inducibility of atrial flutter/fibrillation. There were no complications of heart block, ventricular fibrillation or pathologic evidence of severe shock-induced atrial injury. Thus, low energy synchronous transcatheter cardioversion of atrial flutter/fibrillation is considered feasible and effective. This technique may also be useful in managing the atrial flutter/fibrillation that can occur during electrophysiologic studies.     

Relation of the intraoperative defibrillation threshold to successful postoperative defibrillation with an automatic implantable cardioverter defibrillator

To determine the relation between the intraoperative defibrillation threshold and successful postoperative termination of induced ventricular fibrillation (VF) with the automatic implantable cardioverter defibrillator (AICD), 33 patients who underwent AICD implantation were studied. The defibrillation threshold, determined after at least 10 seconds of VF, was 5 J in 2, 10 J in 6, 15 J in 10, 20 J in 10 and 25 J in 5 patients. The AICD energy rating on the first discharge was 28 +/- 1.8 J. Defibrillation of induced VF was demonstrated postoperatively in 29 of 33 (88%) patients. The AICD terminated VF postoperatively in all 18 patients with a defibrillation threshold less than or equal to 15 J. Only 11 of the 15 (73%) patients with a defibrillation threshold greater than or equal to 20 J (p less than 0.04) had VF terminated postoperatively. In all 4 patients in whom the AICD failed to terminate induced VF, the energy difference between the AICD rating and the defibrillation threshold was less than or equal to 10 J. Among the 14 patients with a difference of less than or equal to 10 J between the AICD energy rating and the defibrillation threshold, there were no significant differences between the 4 patients with and the 10 without successful VF termination with respect to the duration of VF induced postoperatively or the AICD lead system. In summary, failure to terminate VF with the AICD is not uncommon (27%) when the defibrillation threshold approaches the energy delivering capacity of the AICD.(ABSTRACT TRUNCATED AT 250 WORDS)     

Transesophageal low-energy cardioversion in an animal model of life-threatening tachyarrhythmias

The purpose of this study was to determine the feasibility and efficacy of terminating life-threatening ventricular tachyarrhythmia by low-energy synchronous or asynchronous shocks delivered through a transesophageal catheter that had both an anode and a cathode. Forty-three episodes of ventricular fibrillation or flutter (Vf or VF) were provoked by transesophageal asynchronous random shocks occurring during the vulnerable period of the ventricular cycle in seven dogs and seven pigs that were healthy adults. The 43 episodes of Vf or VF were terminated by the transesophageal technique. The defibrillation energy thresholds were 23.11 +/- 6.28 J (range, 5-30 J). Seven episodes of ventricular tachycardia (VT) with a cycle length of 360 msec or less (330 +/- 27 msec) were provoked by ventricular pacing stimuli during acute myocardial ischemia resulting from delayed resuscitation in two dogs and three pigs. Five of the seven VTs had a duration of 31 seconds or more, and they were all terminated by transesophageal synchronous shocks, the cardioversion thresholds being 1.71 +/- 2.25 J (range, 0.25-5 J). Fourteen episodes of idioventricular tachycardia (IVT) with a cycle length of 400 msec or more (445 +/- 33.5 msec) spontaneously occurred after the use of adrenaline and after defibrillation in four dogs and five pigs. We also succeeded in terminating seven episodes of IVT with a duration of 34 seconds or more by the same means of treating VT, although IVT is not an indication for cardioversion in the clinical setting.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of lidocaine on relation between defibrillation threshold and upper limit of vulnerability in open-chest dogs.

BACKGROUND. The purpose of the present study was to test the effects of lidocaine on the relation between the defibrillation threshold and the upper limit of vulnerability. METHODS AND RESULTS. The shock strength associated with a 50% probability of successful defibrillation (DFT50) and the shock strength associated with a 50% probability of reaching the upper limit of vulnerability (ULV50) were determined in 11 open-chest dogs by using the delayed up-down method before and during lidocaine (seven dogs) or normal saline (four dogs) infusion. The ventricles were paced at a cycle length of 300 msec. Shocks of various strengths were then given via a patch-patch electrode configuration on the anterior and posterior surfaces of the ventricle to determine the ULV50. Once ventricular fibrillation was induced, shocks were given 15-20 seconds later via the same electrode configuration to determine the DFT50. Lidocaine infusion resulted in a serum level of 15 +/- 4 micrograms/ml. This was associated with a lengthening of the QT interval but not with the widening of the QRS complex. In all dogs, both the ULV50 and the DFT50 increased significantly when tested during lidocaine infusion. Mean ULV50 during lidocaine infusion was 496 +/- 70 V or 13.1 +/- 4.3 J, which were significantly higher than the baseline values of 333 +/- 67 V or 5.3 +/- 2.2 J (p less than 0.001 for both voltage and energy). Mean DFT50 during lidocaine infusion was 407 +/- 41 V or 8.7 +/- 1.7 J, which were significantly higher than the baseline values of 300 +/- 38 V and 4.4 +/- 1.1 J (p = 0.004 for voltage and p = 0.013 for energy). The r values between the ULV50 and the DFT50 were 0.79 (p = 0.037) for voltage and 0.80 (p = 0.030) for energy at baseline and 0.85 (p = 0.016) for voltage and 0.88 (p = 0.009) for energy during the lidocaine infusion. However, the increments of the ULV50 (163 +/- 88 V or 7.8 +/- 4.6 J) were significantly greater than the increments of the DFT50 (107 +/- 51 V or 4.4 +/- 1.9 J, p = 0.035 for voltage and p = 0.023 for energy). Normal saline infusion did not alter DFT50 or ULV50. CONCLUSIONS. Lidocaine infusion significantly increases both ULV50 and DFT50. These results are compatible with the upper limit of vulnerability hypothesis of defibrillation. However, the greater increase of the upper limit of vulnerability than the defibrillation threshold with lidocaine infusion indicates that other factors may also need to be considered to explain the results.     

Defibrillator electrode-chest wall coupling agents: influence on transthoracic impedance and shock success

The purpose of this study was to determine if the difference in transthoracic impedance produced by different coupling agents affects the success of shocks for defibrillation. Three different coupling agents, Harco pads (Hewlett-Packard), Littman pads (3M) and Redux paste (Hewlett-Packard), were assessed in 10 anesthetized dogs in which ventricular fibrillation was induced by electrical stimulation of the right ventricle. Defibrillation was attempted 15 seconds later, using 50, 100 and 150 joules (selected energy). Actual delivered energy, current, impedance and the percent of the shocks that achieved defibrillation were determined for the three coupling agents. Redux paste gave significantly lower impedance and higher current than the two disposable performed coupling pads tested. Despite this, there were no significant differences in shock success among the three coupling agents. Thus, in this experimental model, over a three-fold energy range, disposable coupling pads were as effective as electrode paste for defibrillation despite the slightly higher impedance of the disposable pads.