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PubMed1996

Upper limit of vulnerability is a good estimator of shock strength associated with 90% probability of successful defibrillation in humans with transvenous implantable cardioverter-defibrillators.

OBJECTIVES: The goals of this study were to determine the probability of successful defibrillation at the upper limit of vulnerability and to evaluate a minimal safety margin for implantable cardioverter-defibrillator first shocks based solely on the upper limit of vulnerability. BACKGROUND: The upper limit of vulnerability is the strength at or above which ventricular fibrillation is not induced when a stimulus is delivered during the vulnerable phase of the cardiac cycle. It has been proposed as an estimate of defibrillation efficacy because it correlates with the defibrillation threshold and can be determined with a single episode of fibrillation. METHODS: We studied 40 patients prospectively at implantation of transvenous cardioverter-defibrillators. Defibrillation threshold was defined as the weakest biphasic shock that defibrillated after 10 s of ventricular fibrillation. The upper limit of vulnerability was defined as the weakest biphasic shock that did not induce ventricular fibrillation when given at 0, 20 and 40 ms before the peak of the T wave in ventricular paced rhythm at cycle length 500 ms. After determination of the upper limit of vulnerability and defibrillation threshold, patients underwent six additional fibrillation-defibrillation episodes. The strength of five of the defibrillation shocks was equal to the upper limit of vulnerability; the strength of one of the six shocks was randomly selected to be equal to the upper limit of vulnerability plus 3 J. The implantable cardioverter-defibrillator was tested at the upper limit of vulnerability plus 3 J in 28 patients. RESULTS: The defibrillation threshold was 8.8 +/- 5.0 J (mean +/- SD), and upper limit of vulnerability was 11.3 +/- 4.6 J; the defibrillation threshold and upper limit of vulnerability were highly correlated (r = 0.89, p < 0.001). The success rate for the 200 defibrillation shocks with strength equal to the upper limit of vulnerability was 90% (95% confidence intervals based on proportion of successes in 40 patients: 86% to 94%). All five defibrillation test shocks at the upper limit of vulnerability were successful in 24 patients (60%); four of five were successful in 12 patients (30%); and three of five were successful in 4 patients (10%). All 40 test shocks and 28 implantable cardioverter-defibrillator shocks with a strength equal to the upper limit of vulnerability plus 3 J were successful. CONCLUSIONS: The upper limit of vulnerability is a good estimator of the shock strength associated with 90% probability of successful defibrillation (DFT90). A strength of 3 J above the upper limit of vulnerability is a good estimate of the minimal acute safety margin for implantable cardioverter-defibrillator first shocks.

Adult↗

PubMed1998

A prospective evaluation of two defibrillation safety margin techniques in patients with low defibrillation energy requirements.

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 (< or = 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 < or = 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.

Aged↗

PubMed1995

Clinical predictors of the defibrillation threshold with the unipolar implantable defibrillation system.

OBJECTIVES: The purpose of this study was to determine the relation between clinical variables and the defibrillation threshold by using a standardized testing protocol and a uniform implantable defibrillator system. BACKGROUND: Past studied have not revealed useful correlations between clinical variables and the energy required to terminate ventricular fibrillation. Most of these studies did not use a uniform implantable defibrillator system or a standardized protocol to measure the defibrillation threshold and, thus, did not control for the influence of these technical influences. We postulated that defibrillator and defibrillation threshold measurement-based sources of variability overshadowed important clinical predictors. METHODS: The defibrillation threshold was measured by using a standardized protocol in 101 consecutive patients. We used a transvenous unipolar pectoral defibrillation system that employed a single endocardial right ventricular defibrillation coil as the anode and the shell of an 80-cm3 pulse generator as the cathode to deliver a 65% tilt biphasic pulse. RESULTS: Several clinical variables were found to be significantly associated with the defibrillation threshold: patient gender, height, weight, body surface area, heart rate at rest, QRS and corrected QT (QTc) intervals, left ventricular mass and several measures of heart and chest size by chest roentgenogram. None of these variables had a correlation coefficient > 0.45 with the defibrillation threshold. On multivariate analysis, left ventricular mass and heart rate at rest were the only independent predictors of the defibrillation threshold and explained only 25% of the observed variability. CONCLUSIONS: Despite the use of a uniform transvenous defibrillation system and a standardized protocol to measure the defibrillation threshold, no clinically relevant correlation was found between clinical variables and the defibrillation threshold. The defibrillation threshold is probably a function of a complex interaction of anatomic, physiologic and cellular variables that are not adequately represented by easily obtainable clinical information. It is probably not possible to predict defibrillation outcome from standard clinical variables.

