Upper Limit of Vulnerability Predicts Chronic Defibrillation Threshold for Transvenous Implantable Defibrillators

ULV Predicts Chronic DFT. Introduction: The upper limit of vulnerability (ULV) is the shock strength at or above which ventricular fibrillation cannot be induced when delivered in the vulnerable period. It correlates acutely with the acute defibrillation threshold (DFT) and can be determined with a single episode of fibrillation. The goal of this prospective study was to determine the relationship between the ULV and the chronic DFT.
Methods and Results: We studied 40 patients at, and 3 months after, implantation of transvenous cardioverter defibrillators. The ULV was defined as the weakest biphasic shock that failed to induce fibrillation when delivered 0,20, and 40 msec before the peak of the T wave. Patients were classified as clinically stable or unstable based on prospectively defined criteria. There were no significant differences between the group means for the acute and chronic determinations of ULV (13.5 ± 5.3 J vs 12.4 ± 6.8 J, P = 0.25) and DFT (10.1 ± 5.0 J vs 9.9 ± 5.7 J, P = 0.74). Five patients (15%) were classified as unstable. The strength of the correlation between acute ULV and acute DFT (r = 0.74, P < 0.001) was similar to that between the chronic ULV and chronic DFT (r = 0.82, P < 0.001). There was a correlation between the change in ULV from acute to chronic and the corresponding change in DFT (r = 0.67, P < 0.001). The chronic DFT was less than the acute ULV + 3 J in all 35 stable patients, but it was greater in 2 of 5 unstable patients (P = 0.04).
Conclusions: The strength of the correlation between the chronic ULV and the chronic DFT is comparable to that between the acute ULV and the acute DFT. Temporal changes in the ULV predict temporal changes in the DFT. In clinically stable patients, a defibrillation safety margin of 3 J above the acute ULV proved an adequate chronic safety margin.     

The Ventricular Defibrillation and Upper Limit of Vulnerability Dose-Response Curves

DF and ULV Dose-Response Curves. Introduction: A stimulus delivered in the T wave of a paced cardiac cycle can induce ventricular fibrillation (VF). If the stimulus strength is increased, the probability of inducing VF decreases. This study determines an ideal mathematical model (a dose-response curve) for the relationship between the shock strength and the probability of inducing VF or defibrillating.
Methods and Results : Defibrillating electrodes were implanted in the right ventricle and superior vena cava in 16 pigs. The electrode in the vena cava was electrically connected to a cutaneous patch. The same electrodes were used for both VF induction and defibrillation. T wave stimuli were given at the peak of the T wave according to a modified up-down protocol (40 V up, 20 V down). When a T wave stimulus induced VF, a defibrillation stimulus was delivered 10 seconds later, also according to the modified up-down protocol. Exponential, logistic, log-dose logistic, piecewise linear, and Box-Tiao dose-response curves were fit to the resulting data using the maximum likelihood method. For the defibrillation data, it was found that only the logistic and Box-Tiao curves fit all of the animals (P < 0.05). For VF induction, only the Box-Tiao curve fit all of the animals (P < 0.05). Extrapolating along a dose-response curve that did not fit to a shock strength with a very low probability of inducing VF or a very high probability of defibrillating yielded errors as great as 610 V.
Conclusion : The Box-Tiao dose-response curve is the best single choice for fitting VF induction or defibrillation datasets     

Effect of Shock Waveform on Relationship Between Upper Limit of Vulnerability and Defibrillation Threshold

