Current-based versus energy-based ventricular defibrillation: a prospective study

Abuse of defibrillator pads: Suicide by electrocution

We report a Case of a medical professional, with a very good knowledge of emergency medical devices, who committed suicide using defibrillator pads to electrocute. The decedent attached the defibrillator pads in the standard position on his chest and manually connected the terminals to an extension cord through an additional switching device. When he switched on the device, electricity passed through his heart. The autopsy showed attached defibrillator pads and severe skin burns beneath it. Scene findings were consistent with suicide. The cause of death was electrocution and manner of death was suicide. Suicide by electrocution is uncommon and suicide by electrocution using defibrillator pads has not been reported in English literature.

A framework of current based defibrillation improves defibrillation efficacy of biphasic truncated exponential waveform in rabbits

Defibrillation is accomplished by the passage of sufficient current through the heart to terminate ventricular fibrillation (VF). Although current-based defibrillation has been shown to be superior to energy-based defibrillation with monophasic waveforms, defibrillators with biphasic waveforms still use energy as a therapeutic dosage. In the present study, we propose a novel framework of current-based, biphasic defibrillation grounded in transthoracic impedance (TTI) measurements: adjusting the charging voltage to deliver the desired current based on the energy setting and measured pre-shock TTI; and adjusting the pulse duration to deliver the desired energy based on the output current and intra-shock TTI. The defibrillation efficacy of current-based defibrillation was compared with that of energy-based defibrillation in a simulated high impedance rabbit model of VF. Cardiac arrest was induced by pacing the right ventricle for 60 s in 24 New Zealand rabbits (10 males). A defibrillatory shock was applied with one of the two defibrillators after 90 s of VF. The defibrillation thresholds (DFTs) at different pathway impedances were determined utilizing a 5-step up-and-down protocol. The procedure was repeated after an interval of 5 min. A total of 30 fibrillation events and defibrillation attempts were investigated for each animal. The pulse duration was significantly shorter, and the waveform tilt was much lower for the current-based defibrillator. Compared with energy-based defibrillation, the energy, peak voltage, and peak current DFT were markedly lower when the pathway impedance was > 120 Ω, but there were no differences in DFT values when the pathway impedance was between 80 and 120 Ω for current-based defibrillation. Additionally, peak voltage and the peak current DFT were significantly lower for current-based defibrillation when the pathway impedance was < 80 Ω. In sum, a framework of adjusting the charging voltage and shock duration to deliver constant energy for low impedance and constant current for high impedance via pre-shock and intra-shock impedance measurements, greatly improved the defibrillation efficacy of high impedance by lowering the energy DFT.

Role of peak current in conversion of patients with ventricular fibrillation

INTRODUCTION

Peak currents are the final arbiter of defibrillation in patients with ventricular fibrillation (VF). However, biphasic defibrillators continue to use energy in joules for electrical conversion in hopes that their impedance compensation properties will address transthoracic impedance (TTI), which must be overcome when a fixed amount of energy is delivered. However, optimal peak currents for conversion of VF remain unclear. We aimed to determine the role of peak current and optimal peak levels for conversion in collapsed VF patients.

METHODS

Adult, non-pregnant patients presenting with non-traumatic VF were included in the study. All defibrillations that occurred were included. Impedance values during defibrillation were used to calculate peak current values. The endpoint was return of spontaneous circulation (ROSC).

RESULTS

Of the 197 patients analysed, 105 had ROSC. Characteristics of patients with and without ROSC were comparable. Short duration of collapse < 10 minutes correlated positively with ROSC. Generally, patients with average or high TTI converted at lower peak currents. 25% of patients with high TTI converted at 13.3 ± 2.3 A, 22.7% with average TTI at 18.2 ± 2.5 A and 18.6% with low TTI at 27.0 ± 4.7 A (p = 0.729). Highest peak current conversions were at < 15 A and 15-20 A. Of the 44 patients who achieved first-shock ROSC, 33 (75.0%) received < 20 A peak current vs. > 20 A for the remaining 11 (25%) patients (p = 0.002).

CONCLUSION

For best effect, priming biphasic defibrillators to deliver specific peak currents should be considered.

Average current is better than peak current as therapeutic dosage for biphasic waveforms in a ventricular fibrillation pig model of cardiac arrest

OBJECTIVE

Defibrillation current has been shown to be a clinically more relevant dosing unit than energy. However, the effects of average and peak current in determining shock outcome are still undetermined. The aim of this study was to investigate the relationship between average current, peak current and defibrillation success when different biphasic waveforms were employed.

METHODS

Ventricular fibrillation (VF) was electrically induced in 22 domestic male pigs. Animals were then randomized to receive defibrillation using one of two different biphasic waveforms. A grouped up-and-down defibrillation threshold-testing protocol was used to maintain the average success rate of 50% in the neighborhood. In 14 animals (Study A), defibrillations were accomplished with either biphasic truncated exponential (BTE) or rectilinear biphasic waveforms. In eight animals (Study B), shocks were delivered using two BTE waveforms that had identical peak current but different waveform durations.

RESULTS

Both average and peak currents were associated with defibrillation success when BTE and rectilinear waveforms were investigated. However, when pathway impedance was less than 90Ω for the BTE waveform, bivariate correlation coefficient was 0.36 (p=0.001) for the average current, but only 0.21 (p=0.06) for the peak current in Study A. In Study B, a high defibrillation success (67.9% vs. 38.8%, p<0.001) was observed when the waveform delivered more average current (14.9±2.1A vs. 13.5±1.7A, p<0.001) while keeping the peak current unchanged.

CONCLUSION

In this porcine model of VF, average current was better than peak current to be an adequate parameter to describe the therapeutic dosage when biphasic defibrillation waveforms were used. The institutional protocol number: P0805.