Amiodarone↗

PubMed1987

Defibrillation threshold: a simple and quantitative estimate of the ability to defibrillate.

It has recently been shown that the probability of successful defibrillation as a function of energy has a sigmoidal dose-response relationship. Determination of a defibrillation "dose-response curve" is time consuming and requires multiple defibrillation attempts. On the other hand, determination of a defibrillation threshold is achieved rapidly and would be better suited to study the effect of interventions on the ability to defibrillate patients. We assessed the relationship of defibrillation threshold to the defibrillation "dose-response curve" in twelve open chest, halothane anesthetized pigs. Ventricular fibrillation was induced electrically, and defibrillation was attempted by passing sequential pulse shocks through an indwelling catheter and plaque electrodes. Defibrillation threshold was determined by decreasing the stored voltage of the initial shock until it failed to defibrillate the heart. Five different stored voltage levels distributed around defibrillation threshold were then randomly administered, six times for each level. A "dose-response curve" was obtained for each animal. Defibrillation threshold superimposed on the "dose-response curve" at 76 +/- 7.2 percent (mean +/- SEM) defibrillation success. Energy delivered at 1.5 times average defibrillation threshold was predicted to achieve 100 percent defibrillation success for a single shock in all animals. We conclude that defibrillation threshold provides a simple and quantitative estimate of the ability to defibrillate with a predictable relationship to the "dose-response curve."

Animals↗

PubMed1998

Randomized comparison of a 90 uF capacitor three-electrode defibrillation system with a 125 uF two-electrode defibrillation system.

INTRODUCTION: A variety of factors, including the number of defibrillation electrodes and shocking capacitance, may influence the defibrillation efficacy of an implantable defibrillator system. Therefore, the purpose of this study was to compare the defibrillation energy requirement using a 125 uF two-electrode defibrillation system and a 90 uF three-electrode defibrillation system. METHODS AND RESULTS: The defibrillation energy requirements measured with both systems were compared in 26 consecutive patients. The two-electrode system used a single transvenous lead with two defibrillation coils in conjunction with a biphasic waveform from a 125 uF capacitor. The three-electrode system used the same transvenous lead, utilized a pectoral implantable defibrillator generator shell as a third electrode, and delivered the identical biphasic waveform from a 90 uF capacitor. The two-electrode system was associated with a higher defibrillation energy requirement (10.8 +/- 5.5 J) than was the three-electrode system (8.9 +/- 6.7 J, p < 0.05), however, the leading edge voltage was not significantly different between systems (361 +/- 103 V vs. 397 +/- 123 V, P = 0.07). The two-electrode system also had a higher shocking resistance (49.0 +/- 9.0 ohms vs. 41.4 +/- 7.3 ohms, p < 0.001) and a lower peak current (7.7 +/- 2.6 A vs. 10.1 +/- 3.7 A, p < 0.001) than the three-electrode system. CONCLUSIONS: A three-electrode defibrillation system that utilizes a dual coil transvenous lead and a subcutaneous pectoral electrode with lower capacitance is associated with a lower defibrillation energy requirement than is a dual coil defibrillation system with higher capacitance. This finding suggests that the utilization of a pectoral generator as a defibrillation electrode in conjunction with smaller capacitors is a more effective defibrillation system and may allow for additional miniaturization of implantable defibrillators.

Aged↗

PubMed1993

Is the second phase of a biphasic defibrillation waveform the defibrillating phase?