ULV-DFT Waveform. Introduction: The upper limit of vulnerability (ULV) correlates with the defibrillation threshold (DFT). The ULV can he determined with a single episode of ventricular fibrillation and is more reproducible than the single-point DFT. The critical-point hypothesis of defibrillation predicts that the relation between the ULV and the DFT is independent of shock waveform. The principal goal of this study was to test this prediction. Methods and Results: We studied 45 patients at implants of pectoral cardioverter defibrillators. In the monophasic-biphasic group (n = 15), DFT and ULV were determined for monophasic and biphasic pulses from a 120-μF capacitor. In the 60- to 110-μF group (n = 30), DFT and ULV were compared for a clinically used 110-μF waveform and a novel 60-μF waveform with 70% phase 1 tilt and 7-msec phase 2 duration. In the monophasic-biphasic group, all measures of ULV and DFT were greater for monophasic than biphasic waveforms (P < 0.0001). In the 60- to 110-/tF group, the current and voltage at the ULV and DFT were higher for the 60-μF waveform (P < 0.0001), hut stored energy was lower (ULV 17%, P < 0.0001; DFT 19%, P = 0.03). There was a close correlation between ULV and DFT for both the monophasic-biphasic group (monophasic r²= 0.75, P < 0.001; hiphasic r²= 0.82, P < 0.001) and the 60- to 110-μF group (60 μF r²= 0.81 P < 0.001; 110 μF r²= 0.75, P < 0.001). The ratio of ULV to DFT was not significantly different for monophasic versus biphasic pulses (1.17 ± 0.12 vs 1.14 ± 0.19, P = 0.19) or 60-μF versus 110-μF pulses (1.15 ± 0.16 vs 1.11 ± 0.14, P = 0.82). The slopes of the ULV versus DFT regression lines also were not significantly different (monophasic vs biphasic pulses, P = 0.46; 60-μF vs UO-μF pulses, P = 0.99). The sample sizes required to detect the observed differences between experimental conditions (P < 0.05) were 4 for ULV versus 6 for DFT in the monophasic-biphasic group (95% power) and 11 for ULV versus 31 for DFT in the 60- to 110-μF group (75% power). Conclusion: The relation between ULV and DFT is independent of shock waveform. Fewer patients are required to detect a moderate difference in efficacy of defibrillation waveforms by ULV than by DFT. A small-capacitor biphasic waveform with a long second phase defibrillates with lower stored energy than a clinically used waveform.     

The Effect of Cardiac Compression on Defibrillation Efficacy and the Upper Limit of Vulnerability

Compression Affects Defibrillation and ULV. Introduction: We determined the effects of decreasing the ventricular blood volume and altering cardiac geometry on defibrillation, the upper limit of vulnerability (ULV), and the relationship between them. Methods and Results: In six pigs, fibrillation/defibrillalion trials were performed with a left ventricular apex patch to a superior vena cava catheter electrode configuration and a biphasic waveform. Thirty trials each were performed on a compressed versus noncompressed (normal) heart. Compression was achieved using direct mechanical ventricular actuation. Dose-response curves were constructed, and the 50% probability points (KD50) were compared for leading edge voltage (LEV), leading edge current (LEI), and total energy (TE). In another 12 pigs, triplicate defibrillation thresholds (DFTs) and ULVs were determined for each heart state. The T wave was scanned with shocks in 10-msec steps for determining the ULV. Compression resulted in decreased ED50s for LEV (δ= 138 ± 77 V, P < 0.05, mean ± SD), LEI (A = 1.57 ± 0.7 A, P < 0.05), and TE (δ= 4.9 ± 3.6 J, P < 0.05) compared to normal. In the second study, compression significantly reduced DFT (P < 0.02) and ULV (P < 0.02) for LEV, LEI, and TE compared to normal. The ULV tended to be lower than the DFT for the normal heart state (δ= 23 ± 46 V LEV; P = NS). However, the ULV was significantly greater than the DFT for the compressed heart state (A = 19 ± 25 V LEV; P < 0.03). Conclusions: Shock delivery during cardiac compression improves defibrillation efficacy. Additionally, cardiac compression decreases both DFT and ULV, which supports the ULV hypothesis of defibrillation. Finally, maintaining the heart's geometric and volumetric state during ULV testing in paced rhythm and DFT testing in ventricular fibrillation moves the ULV higher than the DFT—the position predicted by the ULV hypothesis for defibrillation.     

Effect of Ventricular Dilatation on Defibrillation Threshold in the Isolated Perfused Rabbit Heart

Ventricular Dilatation and DFT. Introduction: Ventricular dilatation has Important electrophysiologic effects, but its effect on ventricular defibrillation threshold (DFT) is unknown.
Methods and Results: A fluid-filled, latex balloon was placed in the left ventricular cavity of 19 isolated rabbit hearts. In each experiment, an undilated volume (equivalent to a left ventricular end-diastolic pressure of approximately 0 mmHg) was compared to a dilated volume achieved by adding 1.0 mL of saline (n = 10) or 5% dextrose (n = 9) to the intracavitary balloon. Left ventricular effective refractory period (ERP) and DFT were determined at each volume. Defibrillation was attempted with a monophasic shock delivered between a patch electrode positioned over the posterior left ventricle (cathode) and a metallic aortic cannula (anode). DFT was determined using a modified "down/up" protocol with 10-V steps. Ventricular dilatation increased the left ventricular end-diastolic pressure from 0 ± 0.5 mmHg to 35 ± 3 mmHg (P < 0.001), decreased the average left ventricular ERP 15% (from 116 ± 3 msec to 99 ± 3 msec; P < 0.001), and increased the average DFT 30% (from 96 ± 4 V to 125 ± 7 V; P < 0.001). In one third of experiments, the dilated DFT was ≥ 150% of the DFT at zero volume. The mechanism of the observed increase in DFT is unknown but may be related to the decrease in refractoriness observed with ventricular dilatation.
Conclusion: Acute ventricular dilatation in this model increased DFT an average of 30%, an effect not previously described. This observation may have implications for patients with implantable cardioverter defibrillators.     

