Acute effects of intravenous propafenone on the internal ventricular defibrillation threshold in the anesthetized dog

The application of artificial neural networks in metabolomics: a historical perspective

BACKGROUND

Metabolomics data, with its complex covariance structure, is typically modelled by projection-based machine learning (ML) methods such as partial least squares (PLS) regression, which project data into a latent structure. Biological data are often non-linear, so it is reasonable to hypothesize that metabolomics data may also have a non-linear latent structure, which in turn would be best modelled using non-linear equations. A non-linear ML method with a similar projection equation structure to PLS is artificial neural networks (ANNs). While ANNs were first applied to metabolic profiling data in the 1990s, the lack of community acceptance combined with limitations in computational capacity and the lack of volume of data for robust non-linear model optimisation inhibited their widespread use. Due to recent advances in computational power, modelling improvements, community acceptance, and the more demanding needs for data science, ANNs have made a recent resurgence in interest across research communities, including a small yet growing usage in metabolomics. As metabolomics experiments become more complex and start to be integrated with other omics data, there is potential for ANNs to become a viable alternative to linear projection methods.

AIM OF REVIEW

We aim to first describe ANNs and their structural equivalence to linear projection-based methods, including PLS regression. We then review the historical, current, and future uses of ANNs in the field of metabolomics.

KEY SCIENTIFIC CONCEPT OF REVIEW

Is metabolomics ready for the return of artificial neural networks?

Clinical study of the acute effects of intravenous nifekalant on the defibrillation threshold in patients with persistent and paroxysmal atrial fibrillation

BACKGROUND

Antiarrhythmic agents are considered to have significant effects on the defibrillation energy requirement, so this study investigated the effects of nifekalant on defibrillation.

METHODS AND RESULTS

Forty-two patients with persistent atrial fibrillation (AF) underwent electrical cardioversion via intracardiac electrode catheters prior to and after the intravenous administration of nifekalant. The success rate of the defibrillation and change in the defibrillation threshold using sequential incremental defibrillation energy deliveries was investigated. In addition, the parameters that could predict the beneficial effects of nifekalant were also assessed. Nifekalant significantly decreased the defibrillation energy requirement in 13 of the 42 cases, and nifekalant also converted AF to sinus rhythm with an identical energy to that of the last unsuccessful defibrillation in 21 of 42 cases. The success of defibrillation seemed to be dependent on significant prolongation of the intracardiac atrial electrogram intervals during AF by the nifekalant.

CONCLUSIONS

Intravenous nifekalant significantly improved the electrical defibrillation efficacy in patients with persistent AF that was resistant to defibrillation, without any serious adverse effects.

Combining antiarrhythmic drugs and implantable devices therapy: benefits and outcome

At least 50% of patients who received an ICD have been treated with antiarrhythmic drugs (AAD). The potential indications for combining antiarrhythmic drugs and ICD are generally the following: reduction of the number of episodes of ventricular tachycardia or ventricular fibrillation and therefore of the number of shocks, improving patient's quality of life and extending the battery life of the ICD, prevention of supraventricular arrhythmias and/or control of their rate, lengthening of the tachycardia cycle length to allow ventricular tachycardia conversion by antitachycardia pacing and reduction of the number of episodes of syncope. Although previous papers reported conflicting results about pharmacologic therapy in reducing the frequency of iCD shocks, some recent randomized prospective studies showed the efficacy of pharmacologic therapy in reducing the frequency of ICD shocks. The use of antiarrhythmic drugs can have also adverse effect: an increase in the defibrillation threshold, an increase in the pacing threshold and an increase in the VT cycle length leading to detection failure. We have also to consider that some advantages derived from antiarrhythmic drugs can be reached by the new devices with atrial sensing and pacing and/or the possibility of atrial defibrillation or by using catheter ablation as adjunctive therapy to ICD. For these reasons, the concomitant use of antiarrhythmic drugs and ICD should be evaluated in each patient in relation to specific clinical and electrophysiologic features including: the frequency, the rate and the clinical presentation of the ventricular arrhythmia, the effect of the selected drug on the defibrillation threshold, the defibrillation threshold at the implant, the effect of the selected drug on the ventricular function and the likelihood of proarrhythmic events.