Why some biphasic waveforms defibrillate with lower energies than monophasic waveforms of similar duration is unknown. One hypothesis is that the first phase of a biphasic waveform acts as a conditioning, hyperpolarizing prepulse to prepare for defibrillation by a second depolarizing phase. To test whether the second phase of a biphasic waveform is the defibrillating phase, three monophasic waveforms, an ascending ramp (A), a square wave (S), and a descending ramp (D), were compared to three biphasic waveforms with A, S, or D in the first phase (biphasic first phase) and three biphasic waveforms with A, S, or D in the second phase (biphasic second phase). Two defibrillation thresholds for each waveform were performed in 18 open chest pigs and mean defibrillation thresholds were compared. In nine pigs 16-msec monophasic and 16/16-msec biphasic waveforms were ranked by mean current and energy at defibrillation threshold. The ranks were the same for monophasic and biphasic second phase waveforms: for mean current A < S = D and for energy A < S < D. The ranks were different for the biphasic first phase waveforms: for mean current S < A = D and for energy S < A = D. Although ranks for the 16-msec monophasic waveforms matched those for the 16/16-msec biphasic second phase waveforms, the biphasic waveforms had higher mean currents and energies at defibrillation threshold. In nine pigs defibrillation thresholds for 6-msec monophasic and 6/6-msec biphasic waveforms were ranked. For mean current the ranks were monophasic: A < S = D; biphasic first phase: A = S = D; and biphasic second phase: S = D < A. For energy the ranks were monophasic: A = S < D; biphasic first phase: A = S = D; and biphasic second phase: S = D < A. Thus, ranks for the 6-msec monophasic waveforms differed from those for the 6/6-msec biphasic second phase waveforms. For 16/16-msec biphasic waveforms, less effective for defibrillation than corresponding 16-msec monophasic waveforms, these results support the hypothesis that the second phase of a biphasic waveform defibrillates since the defibrillation efficacy of a 16/16-msec biphasic waveform is related to the defibrillation efficacy of its second phase waveshape. However, for clinically useful 6/6-msec biphasic waveforms, more effective for defibrillation than 6-msec monophasic waveforms, the hypothesis is not supported because the ability of a 6/6-msec biphasic waveform to defibrillate is unrelated to the defibrillation efficacy of its second phase waveshape.

Animals↗

PubMed1993

Successful implantation of cardioverter-defibrillator systems in patients with elevated defibrillation thresholds.

OBJECTIVES: . The purpose of this study was to conduct a retrospective analysis of 16 patients with high initial defibrillation thresholds in whom a three-electrode system was used to lower defibrillation thresholds and permit implantation of a cardioverter-defibrillator system. BACKGROUND: Patients with high defibrillation thresholds (> 25 J) are uncommon but may be problematic to physicians implanting cardioverter-defibrillator systems. Most conventional systems use two defibrillating electrodes, most commonly two epicardial patches. When defibrillation thresholds remain elevated despite extensive testing of a two-electrode system, a third electrode can be incorporated and tested. However, few published data exist on the use of a three-electrode system in patients with high defibrillation thresholds. METHODS: After failure to achieve satisfactory defibrillation thresholds < 25 J with a two-patch electrode system, a third electrode was incorporated and tested. In all cases, two electrodes were joined to form a common cathode or anode, while a single electrode was used as the opposite polarity electrode. Various three-electrode configurations were then tested. RESULTS: In all 16 patients, satisfactory defibrillation thresholds were achieved and a cardioverter-defibrillator was implanted (95% confidence interval [CI] = 0% to 21%). The mean final defibrillation threshold using the revised three-electrode system was 19.5 +/- 3.7 J (p < 0.0001). A mean of 6 +/- 3 electrode configurations/patient were tested before the final configuration was selected. A total of nine different electrode configurations were used in the 16 study patients; the most common of these incorporated left and right ventricular patches as combined cathode and a superior vena cava coil (n = 5) or right atrial patch electrode (n = 3) as single anode. CONCLUSION: Patients with high initial defibrillation thresholds can generally undergo successful cardioverter-defibrillator implantation with a three-electrode system if enough electrode configurations are tested after a third electrode is incorporated.

Aged↗

PubMed1996

Nonthoracotomy implantation of cardioverter defibrillators: preliminary experience with a defibrillation lead placed at the right ventricular outflow tract.