An Experimental Study of Transvenous Defibrillation Using a Coronary Sinus Catheter

The efficacy of a transvenous defibrillating system, utilizing bipolar right ventricular and coronary sinus catheters was evaluated in 14 normal mongrel dogs. Two groups of seven animals each were studied. During all shocks, the right ventricular apex electrode served as the anode. In both groups, defibrillation was performed using the proximal pole of the right ventricular catheter (superior vena cava), as the cathode served as a control (configuration A). In group 1, a coronary sinus cathode (configuration B) was compared to control. The mean energy at which 50% or more of the shocks were successful was similar for configuration B (20.7 ± 7.9 joules) and for configuration A (18.8 ± 9.4 joules). In group 2, the superior vena cava and coronary sinus electrodes served as a common cathode (configuration C). Mean defibrillation energy at which 50% or more of the shocks was successful was 21.4 ± 9.0 joules for configuration C and 27.1 ± 9.5 joules for configuration A (P < 0.01). Leading edge voltage was similar for all three configurations, hut shock duration was longer for configuration A (11.3 ± 2.8 msec) than configuration B (6.6 ± 1.8 msec) or C (6.1 ± 1.5; P < 0.05). Nonsustained ventricular tachycardia and transient heart block were common, but no damage to the coronary sinus was noted despite the delivery of up to 38 shocks. Conclusions: (1) With the catheter system used, coronary sinus to right ventricular apex defibrillation system offered no advantages over a superior vena cava to right ventricular apex system; (2) A three-electrode system with the high right atrium and coronary sinus serving as the common cathode reduced defibrillation thresholds significantly without any severe short-term adverse consequences; and (3) Improvements in catheter design may make a coronary sinus catheter part of a feasible transvenous defibrillating system.     

Choosing the Optimal Monophasic and Biphasic Waveforms for Ventricular Defibrillation

Optimal Monophasic and Biphasic Waveforms. Introduction: The truncated exponential waveform from an implantable cardioverter defibrillator can be described by three quantities: the leading edge voltage, the waveform duration, and the waveform time coastant (τ_s). The goal of this work was to develop and test a mathematical model of defibrillation that predicts the optimal durations for monophasic and the first phase of biphasic waveforms for different τ_s values. In 1932, Blair used a parallel resistor-capacitor network as a model of the cell membrane to develop an equation that describes stimulation using square waves. We extended Blair's model of stimulation, using a resistor-capacitor network time constant (τ^m), equal to 2.8 msec, to explicitly account for the waveform shape of a truncated exponential waveform. This extended model predicted that for monophasic waveforms with τ^s of 1.5 msec, leading edge voltage will be constant for waveforms 2 msec and longer; for τ^s of 3 msec, leading edge voltage will be constant for waveforms 3 msec and longer; for τ^s of 6 msec, leading edge voltage will be constant for waveforms 4 msec and longer. We hypothesized that the best phase 1 of a biphasic waveform is the best monophasic waveform. Therefore, the optimal first phase of a biphasic waveform for a given τ^s is the same as the optimal monophasic waveform. Methods and Results: We tested these hypotheses in two animal experiments. Part I: Defibrillation thresholds were determined for monophasic waveforms in eight dogs. For τ^s of 1.5 msec, waveforms were truncated at 1, 1.5, 2, 2.5, 3, 4, 5, and 6 msec. For τ^s of 3 msec, waveforms were truncated at 1, 2, 3, 4, 5, 6, and 8 msec. For τ^s of 6 msec, waveforms were truncated at 2, 3, 4, 5, 6, 8, and 10 msec. For waveforms with τ_s, of 1.5, leading edge voltage was not significantly different for the waveform durations of 1.5 msec and longer. For waveforms witb τ^s of 3 msec, leading edge voltage was not significantly different for waveform durations of 2 msec and longer. For waveforms with τ^s of 6 msec, there was no significant difference in leading edge voltage for the waveforms tested. Part II: Defibrillation thresholds were determined in another eight dogs for the same three τ^s values For each value of τ^s, six biphasic waveforms were tested: 1/1, 2/2, 3/3, 4/4, 5/5, and 6/6 msec. For waveforms with τ^s of 1.5 msec, leading edge voltage was a minimum for the 2/2 msec waveform. For waveforms with τ^s of 3 msec, leading edge voltage was a minimum for the 3/3 msec waveform. For waveforms with τ^s of 6 msec, leading edge voltage was a minimum and not significantly different for the 3/3, 4/4, 5/5, and 6/6 msec waveforms. Conclusions: The model predicts the optimal monophasic duration and the first phase of a biphasic waveform to within 1 msec as τ^s varies from 1.5 to 6 msec: for τ^s equal to 1.5 msec, the optimal monophasic waveform duration and the optimal first phase of a biphasic waveform is 2 msec, for τ_s equal to 3.0 msec, the optimal duration is 3 msec, and for τ^s equal to 6 msec, the optimal duration is 4 msec. For both monophasic and biphasic waveforms, optimal waveform duration shortens as the waveform time constant shortens.     

