Subthreshold Shock Research Articles

Reactive atomistic simulations of shock-induced initiation processes in mixtures of ammonium nitrate and fuel oil

Ammonium nitrate mixed with fuel oil (ANFO) is a commonly used blasting agent. In this paper we investigated the shock properties of pure ammonium nitrate (AN) and two different mixtures of ammonium nitrate and n-dodecane by characterizing their Hugoniot states. We simulated shock compression of pure AN and ANFO mixtures using the Multi-scale Shock Technique, and observed differences in chemical reaction. We also performed a large-scale explicit sub-threshold shock of AN crystal with a 10 nm void filled with 4.4 wt% of n-dodecane. We observed the formation of hotspots and enhanced reactivity at the interface region between AN and n-dodecane molecules.

Activation sequences following failed atrial defibrillation

The purposes of this study were to examine the first activations following atrial defibrillation shocks to help understand how and where atrial fibrillation (AF) relapsed following failed shocks and to assess the difference in postshock activation between failed and successful shocks. While many studies have investigated the mechanism of ventricular defibrillation, much less is known about the mechanisms of AF. Sustained AF was induced electrically after pericardial infusion of methylcholine in 10 sheep. Biphasic subthreshold shocks were delivered to three configurations: right atrium to distal coronary sinus (RA-CS), sequential shocks with RA-CS as the first pathway followed by proximal CS to superior vena cava as the second pathway (Sequential), and right ventricle to superior vena cava plus can (V-triad). In eight sheep, global atrial mapping was performed with 504 electrodes spaced 3 to 4 mm apart. Earliest postshock activations mostly arose from the left atrium for V-triad but arose from either atrium for RA-CS and Sequential. Preshock AF cycle lengths were significantly shorter at the earliest activation sites than at seven of eight other sites globally distributed over both atria. In all type B successful episodes in which one or more rapid activations occurred after the shock and in 50 of the 72 failed episodes analyzed, activation fronts spread away from the earliest site in a focal pattern, and discrete nonfragmented activation complexes were present in the first derivatives of the electrograms. In the other 22 failed episodes, earliest activation fronts spread in a nonfocal pattern, and earliest postshock electrogram derivatives were fractionated. To better interpret the activation pattern in the fragmented regions, a 504 electrode plaque with 1.5-mm electrode spacing was placed on the right atrial appendage in two additional sheep. In 11 of 108 failed episodes, earliest postshock activation appeared inside the plaque and spread in a focal pattern with nonfragmented electrogram derivatives in 10 episodes and in a reentrant pattern with fragmented electrogram derivatives in the other. (1) The electrode configuration influenced the location of earliest postshock activation. (2) Earliest postshock activation occurred where the preshock AF cycle length was short. (3) Earliest activations following all type B successful and most failed episodes were not fragmented and spread in a focal pattern. (4) The region of earliest postshock activation in the failed episodes without a focal postshock activation pattern exhibited regions of fragmented electrogram derivatives that may represent conduction block and possibly reentry.

Predetonation conductivity of a TATB-based explosive

We have measured the electric conductivity of a low-sensitivity TATB-based explosive compressed by a shock wave. The results confirm the hypothesis that subthreshold shock waves induce the transition of crystalline explosives into a semiconductor state.

Effects of Subthreshold Shocks on Wavelet Propagation during Atrial Fibrillation in Humans

Our objectives are: first to investigate the effects of internal cardioversion energies on the wave fronts propagation in the right atrium immediately after the energy delivery; second, to track the time course of these effects. The study is based on a measure of organization of the endoatrial electrograms obtained by a multipolar basket catheter inserted in the right atrium. We estimated the level of organization by computing the percentage of points laying on the signal baseline (i.e., number of occurrences, NO). NO values were computed on two-second long windows. Six non-overlapped windows were selected, one just before and five just after the last unsuccessful shock. Immediately after the shock most of the patients exhibited an increase in the organization patterns. This increase was more evident in those patients with rather disorganized patterns and higher energy threshold. This effect fades within a few seconds after the shock delivery. Our data confirm the idea that the electrical shock causes a widespread extinction of electrical wavefronts, which regenerates after the shock. Since an increase of organization may lead to a reduction of energy threshold, a potential application of these findings might consist in the delivery of multiple subthreshold shocks instead of a single one.