UNLABELLED: Although morbidity and mortality associated with defibrillator implantation using a nonthoracotomy approach have decreased as compared with a thoracotomy approach, defibrillation thresholds have been higher and fewer patients satisfied implant criteria. It may be possible to improve on the success of nonthoracotomy defibrillator implantation by the placement of a right ventricular (RV) outflow defibrillation lead. Implantable cardioverter defibrillator implantation data of 30 consecutive patients with clinical VT or VF were reviewed. Three defibrillation leads were routinely used. When either pacing threshold at the RV apex was inadequate (n = 2) or 18-J shocks were not successful in terminating VF in 3 of 4 trials (n = 8), the RV apex lead was positioned to the RV outflow tract attaching to the septum. Defibrillation testing was first performed with the RV apex lead in combination with CS, SVC, and/or subcutaneous leads. Twenty patients satisfied implant criteria with a defibrillation threshold of 13.5 +/- 3.6 J. In 7 of the 10 patients, whose RV lead was repositioned to the RV outflow tract, this lead in combination with SVC, CS, or subcutaneous leads produced successful defibrillation at < or = 18 J or in 3 of 4 trials. This approach improved the overall success of nonthoracotomy implantation of defibrillators from 69% to 90%. After a follow-up of 27 +/- 6 months, there was no dislodgment of the RV outflow tract defibrillation leads. CONCLUSIONS: This article reports the preliminary observation that placement of defibrillation leads to the RV outflow tract in humans was possible and without dislodgment. RV outflow tract offers an alternative for placement of defibrillation leads, which may improve on the success of nonthoracotomy defibrillator implantation.

Adult↗

PubMed1988

Prospective evaluation of initially ineffective defibrillation pulses on subsequent defibrillation success during ventricular fibrillation in survivors of cardiac arrest.

The effect of initially ineffective defibrillation pulses on subsequent defibrillation success is not known. Therefore, the voltage, current and energy at the defibrillation threshold were compared with the defibrillation rescue pulse voltage, current and energy that terminated ventricular fibrillation when an ineffective pulse just below the defibrillation threshold had been used initially. This lower amplitude ineffective pulse was termed a "subdefibrillation threshold" pulse. The pulse that restored sinus rhythm after the subdefibrillation threshold pulse was delivered was termed the "subdefibrillation threshold rescue pulse." This comparison was undertaken, intraoperatively, in 14 out-of-hospital cardiac arrest survivors using a sequential-pulse catheter-patch defibrillation system. Each of the 14 patients required higher voltage, current, delivered energy and stored energy for defibrillation with the subdefibrillation threshold rescue pulse than with the defibrillation threshold pulse. The defibrillation threshold voltage was 451 +/- 127 volts compared with a subdefibrillation threshold rescue voltage of 585 +/- 147 volts (p less than 0.00002). The defibrillation threshold current was 5.5 +/- 2.4 amps compared with a subdefibrillation threshold rescue current of 7.2 +/- 2.7 amps (p less than 0.00001). Delivered and stored energies were 10.9 +/- 7.4 and 12.3 +/- 7.2J, respectively, for the defibrillation threshold pulse and were 17.6 +/- 9.4 J (p less than 0.00002) and 20.5 +/- 9.3 J (p less than 0.00005), respectively, for the subdefibrillation threshold rescue pulse. It is concluded that the risk of requiring considerably higher energies than anticipated for defibrillation must be incurred, should defibrillation fail because of an initially insufficient defibrillation pulse.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed2001

Atrial defibrillation thresholds of electrode configurations available to an atrioventricular defibrillator.

INTRODUCTION: Little investigation has been conducted to assess the atrial defibrillation thresholds of electrode configurations using electrodes designed for internal ventricular defibrillation (right ventricle [RV], superior vena cava [SVC], and pulse generator housing [Can]) combined with coronary sinus (CS) electrodes. We hypothesized that a CS-->SVC+Can electrode configuration would have a lower atrial defibrillation threshold than a standard configuration for defibrillation, RV-->SVC+Can. We also tested the atrial defibrillation thresholds of five other configurations. METHODS AND RESULTS: In 12 closed chest sheep, we situated a two-coil (RV, SVC) defibrillation catheter, a left-pectoral subcutaneous Can, and a CS lead. Atrial fibrillation was burst induced and maintained with continuous infusion of intrapericardial acetyl-beta-methylcholine chloride. Using fixed-tilt biphasic shocks, we determined the atrial defibrillation thresholds of seven test configurations in random order according to a multiple-reversal protocol. The peak voltage and delivered energy atrial defibrillation thresholds of CS-->SVC+Can (168+/-67 V, 2.68+/-2.40 J) were significantly lower than those of RV-->SVC+Can (215+/-88 V, 4.46+/-3.40 J). The atrial defibrillation thresholds of the other test configurations were RV+CS-->SVC+Can: 146+/-59 V, 1.92+/-1.45 J; RV-->CS+SVC+Can: 191+/-89 V, 3.53+/-3.19 J; CS-->SVC: 188+/-98 V, 3.77+/-4.14 J; SVC-->CS+ Can: 265+/-145 V, 7.37+/-9.12 J; and SVC-->Can: 516+/-209 V, 24.5+/-15.0 J. CONCLUSIONS: The atrial defibrillation threshold of CS-->SVC+Can is significantly lower than that of RV-->SVC+Can. In addition, the low atrial defibrillation threshold of RV+CS-->SVC+Can merits further investigation. Based on corroboration of low atrial defibrillation thresholds of CS-based configurations in humans, physicians might consider using CS leads with atrioventricular defibrillators.