Comparison of Coronary Venous Defibrillation with Conventional Transvenous Internal Defibrillation in Man

Objective: Animal studies have shown that defibrillation in coronary veins is more effective than in the right ventricle. We aimed to assess the feasibility of placing defibrillation electrodes in the middle cardiac vein (MCV) in man and its impact on defibrillation requirements. Methods: A prospective randomised study conducted in a tertiary referral centre. 10 patients (9 male) undergoing ICD implantation (65 (12) yrs) for NASPE/BPEG indications were studied. Defibrillation thresholds (DFT) were measured, using a binary search and an external defibrillator after 10 seconds of ventricular fibrillation, for the following configurations in each patient (order of testing randomised): RV + MCV Can and RV SVC + Can. Interventions: A dual coil defibrillation electrode was placed transvenously in the right ventricle (RV) in the conventional manner. Using a guiding catheter a 3.2 Fr (67.5 mm length) electrode was placed transvenously in MCV. A test-can was placed subcutaneously in the left pectoral region. Results: Lead placement was possible in 8/10 pts. Time to perform a middle cardiac venogram and place the electrode was 21 (23) mins. No adverse events were observed. Defibrillation current was less (6.7 (2.7) A) with RV + MCV Can compared to the conventional RV SVC + Can configuration (8.9 (3.4) A, p = 0.03). There was no significant difference in defibrillation voltage or energy. However, shock impedance was higher in the former configuration (57 (10) v. 43 (6) , p = 0.001). Conclusions: In the majority of cases placement of a defibrillation lead in MCV is feasible. Defibrillation current requirements are 25% less when the shock is delivered using a MCV electrode.     

A Prospective Evaluation of Two Defibrillation Safety Margin Techniques in Patients with Low Defibrillation Energy Requirements

Low-Energy Defibrillation. Introduction : In patients undergoing defibrillator implantation, an appropriate defibrillation safety margin has been considered to be either 10 J or an energy equal to the defibrillation energy requirement. However, a previous clinical report suggested that a larger safety margin may be required in patients with a low defibrillation energy requirement. Therefore, the purpose of this prospective study was to compare the defibrillation efficacy of the two safety margin techniques in patients with a low defibrillation energy requirement.
Methods and Results : Sixty patients who underwent implantation of a defibrillator and who had a low defibrillation energy requirement (≤ 6 J) underwent six separate inductions of ventricular fibrillation, at least 5 minutes apart. For each of the first three inductions of ventricular fibrillation, the first two shocks were equal to either the defibrillation energy requirement plus 10 J (14.6 ± 1.0 J), or to twice the defibrillation energy requirement (9.9 ± 2.3 J). The alternate technique was used for the subsequent three inductions of ventricular fibrillation. For each induction of ventricular fibrillation, the first shock success rate was 99.5%± 4.3% for shocks using the defibrillation energy requirement plus 10 J, compared to 95.0%± 17.2% for shocks at twice the defibrillation energy requirement (P = 0.02). The charge time (P < 0.0001) and the total duration of ventricular fibrillation (P < 0.0001) were each approximately 1 second longer with the defibrillation energy requirement plus 10 J technique.
Conclusion : This study is the first to compare prospectively the defibrillation efficacy of two defibrillation safety margins. In patients with a defibrillation energy requirement ≤ 6 J, a higher rate of successful defibrillation is achieved with a safety margin of 10 J than with a safety margin equal to the defibrillation energy requirement.     