Effect of a class III antiarrhythmic drug on the configuration of dose response curve for defibrillation

Antiarrhythmic agents with a Class III action are known to increase defibrillation efficacy. We investigated whether a Class III drug simply shifts the dose-response curve for defibrillation or more extensively alters the curve. Forty-five dogs were divided into four groups according to the shock waveform and the presence or absence of treatment with a novel Class III drug, MS-551 (2 mg/kg bolus + 0.02 mg/kg per min). In addition to the conventional transcardiac DFT, dose-response curves were obtained by fitting the results of 40 fibrillation-defibrillation sequences at five shock strengths to a logistic model. MS-551 significantly decreased DFT regardless of the shock waveform (control vs MS-551 = 306 +/- 79 V vs 229 +/- 72 V [monophasic shock, P < 0.05], or 227 +/- 42 V vs 176 +/- 26 V [biphasic shock, P < 0.005]). The dose-response curves in dogs treated with MS-551 had a gentler slope than those without treatment, and the ratio of the voltages corresponding to 50% and 90% defibrillation success (V90/V50) was significantly greater in the MS-551 group (monophasic: 1.21 +/- 0.06 vs. 1.62 +/- 0.42 [P < 0.005], biphasic: 1.20 +/- 0.05 vs 1.37 +/- 0.18 [P < 0.01]). The V90/DFT ratio was also significantly larger in the MS-551 group (monophasic: 1.22 +/- 0.12 vs 1.66 +/- 0.37 [P < 0.001]; biphasic: 1.19 +/- 0.11 vs 1.44 +/- 0.79 [P < 0.005]). Thus, this Class III drug decreased the shock strength corresponding to relatively higher success rate (approximately 90%) less markedly than that for moderate success rate (approximately 50%). These results suggest that a Class III drug does not simply shift the dose response curve in proportion to the change in DFT, but more extensively alters its configuration.

Favorable effects of flecainide in transvenous internal cardioversion of atrial fibrillation

OBJECTIVES

The aim of the study was to evaluate the effects of intravenous (IV) flecainide on defibrillation energy requirements in patients treated with low-energy internal atrial cardioversion.

BACKGROUND

Internal cardioversion of atrial fibrillation is becoming a more widely accepted therapy for acute episode termination and for implantable atrial defibrillators.

METHODS

Twenty-four patients with atrial fibrillation (19 persistent, 5 paroxysmal) underwent elective transvenous cardioversion according to a step-up protocol. After successful conversion in a drug-free state, atrial fibrillation was induced by atrial pacing; IV flecainide (2 mg/kg) was administered and a second threshold was determined. In patients in whom cardioversion in a drug-free state failed notwithstanding a 400- to 550-V shock, a threshold determination was attempted after flecainide.

RESULTS

Chronic persistent atrial fibrillation was converted in 13/19 (68%) patients at baseline and in 16/19 (84%) patients after flecainide. Paroxysmal atrial fibrillation was successfully cardioverted in all the patients. A favorable effect of flecainide was observed either in chronic persistent atrial fibrillation (13 patients) or in paroxysmal atrial fibrillation (5 patients) with significant reductions in energy requirements for effective defibrillation (persistent atrial fibrillation: 4.42+/-1.37 to 3.50+/-1.51 J, p < 0.005; paroxysmal atrial fibrillation: 1.68+/-0.29 to 0.84+/-0.26 J, p < 0.01). In 14 patients not requiring sedation, the favorable effects of flecainide on defibrillation threshold resulted in a significant reduction in the scores of shock-induced discomfort (3.71+/-0.83 vs. 4.29+/-0.61, p < 0.005). No ventricular proarrhythmia was observed for any shock.