Alabama↗

PubMedDate not supplied

Defibrillator failures. Causes of problems and recommendations for improvement. Defibrillator Working Group.

The Defibrillator Working Group of the Food and Drug Administration reviewed data from the Medical Device Reporting System and a recent five-state survey, as well as information presented at two Food and Drug Administration-sponsored conferences. These data include 156 reports of defibrillator problems to the Emergency Care Research Institute Problem Reporting Network, 495 reports of device problems to the Medical Device Reporting System, 676 reports of "defibrillator failure" in the five-state survey, 594 inspections of in-service defibrillators, and site visits to 212 emergency care facilities. The Defibrillator Working Group concluded that the frequency of defibrillator failures during clinical use may be unacceptably high. While some failures are attributable to component malfunctions, evidence suggests that errors in operator use and errors in defibrillator care and maintenance account for a high proportion of defibrillator failures. Inadequate initial training and cursory continuing education increases the chances of operator errors at the moment when correct operation is needed most. Failure of operators to perform daily equipment checks leads to poor familiarity with the equipment and failure to identify component failures or damaged devices. Many defibrillators and batteries are kept in service beyond an expected useful life, given their level of clinical use. In smaller hospitals and emergency medical services systems periodic maintenance responsibilities are not always appropriately delegated between qualified engineering personnel and defibrillator operators, and some systems completely lack services from clinical engineers. The objectives of the Defibrillator Working Group are to make personnel who use defibrillators more aware of the potential for errors in operator performance and in periodic maintenance and to recommend improvements in training, maintenance, and defibrillator design. This article presents the initial observations and recommendations of the working group.

Education, Continuing↗

PubMed1997

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 transvenous 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 transvenous 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 RR 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.

Adult↗

PubMed1995

A prospective randomized comparison in humans of 90-mu F and 120-mu F biphasic pulse defibrillation using a unipolar defibrillation system.

INTRODUCTION: Capacitance is known to influence defibrillation. Optimal biphasic waveform capacitance for transvenous unipolar defibrillation systems in man is currently being defined. In an effort to improve defibrillation efficacy, we examined the relative defibrillation efficacy of a 65% tilt biphasic pulse from a 90-mu F capacitor compared to a 65% tilt biphasic pulse from a 120-mu F capacitor in a prospective, randomized fashion in 16 consecutive cardiac arrest survivors undergoing defibrillator surgery. METHODS AND RESULTS: The transvenous unipolar pectoral defibrillation system uses a single endocardial RV anodal defibrillation coil and the shell of an 80-cc volume (88 cm2 surface area) pulse generator (Medtronic Model 7219C PCD "active CAN") as the cathode for the first phase of the biphasic shock: RV+ --> CAN-. Defibrillation thresholds for each capacitance were determined prospectively in a randomized fashion. The defibrillation threshold results for the 90-mu F capacitance were: leading edge voltage 383 +/- 132 V; stored energy 7.4 +/- 5.0 J; and resistance 57 +/- 10 omega. The results for the 120-mu F capacitance were: leading edge voltage 315 +/- 93 V (P = 0.002); stored energy 6.5 +/- 3.7 J (P = 0.21); and resistance 57.0 +/- 11 omega (P = 0.87). CONCLUSIONS: We conclude that 90-mu F, 65% tilt biphasic pulses used with unipolar pectoral defibrillation systems have equivalent stored energy defibrillation efficacy compared to 120-mu F, 65% tilt pulses. Use of lower capacitance is possible in present implantable defibrillators without compromising defibrillation.

Adult↗

PubMed1988

Prediction of defibrillation success from a single defibrillation threshold measurement with sequential pulses and two current pathways in humans.

