Current-based transthoracic defibrillation

A Systematic Review of the Transthoracic Impedance during Cardiac Defibrillation

For cardiac defibrillator testing and design purposes, the range and limits of the human TTI is of high interest. Potential influencing factors regarding the electronic configurations, the electrode/tissue interface and patient characteristics were identified and analyzed. A literature survey based on 71 selected articles was used to review and assess human TTI and the influencing factors found. The human TTI extended from 12 to 212 Ω in the literature selected. Excluding outliers and pediatric measurements, the mean TTI recordings ranged from 51 to 112 Ω with an average TTI of 76.7 Ω under normal distribution. The wide range of human impedance can be attributed to 12 different influencing factors, including shock waveforms and protocols, coupling devices, electrode size and pressure, electrode position, patient age, gender, body dimensions, respiration and lung volume, blood hemoglobin saturation and different pathologies. The coupling device, electrode size and electrode pressure have the greatest influence on TTI.

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.

Comparison of low-energy versus high-energy biphasic defibrillation shocks following prolonged ventricular fibrillation

INTRODUCTION

Since the initial development of the defibrillator, there has been concern that, while delivery of a large electric shock would stop fibrillation, it would also cause damage to the heart. This concern has been raised again with the development of the biphasic defibrillator.

OBJECTIVE

To compare defibrillation efficacy, postshock cardiac function, and troponin I levels following 150-J and 360-J shocks.

METHODS

Nineteen swine were anesthetized with isoflurane and instrumented with pressure catheters in the left ventricle, aorta, and right atrium. The animals were fibrillated for 6 minutes, followed by defibrillation with either low-energy (n = 8) or high-energy (n = 11) shocks. After defibrillation, chest compressions were initiated and continued until return of spontaneous circulation (ROSC). Epinephrine, 0.01 mg/kg every 3 minutes, was given for arterial blood pressure < 50 mmHg. Hemodynamic parameters were recorded for four hours. Transthoracic echocardiography was performed and troponin I levels were measured at baseline and four hours following ventricular fibrillation (VF).

RESULTS

Survival rates at four hours were not different between the two groups (low-energy, 5 of 8; high-energy, 7 of 11). Results for arterial blood pressure, positive dP/dt (first derivative of pressure measured over time, a measure of left ventricular contractility), and negative dP/dt at the time of lowest arterial blood pressure (ABP) following ROSC were not different between the two groups (p = not significant [NS]), but were lower than at baseline. All hemodynamic measures returned to baseline by four hours. Ejection fractions, stroke volumes, and cardiac outputs were not different between the two groups at four hours. Troponin I levels at four hours were not different between the two groups (12 +/- 11 ng/mL versus 21 +/- 26 ng/mL, p = NS) but were higher at four hours than at baseline (19 +/- 19 ng/mL versus 0.8 +/- 0.5 ng/mL, p < 0.05, groups combined).

CONCLUSION

Biphasic 360-J shocks do not cause more cardiac damage than biphasic 150-J shocks in this animal model of prolonged VF and resuscitation.

The effects of concentric ring electrode electrical stimulation on rat skin

Surface electrodes are commonly used electrodes clinically, in applications such as functional electrical stimulation for the restoration of motor functions, pain relief, transcutaneous electrical nerve stimulation, electrocardiographic monitoring, defibrillation, surface cardiac pacing, and advanced drug delivery systems. Common to these applications are occasional reports of pain, tissue damage, rash, or burns on the skin at the point where electrodes are placed. In this study, we quantitatively analyzed the effects of acute noninvasive electrical stimulation from concentric ring electrodes (CRE) to determine the maximum safe current limit. We developed a three-dimensional multi-layer model and calculated the temperature profile under the CRE and the corresponding energy density with electrical-thermal coupled field analysis. Infrared thermography was used to measure skin temperature during electrical stimulation to verify the computer simulations. We also performed histological analysis to study cell morphology and characterize any resulting tissue damage. The simulation results are accurate for low energy density distributions. It can also be concluded that as long as the specified energy density applied is kept below 0.92 (A2/cm4.s(-1)), the maximum temperature will remain within the safe limits. Future work should focus on the effects of the electrode paste.