CONCLUSIONS

Intravenous flecainide reduces atrial defibrillation threshold in patients treated with low-energy internal atrial cardioversion. This reduction in threshold results in lower shock-induced discomfort. Additionally, flecainide may increase the procedure success rate in patients with chronic persistent atrial fibrillation.

Do the effects of antiarrhythmic drugs on defibrillation efficacy vary among different shock waveforms?

This study was designed to extend our knowledge on how pharmacological modification of defibrillation efficacy is associated with shock waveform. In 35 anesthetized dogs, the baseline transcardiac DFT was determined using 12-ms monophasic and three biphasic waveforms (10 ms-2 ms, 8 ms-4 ms, and 6 ms-6 ms). Twenty-eight dogs were then treated with either lidocaine (n = 7), mexiletine (n = 7), dofetilide (n = 7), or MS-551 (n = 7), while 7 dogs were left untreated to confirm the reproducibility of DFT data. Subsequently, DFT measurements were repeated in all dogs. Waveform related differences of the baseline DFT were significant, and the monophasic DFT was higher than any of the biphasic DFTs. Lidocaine increased DFT by 11% +/- 12% (12-ms monophasic), 20% +/- 20% (10 ms-2 ms, P < 0.05), 13% +/- 20% (8 ms-4 ms), and 12% +/- 10% (6 ms-6 ms, P < 0.05). With infusion of mexiletine, the DFT increased by 17% +/- 16% (P < 0.05), 9% +/- 12%, 10% +/- 10% (P < 0.05), and 4% +/- 15%, respectively. Both dofetilide and MS-551 significantly decreased the DFT regardless of the pulse waveform (dofetilide: from -18% +/- 19% to -24% +/- 19%, MS-551; from -18% +/- 11% to -32% +/- 6%). In all drug groups, waveform related differences in DFT remained significant. These results support the view that the advantages of biphasic shock waveforms are not lessened by treatment with antiarrhythmic drugs.

Effects of amiodarone and its active metabolite desethylamiodarone on the ventricular defibrillation threshold

OBJECTIVES

We evaluated whether the reported difference in the ventricular defibrillation threshold (DFT) between short-term intravenous and oral amiodarone is due to the effect of amiodarone's active metabolite desethylamiodarone (DEA).

BACKGROUND

Amiodarone is frequently used in patients with implantable cardioverter-defibrillator devices (ICD). Long-term oral amiodarone raises the DFT, but intravenous amiodarone has not been shown to have this effect. DEA, an active metabolite of amiodarone, has different electrophysiologic properties than its parent compound and may be responsible for the observed different effects of intravenous and oral amiodarone on DFT.

METHODS

We ascertained the DFT in 24 pigs randomized to receive intravenous amiodarone, DEA or vehicle. Defibrillation was delivered through a transvenous lead system using a biphasic waveform. The DFT was determined using an up-down DFT algorithm and defined as the average minimal energies resulting in successful defibrillation delivered from ascending and descending serial shocks.

RESULTS

Amiodarone caused a dose-response increase in DFT (mean +/- SD) from 22.7 +/- 4.1 (baseline) to 26.1 +/- 2.9 (10 mg/kg body weight), p = 0.11, to 34.9 +/- 8.2 J (after an additional 15 mg/kg), p = 0.035. DEA (10 mg/kg) caused an increase in DFT from 20.5 +/- 6.3 to 33.9 +/- 13.6 J, p < 0.01. Addition of 15 mg/kg of DEA resulted in hemodynamic instability and thus DFT was not obtained. In the control group, DFT decreased from 26.8 +/- 7.7 at baseline to 23.1 +/- 7.4 (dose 1), p = 0.19, to 22.8 +/- 6.2 J (dose 2), p = 0.18.

CONCLUSIONS

DEA increases DFT by a greater amount than its parent drug amiodarone. There is an effect of intravenous amiodarone on DFT that is dose dependent.