The ultimate aim of defibrillation testing is to predict consistent defibrillation. This study tested the hypothesis that defibrillation success could be predicted from a single measurement of defibrillation threshold. We measured defibrillation threshold by using three patch electrodes and a standard protocol intraoperatively in 49 patients undergoing arrhythmia surgery. Each patient was then assigned to one of five energy subgroups (0.5, 1.0, 1.5, 2.0, or 2.5 times defibrillation threshold) for a single shock (followed by a rescue shock if necessary) for a subsequent ventricular fibrillation episode. A curve relating percent success to energy was then constructed for the group. Defibrillation threshold averaged 4.7 +/- 2.98 J for the group (mean +/- SD). There was a curvilinear relation between the energy of the defibrillation threshold ratio test shock and percent success: 33.3%, 58.3%, 81.8%, 91.7%, and 100% at mean defibrillation threshold ratios of 0.56 +/- 0.14, 1.02 +/- 0.07, 1.53 +/- 0.14, 1.88 +/- 0.09, and 2.60 +/- 0.14, respectively. We conclude that consistent defibrillation is predictable from a single measurement of defibrillation threshold. Furthermore, for an individual patient, a safety margin of 2.6 times defibrillation threshold should approximate 100% successful defibrillation for a single test shock.

Adult↗

PubMed1994

Why do some patients have high defibrillation thresholds at defibrillator implantation? Answers from basic research.

Implantable cardioverter defibrillators reduce the risk of sudden cardiac death in patients with ventricular tachyarrhythmias. However, for the few patients with unacceptably high defibrillation thresholds at implantation the risk of sudden death may remain high. If a small number of defibrillation attempts are used to determine a defibrillation threshold, then a high defibrillation threshold may occur in some patients due to the probabilistic nature of defibrillation: a small percentage of shocks will fail even at optimal shock strengths. Basic investigations have suggested mechanisms for high defibrillation thresholds in other patients. The extracellular potential gradients produced by a shock correlate with ability to defibrillate and may be used to classify mechanisms for high defibrillation thresholds. Computerized mapping studies have demonstrated that extracellular potential gradient fields produced by defibrillation shocks are uneven with high gradient areas close to the electrodes and low gradient areas distant from the electrodes. A high defibrillation threshold may occur because: (1) a shock creates a subthreshold potential gradient in the low gradient areas; (2) a patient has a higher minimum potential gradient threshold than other patients; or (3) a shock leads to refibrillation in the high gradient areas. This article reviews experimental evidence to support each of these three possibilities then suggests experimental and clinical investigations that may clarify the causes of high defibrillation thresholds in patients.

Defibrillators, Implantable↗

PubMed1999

[Effect of defibrillation on hemodynamics during cardioverter defibrillator implantation].

This study evaluated the influences of induced ventricular fibrillation and defibrillation on the hemodynamics in patients with malignant ventricular arrhythmias during cardioverter defibrillator implantation. A total of 15 patients were analyzed, divided into 2 groups based on ventricular contractility. Group A included 6 patients with preoperative left ventricular fractional shortening < or = 0.20 and Group B consisted of 9 patients with fractional shortening > 0.20. Mean blood pressure, cardiac output, pulmonary capillary wedge pressure, and left ventricular stroke work index were measured before and after ventricular fibrillation and defibrillation. In addition, end-systolic wall stress and rate-corrected mean velocity of circumferential fiber shortening were obtained from transesophageal echocardiography. In Group B, measured 30 sec after defibrillation, mean blood pressure, cardiac output, and left ventricular stroke work index increased significantly (p < 0.05), whereas only pulmonary capillary wedge pressure indicated a significant increase (p < 0.001) after defibrillation in Group A. Throughout 5 repeated ventricular fibrillations and defibrillations. Group A showed consistently lower postoperative values than preoperative values for mean blood pressure, cardiac output and left ventricular stroke work index. In contrast, in Group B, the postoperative values were constantly higher than preoperative values. Although end-systolic wall stress decreased after defibrillation in both groups, the magnitude of decrease was significantly greater in Group A than in Group B (p < 0.01). While rate-corrected mean velocity of circumferential fiber shortening increased after defibrillation in both groups, the difference of rate-corrected mean velocity of circumferential fiber shortening between Groups A and B was statistically significant at beats one and 2 following defibrillation (p < 0.05) where Group A indicated a more drastic increase than Group B. In both groups, post-defibrillation end-systolic wall stress and rate-corrected mean velocity of circumferential fiber shortening gradually approached the preoperative values as beats proceeded. These results suggest that repeated defibrillations in patients with poor ventricular contractility potentially deteriorates hemodynamics, presumably because the left ventricle is often intolerant to abrupt arterial collapse by ventricular fibrillation and to subsequent rapid preload increase after defibrillation.

