Effect of Increased Parasympathetic and Sympathetic Tone on Internal Atrial Defibrillation Thresholds in Humans

Although changes in autonomic tone affect ventricular defibrillation, little is known about the effect of increased parasympathetic or sympathetic tone on the atrial defbrillation threshold. Methods: To evaluate the effect of reflexly increased parasympathetic and increase α- and β-adrenergic tone on the atrial defibrillation threshold (ADFT), atrial fibrillation was induced in 14 patients. ADFTs, right atrial refractory period (RARP), and monophasic action potential duration (MAPD) were determined before and after autonomic intervention. ADFTs were determined with a step-up protocol using 3/3-ms biphasic shocks delivered through decapolar catheters in the right atrial appendage and coronary sinus. Two groups were studied. Group I (N = 8) had ADFTs determined at baseline, after receiving phenylephrine (PE), and with PE plus atropine (A). Group 2 (N = 6) had ADFTs determined at baseline and after receiving isoproterenol (ISO). Results: Group I: PE significantly increased sinus cycle length (SR-CL) compared to baseline (742 ± 123 to 922 ± 233 ms) without significantly changing RARP, MAPD, or ADFT (2.3 ± 1.3 J vs 2.3 ± 0.8 J). With PE + A, SR-CL significantly decreased (529 ± 100 ms vs 742 ± 123 ms) and MAPD shortened (231 ± 41 ms vs 279 ± 49 ms) without altering RARP or ADFT (1.94 ± 0.9 J vs 2.25 ± 1.25 J). Group 2: ISO decreased SR-CL (486 ± 77 ms vs 755 ± 184 ms) and MAPD (169 ± 37 ms vs 226 + 58 ms) but not RARP or ADFT (2.25 ± 1.21 J vs 2.33 ± 1.75 J). Conclusions: Increasing parasympathetic, α-, or β-adrenergic tone does not affect the ADFT despite causing significant electrophysiological changes in the atria.     

Internal Atrial Defibrillation During Electrophysiological Studies and Focal Atrial Fibrillation Ablation Procedures

Induction of sustained AF during electrophysiological studies requires electrical cardioversion to restore sinus rhythm for continuation of the electrophysiological study and mapping procedure. The study included 104 consecutive patients (age 59 +/- 12 years, 74 men), who were in stable sinus rhythm at the beginning of the electrophysiological study, underwent internal atrial defibrillation (IAD) of AF (> 15 minutes) that was induced during electrophysiological study. In 21 patients, AF was regarded to be the clinical problem (group I), and in the remaining 83 patients other arrhythmias represented the primary target of the electrophysiological study (group II). A 7.5 Fr cardioversion catheter (EP Medical) equipped with a distal array was used and placed in the left pulmonary artery and a proximal array of the same size was located along the lateral right atrial wall. All patients were successfully cardioverted with a mean energy of 6.2 +/- 4.0 1. In 18 (78%) of 21 group I patients and in 12 (14%) of 81 group II patients, AF recurred 3.7 +/- 3.4 and 2.4 +/- 1.4 times during electrophysiological study, respectively. The IAD shock did not suppress focal activity, thus the mapping of atrial foci responsible for AF could be continued even after several IADs. No IAD related complications occurred during the study. In conclusion, (1) IAD can be safely and successfully performed during electrophysiological study without using narcotic drugs or high electric energies; (2) IAD does not suppress focal activity; and (3) even if AF recurs frequently during the electrophysiological study, IAD can be performed several times without significant time delay.     