[Influence of waveform and configuration of electrodes on the defibrillation threshold of implantable cardioverter-defibrillators]

The defibrillation threshold (DFT) is no threshold in the true sense. Between energy levels which defibrillate in all cases and energy levels which never defibrillate, a broad range of energies exists which might or might not defibrillate. Thus, the value of the DFT is dependant on the protocol used for its determination. Usually the DFT presents an energy at which the implantable cardioverter-defibrillator (ICD) will defibrillate successfully at a rate of approximately 75%. To achieve a 100% success rate the energy has to be programmed 15 J above the DFT or twice the DFT.Using DFT measurements the energy needed for internal defibrillation could be gradually reduced in the last years. Major break throughs have been the introduction of the biphasic defibrillation waveform and the use of pectorally implanted ICD shells as defibrillation electrodes. The shortening of the defibrillation impulse by the use of lower capacitances could not improve DFTs but allowed to construct ICDs of smaller volume. Addition of a superior vena cava electrode or a subcutaneous array electrode at the left lateral chest to the standard bipolar electrode system (right ventricle, pectoral ICD can) allowed for tri- and quadripolar lead configurations which reduced DFTs on average only slightly but reduced the standard deviation of DFTs significantly and thus helped to avoid high DFTs. Besides building smaller ICDs, reduction of DFTs and thus programming of lower defibrillation ICD energies allows for improved battery longevities and reduced capacitor charging times and thus a lower incidence of syncopes.

Circadian variation in defibrillation energy requirements

BACKGROUND

Reports have demonstrated a circadian variation in the incidence of acute myocardial infarction, ventricular arrhythmias, and sudden cardiac death. We tested the hypothesis that a similar circadian variation exists for defibrillation energy requirements in humans.

METHODS AND RESULTS

We reviewed the time of defibrillation threshold (DFT) measurements in 134 patients with implantable cardioverter-defibrillators (ICDs) who underwent 345 DFT measurements. The DFT was determined in 130 patients at implantation, in 121 at a 2 months, and in 94 at 6 months. All patients had nonthoracotomy systems. The morning DFT (8 AM to 12 noon) was 15.1 +/- 1.2 J compared with 13.1 +/- 0.9 J in the midafternoon (12 noon to 4 PM) and 13.0 +/- 0.7 J in the late afternoon (4 to 8 PM), P < .02. In a separate group of 930 patients implanted with an ICD system with date and time stamps for each therapy, we reviewed 1238 episodes of ventricular tachyarrhythmias treated with shock therapy. To corroborate the hypothesis that energy requirements for arrhythmia termination vary during the course of the day, we plotted the failed first shock frequency for all episodes per hour. There was a significant peak in failed first shocks in the morning compared with other time intervals (P = .02).

CONCLUSIONS

There is a morning peak in DFT and a corresponding morning peak in failed first shock frequency. This morning peak resembles the peaks seen in other cardiac events, specifically sudden cardiac death. These findings have important implications for appropriate ICD function, particularly in patients with marginal DFTs.

Effects of oral propafenone administration before electrical cardioversion of chronic atrial fibrillation: a placebo-controlled study

OBJECTIVES

Our aim was to evaluate the benefits and risks of administering propafenone before electrical defibrillation for chronic atrial fibrillation.

BACKGROUND

In this context, an antiarrhythmic drug-although potentially useful in preventing early recurrence of arrhythmia-could adversely affect the defibrillation threshold and reduce the cardioversion success rate.

METHODS

We randomly assigned 100 patients with chronic atrial fibrillation to oral treatment with either placebo (51 patients) or 750 mg/day of propafenone (49 patients) for 48 h before administration of direct current shock. After successful cardioversion, all patients received propafenone therapy and were followed up for 48 h.