Adult↗

PubMed1994

Effects of procainamide and lidocaine on defibrillation energy requirements in patients receiving implantable cardioverter defibrillator devices.

INTRODUCTION: In acute canine studies, lidocaine, but not procainamide, increases defibrillation energy requirements. We evaluated the effects of lidocaine or procainamide on defibrillation energy requirements in 27 patients undergoing intraoperative testing for implantable cardioverter defibrillator device placement. METHODS AND RESULTS: Patients were tested off antiarrhythmic drugs and again following either lidocaine (200 to 250 mg loading and 3 mg/min maintenance infusions) or procainamide (1 gm loading and 3 to 4 mg/min maintenance infusions). The defibrillation testing protocol consisted of initial testing at 15 J, followed by higher or lower energies to determine the lowest energy producing three consecutive successful defibrillations. Overall, the mean defibrillation energy increased from 14 +/- 5 J to 18 +/- 7 J during lidocaine (plasma concentration 5.1 +/- 1.6 micrograms/mL; P < 0.02) but were similar at baseline (12 +/- 5 J) and during procainamide infusion (13 +/- 6 J) (plasma concentration: procainamide 10.7 +/- 7.2 micrograms/mL; N-acetyl procainamide 1.0 +/- 0.4 micrograms/mL). A positive linear correlation was found between lidocaine plasma concentration and percent change in defibrillation energy (lidocaine: r = 0.61; P = 0.01). Procainamide raised the defibrillation energy in three patients, two with supratherapeutic plasma concentrations. The increase in defibrillation energy equaled or exceeded 25 J in four patients after lidocaine and in one patient after procainamide. CONCLUSION: The data suggest that at high plasma concentrations, lidocaine and procainamide adversely affect defibrillation energy requirements consistent with an adverse, concentration-dependent effect of sodium channel blockade on defibrillation energy requirements in patients.

Aged↗

PubMed1995

Elevated defibrillation threshold when right-sided venous access is used for nonthoracotomy implantable defibrillator lead implantation. The Endotak Investigators.

INTRODUCTION: Although myriad factors influence the defibrillation threshold, the relation between the site of transvenous lead entry into the vascular system and the defibrillation threshold has not been reported. This study examines the influence that venous entry site has on defibrillation success for a transvenous implantable cardioverter defibrillator lead with two defibrillating coils. METHODS AND RESULTS: The study population comprised 345 patients. Their mean age was 61 +/- 13 years and, left ventricular ejection fraction was 0.33 +/- 0.13. A left-sided approach was used in 324 (93.9%) of the patients, and a right-sided approach was used in the remaining 21 (6.1%) patients. There was no difference in the gender, age, left ventricular ejection fraction, or underlying cardiac disease in the two groups. For all patients, with a transvenous lead used either alone or with a submuscular or subcutaneous patch, the biphasic defibrillation threshold was 9.9 +/- 4.8 J when a left-sided approach was used, and 14.0 +/- 7.3 J when a right-sided approach was used (P = 0.02). When a transvenous lead was used with a submuscular or subcutaneous patch (115 patients), the biphasic defibrillation threshold was 9.5 +/- 4.3 J when a left-sided approach was used, and 12.0 +/- 10.0 J when a right-sided approach was used (P = 0.98). When a transvenous lead was used without a submuscular or subcutaneous patch (230 patients), the biphasic defibrillation threshold was 10.1 +/- 5.0 J when a left-sided approach was used, and 14.6 +/- 6.6 J when a right-sided approach was used (P < 0.01). For the entire group of patients and for each specific lead arrangement, there was no significant difference in the defibrillating lead system impedance when right-sided versus left-sided approaches were compared. CONCLUSION: Left-sided approaches to implant transvenous leads with two coils for defibrillation result in lower biphasic defibrillation thresholds than when right-sided approaches are used.

Defibrillators, Implantable↗