Electrical features of eighteen automated external defibrillators: a systematic evaluation

AIM

Assessment and comparison of the electrical parameters (energy, current, first and second phase waveform duration) among eighteen AEDs.

METHOD

Engineering bench tests for a descriptive systematic evaluation in commercially available AEDs. AEDs were tested through an ECG simulator, an impedance simulator, an oscilloscope and a measuring device detecting energy delivered, peak and average current, and duration of first and second phase of the biphasic waveforms. All tests were performed at the engineering facility of the Lombardia Regional Emergency Service (AREU).

RESULTS

Large variations in the energy delivered at the first shock were observed. The trend of current highlighted a progressive decline concurrent with the increases of impedance. First and second phase duration varied substantially among the AEDs using the exponential biphasic waveform, unlike rectilinear waveform AEDs in which phase duration remained relatively constant.

CONCLUSIONS

There is a large variability in the electrical features of the AEDs tested. Energy is likely not to be the best indicator for strength dose selection. Current and shock duration should be both considered when approaching the technical features of AEDs. These findings may prompt further investigations to define the optimal current and duration of the shock waves to increase the success rate in the clinical setting.

Current is better than energy as predictor of success for biphasic defibrillatory shocks in a porcine model of ventricular fibrillation

OBJECTIVE

The evidence that monophasic defibrillation success is mainly determined by current is secure. However, modern defibrillators use biphasic waveforms. The aim of this study was to compare energy, peak voltage and peak current in predicting biphasic shock success in a porcine model of ventricular fibrillation (VF) where the impedance varies within a wide of ranges.

METHODS

In 14 domestic male pigs weighing between 27 and 38 kg, VF was electrically induced and untreated for 15 s. Animals were randomized to receive defibrillation attempts from one of two defibrillators with different impedance compensation methods. A grouped up-and-down defibrillation threshold testing protocol was used to maintain the average success rate in the neighborhood of 50%. After a recovery interval of 5 min, the testing sequence was repeated for a total of 60 test shocks for each animal.

RESULTS

A high defibrillation success was observed when high peak current was delivered. The area under ROC curve for predicting shock success was 0.681 for peak current, 0.585 for peak voltage and 0.562 for energy. The odds ratio revealed that peak current was a better predictor (OR=1.321, p<0.001) for defibrillation outcome compared with energy (OR=0.979, p<0.001) and peak voltage (OR=1.000, p=0.69) when multivariable logistic regression was conducted.

CONCLUSION

In this porcine model of VF within a wide range of transthoracic impedance, peak current was a better indicator for shock success than the currently used energy for biphasic defibrillatory shocks. This finding may encourage design of new current-based biphasic defibrillators.

Comparison of defibrillation efficacy and survival associated with right versus left pectoral placement for implantable defibrillators

The preferred location for an implantable cardioverter-defibrillator (ICD) generator is the left pectoral region as a result of the shock vector formed by the active can and the lead system. However, a right pectoral site is necessary when left-sided implantation is contraindicated. The Low Energy Safety Study was a prospective, randomized trial conducted to assess chronic defibrillation efficacy in 627 patients, including 37 (5.9%) who received right pectoral implants and 590 (94.1%) who received left pectoral implants. Patients were followed for a mean of 24 +/- 13 months. There were no significant differences observed between patients who received left versus right pectoral implants in age, gender, indications, New York Heart Association classification, or ejection fraction. Patients who received a right pectoral implant had higher defibrillation thresholds at implantation (10.6 +/- 3.8 J) than those who received a left pectoral implant (8.9 +/- 4.2 J, p = 0.01) despite similar shock impedances. The conversion efficacy for spontaneous arrhythmia episodes among patients who received right and left pectoral implants were not significantly different (33 of 33 [100%] vs 255 of 263 [97%], respectively; p = 0.31). In addition, the conversion efficacy for induced ventricular fibrillation episodes were also similar (187 of 188 [99%] on the right vs 2429 of 2475 [98%] on the left, p = 0.18). However, the all-cause mortality rate was higher for patients who received right-sided implants (hazard ratio 1.93, p <0.004). In conclusion, defibrillation thresholds are higher with right pectoral implants compared with left-sided implants, but with a proper energy safety margin, there are no significant differences in spontaneous or induced shock conversion efficacy. However, the near doubling of the mortality rate among patients with right-sided implants needs to be considered when recommending such device therapy.

Examination of a middle cardiac vein defibrillation coil as stand-alone anode, auxiliary anode, and bystander electrode in a transvenous defibrillation circuit

In porcine studies anodes in the middle cardiac vein compare favorably with those in the RV. It has not been demonstrated whether the RV and middle cardiac vein or the middle cardiac vein alone anodes are superior when shocking to a conventional SVC and active housing cathode nor whether a bystander middle cardiac vein electrode exerts a passive electrode affect. Twelve pigs were anesthetized and had an active housing implanted in the left pectoral region and defibrillation coils placed at the RV apex and in the SVC. A custom-made defibrillation coil (Ela Medical) was advanced into the middle cardiac vein through a 9 Fr transvenous catheter. The DFT for three anodes (RV; RV and middle cardiac vein; middle cardiac vein) to the SVC and active housing was then assessed by a three reversal binary search, the order of testing was randomized. In seven animals DFT was assessed in the same way for the configuration of RV to SVC and active housing twice more, with and without a bystander middle cardiac vein coil electrode in place. The results were middle cardiac vein 7.5 +/- 1.7 J, RV and middle cardiac vein 7.3 +/- 1.7 J reduced DFT significantly compared to RV 13.8 +/- 4.2 J (both P < 0.000). There was no significant difference between the middle cardiac vein and the middle cardiac vein and RV (P = 0.67, 95% CI for difference -0.64-0.96). The DFT of RV to SVC and the active housing was the same with (13.2 +/- 4.0) and without (13.7 +/- 4.2) the middle cardiac vein bystander coil in place (P = 0.177, 95% CI for difference -0.33-1.33 J). Shocking to a SVC and active housing cathode, middle cardiac vein, and RV and middle cardiac vein anodes are equally effective in lowering DFT compared to the RV. The middle cardiac vein coil electrode does not exert a passive electrode affect on the RV to the SVC and active housing defibrillation.