Reliability and clinical benefits of a new subthreshold noninvasive shock lead integrity test

Reliability and clinical benefits of a new subthreshold noninvasive shock lead integrity test

Subthreshold test pulse versus shock delivery to evaluate high voltage impedance in ICD patients

Subthreshold test pulse versus shock delivery to evaluate high voltage impedance in ICD patients

Do unsuccessful subthreshold defibrillation shocks enable subsequent successful defibrillation with lower energy?

Do unsuccessful subthreshold defibrillation shocks enable subsequent successful defibrillation with lower energy?

901–42 Redetection of Ventricular Fibrillation After Subthreshold Shocks: Prospective Analysis with Standard Bipolar and Integrated Bipolar Sensing Systems

We prospectively analyzed detection (D) and redetection (R) characteristics between transvenous defibrillators (ICD) using standard bipolar and integrated bipolar sensing. Monophasic and biphasic ICDs were included. Subthreshold shocks were intentionally delivered and redetection assessed at the predischarge, 1, 3, and 6 months follow-up retesting. Among the 160 ICDs with standard bipolar sensing 530 VF inductions were analyzed. After the failed shocks undersensing was more frequent (3% vs 20%, p < 0.01) but did not remarkably prolong redetection (3.13 ± 0.8 sec vs 3.3 ± 1.1 sec). Among the 150 ICDs with integrated bipolar sensing 60 were CPI and 90 Ventritex. After 200 failed shocks redetection prolonged with the CPI system (3.1 ± 1.4 sec vs 4.6 ± 3.6 sec, p < 0.05) while did not change after 270 failed shocks with the Ventritex ICD (5.3 ± 2.8 sec vs 4.9 ± 1.7 sec). This may reflect different nominal settings for 0 and R. In 6 out of 90 patients with Ventritex and lout of 60 with the CPI ICDs the device failed to redetect VF. None of those patients had sudden death at follow-up. Analysis of endocardial electrograms showed that failure to redetect VF is not associated with a uniform reduction, but with a rapid and repetitive change of electrogram amplitude. 1) Standard bipolar sensing more effectively redetects VF; 2) endocardial electrogram analysis provide insights in understanding the mechanism of undersensing; 3) the clinical relevance of this phenomenon remains unknown.

Comparison of initial detection and redetection of ventricular fibrillation in a transvenous defibrillator system with automatic gain control

Objectives. The purpose of this study was to prospectively evaluate postshock redetection of ventricular fibrillation by a system that coupled an implantable cardioverter-defibrillator with an automatic gain control sense amplifier and a transvenous lead system.Background. Redetection of ventricular fibrillation after an unsuccessful first shock has not been systematically evaluated. Previous studies have suggested that sensing performance of some-lead systems may be adversely affecter by the delivery of subthreshold shocks.Methods. The time required for both initial detection and redetection of ventricular fibrillation was compared in 22 patients. These times were estimated by subtracting the capacitor charge time from the total event time.Results. A total of 113 successful and 57 unsuccessful initial shocks were delivered during induced ventricular fibrillation. The mean ± SD initial time to detection of ventricular fibrillation was 5.5 ± 1.7 s (range 2.4 to 10.8); the time to redetection ranged from 1.5 to 18.5 s (mean 4.5 ± 2.8, p = NS vs. detection time). Abnormal redetection episodes, defined as a redetection time >10.2 s (i.e., >2 SD above the mean redetection time), were observed in 4 (18%) of 22 patients.Conclusions. Redetection of ventricular fibrillation after a subthreshold first shock may be delayed. Device testing with intentional delivery of subthreshold shocks to verify successful postshock redetection of ventricular fibrillation should be performed routinely in all patients.