Rapid Atrial Pacing Does Not Decrease the Atrial Defibrillation Threshold

The aim of the study was to evaluate the effect of preshock atrial pacing on the atrial defibrillation threshold (DFT) during internal cardioversion of AF. The implantable atrial defibrillator has been added to the therapeutic options for patients with recurrent episodes of persistent AF. Although the device is efficient in restoring sinus rhythm, patient discomfort is a limitation. Methods that lower the ADFT are needed. Eleven patients with AF underwent internal cardioversion. In a randomized, crossover design, ADFT testing was performed, applying a step-up protocol starting at 100 V. Rapid atrial pacing was performed with a right atrial catheter for 20 seconds at 90% of the average cycle length of the fibrillatory waves and was immediately followed by a biphasic defibrillation shock. At each energy level, pacing + shock was compared to shock only, until the level at which sinus rhythm was restored by both modes. The step-up protocol was thereafter repeated using the inverse sequence of the two modes. A total of 19 ADFTs were obtained. For 10 the ADFT was lower with pacing + shock, in 4 equal and in 5 higher, than with shock only. The ADFT (mean ± SD) with pacing + shock was   260 ± 84   V   (3.4 ± 2.9 J)   and did not differ from shock only:   268 ± 85   V   (3.8 ± 3.0 J) (P > 0.05)   . The coefficient of variation and the coefficient of reproducibility for pacing + shock was 16% and 60 V, respectively, and for shock only 17% and 61 V. Rapid atrial pacing did not influence the internal ADFT in AF. The randomized, crossover protocol used was reproducible between different modes, and seems useful when testing the impact of different interventions on the ADFT. (PACE 2003; 26[Pt. I]:1461–1466)     

Spontaneous Reinitiation of Atrial Fibrillation Following Transvenous Atrial Defibrillation

Spontaneous reinitiation of atrial fibrillation (AF) has not been systematically looked at in patients undergoing transvenous AF. This study involved 11 patients, the mean age 60 ± 8 years. 3 male and 8 female, in whom transvenous atrial defibrillation successfully converted AF to sinus rhythm. Eight patients had paroxysmal AF and three patients had chronic persistent AF for 4 weeks or more. Four patients were taking antiarrhythmic medications at the time of testing. Multipolar transvenous catheters were positioned inside the coronary sinus, right atrium, and the right ventricle. Atrial defibrillation testing was performed using the METRIX atrial defibrillation system in nine patients and the Ventritex HVSO2 in the remaining two patients. A total of 64 therapeutic shocks (range 3–11) were delivered in the 11 patients, and 31 of these successfully converted AF to sinus rhythm. In four patients spontaneous AF was reinitiated following 12 successful transvenous atrial defibrillation episodes. The mean time to reinitiation of AF following shock delivery and restoration of sinus rhythm was 8.26 ± 5.25 seconds, range 1.8–19.9 seconds. All 12 episodes of spontaneous AF were preceded by a spontaneous premature atrial complex. The coupling interval of the premature atrial complexes was 443 ± 43 ms, range 390–510 ms. None of the patients taking antiarrhythmic medications or those demonstrating no premature atrial complexes had spontaneous reinitiation of AF. In conclusion, spontaneous reinitiation of AF can occur in a significant proportion of patients with AE undergoing transvenous atrial defibrillation. This phenomenon is preceded by the occurrence of atrial premature complex. Findings of this study may have significant clinical implications.(PACE 1998; 21:1105–1110)     

A Comparison of Transvenous Atrial Defibrillation of Acute and Chronic Atrial Fibrillation and the Effect of Intravenous Sotalol on Human Atrial Defibrillation Threshold

The comparative efficacy and safety of transvenous defibrillation for acute and chronic AF and the effect of antiarrhythmic agents on this therapy have not been evaluated. Transvenous atrial defibrillation was performed in 25 patients with chronic AF and 13 patients with acute AF by delivering R wave synchronized, biphasic shocks between the right atrium and coronary sinus. The lowest energy and voltage resulting in successful defibrillation were considered to be atrial defibrillation threshold (ADFT). Intravenous sotalol (1.5 mg/kg) was thengiven over 15 minutes and ADFT was determined again. The mean ADFT was 1.5 /and 3.6 J for acute and chronic AF, respectively, and the threshold was highly reproducible. Sotaloi reduced ADFT in patients with acute AF while the reduction in chronic AF group was not significant. There was no significant increase in creatinine kinase nor reduction in blood pressure, but prolonged pause after successful defibrillation required ventricular supporting pacing. We conclude that transvenous atrial defibrillation is a safe and effective means for defibrillating both acute and chronic AF. ADFT was lower in acute AF than in chronic AF. ADFT was highly reproducible during repeated defibrillation. Sotalol reduced ADFT in acute AF and to a lesser extent in chronic AF, and increased the defibrillation success rate. Ventricular pacing will often be required because of prolonged pause after successful defibrillation.     