RESULTS

Before defibrillation, three patients in the propafenone group (6.1%) had reversion to sinus rhythm and one had sustained ventricular tachycardia. Shock efficacy (82.4% vs. 84.4%) and the cumulative effective energy (395 +/- 258 vs. 421 +/- 236 J) were not different between the placebo and propafenone groups. In the propafenone group, 11 patients had their arrhythmia transformed into atrial flutter and required a lower energy level for arrhythmia conversion than did the other patients with continued atrial fibrillation (245 +/- 197 vs. 493 +/- 215 J, p < 0.01); the latter patients showed a trend (p < 0.10) toward higher energy requirements than that of patients who received placebo. The incidence of asymptomatic bradyarrhythmias was higher in the propafenone group (28.9% vs. 7.1%, p < 0.02), but more patients who received placebo had early recurrence of atrial fibrillation (16.7% vs. 0%, p < 0.02). Two days after cardioversion, more patients given propafenone (73.5% vs. 52.9%, p < 0.05) were discharged from the hospital with sinus rhythm. During the in-hospital stay, propafenone was withdrawn from six patients (6.6%) because of side effects.

CONCLUSIONS

Propafenone, given before electrical cardioversion for chronic atrial fibrillation does not affect the mean defibrillation threshold or the rate of successful arrhythmia conversion. It decreases the recurrence of atrial fibrillation early after shock, thus allowing more patients to be discharged from the hospital with sinus rhythm.

Effects of oral propafenone on defibrillation and pacing thresholds in patients receiving implantable cardioverter-defibrillators. Propafenone Defibrillation Threshold Investigators

OBJECTIVES

The effects of propafenone, a predominantly class IC antiarrhythmic drug, on defibrillation and pacing thresholds were evaluated in patients undergoing cardioverter-defibrillator implantation.

BACKGROUND

Previous studies have shown that the class IC agents encainide and flecainide may increase the energy requirements for pacing and defibrillation. Animal studies with propafenone have shown inconsistent results regarding its effect on defibrillation energy requirements. This report investigated the effects of propafenone on defibrillation and pacing thresholds in humans.

METHODS

After cardioverter-defibrillator implantation, 47 patients were enrolled in a double-blind, three-way parallel, randomized trial of 450 mg/day (Group 1) or 675 mg/day (Group 2) of oral propafenone or placebo (Group 3) for 3 to 7 days. Predischarge defibrillation and pacing thresholds after treatment were compared with baseline thresholds obtained at implantation.

RESULTS

There was no statistically significant difference between implantation and predischarge defibrillation thresholds in the three groups (Group 1: [mean +/- SE] 11.0 +/- 1.3 vs. 12.1 +/- 1.5 J; Group 2: 11.5 +/- 1.1 vs. 13.6 +/- 1.3 J; Group 3: 12.5 +/- 1.2 vs. 13.3 +/- 1.6 J), and no significant difference between treatment groups was found with a 0.86 power to detect a 5-J difference between groups. Paired pulse width pacing thresholds at 2.8 V were compared in 14 patients. A small increase of 0.02 ms was noted at predischarge testing in patients treated with propafenone and placebo.

CONCLUSIONS

Short-term oral propafenone (450 and 675 mg/day) does not significantly affect defibrillation or pacing thresholds. Concomitant use of propafenone in patients with implantable cardioverter-defibrillators with recurrent ventricular or atrial tachyarrhythmias should not interfere with proper device function.

Mechanism of antiarrhythmic drug-induced changes in defibrillation threshold: role of potassium and sodium channel conductance

OBJECTIVES

We sought to determine which ion current predominantly affects defibrillation outcomes by using specific pharmacologic probes (lidocaine [a sodium channel blocking agent] and cesium [an outward potassium channel blocking agent]) in 26 swine.

BACKGROUND

The effect of a drug on sodium or potassium channel conductance, or both, may affect defibrillation threshold values. However, it is unknown which ion channel predominates.

METHODS

Each pig was randomly assigned to one of four treatment groups with two treatment phases: group 1 = placebo (D5W) in treatment phase I followed by placebo plus cesium in treatment phase II (n = 6); group 2 = lidocaine followed by lidocaine plus placebo (n = 7); group 3 = lidocaine followed by lidocaine plus cesium (n = 7); group 4 = placebo followed by placebo plus placebo (n = 6). Defibrillation threshold values and electrocardiographic measurements were obtained at baseline and at treatment phases I and II.