[Transthoracic defibrillation. Physiologic and pathophysiologic principles and their role in the outcome of resuscitation]

As one major link in the chain of survival, early transthoracic (external) cardiac defibrillation is aimed at the termination of ventricular flutter and ventricular fibrillation. Most important to the success of defibrillation is the passage of a defined amount of current through a critical mass of heart muscle. Different transthoracic resistances reduce the effective density of the current within the heart. As for other therapeutic intervention procedures, recommendations for the optimal strength of current to be applied to the fibrillating heart need to be evaluated and defined for therapeutical defibrillation too. Unnecessarily high current density causes damage to the heart and should be prevented. By using biphasic waveforms in contrast to monophasic impulses, the amount of current can be reduced but the same or even higher efficacy is attained. Therefore possible myocardial damage might be clearly reduced. Even with individually altered thoracic impedance effective conversion of cardiac rhythm can be achieved by device-controlled compensation and biphasic waveforms. According to their different mechanisms or origin (electrically induced or spontaneously caused by organic heart disease) the probability of successful conversion of the cardiac rhythm by one single electrical impulse varies. The optimum point in time for defibrillation during resuscitation needs to be redefined. In order to improve comparability, further studies should use standardized definitions for successful defibrillation relating to the resulting cardiac rhythm.

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) Omega, 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.

Temporal decline in defibrillation thresholds with an active pectoral lead system

OBJECTIVES

The objective of this study was to characterize temporal changes in defibrillation thresholds (DFTs) after implantation with an active pectoral, dual-coil transvenous lead system.

BACKGROUND

Ventricular DFTs rise over time when monophasic waveforms are used with non-thoracotomy lead systems. This effect is attenuated when biphasic waveforms are used with transvenous lead systems; however, significant increases in DFT still occur in a minority of patients. The long-term stability of DFTs with contemporary active pectoral lead systems is unknown.

METHODS

This study was a prospective assessment of temporal changes in DFT using a uniform testing algorithm, shock polarity and dual-coil active pectoral lead system. Thresholds were measured at implantation, before discharge and at long-term follow-up (70 +/- 40 weeks) in 50 patients.

RESULTS

The DFTs were 9.2 +/- 5.4 J at implantation, 8.3 +/- 5.8 J before discharge and 6.9 +/- 3.6 J at long-term follow-up (p < 0.01 by analysis of variance; p < 0.05 for long-term follow-up vs. at implantation or before discharge). The effect was most marked in a prespecified subgroup with high implant DFTs (> or =15 J). No patient developed an inadequate safety margin (< 9 J) during follow-up.

CONCLUSIONS

The DFTs declined significantly after implantation with an active pectoral, dual-coil transvenous lead system, and no clinically significant increases in DFT were observed. Therefore, routine defibrillation testing may not be required during the first two years after implantation with this lead system, in the absence of a change in the cardiac substrate or treatment with antiarrhythmic drugs.

ICD therapy in the new millennium

Remarkable progress has been made in the 15 years since ICD therapy was approved for human use. The early "shock boxes" had almost no diagnostic capabilities and required thoracotomy for epicardial patch implantation with typical duration of hospitalization of about a week. Pulse-generator longevity was less than 2 years. Modern devices provide detailed information about the morphology and rate of electrocardiographic signals before, during, and after arrhythmia therapy. The down-sizing of pulse generators and improvements in lead design and shock waveforms allow the simplicity of defibrillator implantation to approach that of pacemakers, with defibrillation thresholds comparable with those initially observed with epicardial patches. Despite the marked reduction in size and increase in diagnostic capabilities, device longevity is now longer than 6 years. Routine outpatient ICD implantation is presently feasible and will increase in frequency if ongoing primary prevention trials prove beneficial. Further advances in lead technology and arrhythmia discrimination should increase the efficacy and reliability of therapy. Finally, devices have the capabilities to treat multiple problems in addition to life-threatening ventricular arrhythmias including atrial arrhythmias and congestive heart failure.

Transthoracic cardioversion of atrial fibrillation: comparison of rectilinear biphasic versus damped sine wave monophasic shocks

BACKGROUND

Clinical studies have shown that biphasic shocks are more effective than monophasic shocks for ventricular defibrillation. The purpose of this study was to compare the efficacy of a rectilinear biphasic waveform with a standard damped sine wave monophasic waveform for the transthoracic cardioversion of atrial fibrillation.

METHODS AND RESULTS

In this prospective, randomized, multicenter trial, patients undergoing transthoracic cardioversion of atrial fibrillation were randomized to receive either damped sine wave monophasic or rectilinear biphasic shocks. Patients randomized to the monophasic protocol (n=77) received sequential shocks of 100, 200, 300, and 360 J. Patients randomized to the biphasic protocol (n=88) received sequential shocks of 70, 120, 150, and 170 J. First-shock efficacy with the 70-J biphasic waveform (60 of 88 patients, 68%) was significantly greater than that with the 100-J monophasic waveform (16 of 77 patients, 21%, P<0.0001), and it was achieved with 50% less delivered current (11+/-1 versus 22+/-4 A, P<0.0001). Similarly, the cumulative efficacy with the biphasic waveform (83 of 88 patients, 94%) was significantly greater than that with the monophasic waveform (61 of 77 patients, 79%; P=0.005). The following 3 variables were independently associated with successful cardioversion: use of a biphasic waveform (relative risk, 4.2; 95% confidence intervals, 1.3 to 13.9; P=0.02), transthoracic impedance (relative risk, 0.64 per 10-Omega increase in impedance; 95% confidence intervals, 0.46 to 0.90; P=0.005), and duration of atrial fibrillation (relative risk, 0.97 per 30 days of atrial fibrillation; 95% confidence intervals, 0.96 to 0.99; P=0.02).