Effect of Transvenous Atrioventricular Nodal Ablation on the Function of Implanted Pacemakers

We report on seven patients with implanted pacemakers who underwent transvenous ablation of the atrioventricular junction using direct current shocks of 200 to 350 joules. Pacemaker impulse duration and rate were unaffected, but one rate responsive (TX) pacemaker was reprogrammed by a 300-joule shock. Transient increases in stimulation threshold did occur in two patients, and exit block for 2-15 seconds developed on four occasions. Chronic stimulation thresholds were unaffected. We conclude that it is preferable to carry out ablation before pacemaker implantation, but it is possible to perform transvenous ablation of the atrioventricular junction without damage to an implanted pacemaker; however, a transient rise in stimulation threshold or even exit block may occur, and pacemaker function should be carefully assessed after the procedure.     

The Effects of Atrial Electrical Remodeling on Atrial Defibrillation Thresholds

EVERETT, T.H. et al. : The Effects of Atrial Electrical Remodeling on Atrial Defibrillation Thresholds. Electrical remodeling of atrial fibrillation may account for the increase in atrial defibrillation thresholds over time. The aim of this study was to examine the time course of electrical remodeling and the benefit of early defibrillation on the defibrillation threshold. Twenty‐six mongrel dogs weighing 27.6 ± 3.3 kg were induced into AF by repeated high output burst atrial pacing. Eight dogs were paced for multiple time periods of 5, 20, 40, and 60 minutes. Five dogs each had burst pacing for 4 hours and 8 hours, and eight dogs were paced at a high rate (640 beats/min ) for 48 hours. Biphasic atrial defibrillation shocks with a pulse width of 3/3 ms synchronized to the left apical electrogram were delivered to coil electrode catheters positioned in the lateral left and right atria. Defibrillation voltage was increased from 50 V in 20‐ to 30‐V steps until defibrillation was successful. As the pacing period increased, a decrease in atrial fibrillation cycle lengths and atrial effective refractory period was not observed before 8 hours. Similarly, the defibrillation threshold did not change significantly until the 8‐hour pacing period was reached. The defibrillation thresholds were 69 ± 28 V for 5 minutes, 64 ± 20 V for 20 minutes , 99 ± 85 V for 40 minutes , 78 ± 51 V for 60 minutes , 78 ± 38 V for 4 hours , 124 ± 33 V for 8 hours , and 133 ± 32 V for 48 hours (mean ± SD ) (P < 0.05 ). Atrial electrical remodeling in a rapid atrial pacing canine model is not observed until after 4 hours of burst atrail pacing. The atrial defibrillation threshold increases with increasing duration of burst atrial pacing, and follows a similar time course to other parameters of electrical remodeling.     

Diagnosis of Paroxysmal Atrial Fibrillation in Patients with Implanted Pacemakers: Relationship to Symptoms and Other Variables