RESULTS

Lidocaine increased defibrillation threshold values from baseline by 71% in group 2 (p = 0.02) and by 92% in group 3 (p < 0.01). There were no changes in defibrillation threshold values from baseline to D5W in groups 1 and 4. When D5W was added to lidocaine in group 2 and D5W in group 4, there were no significant changes in defibrillation threshold values. However, when cesium was added to lidocaine in group 3, the elevated defibrillation threshold values (mean +/- SD) returned to baseline values (from 15.7 +/- 3.46 to 7.55 +/- 3.19 J, p < 0.01). Cesium added to D5W in group 1 also significantly reduced defibrillation threshold values from 7.10 +/- 1.27 to 4.14 +/- 1.75 J (p < 0.01). The effect of cesium on defibrillation threshold values was similar between groups 1 and 3, regardless of lidocaine, such that these values were reduced by 40 +/- 14% and 51 +/- 18%, respectively (p = 0.28).

CONCLUSIONS

Cesium, through potassium blockade, reverses lidocaine-induced elevation in defibrillation threshold values. The magnitude of defibrillation threshold reduction when cesium was added to lidocaine was similar to the defibrillation threshold reduction when cesium was added to placebo. Thus, inhibiting outward potassium conductance and prolonging repolarization decreases defibrillation threshold values independent of sodium channel blockade.

Differential effects of lidocaine on defibrillation threshold with monophasic versus biphasic shock waveforms

BACKGROUND

Defibrillation waveforms and antiarrhythmic drugs have disparate effects on myocardial excitability and refractoriness, making it likely that antiarrhythmic drugs will interact with one waveform differently than with another. The aim of the present study was to determine if the increase in defibrillation threshold (DFT) induced by lidocaine is similar for electrical shocks with monophasic and biphasic waveforms.

METHODS AND RESULTS

Twenty-six pentobarbital-anesthetized farm-raised pigs were instrumented with pacing catheters and epicardial defibrillation electrodes. Each pig was assigned to one of four groups: (1) monophasic shock waveform and placebo (5% dextrose in water [D5W]) (n = 7), (2) monophasic shock waveform and lidocaine (n = 7), (3) biphasic shock waveform and placebo (D5W) (n = 5), or (4) biphasic shock waveform and lidocaine (n = 7). DFT was measured at baseline and subsequently during treatment (D5W or lidocaine). In the monophasic waveform groups, DFT increased from baseline in response to lidocaine by 92% (P < .0001), whereas DFT values in response to D5W did not change. In the biphasic waveform groups, DFT values did not change from baseline in response to lidocaine (P = NS), whereas DFT values from baseline in response to D5W significantly decreased by 29% (P = .04). In the monophasic waveform groups, the change in DFT from baseline in response to lidocaine was significantly different than the change from baseline in response to D5W (92 +/- 29% versus -0.5 +/- 29%, respectively) (P < .0002). In the biphasic waveform groups, however, the change in DFT from baseline in response to lidocaine was similar to the change from baseline in response to D5W (-5.66 +/- 15% versus -29 +/- 17%, respectively) (P = .48). Furthermore, the change in DFT from baseline in response to lidocaine differed significantly between monophasic and biphasic waveform groups (92 +/- 29% versus -5.66 +/- 15%) (P < .0002), whereas the change from baseline in response to D5W did not differ between monophasic and biphasic waveforms (-0.5 +/- 29% versus -29 +/- 17%) (P = .34).

CONCLUSIONS

Compared with placebo groups, DFT values increased during lidocaine treatment to a much greater degree in the monophasic waveform group than in the biphasic waveform group receiving lidocaine. These data support our hypothesis that antiarrhythmic drugs can affect the defibrillation efficacy of monophasic waveforms differently than that of biphasic waveforms.