Basic Mechanisms of Ventricular Fibrillation and Defibrillation

Recordings made simultaneously from many electrodes placed on and in the hearts of animals have provided much new information about the basic principles of ventricular fibrillation and defibrillation. The findings are listed below. Fibrillation is maintained predominantly by wandering wavelet reentry and secondarily by repeating spiral rotor reentry. Activation sequences become more organized after 1 min of fibrillation than during the first few seconds of fibrillation and become less organized again after 5 min of fibrillation. Earliest activation following a shock slightly lower in strength than needed to defibrillate (a subthreshold defibrillation shock) occur in those cardiac regions in which the potential gradients generated by the shock are weakest. Activation fronts after subthreshold shocks are not continuations of activation fronts present just before the shock. An upper limit exists to the stength of shocks that induce fibrillation when given during the “vulnerable period” of regular rhythm. This upper limit of vulnerability correlates with and is similar in strength to the defibrillation threshold. To defibrillate, a shock must halt the activation fronts of fibrillation without giving rise to new activation fronts that reinduce fibrillation.The response to shocks during regular rhythm just below the upper limit of vulnerability is similar to the response to subthreshold defibrillation shocks. Shocks during regular rhythm initiate rotors of reentrant activation leading to fibrillation when a critical point is formed at which a certain critical value of shock potential gradient field strength intersects a certain critical degree of myocardial refractoriness. This critical point may explain the existence of the upper limit of vulnerability. The critical point may also partially explain the finding that the relationship between shock strength and the success of the shock in halting fibrillation is better represented by a probability function rather than by a discrete threshold value. Very high potential gradients, approximately an order of magnitude greater than needed for defibrillation, have detrimental effects on the heart, including conduction block, induction of arrhythmias, decreased wall motion, and tissue necrosis. Biphasic waveforms are superior to monophasic waveforms for defibrillation because they require a lower potential gradient to halt fibrillation activation fronts and require a higher potential gradient to cause detrimental effects. These results suggest that, for defibrillation, the best shock waveform is biphasic and the best electrode configuration is the one that creates a certain minimum potential gradient throughout the ventricles with the smallest strength shock.

The effect of an unsuccessful subthreshold shock on the energy requirement for the subsequent defibrillation

The effect of an unsuccessful subthreshold shock on the energy requirement for the subsequent defibrillation was studied in 10 anesthetized dogs. Defibrillation was achieved with a spring catheter electrode in the superior vena cava and a patch electrode on the anteroapical ventricular wall. Success rates of defibrillation 20 seconds from the onset of ventricular fibrillation were determined at three energy levels with and without a preceding subthreshold shock. Altogether, 637 episodes of fibrillation-defibrillation were performed (63.7 ± 6.7 per dog). Predicted energy levels for defibrillation success rates of 50% and 80% (E50 and E80) acquired from a logistic regression curve were 0.0303 ± 0.0064 and 0.0367 ± 0.0069 joule/gm, respectively, without subthreshold shocks. E50 and E80 with an unsuccessful subthreshold shock resulted in comparable values (E50: 0.0325 ± 0.0041 joule/gm; E80: 0.0.380 ± 0.0100 joule/gm). Our results suggest that an unsuccessful low-energy shock does not alter the energy requirement for subsequent defibrillation with an implantable defibrillator.

Clinical transvenous cardioversion of recurrent life-threatening ventricular tachyarrhythmias: Low energy synchronized cardioversion of ventricular tachycardia and termination of ventricular fibrillation in patients using a catheter electrode