Background: Atrial fibrillation (AF) is not always accompanied by clear-cut symptoms and symptoms suggestive of AF may not correspond to a genuine AF episode. The study prospectively evaluated the burden of asymptomatic AF episodes in pacemaker patients (for sick sinus syndrome) with a history of documented paroxysmal AF.
Methods: Consecutive patients were enrolled and implanted with dual-chamber pacemakers equipped with diagnostic features for AF monitoring. Each patient was instructed about typical AF symptoms and was asked to keep a detailed log of symptoms. Stored pacemaker data were analyzed using only AF episodes >30 s.
Results: The mean follow-up was 16 ± 6 months and 102 patients were enrolled (73 ± 7 years, 59 M). Thirteen patients (13%) dropped out with the development of permanent AF and their data were discarded. Twenty-three patients (26%) without device-stored AF episodes all reported at least one annotated AF episode. There were 1,245 device-stored AF episodes in 66 (74%) out of 89 patients. Patients reported 1,141 episodes of AF-related symptoms. Only 240 (21%) corresponded to a genuine device-stored AF event. The sensitivity and positive predictive value of symptoms to detect AF were respectively 19% and 21%. Episode duration, rate increase at the onset of the arrhythmia, heart disease, or antiarrhythmic drug therapy showed no statistically significant differences comparing symptomatic and asymptomatic episodes.
Conclusions: Many pacemaker patients with paroxysmal AF can develop AF-like symptoms in the absence of device-stored AF. AF-related symptoms have low sensitivity and low positive predictive value in patients with permanent pacemakers.     

Long-Term Internal Cardiac Defibrillation Threshold Stability

The automatic implantable cardioverter-defibrillator is tested intraoperatively with defibrillation trials to ensure effectiveness. It is unknown if the energy requirement for internal defibrillation remains stable and that once demonstrated effective, if the device will continue to be effective in terminating lethal ventricular arrhythmias. In this study, the defibrillation energy requirement was compared in 56 patients at the time of lead implantation to that obtained at the time of generator replacement. Mean time to generator replacement was 17. +/- 6.6 months. The defibrillation threshold was stable over that time (11.9 +/- 6.7 joules compared to 12.7 +/- 8.4 joules, NS). There was no relation between transmyocardial impedance and defibrillation threshold. In addition, no effect on defibrillation threshold was demonstrated by the use of various cardiac medications, concomitant surgery or the occurrence of clinical shocks during follow-up.     

The Effect of Multiple Shocks on Canine Cardiac Defibrillation

FLAKER, G., ET AL.: The Effect of Multiple Shocks on Canine Cardiac Defibrillation. To determine if multiple shocks adversely affect the success of later shocks compared with early shocks, we analyzed the success rates of initial shocks (defibrillation attempts 1–5), first half shocks (defibrillation attempts 1–20) and second half shocks [defibrillation attempts 21–40) in a canine model. Epicardial patches were placed on the right and left ventricle in 28 dogs. Ventricular fibrillation was induced by a 60-Hz shock. After 30 seconds, defibrillation was attempted using 7, 12, 13, or 18 joules with either a uniphasic or biphasic rectangular waveform. The uniphasic waveform was 5 msec in duration; the biphasic waveform was 10 msec, with the lagging 5-msec pulse one-half the amplitude of the leading 5-msec pulse. For uniphasic shocks, the right ventricular patch was positive; for biphasic shocks, the right ventricular patch was positive during the leading 5 msec of the shock and negative during the lagging milliseconds. A total of 960 fibrillation episodes were evaluated; no dog was involved in more than 40 fibrillation episodes. The success rates of defibrillation attempts 1–5, defibrillation attempts 1–20, and defibrillation attempts 21–40 were similar at 12, 13, and 18 joules. This information supports the continued use of up to 40 fibrillation trials in canine cardiac defibrillation. However, at 7 joules defibrillation attempts 21–40 were more successful than defibrillation attempts 1–5, and 1–20. With our methodology, these data are consistent with the hypothesis that low energy shocks create a "sensitizing" effect on cardiac tissue, allowing more successful defibrillation with repeated shocks.     