Serial Defibrillation Threshold Measures in Man: A Prospective Controlled Study

Serial DFT Measures in Man. Introduction; The defibrillation threshold (DFT) may change throughout the first year following implantation of a cardioverter defibrillator, but it remains uncertain if changes are a consequence of changes in clinical condition or are related to fundamental alterations at the electrode-tissue interface. The purpose of this study was to evaluate the extent and time course of DFT changes over the first year following implantable cardioverter defibrillator (ICD) surgery when extraneous clinical and device variables potentially affecting the DFT were excluded. Methods and Results.: We prospectively enrolled 61 patients undergoing epicardial or non-thoracotomy/transvenous ICD therapy into a series of follow-up studies where the DFT was measured at implant and at 1, 6,12, and 52 weeks following implantation in a uniform manner. Stored energy DFT was measured and recorded for all patients. Patient exclusion criteria were: (1) inability to complete all five measures of the DFT; (2) institution of Class I or Class III antiarrhythmic drugs at any time during the study; (3) lead system changes (relocation or new leads) or programming changes in pulse width or current pathway; or (4) development of a significant change in their clinical status, such as decompensated congestive heart failure or acute ischemia. Only 20 of the 61 patients satisfied the criteria required to complete the study. Two of the excluded patients developed high DFTs, which required reprogramming of the current pathway. Eight patients had an epicardial lead system, and 12 had a nonthoracotomy lead system. The rise in DFT over the first 12 weeks was significant for the eight epicardial lead system patients (P = 0.05) and for the 12 nonthoracotomy lead system patients (P = 0.004). The peak rise in DFT occurred at 1 week for the patients with an epicardial lead system (3.4 ± 1.8 J to 7.9 ± 3.8 J) and at 12 weeks for the patients with a transvenous lead system (10.3 ± 5.3 J to 16.1 ± 7.4 J). Conclusions: This study confirms a transient significant rise in the DFT in the first 12 weeks following ICD surgery that partially returns to the implant value over the remainder of the year. Because specific clinical and technical variables were excluded from this study, the observations made in this patient population suggest that the rise in DFT may be a consequence of changes at the electrode-tissue interface.     

Pharmacological Atrial Defibrillation via Temporarily Occluded Coronary Sinus: First clinical experience and implications for an implantable device

The aim of this paper is to report the first experience of pharmacological atrial defibrillation in humans via a temporarily occluded coronary sinus.Patients and methods: In 6 patients (3 women, 3 men; mean age 57.8y, min 31, max 71), with clinical recurrences of atrial fibrillation, an occlusive coronary venogram was carried out in order to establish the origin of the Vein of Marshall. Atrial fibrillation was then induced by atrial pacing in all the patients and after an adequate waiting period to assure that the atrial fibrillation episode was persistent and stable, a bolus of a very low dose of an antiarrhythmic drug was delivered in 3–4 seconds into the temporarily balloon occluded coronary sinus near the orifice of the vein of Marshall. For both the venogram and the pharmacological test a Baim-Turi (USCI-Bard, Billerica MA) or a Vueport (Cardima, Fremont CA) catheter was used.Results and comments: In five patients a single dose of 7mg of propafenone was immediately effective in restoring the sinus rhythm. In the remaining patient 2 doses of 7mg of propafenone failed to interrupt the arrhythmia, which was subsequently interrupted by a bolus of 0.1mg of ibutilide fumarate given after a waiting period of 20 minutes. Retroperfusion of the left atrium could account for these results; in fact the Vein of Marshall has no valvular apparatus in contrast with other coronary sinus tributary veins which are equipped with an uni- or bicuspidal valve.Conclusions: Pharmacological atrial defibrillation with a minimal dose of an antiarrhythmic drug delivered near the orifice of the Vein of Marshall via the temporarily occluded coronary sinus is feasible and effective. This new pharmacological atrial defibrillation can offer interesting opportunities in developing an implantable pharmacological atrial defibrillator.     