CONCLUSIONS

For transthoracic cardioversion of atrial fibrillation, rectilinear biphasic shocks have greater efficacy (and require less energy) than damped sine wave monophasic shocks.

Optimization of transvenous coil position for active can defibrillation thresholds

INTRODUCTION

Lead systems that include an active pectoral pulse generator are now standard for initial defibrillator implantations. However, the optimal transvenous lead system and coil location for such active can configurations are unknown. The purpose of this study was to evaluate the benefit and optimal position of a superior vena cava (SVC) coil on defibrillation thresholds with an active left pectoral pulse generator and right ventricular coil.

METHODS AND RESULTS

This prospective, randomized study was performed on 27 patients. Each subject was evaluated with three lead configurations, with the order of testing randomized. Biphasic shocks were delivered between the right ventricular coil and an active can alone (unipolar), or an active can in common with the proximal coil positioned either at the right atrial/SVC junction (low SVC) or in the left subclavian vein (high SVC). Stored energies at defibrillation threshold were higher for the single-coil, unipolar configuration (11.2 +/- 6.6 J) than for the high (8.9 +/- 4.2 J) or low (8.5 +/- 4.2 J) SVC configurations (P < 0.01). Moreover, 96% of subjects had low (< or = 15 J) thresholds with the SVC coil in either position compared with 81% for the single-coil configuration. Shock impedance (P < 0.001) was increased with the unipolar configuration, whereas peak current was reduced (P < 0.001).

CONCLUSION

The addition of a proximal transvenous coil to an active can unipolar lead configuration reduces defibrillation energy requirements. The position of this coil has no significant effect on defibrillation thresholds.

Comparison of a novel rectilinear biphasic waveform with a damped sine wave monophasic waveform for transthoracic ventricular defibrillation. ZOLL Investigators

OBJECTIVES

We compared the efficacy of a novel rectilinear biphasic waveform, consisting of a constant current first phase, with a damped sine wave monophasic waveform during transthoracic defibrillation.

BACKGROUND

Multiple studies have shown that for endocardial defibrillation, biphasic waveforms have a greater efficacy than monophasic waveforms. More recently, a 130-J truncated exponential biphasic waveform was shown to have equivalent efficacy to a 200-J damped sine wave monophasic waveform for transthoracic ventricular defibrillation. However, the optimal type of biphasic waveform is unknown.

METHODS

In this prospective, randomized, multicenter trial, 184 patients who underwent ventricular defibrillation were randomized to receive a 200-J damped sine wave monophasic or 120-J rectilinear biphasic shock.

RESULTS

First-shock efficacy of the biphasic waveform was significantly greater than that of the monophasic waveform (99% vs. 93%, p = 0.05) and was achieved with nearly 60% less delivered current (14 +/- 1 vs. 33 +/- 7 A, p < 0.0001). Although the efficacy of the biphasic and monophasic waveforms was comparable in patients with an impedance < 70 ohms (100% [biphasic] vs. 95% [monophasic], p = NS), the biphasic waveform was significantly more effective in patients with an impedance > or = 70 ohms (99% [biphasic] vs. 86% [monophasic], p = 0.02).

CONCLUSIONS

This study demonstrates a superior efficacy of rectilinear biphasic shocks as compared with monophasic shocks for transthoracic ventricular defibrillation, particularly in patients with a high transthoracic impedance. More important, biphasic shocks defibrillated with nearly 60% less current. The combination of increased efficacy and decreased current requirements suggests that biphasic shocks as compared with monophasic shocks are advantageous for transthoracic ventricular defibrillation.

The effect of shock configuration and delivered energy on defibrillation impedance

Shock impedance is an important determinant of defibrillation efficacy. Lead configuration, shock polarity, and delivered energy can affect shock impedance, but these variables have not been studied in active can lead systems. The present study was a prospective evaluation of 25 patients undergoing initial transvenous defibrillator implantation. In all patients, a dual coil lead and pectoral emulator were placed and three lead configurations were tested in random order: Lead (distal to proximal coil), unipolar (distal coil to can), and triad (distal coil to can + proximal coil). Shock energies of 0.1- to 15-J shock were evaluated. Impedance increased a mean of 21% as delivered energy was decreased (P < 0.001), an effect independent of lead configuration. At all delivered energies, impedances in the unipolar configuration were about 40% higher than triad, while the lead configuration was about 20% higher than triad (ps < 0.001). Polarity did not affect impedance. These results indicate that transvenous lead configurations and delivered energy, but not polarity, significantly influence shock impedance. The magnitude of the increase of impedance at low energies is independent of the shocking pathway. This effect has important implications for low energy shocks used to terminate atrial fibrillation or ventricular tachycardia.