Waveform Optimization for Internal Atrial Defibrillation: Effects of Waveform Rounding, Phase Duration, and Voltage Swing

KIDWAI, B.J., et al. : Waveform Optimization for Internal Atrial Defibrillation: Effects of Waveform Rounding, Phase Duration, and Voltage Swing. The aim of this study was to compare the efficacy of internal atrial defibrillation by conventional truncated exponential biphasic waveforms with and without waveform rounding (1–2 phases) and to determine optimal duration for this novel double rounded waveform. Atrial fibrillation, induced by rapid electrical stimulation, was converted by internal shocks through defibrillation catheters (lateral right atrium and coronary sinus) in anesthetised sheep. Rounding the leading edges of the conventional biphasic waveform (Ventritex HVS‐02; settings 100/–50 V, 150/–70 V , and 200/–100 V; n = 8 ) reduced delivered peak and trough voltages, currents, and energy (by ≥ 21%, P < 0.001 ; for double (both phases) rounded) without decreasing cardioversion success. At 100/–50 V the efficacy of single (first phase) rounded (53 ± 13%; mean ± SEM ) and double rounded (59 ± 11% ) shocks was similar to the conventional waveform (56 ± 14% ). Double rounded waveform (phase durations 1–20 ms) efficacy was optimum at 6–10 ms phase duration (100% success at 10–ms phase duration; 1.52 ± 0.04 J delivered energy; n = 6 ). Successful cardioversion by conventional, single rounded, and double rounded biphasic waveforms (duration 6 ms each phase), conventional monophasic, rounded monophasic (duration 12 ms), and a damped sine waveform correlated strongly with peak‐to‐trough voltage swing within the waveform (r = 0.882; P < 0.01; n = 8 ). For internal atrial defibrillation, rounding both phases of the conventional biphasic waveforms, the double rounded waveform, permits similar efficacy to the conventional truncated exponential biphasic waveform at reduced peak voltage, current, and delivered energy. Optimum phase duration is 6–10 ms (tested range 1–20 ms).     

The Effects of Reverse Atrial Electrical Remodeling on Atrial Defibrillation Thresholds

MANGRUM, J.M., et al. : The Effects of Reverse Atrial Electrical Remodeling on Atrial Defibrillation Thresholds. The implantable atrial defibrillator is a developing therapeutic option for paroxysmal atrial fibrillation, but shock related discomfort continues to be a limiting factor. To further characterize successful defibrillation, the relationship between reverse atrial electrical remodeling and internal atrial defibrillation thresholds in canines with chronic atrial fibrillation was examined. Testing was performed in 21 dogs. Chronic atrial fibrillation was induced in eight dogs by creating moderate mitral regurgitation and rapidly pacing the right atrium for ≥ 6 weeks. The atrial fibrillation cycle length, atrial effective refractory period, refractory period dispersion, and internal atrial defibrillation thresholds were determined after establishment of chronic atrial fibrillation after 4 hours of sinus rhythm postcardioversion and 7 days of sinus rhythm postcardioversion. These measurements were then compared to a normal population of 13 dogs. The atrial defibrillation thresholds were 6.6 J (1.9–10.1 J) initially, 2.9 J (1.5–3.7 J) after 4 hours of sinus rhythm, and 0.9 J (0.4 – 1.3 J) after 7 days of sinus rhythm (  P = 0.04  ). This decrease was associated inversely with the atrial effective refractory period (  P < 0.03  ), and atrial fibrillation cycle length (  P < 0.05  ), and with a decrease in atrial refractory period dispersion after 7 days of sinus rhythm (  P = 0.04  ). These electrophysiological measurements reached normal population levels by 7 days. Atrial defibrillation thresholds decrease as atrial reverse electrical remodeling occurs and this reduction corresponds to increased atrial fibrillation cycle length, increased atrial refractoriness, and decreased refractory period dispersion.     

Significant Reduction of Atrial Defibrillation Threshold and Inducibility by Catheter Ablation of Atrial Fibrillation