Discrimination of Sinus Rhythm, Atrial Flutter, and Atrial Fibrillation Using Bipolar Endocardial Signals

Discrimination of NSR, AFL, and AF. Introduction : Analysis of endocardial signals obtained from an electrode located in the right atrium as realized in newly designed dual chamber, implantable cardioverter defibrillators might be used to provide additional therapeutic options, such as overdrive pacing or low-energy atrial cardioversion for the treatment of concomitant atrial flutter (AFL) or atrial fibrillation (AF). Therefore, we developed a computer algorithm for discrimination of normal sinus rhythm (NSR), AFL, and AF that may lead to adequate differential therapy of atrial tachyarrhythmias in an automated mode.
Methods and Results : During an electrophysiologic study, bipolar endocardial signals from the high right atrium were obtained in 28 patients during sustained AFL or AF and after restoration of NSR. A total of 286 data segments of 5-second duration were recorded (NSR: 96, AFL: 86, AF: 104). Mean atrial cycle length (MCL), standard deviation of mean atrial cycle length (SDCK), and index of irregularity (IR). defined as the ratio between MCL and SDCL, were calculated for each data segment. A cutoff of 315 msec for MCL allowed discrimination of NSR from atrial tachyarrhythmias with 100% sensitivity and specificity. For discrimination of AF from AFI- by using SDCL, a cutoff value of 11.5 msec led to a sensitivity of 99% and a specificity of 90%. Best discrimination of AF from AFL was found for the criterion IR ≥ 7.5%, resulting in a sensitivity of 100% with a specificity of 95% for AF detection.
Conclusion : The investigated algorithm provides discrimination of NSR, AFL, and AF with high sensitivity and specificity. Incorporation of this algorithm in an implantable automated antitachycardia device may lead to adequate differential therapy in patients suffering from spontaneous episodes of AF and AFL.     

Ventricular Fibrillation Resulting from Synchronized Internal Atrial Defibrillation in a Patient with Ventricular Preexcitation

VF After Synchronized Internal Atrial Defibrillation. This case describes ventricular proarrhythmia as a result of a synchronized internal atrial defibrillation shock in a 29-year-old man with Ebstein's anomaly referred for radiofrequency ablation of a right posterior accessory pathway. During the electrophysiologic study, atrial fibrillation was induced and 3/3 msec shocks of various strengths were delivered between two decapolar defibrillation catheters in the coronary sinus and right atrial appendage. A 2.0-J biphasic shock synchronized to an R wave after a short-long-short ventricular cycle length pattern with a preshock coupling interval of 245 msec induced ventricular fibrillation, which was externally defibrillated with 200 J. This observation has implications for the development of implantable atrial defibrillators.     

Early Postoperative Rise in Defibrillation Threshold in Patients with Nonthoracotomy Defibrillation Lead Systems:

DFT Rise with Nonthoracotomy Lead Systems. introduction: In patients with nonthoracotomy defibrillation lead (NTL) systems coupled with monophasic shock waveforms, the defibrillation threshold (DFT) rises early after implantation. There is little information regarding features predictive of the DFT rise, or DFT changes early after implantation of NTL systems coupled with biphasic shock waveforms. Methods and Results: DFT measurements were performed serially at implantation, prior to hospital discharge (mean 4 ± 3 days), and at follow-up (mean 49 ± 22 days) in 146 patients with an NTL system. Factors were assessed for association with a “clinically important” early postimplantation DFT rise, defined as a rise of ≥ 2 energy steps (2 to 4 J per step; ≥ 5 J total). A clinically important early postimplantation DFT rise occurred in 48 patients (33%). Univariate predictors of the rise included the monophasic shock waveform, the Medtronic Transvene lead system, the presence of a subcutaneous defibrillation patch, and the number of shocks delivered during the implantation procedure. However, the only independent predictor of a clinically important DFT rise was the monophasic shock waveform (F = 18, P < 0.001). For the monophasic patient group (n = 79), the incidence of a DFT rise was 53% (n = 42). For the biphasic patient group (n = 67), the incidence of a DFT rise was 9% (n = 6). The clinical characteristics of the monophasic and biphasic groups were not significantly different, nor were their DFTs at implantation. Among a subgroup of 18 consecutive patients who underwent serial DFT testing utilizing both monophasic and biphasic waveforms, the incidence of a clinically important DFT rise with monophasic (n = 9, 50%) was higher than with biphasic shocks (n = 3, 17%; P = 0.05). Conclusion: NTL systems coupled with biphasic shock waveforms have an attenuated incidence of a clinically important DFT rise early after implantation, relative to patients with NTL systems coupled to monophasic waveforms.     