The effect of delivered energy on defibrillation shock impedance

The impedance of internal defibrillator shocks is an important determinant of defibrillation efficacy. To assess the effect of delivered energy on impedance, we studied 97 patients with 4 different lead systems. The lead systems evaluated were two epicardial patches, a hybrid system of a patch and right atrial coil, a dual coil transvenous lead and a transvenous lead with a subcutaneous patch. Impedances were measured for 6 shock energies between 0.1 and 30 J. Shock impedance increased at low energies for all lead systems (p < 0.001), although the rate of increase varied markedly between systems. The energy factor (FE), which is the ratio of impedances for the 0.1 and 10 J shocks, was least for the platinum transvenous lead (1.2 +/- 0.02) and greatest for the titanium hybrid lead (4.2 +/- 0.2). Reversing the polarity of the hybrid lead markedly attenuated the impedance rise. These findings indicate that there is at least a modest rise (20%) of shock impedance at very low delivered energies. The largest increases noted with titanium lead systems are primarily due to polarization. Titanium transvenous leads should be avoided when low energy shocks are utilized such as for the cardioversion of ventricular tachycardia or atrial fibrillation.

Effects of transthoracic impedance and peak current flow on defibrillation success in a prehospital setting

STUDY OBJECTIVE

To assess whether transthoracic impedance and peak current are determinants of defibrillation success in patients with out-of-hospital ventricular fibrillation (VF).

METHODS

A retrospective cohort study was carried out in a suburban Canadian EMS system. Participants were patients who experienced out-of-hospital cardiac arrest in the regional municipality of Ottawa-Carleton, had VF rhythm at presentation, and received countershocks from the Laerdal Heartstart 2000 automated external defibrillator.

RESULTS

A total of 310 patients met the inclusion criteria. Collectively they received 717 countershocks. The first shocks were successful in converting VF rhythm 25.5% of the time. The most important determinant of shock success was the interval from when the call was received until delivery of the first shock (P<.01). Length of time at scene, current, impedance, and patient age were not significant determinants of success of first shock. The time interval until first shock was also a determinant of survival (P<.01). EMS response time, whether the arrest was witnessed, initial impedance, and current were not determinants of survival.

CONCLUSION

OHCA shock success and survival are associated with EMS system factors such as the interval from when the call was received until delivery of the first shock. The importance of impedance and peak current remain theoretic for out-of-hospital defibrillation success and did not influence defibrillation success in this study.

Comparison of single- and dual-coil active pectoral defibrillation lead systems

OBJECTIVES

The purpose of this study was to compare defibrillation thresholds with lead systems consisting of an active left pectoral electrode and either single or dual transvenous coils.

BACKGROUND

Lead systems that include an active pectoral pulse generator reduce defibrillation thresholds and permit transvenous defibrillation in nearly all patients. A further improvement in defibrillation efficacy is desirable to allow for smaller pulse generators with a reduced maximal output.

METHODS

This prospective study was performed in 50 consecutive patients. Each patient was evaluated with two lead configurations with the order of testing randomized. Shocks were delivered between the right ventricular coil and either an active can alone (single coil) or an active can with the proximal atrial coil (dual coil). The right ventricular coil was the cathode for the first phase of the biphasic defibrillation waveform.

RESULTS

Delivered energy at the defibrillation threshold was 10.1+/-5.0 J for the single-coil configuration and 8.7+/-4.0 J for the dual-coil configuration (p < 0.02). Moreover, 98% of patients had low (<15 J) thresholds with the dual-coil lead system, compared with 88% of patients with the single-coil configuration (p=0.05). Leading edge voltage (p < 0.001) and shock impedance (p < 0.001) were also decreased with the dual-coil configuration, although peak current was increased (p < 0.001).

CONCLUSIONS

A dual-coil, active pectoral lead system reduces defibrillation energy requirements compared with a single-coil, unipolar configuration.

Cardiopulmonary resuscitation: a review for clinicians

Attempts at cardiopulmonary resuscitation (CPR) date from antiquity, but it is only in the last 50 years that a scientifically-based methodology has been developed. External chest compressions is the standard method for managing circulatory arrest, however, numerous alterations of this technique have been proposed in attempts to improve outcome from CPR. Defibrillation is the single most important therapy for the management of ventricular fibrillation or pulseless ventricular tachycardia. Adrenergic agents used to improve myocardial and cerebral perfusion are also the subject of considerable investigation with new agents entering clinical study. This paper reviews the history, current techniques and pharmacotherapy as well as controversial issues in the management of patients with cardiac arrest.

Iridium oxide-coated defibrillation electrode: reduced shock polarization and improved defibrillation efficacy

BACKGROUND

Transvenous implantable cardioverter-defibrillator (ICD) leads are designed to deliver electric shocks to the heart for termination of ventricular dysrhythmias. However, the efficiency of different lead materials has not been well studied. This study compares an ICD lead coated with iridium oxide (IROX), a material that reduces shock-induced polarization, with an otherwise identical, uncoated lead.

METHODS AND RESULTS

The defibrillation threshold (DFT) was determined in 13 swine with both IROX-coated and uncoated ICD leads paired with an uncoated "can" electrode. The leads were exchanged through a Teflon sheath to reproduce the intracardiac position. The delivered energy DFT of the IROX-coated lead was 15.9+/-5.4 J and was significantly lower than the delivered energy DFT of the uncoated lead (19.1+/-5.1 J; P<.006). The initial lead impedance was equivalent in both leads (IROX, 41.7+/-5.8 omega; uncoated, 41.3+/-4.7 omega; P=NS) at DFT. However, the impedance rose by 7.3+/-2.0 omega during the first phase and by 3.7+/-2 omega during the second phase with the uncoated lead, whereas the corresponding impedance change was 1.0+/-0.3 omega during phase 1 and 1.6+/-0.5 omega during phase 2 (P<.01 each phase) when the IROX-coated lead was used.

CONCLUSIONS

This study shows that an IROX coating of this lead system significantly lowers the DFT energy in the swine model. The blunting of the impedance rise by the IROX coating that is seen is consistent with a reduction in electrode polarization.