Background: Radiofrequency catheter ablation (RFCA) of atrial fibrillation (AF) has antiarrhythmic effects by multiple mechanisms. We hypothesized that RFCA curtails atrial defibrillation threshold (A‐DFT) and postablation induction pacing cycle length (iPCL), making critical mass reduction one potential mechanism by which antiarrhythmic effect is achieved. Methods: We included 289 patients with AF (male 77.9%, 55.7 ± 10.8 years old; 197 paroxysmal AF: 92 persistent AF) who underwent RFCA. A‐DFT (serial internal cardioversion 2, 3, 5, 7, and 10 J) and iPCL (serial 10 mA 10‐second atrial pacing with pacing cycle length 250, 200, 190, 180, 170, 160, and 150 ms) were evaluated before and after RFCA. Results: (1) RFCA of AF reduced the A‐DFT from 6.7 ± 3.7 J to 3.0 ± 3.0 J (P < 0.001). (2) AF ablation reduced AF inducibility from 95.4% before the procedure to 56.3% after the procedure (P < 0.001), and the iPCL from 194.8 ± 32.6 to 160.9 ± 26.2 ms (P < 0.001). (3) In patients who underwent a greater number of ablation lines, the post‐RFCA A‐DFT (P < 0.001) was lower, and %ΔA‐DFT (P = 0.003) and proportion of atrial tachycardia (P = 0.022) were higher than those with a lower number of ablation lines. Conclusion: AF ablation significantly reduced A‐DFT, AF inducibility, and iPCL, and the degree of their reduction was related to the number of ablation lines. (PACE 2012;35:1428–1435)     

Transvenous Single Lead Atrial Defibrillation: Efficacy and Risk of Ventricular Fibrillation in an Ischemic Canine Model

Transvenous atrial defibrillation with multiple atrial lead systems has been shown to be effective in models without the potential for ventricular arrhythmias. The specific aim of this study was to evaluate the efficacy and safety of transvenous single lead atrial defibrillation in a canine model of ischemia cardiomyopathy. Ten dogs had ischemia cardiomyopathy induced by repeated intracoronary micmsphere injections. The mean LV ejection fraction decreased from 71%± 9% to 38%± 14% (P = 0.003). Spontaneous atrial fibrillation (AF) developed in four dogs, and in six AF was induced electrically. Atrial defibrillation thresholds (ADFTs) were determined with synchronous low energy shocks using a transvenous tripolar lead with two defibrillation coils (right ventricle, superior vena cava) and an integrated sensing lead (RV coil vs electrode tip). The ADFTs derived by logistic regression were compared at 50% and 90% probability of success (ED₅₀, ED₉₀): ED₅₀ was 2.4 ±1.7 J and 2.9 ±2.1 J, respectively, for 5- and 10-ms monophasic shocks, and 1.8 ± 0.9 J and 2.1 ± 1.3 J, respectively, for 5- and 10-ms biphasic shocks. Immediately after 3 of 2,179 (0.1%) synchronized shocks, ventricular fibrillation (VF) developed. VF was induced in 3 of 1,062 (0.3%) shocks with integrated sensing (RV coil vs electrode tip) compared to 0 of 1,117 shocks when a separate bipolar RV sensing electrode was used for synchronization. In our canine model of ischemic cardiomyopathy, low energy atrial defibrillation via a transvenous single lead system was highly effective. However, there was a small but definite risk of VF induction, which seemed to be greater when an integrated as opposed to a true bipolar RV sensing was used.     

The Effect of Biphasic Waveform Tilt in Transvenous Atrial Defibrillation

Atrial defibrillation can be accomplished using low energy shocks and transvenous catheters. The biphasic waveform tilt required to achieve optimal atrial defibrillation thresholds (ADFTs) is, however, not known. The effect of single capacitor biphasic waveform tilt modification on ADFT was assessed in 20 patients. Following AF induction the defibrillation pulses were delivered between the catheters positioned in the coronary sinus and the right atrium.
The single capacitor biphasic waveform shocks, delivered over the same pathways, consisted of 65% tilt (65/65 biphasic waveform) to produce an overall tilt of 88%, or 50% tilt (50/50 biphasic waveform) to produce an overall tilt of 75%. Although 65/65 biphasic waveform delivers more energy, the shorter duration 50/50 biphasic waveform reduced stored energy ADFT 21%, from 1.34 ± 0.82 J with 65/65 biphasic to 2.06 ± 0.81 J. These differences were not statistically significant. Nine patients had lower ADFT with 50/50 biphasic waveform while five patients had lower ADFT with 65/65 biphasic waveform. Equivalent reduction in ADFT was seen in the remaining six patients. The ADFT was 0.83 ± 0.65 J when both tilts were considered. In conclusion, biphasic waveform tilt modification may affect the ADFT in an individual patient. The optimal biphasic waveform for ADFT is not known.     