The purpose of this study was to test the efficacy, safety, and patient tolerance of transvenous cardioversion and defibrillation in patients who had recurrent ventricular tachyarrhythmias. In five of seven patients, a truncated exponential shock of 0.025 to 2.0 joules synchronized to the QRS complex terminated 47 episodes of recurrent sustained ventricular tachycardia (VT). Cardioversion threshold was ≤ 0.25 joule in three patients and 0.75 to 2.0 joules in two patients. Shocks of 0.75 joule and 2.0 joule failed to terminate VT in one patient each; higher energies were not tried because of hemodynamic decompensation. In one patient, a shock of 25 joules terminated ventricular fibrillation (VF) on three occasions, and in another patient a shock of 1.0 joule terminated atrial fibrillation on one occasion. Shocks ≤ 0.5 joule were well tolerated by the awake unsedated patient. One hundred forty of 141 synchronized shocks (including subthreshold shocks) produced no repetitive ventricular activity. In one seriously ill patient who had received multiple antiarrhythmic drugs and required balloon counterpulsation for hemodynamic support, on a single occasion each a synchronized transvenous shock and a synchronized conventional transthoracic shock produced ventricular flutter and ventricular fibrillation (VF), respectively. We conclude that synchronized transvenous cardioversion by a catheter electrode offers promise as a new therapeutic approach.

Patterns of ventricular activity during catheter defibrillation.

In order to clarify the mechanism of ventricular catheter defibrillation in which the electrode distribution and the low energy requirements make a simultaneous depolarization of the entire myocardium unlikely, the electrocardiograms recorded during 120 catheter fibrillation-defibrillation episodes in 39 dogs were analyzed. Three distinct, equally distributed defibrillation patterns were observed: 1) immediate resumption of a coordinated rhythm, thought to reflect complete depolarization of the myocardium; 2) increasing coarsening of the fibrillation waveforms interpreted as progressive reduction in the number of fibrillating fibers with reversion when a critical mass of myocardium with synchronized activity is reached, and 3) production of more coordinated "flutter-like" ventricular complexes probably representing a rhythm distinct from fibrillation and convertible to sinus rhythm by a second subthreshold shock. These observations suggest that total depolarization of the entire myocardium is not a prerequisite for ventricular defibrillation.

Effects of mechanical stimulation on the nerve fiber.

1. The effects of the mechanical stimulations on the nerve trunk were exarnined by recording propagated action currents from a single nerve fiber.2. In the case of single brief mechanical shocks, the nerve fiber responded to them in the ratio of 1:1. A single strong shock produced invariably, repetitive excitation.3. The response time of the mechanical stimulation was found to lie between those of short electrical pulse and of rheobasic stimulating current.4. The effect of mechanical pressure on the nerve fiber was identified with the action of catelectrotonus. The pressure on the nerve fiber produced an increase in excitability, and the increased excitability in the portion stimulated with a brief mechanical subthreshold shock lasted about 15msec.

The excitatory and recovery processes in the nerve fibre as modified by temperature changes

1. 1. An exponentially rising current excites an isolated single nerve fibre when, and only when, it rises above and crosses the rheobase at a rate of rise greater than the minimal gradient of the fibre. The rheobase is not affected by changes in temperature. The minimal gradient shows a tendency to be increased by cold, but there is a marked hysteresis in the effect of temperature changes on the minimal gradient. 2. 2. A brief subthreshold shock produces an excitatory state which first rises and then falls after the end of the shock. The time course of this process is retarded by cold. The minimum quantity of electricity needed to excite a single nerve fibre is increased by cold, its temperature coefficient being 1.7 for a change of 10°C. 3. 3. At every temperature, the earliest return of excitability occurs immediately after the end of the spike. The process of recovery is affected by temperature changes to the same extent as the spike duration.

The subthreshold potentials in a crustacean nerve fibre

In a recent paper Katz (1937) has described a number of observations which are inconsistent with classical theories of excitation, but which may be explained by assuming that a subthreshold shock can elicit a small and localized action potential in the cathodic part of a nerve fibre. At first the assumption of a subthreshold response seems to be in conflict with the allor-nothing law, but there is in reality no contradiction, since the all-ornothing principle refers to the propagated disturbance, and does not exclude the possibility of graded reactions in the stimulated region. Indeed, the existence of some kind of subliminal action potential is to be expected from the local circuit theory of nervous transmission; for the current generated by the activity of a very short length of nerve would be too weak to excite surrounding regions, so that a shock which activated less than a certain length would fail to produce a self-propagating impulse and would leave behind only a small and localized response (cf. Rosenberg 1937, Rushton 1937).