Lead system optimization for transvenous defibrillation

Lead systems that include an active pectoral shell reduce defibrillation thresholds and permit transvenous defibrillation in nearly all patients. A further improvement in defibrillation efficacy is desirable to allow for smaller pulse generators with a reduced maximum output. Accordingly, the purpose of this study was to compare defibrillation thresholds with multiple transvenous lead systems including those with an active pectoral shell to determine which system would optimize defibrillation energy requirements. This prospective study was performed on 21 consecutive patients. Each subject was evaluated with 3 lead configurations with the order of testing randomized. The configurations were a dual coil transvenous lead (lead), the distal right ventricular coil and pectoral pulse generator shell (unipolar), and all 3 components (triad). The right ventricular coil was the cathode for the first phase of the biphasic defibrillation waveform. Delivered energy at defibrillation threshold was 11.2 +/- 3.4 J for the lead configuration, 10.1 +/- 5.2 J for the unipolar configuration, and 7.8 +/- 3.6 J for the triad configuration (p <0.01). Leading edge voltage (p <0.01) and shock impedance (p <0.001) were also decreased for the triad configuration compared with the lead or unipolar configurations, whereas peak current was minimized with the unipolar configuration (p <0.01). We conclude that the combination of a dual coil, transvenous lead and an active pectoral shell reduces defibrillation energy requirements compared with either the lead alone or unipolar configuration. Moreover, the defibrillation thresholds were < or =15 J in all patients using the triad lead system.

Importance of electrode conductive surface area and edge effects on ventricular defibrillation efficacy

INTRODUCTION

The role of edge effects and electrode surface area of the right ventricular (RV) transvenous lead (TVL) on defibrillation efficacy is unknown.

METHODS AND RESULTS

Defibrillation threshold (DFT) testing was conducted randomly in 12 dogs using ring electrode leads in an RV/SVC (superior vena cava) or RV/SVC/patch system. The leads (RV-4, RV-8t, RV-8, RV-15) had electrode surface areas of 20%, 20%, 40%, and 70%, respectively. A computer model predicted the magnitude of electrode surface current (RV-8t > RV-4 > RV-8 > RV-15) and the potential distribution (PD) at four sites: electrode surface (site a) and at 2 mm (b), 4 mm (c), and 8 mm (d) away from the surface. Despite different near-field PDs (sites a, b, c), PDs were nearly identical at site d. Resistance decreased as the surface area increased. DFT energy for the RV-15 lead was lower than the RV-4 and RV-8t. There was no difference between energy requirements for the RV-15 and RV-8 leads. No difference was found in DFT current for each lead. Comparison of the RV-8t and RV-4 leads showed no difference in DFT energy despite a lower resistance and a greater number of edges.

CONCLUSIONS

Increasing the RV TVL surface area lowered the resistance. However, surface area coverages > or = 40% did not lower DFT energy. No significant change in DFT current occurred despite different predicted near-field current densities. PDs were nearly identical 8 mm from the electrode surface. Thus, the far-field current density appears to play a more important role in determining defibrillation success.

Biphasic waveforms prevent the chronic rise of defibrillation thresholds with a transvenous lead system

OBJECTIVES

The purpose of this study was to compare chronic changes in monophasic and biphasic defibrillation thresholds using a uniform transvenous lead system and testing protocol.

BACKGROUND

Defibrillation thresholds increase over time in patients with nonthoracotomy lead systems. This increase can result in an inadequate chronic defibrillation safety margin and could limit the safety of smaller pulse generators, which have a reduced maximal output. However, previous studies of the temporal changes of defibrillation thresholds evaluated complex lead systems or monophasic shock waveforms, neither of which are used with current technology.

METHODS

This study was a prospective, randomized assessment of the effects of shock waveforms on the changes of transvenous defibrillation thresholds over time. Paired monophasic and biphasic thresholds were measured both at implantation and at follow-up (250 +/- 105 days) in 24 consecutive patients who were not receiving antiarrhythmic drugs. The lead system was a dual-coil Endotak C lead, and reverse polarity shocks (distal coil = anode) were delivered.

RESULTS

Monophasic defibrillation thresholds increased from (mean +/- SD) 13.7 +/- 6.0 J to 16.8 +/- 6.7 J (p = 0.02), whereas biphasic thresholds were unchanged (10.4 +/- 4.3 J to 10.2 +/- 4.8 J, p = 0.86) in the same patients. Shock impedance chronically increased (47.0 omega to 50.5 omega, p = 0.02) and was unaffected by waveform.

CONCLUSIONS

These results indicate that biphasic shocks prevent the chronic increase in defibrillation thresholds with a transvenous lead system.

Chronic rise in monophasic defibrillation thresholds with a transvenous lead system

This study was a prospective evaluation of chronic changes of defibrillation thresholds in 31 clinically stable patients with a single transvenous lead, optimal shock polarity, and uniform testing protocol. At a mean follow-up of 273 +/- 146 days, defibrillation thresholds increased 26%, from 13.2 +/- 5.6 J to 17.1 +/- 6:0 J (p < 0.001), and shock impedance increased from 46.2 +/- 7.0 omega to 51.2 +/- 6.2 omega (p < 0.001).