Effects of Encainide on the Function of Implanted Pacemakers

The effect of encainide on chronic pacing thresholds was evaluated in 10 patients, age 64-89, who were exposed to weekly increased encainide dosing (25 mg TID, 50 mg TID, 75 mg TID). Median pacing threshold (mujoules) increased modestly at each period compared to placebo and returned rapidly to baseline after discontinuation. (table; see text) No patient experienced a pacing-related clinical event. One patient had a large threshold increase (566%), but no failure to capture on 24-hour ECG, and one patient whose threshold increased minimally had clinically insignificant capture failure, the longest event being 3 beats. No other failure to capture was noted. Thus, encainide, like a number of other antiarrhythmic drugs, appears to affect pacing thresholds. At the highest dose of 225 mg/day (75 mg TID, which exceeds the generally recommended dose of 50 mg TID), but not at lower doses, some patients may experience loss of capture that does not appear related to the change in threshold energy required. Increases in the duration of the paced QRS induced by encainide did not predict increases in threshold. Therefore, when higher doses of encainide are required in patients with pacemakers, clinical observation and ambulatory electrocardiographic monitoring should be carried out.     

Preimplant Atrial Defibrillation Testing with a Temporary Catheter in Sheep

Prior to implantation of an atrial defibrillator, its effectiveness should be tested in each patient. A new catheter design for temporary use with electrodes for atrial defibrillation, electrogram sensing, and pacing was tested in this study. Atrial defibrillation thresholds defined using this temporary catheter were compared to the ones defined by catheters intended for chronic use with an implantable atrial defibrillator. Atrial defibrillation threshold was determined in six sheep using both types of catheters. Each animal was subjected to studies on 2 consecutive days. On the first day, shocks were applied between two of the temporary catheters. On the following day, permanent leads were inserted and atrial defibrillation threshold was redetermined. In both cases, defibrillation electrodes were positioned in the same heart location with one electrode in the distal coronary sinus and the second electrode in the right atrium. Atrial defibrillation threshold was obtained using 10 V increments or decrements to determine the lowest shock intensity needed to defibrillate the atria. Threshold was defined as the shock intensity at which 20 shock percent success was at or between 15 % and 85%. Statistical analysis showed no significant difference (P < 0.05) between atrial defibrillation threshold energy (0.53 J vs 0.55 J), voltage (122 V vs 120 V) or current (2.2 A vs 2.6 A) measured with the temporary catheters and the permanent leads, respectively. These data indicate that temporary catheters can be used for efficacy testing prior to implant of an atrial defibrillator, and that they predict atrial defibrillation threshold adequately for chronic leads.     

Internal Cardiac Defibrillation Threshold: Effects of Acute Ischemia

The influence of myocardial ischemia on defibrillation success was studied using two different lead orientations in halothane-anesthetized pigs. Ischemia was induced by ligating the left anterior descending artery in its distal third. Controls had loosely tied ligatures placed around the artery at the same site. Ventricular fibrillation was induced by electrical stimulation 30 minutes after coronary artery ligation. Defibrillation used a single truncated pulse of approximately 6 ms duration passed to either: a transvenous electrode catheter (Medtronic, 6880) with the cathode in the apex of the right ventricle and the anode in the superior vena cava-atrial junction region, or the cathode in the apex of the right ventricle and a mesh plaque on the epicardium of the basal lateral left ventricle as anode. Ten seconds after the onset of ventricular fibrillation, defibrillation was attempted with increasing incremental energies until defibrillation was achieved. Fibrillation episodes were repeated at 15-minute intervals until the minimum first shock was successful in defibrillating the animal (i.e., defibrillation threshold). The number of animals successfully defibrillated with a minimum energy above or below 30 J was not different between normal and ischemic animals for either electrode configuration (i.e., 3 out of 20 vs 1 out of 13 for the catheter and 5 out of 6 vs 6 out of 7 for the epicardial plaque, respectively). Also, the cumulative percent success as a function of defibrillation energy was similar in both the normal and ischemic groups. There was a significant reduction in the minimum energy necessary for defibrillation when passing current between the right ventricular apex and the left ventricular epicardial plaque. The present results indicate that, despite differences in lead orientations, acute ischemia in the anesthetized pig does not appear to influence defibrillation success.     