Internal Defibrillation with Smaller Capacitors: A Prospective Randomized Cross-Over Comparison of Defibrillation Efficacy Obtained with 90-μF and 125-μF Capacitors in Humans

Defibrillation with Small Capacitance. Introduction: The size of current implantable cardioverter defibrillators (ICD) is still large in comparison to pacemakers and thus not convenient for pectoral implantation. One way to reduce ICD size is to defibrillate with smaller capacitors. A trade-off exists, however, since smaller capacitors may generate a lower maximum energy output.
Methods and Results: In a prospective randomized cross-over study, the step-down defibrillation threshold (DFT) of an experimental 90-μF biphasic waveform was compared to a standard 125-μF biphasic waveform. The 90-μF capacitor delivered the same energy faster and with a higher peak voltage but provided only a maximum energy output of 20 instead of 34 J. DFTs were determined intraoperatively in 30 patients randomized to receive either an endocardial (n = 15) or an endocardial-subcutaneous array (n = 15) defibrillation lead system. Independent of the lead system used, energy requirements did not differ at DFT for the experimental and the standard waveforms (10.3 ± 4.1 and 9.5 ± 4.9 J, respectively), but peak voltages were higher for the experimental waveform than for the standard waveform (411 ± 80 and 325 ± 81 V, respectively). For the experimental waveform the DFT was 10 J or less using an endocardial lead-alone system in 10 (67%) of 15 patients and in 12 (80%) of 15 patients using an endocardial-subcutaneous array lead system.
Conclusions : A shorter duration waveform delivered by smaller capacitors does not increase defibrillation energy requirements and might reduce device size. However, the smaller capacitance reduces the maximum energy output. If a 10-J safety margin between DFT and maximum energy output of the ICD is required, only a subgroup of patients will benefit from 90-μF ICDs with DFTs feasible using current defibrillation lead systems.     

Right Atrial Appendage Thrombus Found in a Patient in Normal Sinus Rhythm with Normal Right Ventricular Systolic Function

A 79‐year‐old woman underwent transesophageal echocardiography to evaluate the severity of her mitral regurgitation prior to urgent bypass. Evaluation of the right‐sided chambers was notable for a mass in the right atrial appendage (RAA). Surgical excision and pathologic examination proved this to be a thrombus. This is the first reported case of a RAA thrombus in a patient with normal sinus rhythm and normal right ventricular (RV) function. It illustrates that complete transesophageal studies may sometimes demonstrate incidental findings, and that right atrial thrombus can (rarely) be found in patients in sinus rhythm with normal RV function.     

Incidence of Atrial Fibrillation Following Ventricular Defibrillation with Transvenous Lead Systems in Man

Atrial Fibrillation After Ventricular Defibrillation. Introduction: The induction of atrial fibrillation (AF) following implantable defibrillator therapy of ventricular fibrillation carries multiple risks. The frequency of shock-induced AF may be more problematic in patients with transvenous defibrillators because current is often delivered through atrial tissue. Thus, the purpose of this study was to determine the incidence of AF following transvenous ventricular defibrillation. Methods and Results: Atrial electrograms were recorded before and after energy delivery in patients undergoing intraoperative testing of transvenous defibrillation lead systems. A total of 114 tracings were examined from 21 patients following ventricular defibrillation. Transvenous deflbrillation shock strength ranged between 200–800 volts (2–40 joules). Bipolar atrial electrograms were obtained from atrial electrodes with 1-cm interelectrode spacing located on one of the defibrillation catheters. The timing of the ventricular defibrillation shock was expressed as a percentage of the preceding sinus PP interval. Three of the 114 transvenous shocks (2.6%) generated AF. Each episode of AF occurred in a different patient. The shocks responsible for AF occurred at 21%, 43%, and 84% of the preceding sinus PP interval. No relation was found between AF induction and the timing of pulse delivery, pulse strength, or pulse number. Conclusion: We conclude that transvenous ventricular defibrillation infrequently causes AF and that timing shock delivery to the atrial cycle is likely to be of marginal or no benefit in the prevention of shock-induced AF. (J Cardiovasc Electrophysiol, Vol. 3, pp. 411–417, October 1992)     