Current-based transthoracic defibrillation

This study examines in a prospective, multicenter trial the feasibility and advantage of current-based, transthoracic defibrillation. Current-based, damped, sinusoidal waveform shocks of 18, 25, 30, 35, or 40 amperes (A) were administered beginning with 25 A for polymorphic ventricular tachycardia (VT) and ventricular fibrillation (VF) or 18 A for monomorphic VT; success rates were compared with those of energy-based shocks beginning at 200 J for VF/polymorphic VT and 100 J for VT. The current-based shocks were delivered from custom-modified defibrillators that determined impedance in advance of any shock using a "test-pulse" technique; the capacitor then charged to the exact energy necessary to deliver the operator-selected current against the impedance determined by the defibrillator. Three hundred sixty-two patients received > 1 shock for VF, polymorphic VT, or monomorphic VT: 569 current- based shocks and 420 energy-based shocks. Current-based shocks of 35/40 A achieved success rates of up to 74% for VF/polymorphic VT; 30 A shocks terminated 88% of monomorphic VT episodes. Energy-based shocks of 300 J terminated 72% of VF/polymorphic VT; 200-J shocks terminated 89% of monomorphic VT. We could not demonstrate a significant increase in the success rate of current-based shocks over energy-based shocks for patients with high transthoracic impedance; this may be due to inadequate sample size. Thus, current-based defibrillation is clinically feasible and effective. A larger study will be needed to test whether current-based defibrillation is superior to energy-based defibrillation.

Chronic rise in defibrillation threshold with a hybrid lead system

Nonthoracotomy leads have become standard for implantable cardioverter-defibrillators (ICD) because of low perioperative morbidity, mortality, and expense. Reported increases in defibrillation thresholds (DFTs) with these lead systems, however, have raised the possibility of an eventual loss of defibrillation efficacy. The mechanism of this increase is unknown. In contrast, defibrillation efficacy of traditional epicardial lead systems has been demonstrated to remain relatively stable. In the present study, we examined the implantation and chronic DFTs in 45 patients with a hybrid system (a high right atrial coil and an extrapericardial patch) that combines elements from both the thoracotomy and nonthoracotomy approach. The mean threshold increased from 11.7 +/- 3.0 to 15.8 +/- 10.0 J (p < 0.001) and mean impedance increased from 37.0 +/- 7.7 to 48.8 +/- 9.0 ohms (p < 0.0001). There was a marked (> or = 10 J) increase in DFT in 11 patients (24%) including 4 who required reoperation to obtain an adequate safety margin. The increase in DFT was unrelated to any of the analyzed variables. We conclude that the presence of an extrapericardial patch does not prevent the increase in DFT reported with nonthoracotomy lead systems. This increase is unpredictable and occurs in almost 25% of patients.

Impedance to defibrillation countershock: does an optimal impedance exist?

Defibrillation is thought to occur because of changes in the transmembrane potential that are caused by current flow through the heart tissue. Impedance to electric countershock is an important parameter because it is determined by the magnitude and distribution of the current that flows for a specific shock voltage. The impedance is comprised of resistive contributions from: (1) extra-tissue sources, which include the defibrillator, leads, and electrodes; (2) tissue sources, which include intracardiac and extra-cardiac tissue; and (3) the interface between electrode and tissue. Tissue sources dominate the impedance and probably contribute to the wide range of impedance values presented to the defibrillation pulse. Because impedance is not constant within or between subjects, defibrillators must be designed to accommodate these differences without compromising patient safety or therapeutic efficacy. Experimental investigations in animals and humans suggest that impedance changes at several different time scales ranging from milliseconds to years. These alterations are believed to be a result of both electrochemical and physiological mechanisms. It is commonly thought that impedance is optimized when it has been decreased to a minimum, since this allows the most current flow for a given voltage shock. However, if the impedance is lowered by changing the location or size of the electrodes in such a way that current flow is decreased in part of the heart even though current flow is increased elsewhere, then the total voltage, current, and energy needed for defibrillation may increase, not decrease, even though impedance is decreased. A simple boundary element computer model suggests that the most even distribution of current flow through the heart is achieved for those electrode locations in which the impedance across the heart is at or near the maximum cardiac impedance for any location of these particular electrodes. Thus, the optimum shock impedance is achieved when impedance is minimized for extra-tissue and extra-cardiac tissue sources and is at or near a maximum for intracardiac tissue sources.

A three-dimensional finite element model of human transthoracic defibrillation: paddle placement and size

A detailed 3-D finite element model of the conductive anatomy of the human thorax has been constructed to quantitatively assess the current density distribution produced in the heart and thorax during transthoracic defibrillation. The model is based on a series of cross-sectional CT scans and incorporates isotropic conductivities for eight tissues and an approximation of the anisotropic conductivity of skeletal muscle. Current density distributions were determined and compared for four paddle pairs and two paddle sizes. Our results show that the myocardial current density distributions resulting from a defibrillation shock were fairly uniform for the paddle pairs and sizes examined in this study. Specific details of the spatial distribution of the current density magnitudes in the heart were found to depend on paddle placement and size. When the minimum current necessary to defibrillate was delivered, the maximum myocardial current density produced with any of the paddle sizes and positions examined was less than four times the minimum current density necessary to render a myocyte in a fibrillating heart inexcitable, and less than 40% of the damage threshold. These results suggest that common clinically used defibrillation paddle positions have a safety margin as large as 2.5 for current and approximately 6 for energy.

Electrical treatment of cardiac arrhythmias: defibrillation and cardioversion

The passage of electrical current across the chest and heart to terminate ventricular fibrillation (defibrillation) or organized supraventricular or ventricular arrhythmias (cardioversion) is effective and safe. This paper reviews the physiology, techniques, and rationale for defibrillation and cardioversion. New developments and future directions are discussed. Rapid defibrillation is the most important factor in survival of out-of-hospital ventricular fibrillation.