Hemodynamic Effects and Clinical Determinants of Defibrillation Threshold for Transvenous Atrial Defibrillation Using Biatrial Biphasic Shocks in Patients with Chronic Atrial Fibrillation

We assessed the relationship between the hemodynamic changes and shock intensity in transvenous atrial defibrillation for chronic AF. The correlation between the clinical profile and atrial DFT and the factors predicting maintenance of SR after successful defibrillation were also investigated. Atrial defibrillation using entirely transvenoas leads has been investigated as an alternative means of managing patients with AF. However, the hemodynamic consequence of this technique and the clinical factors predicting defibrillation efficacy have not been evaluated. Thirty-seven patients with chronic AF (4 weeks to 60 months) underwent transvenoas atrial defibrillation. Defibrillation was performed by delivering R wave synchronized, biphasic (3/3 ms) shocks with step-up voltages (20–400 V) between defibrillation catheters in the anterolateral right atrium and the distal coronary sinus. Clinical profile of the patients, the DFT, arterial blood pressure, and RH interval during defibrillation and the 6-month recurrence rate were determined. SR was restored in 33 (89%) of 37 patients and the DFT was 3.7 ± 1.4 J (317 ± 58 V). Transvenous atrial defibrillation resulted in a mild reduction in blood pressure (6 ± 10 mmHg), but substantial prolongation of longest postshock RR intervals (507 ± 546 ms), which were significantly related to the shock intensity (r = 0.5, P < 0.001). There was no ventricular proarrhythmia. The patients'age, body weight, duration of AF, left atrial diameter, and ejection fraction were not related to the success of defibrillation, not the 6-month maintenance rate of SR (39%). However, the patients'age was related to DFT. Apart from transient reduction in blood pressure and shock related pauses that may require backup pacing, transvenous biatrial defibrillation was a highly effective and well-tolerated technique. The absence of clinical determinant for successful defibrillation suggests that restoring SR by transvenous atrial defibrillation could be attempted in most patients with chronic AF.     

Low Energy Internal Atrial Cardioversion in Atrial Fibrillation Lasting More Than a Year

The aim of this study was to evaluate the efficacy of low energy internal atrial cardioversion in restoring sinus rhythm (SR) in patients with chronic atrial fibrillation (AF) persisting > 1 year. Fifteen patients with chronic AF lasting > 1 year (from 13–48 months, mean 24 ± 13 months) were studied. R wave synchronized 3/3 ms biphasic shocks were delivered between right atrial and coronary sinus (left pulmonary artery in five patients) electrodes. Sedatives or anesthetics were administered only at the patient's request. Results: Stable SR was restored in 14 (93%)of 15 patients after shocks with a mean leading edge voltage of 377 ± 77 V (range 260–500) and a mean delivered energy of 7.3 ± 3.4 J (range 2.6–12.9). The procedure was performed without anesthesia in 6 (40%) patients. All successfully cardioverted patients were treated with flecainide, sotalol, or amiodarone. During a follow up of 7.7 ± 7.9 months (range 1–24) AF recurred in five (36%) patients. Three of five AF recurrences occurred within 3 days after conversion to SR. Conclusion: Internal low energy atrial cardioversion is highly effective in restoring SR even in patients with AF lasting > 1 year. The long-term results from the standpoint of freedom from AF recurrences, are satisfactory, although additional antiarrhythmic treatment is required, particularly in the first days after conversion.