心脏复律除颤器植入术中除颤阈值测试的现代观点

早期埋藏式心脏复律除颤器装置可靠性差,除颤失败率高,对快速的室性心律失常（室性心动过速）或心室颤动事件唯一的治疗方法是电击;因此在植入埋藏式心脏复律除颤器时常规进行除颤阈值测试。现代埋藏式心脏复律除颤器的性能较前明显改善,除颤性能提高,经静脉途径植入埋藏式心脏复律除颤器的平均除颤阈值是20～30J,低于埋藏式心脏复律除颤器最大输出能量,且除颤阈值测试可给患者带来一定的危险;因此许多临床心脏电生理学者开始质疑埋藏式心脏复律除颤器植入术中除颤阈值测试的价值。     

Immediate Reproducibility of Upper Limit of Vulnerability Measurements in Patients Undergoing Transvenous Implantable Cardioverter Defibrillator Implantation

Reproducibility of ULV. Introduction : Measurement of the upper limit of vulnerability (ULV) with monophasic T wave shocks has been proposed as a patient-specific measurement of defibrillation efficacy that results in fewer episodes of ventricular fibrillation (VF) than measurement of a defibrillation efficacy curve.
Methods and Results : We sought to determine the magnitude of variance in ULV in 63 consecutive patients undergoing implantation of an implantable cardioverter defibrillator (ICD). We measured ULV as the strength at or above which VF is not induced when a stimulus is delivered at 310 msec after an 8-beat ventricular pacing drive at 400 msec. Defibrillation threshold (DFT) was measured in patients with an active can device using a biphasic waveform and the binary search method beginning at 12 J. Sixty-three patients were studied; they bad a mean age of 62 × 12 years and a mean ejection fraction of 35%± 15%. Three quarters of patients bad an ischemic cardiomyopathy. Each patient underwent 4.5 ± 0.8 measurements f ULV. Monophasic ULV correlated poorly with biphasic DFT (R between 0.19 and 0.28, P = 0.04 to 0.17). There was no change in ULV between second to third, third to fourth, and first to last measurement in 22% to 41% of patients. The reliability coefficient was 0.87. A ULV ≥ 20 J was found in eight patients. The only predictor of high ULV was a high DFT.
Conclusion : Monophasic ULVs do not closely predict biphasic active can DFTs using a standard protocol. High DFTs were predicted by high ULVs. There was little variation in the acute measurement of ULV between trials. These findings have important implications for using ULV measurements to determine changes in DFTs after interventions. The methodology of determining ULV is critical to its use for predicting DFTs and programming ICDs.     

A Prospective Randomized Cross-Over Comparison of Mono- and Biphasic Defibrillation Using Nonthoracotomy Lead Configurations in Humans

Biphasic Defibrillation with Nonthoracotomy Leads. Introduction: For current implantable defibrillators, the nonthoracotomy approach to implantation fails in a substantial number of patients. In a prospective randomized cross-over study the defibrillation efficacy of a standard monophasic and a new biphasic waveform was compared for different lead configurations.
Methods and Results: Intraoperatively, in 79 patients receiving nonthoracotomy defibrillation leads, the defibrillation threshold was determined in the initial lead configuration for the mono-and biphasic waveform. In each patient, both waveforms were used alternately with declining energies (20, 15,10, 5 J) until failure of defibrillation occurred. Three different initial lead configurations were tested in different, consecutive, nonrandomized patients using a bipolar endocardial defibrillation lead alone (A; n = 36) or in combination with a subcutaneous defibrillation patch (B; n = 24) or array (C; n = 19) lead. The lowest successful defibrillation energy with the biphasic waveform was less than, equal to, or higher than with the monophasic waveform in 64%, 28%, and 8% of patients, respectively, and on average significantly lower with the biphasic waveform for all three lead configurations (A: 11.3 ± 4.4 J vs 14.5 ± 4.5.); B: 9.7 ± 4.7 J vs 15.1 ± 4.5 J; C: 7.9 ± 4.5 J vs 12.4 ± 4.9 J). Defibrillation efficacy at 20 J was significantly improved by the biphasic waveform (91% vs 76%).
Conclusion: In combination with nonthoracotomy defibrillation leads, the biphasic waveform of a new implantable cardioverter defibrillator showed superior defibrillation efficacy in comparison to the standard monophasic waveform. Defibrillation thresholds were improved for lead systems with and without a subcutaneous patch or array lead.