Increase in defibrillation threshold in non-thoracotomy implantable defibrillators using a biphasic waveform

We have previously reported a chronic increase in defibrillation threshold in a non-thoracotomy implantable cardioverter-defibrillator (ICD) system using monophasic waveforms. To determine if this phenomenon is related to the lead system or the waveform used, we studied the chronic defibrillation threshold in consecutive patients receiving an ICD capable of delivering biphasic waveforms with the same lead system previously evaluated. Twenty-five patients received an ICD with biphasic shock waveform and have been followed for 4 to 15 months. All have undergone defibrillation threshold measurements using the identical testing protocol with biphasic waveforms at implant and at 2 months. Coronary artery disease was present in 15, idiopathic dilated cardiomyopathy in 9, and valvular heart disease in 1. The presenting arrhythmia was ventricular fibrillation in 11, ventricular tachycardia in 10, and syncope with inducible ventricular tachycardia in 4. The configuration of the shocking electrodes was randomized; the lead-only configuration was used in 14 patients (56%), and a subcutaneous patch was used in the remaining patients. Mean defibrillation threshold using a step-down technique was 9.8 +/- 1.0 J at implant, 13.2 +/- 1.6 J at 2 months, and 12.4 +/- 1.5 J at 6 months (p = 0.01 by analysis of variance). There was no change in clinical status, cardiac size, radiographic lead position, or impedance between implant and 2 months. These findings suggest the increase in defibrillation threshold in this ICD system is not related to the type of waveform used, but rather is a feature of non-thoracotomy as opposed to epicardial electrodes.

Analog-digital recording of current and voltage during high voltage DC shocks

Efficient on-line digitization is the prerequisite for computerized analysis of the electrical phenomena occurring during defibrillation. Conventional hardware presently provides only limited time resolution. The performance of various digitization rates for recording of voltage, current, and calculation of derived quantities like impedance, energy, and defibrillator capacitance was investigated. It was assessed both experimentally and by computer simulation of a trapezoidal discharge (of 9 msec duration into a constant resistive load of 50 omega with a defibrillator capacitance of 132 microF). The accuracy achieved with different digitization rates is given. For example, an accuracy of 1% for analog-digital conversion for impedance calculation during this kind of DC shock requires a sampling rate of 8 kHz without, and a rate of 1 kHz with linear interpolation to correct for the hardware dependent error due to sequential sampling.

CONCLUSION

Highly efficient analog-digital conversion of delivered voltage and current during DC shocks is available within the limits of conventional inexpensive hardware.

Interactions between drugs and devices: experimental and clinical studies

It is important to understand the potential interactions between the implantable cardioverter defibrillator (ICD) and antiarrhythmic therapy in patients who receive pharmacologic therapy as an adjunct to ICD therapy. In our cohort of 101 patients, antiarrhythmic agents were prescribed in 67% of the patients during long-term therapy for the following reasons: to suppress ventricular tachycardia/ventricular fibrillation episodes (50%), to lower the rate of ventricular tachycardia (19%), to prevent supraventricular tachyarrhythmia (21%), and for other reasons (10%). The potential influence of antiarrhythmic drugs on the defibrillation threshold (DFT) is the most important issue. In animal studies lidocaine increased the DFT in a dose-dependent manner. Quinidine, procainamide, propafenone, and flecainide did not affect the DFT or, in some cases, led to a small increase. Sotalol even decreased the energy requirements for internal defibrillation. In a prospective investigation we were able to document a significant increase of DFT (from 14.1 + 3.0 to 20.9 + 5.4 J, p < 0.001) by the use of amiodarone (400 mg/day), whereas this effect was not found in patients who received mexiletine (720 mg/day). In conclusion, the DFT or the safety margin for defibrillation should be known before antiarrhythmic agents are administered to patients with an ICD. In case of a small safety margin, the DFT should be reassessed after antiarrhythmic drug therapy is begun.