Transthoracic impedance study with large self-adhesive electrodes in two conventional positions for defibrillation

External defibrillation requires the application of high voltage electrical impulses via large external electrodes, placed on selected locations on the thorax surface. The position of the electrodes is one of the major determinants of the transthoracic impedance (TTI) which influences the intracardiac current flow during electric shock and defibrillation success. The variety of factors which influence TTI measurements raised our interest to investigate the range of TTI values and the temporal TTI variance during long-term application of defibrillation self-adhesive electrodes in two conventional positions on the patient's chest--position 1 (sub-clavicular/sub-axillar position) and position 2 (antero-posterior position). The prospective study included 86 randomly selected volunteers (39 male and 49 female, 67 patients with normal skin, 13 patients with dry skin and 6 patients with greasy skin, 16 patients with chest pilosity and 70 patients without chest pilosity). The TTI was measured according to the interelectrode voltage drop obtained by passage of a low-amplitude high-frequency current (32 kHz) between the two self-adhesive electrodes (active area about 92 cm2). For each patient, the TTI values were measured within 10 s, 1 min and 5 min after sticking the electrodes to the skin surface, independently for the two tested electrode positions. We found that the expected TTI range is between 58 Omega and 152 Omega for position 1 and between 55 Omega and 149 Omega for position 2. Although the two TTI ranges are comparable, we measured significantly higher TTI mean of about (107.2 +/- 22.3) Omega for position 1 compared to (96.6 +/- 19.2) Omega for position 2 (p = 0.001). This fact suggested that the antero-posterior position of the electrodes is favourable for defibrillation. Within the investigated time interval of 5 min, we observed a significant TTI reduction with about 6.9% (7.4 Omega/107.2 Omega) for position 1 and about 5.3% (5.1 Omega/96.6 Omega) for position 2. We suppose that the long-term application of self-adhesive electrodes would lead to improvement of the physical conditions for conduction of the defibrillation current and to diminution of energy loss in the electrode-skin contact impedance. We found that gender is important when position 1 is used because women have significantly higher TTI (111 +/- 20.3) Omega compared to the TTI of men (102.6 +/- 24) Omega (p = 0.0442). Although we found some specifics of the electrode-skin contact layer, we can conclude that because of the insignificant differences in TTI, the operator of the defibrillator paddles does not need to take into consideration the skin type and pilosity of the patients. Analysis of the correlations between TTI and the individual patient characteristics (chest size, weight, height, age) showed that these patient characteristics are unreliable factors for prediction of the TTI values and optimal defibrillation pulse parameters and energy.

Do clinically relevant transthoracic defibrillation energies cause myocardial damage and dysfunction?

Sufficiently strong defibrillation shocks will cause temporary or permanent damage to the heart. Weak defibrillation shocks do not cause any damage to the heart but also do not defibrillate. A relevant and practical question is what range of shock energies is most likely to defibrillate while not causing damage to the heart. This question is most difficult to answer in the pre-hospital defibrillation setting where the patients' size and shape vary, placement of the defibrillation patches vary, and the etiology of their arrhythmia varies. Unlike internal defibrillators, which are tested at implantation, efficacy of an external defibrillator is determined only once, when it is most needed. This review discusses shock damage and dysfunction caused by monophasic waveforms as well as biphasic waveforms. Evidence is presented suggesting that for perfused hearts, the threshold for damage is well above any shock size delivered clinically. For non-perfused hearts, both in humans and animals, evidence is presented that monophasic shocks of up to 5 J/kg do not cause any more cardiac damage/dysfunction than that associated with smaller shocks and that much of this damage is caused by the ischemic period itself rather than the shock. Although many patients can be defibrillated with 150 J (2.2 J/kg) biphasic shocks, some patients may require biphasic shocks up to 360 J (5 J/kg) to be defibrillated. Studies still need to be performed comparing the efficacy and damaging effects of 360 J biphasic shocks to 150 J biphasic shocks. Until those studies are completed, it seems reasonable to use the same 360 J (5 J/kg) energy limit for biphasic shocks as for monophasic shocks.

Automatic adjustment of biphasic pulse duration in transthoracic defibrillation

Many studies have proven that biphasic defibrillation pulses are more efficient than the damped sinusoid monopolar waveform. Transthoracic resistance was shown to change during the two phases. On the other hand, it was proven that transthoracic resistance plays an important role in the defibrillation process, yielding the current for selected energy or voltage. Pre-shock measurement of the resistance may lead to improved selection. Stabilized current defibrillators are of low stored-to-delivered energy ratio. Therefore, automatic dynamic adjustment of some defibrillator parameters with respect to transthoracic resistance changes seems rational. An approach is known for modifying the pulse duration, in order to deliver a selected energy. A method is proposed here and an experimental defibrillator is developed for dynamic pulse duration adjustment with the purpose of obtaining a desired optimal time-course of the cardiac cell transmembrane potential.

Clinical efficacy of the wearable cardioverter-defibrillator in acutely terminating episodes of ventricular fibrillation

The findings of our initial study demonstrate for the first time the ability to terminate induced VT/VF reliably (100% of all episodes) by a single, monophasic 230-J shock delivered by the Wearable Cardioverter-Defibrillator (WCD). Although limited by sample size, our data suggest the WCD could be used as a feasible bridge to definitive implantation of an implantable cardioverter-defibrillator in patients in whom risk stratification for sudden death is not completed.