Test of four defibrillation dosing strategies using a two-dimensional finite-element model

The most widely used defibrillation dosing strategy is that adopted by the American Heart Association in 1986. However, several alternative dosing strategies have been proposed to match delivered energy to the individual requirements of defibrillation subjects. In this study, two-dimensional finite element methods are used to investigate the performance of four of these dosing strategies applied to three thoracic models representative of men and women of different thoracic aspect ratios. From the resulting current density distributions, the relative effectiveness of the following dosing strategies are evaluated and compared: constant current; current proportional to body weight; constant energy; energy proportional to body weight. Our results show that the strategy of applying current proportional to subject body weight with a current dose of 0.58 A kg-1 was able to defibrillate all three subjects with only minimal overexposure of any one of them. None of the other dosing strategies examined could be made to successfully defibrillate all three subjects without significantly overexposing at least one.

Determinants of successful transthoracic defibrillation and outcome in ventricular fibrillation

OBJECTIVE

To examine factors determining defibrillation success and outcome in patients with ventricular fibrillation.

DESIGN

Observational prospective study of age, sex, transthoracic impedance, site of cardiac arrest, ventricular fibrillation duration and amplitude, primary or secondary ventricular fibrillation, aetiology, number of shocks to correct ventricular fibrillation, and drug treatment.

SETTING

A teaching hospital and a mobile coronary care unit with a physician.

PATIENTS

70 consecutive patients (50 male, 20 female) mean age 66.5 years.

INTERVENTIONS

Before the first countershock was administered transthoracic impedance using a 30 kHz low amplitude AC current passed through 8 cm/12 cm self-adhesive defibrillator electrode pads applied in the anteroapical position was measured. The first two shocks were 200 J delivered energy (low energy) and further shocks of 360 J (high energy) were given if required.

MAIN OUTCOME MEASURES

Countershock success and outcome from ventricular fibrillation.

RESULTS AND CONCLUSIONS

First shock success was significantly greater in inhospital arrests (37/53) than in out-of-hospital arrests (5/17) and in those receiving antiarrhythmic treatment (13/15, 86.7%) v (27/51, 52.9%). Transthoracic impedance was similar in those who were successfully defibrillated with one or two 200 J shocks (98.7 (26) omega) and those who required one or more 360 J shocks (91.4 (23) omega). Success rates with two 200 J shocks were similar in those patients with "high" transthoracic impedance (that is, greater than 115 omega) and those with transthoracic impedance (less than or equal to 115 omega) (8/12 (67%) v 44/58 (76%]. Fine ventricular fibrillation was significantly more common in the patients with a transthoracic impedance of greater than 95 omega (41% (13/32] than in those with a transthoracic impedance less than or equal to 95 omega (13% (5/38]. Death during arrest was significantly more common in patients who needed high energy shocks (14/18 (78%] than in those who needed low energy shocks (16/52 (31%]. Multiple regression analysis identified ventricular fibrillation with an amplitude of greater than or equal to 0.5 mV, age less than or equal to 70 years, and arrests that needed less than or equal to two shocks for defibrillation, in rank order as independent predictors of survival to discharge.

Safety of external cardioversion/defibrillation in patients with internal defibrillation patches and no device

Placement of prophylactic epicardial defibrillation patches at time of open-heart surgery in patients at risk for postoperative arrhythmias has been strongly questioned. Concern has centered on the ability to safely perform subsequent external defibrillation if needed. From 61 patients who were treated with a two-stage strategy we identified 17 who, while wearing epicardial patches and no generator, received external cardioversion/defibrillation for 20 episodes of hemodynamically unstable ventricular arrhythmias. All the patients had one small and one large patch. Eighteen of the episodes were induced during electrophysiological testing (with transthoracic shocks delivered via pad electrodes oriented in an apex-posterior configuration) and two were spontaneous. The episodes occurred at 21 +/- 27 days from patch implant. Thirteen episodes (65%) were converted with one shock at an energy level of 185 +/- 65 J. Seven (35%) required a second shock at 351 +/- 22 J. The accumulated energy requirement was 286 +/- 205 J. No adverse outcomes were noted. The number of episodes requiring more than one shock and the energy requirements were not different from those in a control group of 20 similar arrhythmias treated with the same equipment. Under these conditions, external cardioversion/defibrillation in patients with one large and one small epicardial defibrillation patch was uniformly successful. Further data is needed in the out-of-hospital setting and on the results of external defibrillation in patients with two large patches.

The fundamental law of electrostimulation and its application to defibrillation

Around the turn of the last century, there was an intensive discussion among physiologists as to whether there is a law describing the phenomena of electrostimulation and which formula may best approximate it mathematically. J.L. Hoorweg found in 1892 that the voltage at which a capacitor must be charged to elicit an excitation, was a function of the capacitance in an inverse correlation. G. Weiss reported in 1901 that according to his investigations a linear relationship existed between the duration of a pulse and the corresponding quantity of electricity applied and called it "formule fondamentale." We are now able to give the "fundamental formula" a physical interpretation that yields, as result, the electric field produced by the electrode acting on the excitable membrane. The electric field in the extracellular space is transformed by the cell geometry ratio: cell length to membrane thickness yielding a high transmembrane field capable of reducing the inherent electric field to its threshold level. The consequences drawn from this hypothesis are remarkable and (should) have an influence on all applications of electrostimulation including the discussions on defibrillation. The application of the stimulation theory to defibrillation yields as results: (1) The basic engineering principle of defibrillation is to produce an electric field within the ventricles of 400 V/m or more. An orthogonal pulse application may reduce the energy requirements, as more fibers are longitudinally reached by the electric field; (2) The shape of the defibrillation pulse and its polarity plays no role. Consequently it follows that biphasic pulses must be less efficient than monophasic pulses, if they are close to the chronaxie; and (3) The most serious disadvantage in today's defibrillation practice is its dose characterization in "energy"; but this physical quantity cannot be justified in the light of the fundamental law of electrostimulation.