A pilot study: defibrillation thresholds in dogs are similar with isoflurane, halothane, and pentobarbital

The objective of this pilot study was to determine if three common anesthetic drugs have differing effects on the measurement of defibrillation thresholds (DFT) in dogs. The drugs compared were pentobarbital, isoflurane, and halothane. We used six dogs, which were surgically instrumented, in a chronic study design. Each dog had two internal defibrillation patches placed on its heart, which were used to deliver the defibrillation energy. DFT was determined while each dog was anesthetized under each of the listed drugs in a crossover design. This pilot study suggests that differences in DFT due to the anesthetic drugs is not significant in studies with low numbers of animals (halothane 14.5 +/- 1.0, isoflurane 14.2 +/- 1.0, pentobarbital 12.8 +/- 1.0; P = NS; mean +/- SE). The variation in DFT between individual animals is much larger than the difference in DFT due to the drugs.

Rise in chronic defibrillation thresholds in nonthoracotomy implantable defibrillator

BACKGROUND

To establish the chronic stability of defibrillation thresholds (DFTs) in a transvenous cardioverter/defibrillator (TCD) system, we studied 37 consecutive patients with TCD systems implanted for > 6 months.

METHODS AND RESULTS

DFT was measured by a step-down method at implant and 2 and 6 months later. The mean ejection fraction was 34.5 +/- 14.3%. Coronary artery disease with previous myocardial infarction was present in 31 patients. The mean DFT rose from 13.3 +/- 4.3 J at implant to 16.5 +/- 4.7 J at 2 months (P < .001) and 17.6 +/- 5.4 J at 6 months (P < .0001). ANOVA revealed a statistically significant rise in DFT over time (P < .0005). At 2 months, 25 patients had a rise in DFT, and 14 had a rise > or = 5 J. The observed rise at 2 months persisted in 19 patients. A chronic rise, defined as > or = 5 J rise at 6 months, occurred in 17 patients. Univariate analysis of clinical as well as implant variables revealed no predictors of a rise in DFT in this group.

CONCLUSIONS

We conclude that there is a significant rise in DFT at 2 and 6 months in this TCD system. Although the chronic threshold remained well within the available energy range of the pulse generator, this observation has important implications for implantation guidelines, programming, and future pulse generator development for TCD patients.

Clinical characteristics and outcome of patients with high defibrillation thresholds. A multicenter study

BACKGROUND

Successful defibrillation by an implantable cardioverter-defibrillator (ICD) depends on its ability to deliver shocks that exceed the defibrillation threshold. This study was designed to identify clinical characteristics that may predict the finding of an elevated defibrillation threshold and to describe the outcome of patients with high defibrillation thresholds.

METHODS AND RESULTS

The records of 1,946 patients from 12 centers were screened to identify 90 patients (4.6%) with a defibrillation threshold greater than or equal to 25 J. Excluding three patients who received ICDs that delivered greater than 30 J, there were 81 men and six women with a mean age of 59.5 +/- 10.1 years, a mean left ventricular ejection fraction of 0.32 +/- 0.14, and a 76% prevalence of coronary artery disease. Sixty-one patients (70%) were receiving antiarrhythmic drugs, and 45 (52%) were receiving amiodarone. Seventy-one patients (82%) received an ICD. Death occurred in 27 patients--19 of the 71 (27%) with an ICD (eight arrhythmic), and eight of the 16 (50%) without an ICD (four arrhythmic). Actuarial survival for all patients at 5 years was 67%. Actuarial survival rates at 2 years for patients with and without an ICD were 81% and 36%, respectively (p = 0.0024). Actuarial survival at 5 years for the ICD patients was 73%; no patient without an ICD has lived longer than 32 months. Actuarial survival free of arrhythmic death in the ICD patients at 5 years was 84%. Although the only variable to predict survival was ICD implantation (p = 0.003), it is entirely possible that in this retrospective analysis, clinical selection decisions to implant or to not implant an ICD differentiated patients destined to have better or worse outcomes, respectively.

CONCLUSIONS

Antiarrhythmic drug use may be causally related to the finding of an elevated defibrillation threshold. When patients with high defibrillation thresholds receive an ICD, arrhythmic death remains an important risk (42% of deaths in these patients were arrhythmia related, with 16% actuarial incidence at 5 years). Vigorous testing to optimize patch location can potentially benefit patients by enhancing the margin of safety for effective defibrillation.