The effects of ventricular fibrillation duration and site of initiation on the defibrillation threshold during early ventricular fibrillation

Cardiopulmonary resuscitation in children

PURPOSE OF REVIEW

To summarize recent advances in pediatric cardiopulmonary arrest prevention, resuscitation and postresuscitation management.

RECENT FINDINGS

Pediatric cardiac arrest has traditionally been considered a futile medical condition with dismal outcomes. Data in the 21st century indicate that more than 25% of children treated for in-hospital cardiac arrests survive to hospital discharge and more than 10% of children older than 1 year treated for out-of-hospital cardiac arrests survive to hospital discharge. These data establish that children are more likely to survive to hospital discharge than adults after both in-hospital and out-of-hospital cardiac arrests. Before arrest, exciting new studies demonstrate that the implementation of in-hospital pediatric medical emergency teams is associated with significant decreases in cardiac arrest incidence and overall pediatric hospital mortality. During arrest, ventricular fibrillation or ventricular tachycardia, once thought to be rare in children, occurs during 25% of inhospital pediatric cardiac arrests and at least 7% of out-of-hospital pediatric cardiac arrests. Survival to hospital discharge is much more likely after arrests with a first documented rhythm of ventricular fibrillation or ventricular tachycardia than after pulseless electric activity and asystole. However, ventricular fibrillation or ventricular tachycardia is not always a favorable rhythm, as survival to discharge is much less likely when ventricular fibrillation or ventricular tachycardia occurs during resuscitation from an arrest with the first documented rhythm of pulseless electric activity or asystole. Further, extracorporeal membrane oxygenation cardiopulmonary resuscitation appears promising under special resuscitation circumstances to improve outcome from highly selected in-hospital pediatric cardiac arrest victims. Further, postresuscitation interventions such as goal-directed therapies and therapeutic hypothermia have been demonstrated in adults and infants to improve outcome for selected cardiac arrest victims and are promising candidate targets for study in children.

SUMMARY

Pediatric cardiac arrest is not a futile condition; many children are successfully resuscitated each year. The implementation of new prearrest, intraarrest and postresuscitative therapies has the potential to further improve survival rates following pediatric cardiac arrest.

QRS prolongation is associated with high defibrillation thresholds during cardioverter-defibrillator implantations in patients with hypertrophic cardiomyopathy

BACKGROUND

Although high defibrillation threshold (DFT) is a major and unavoidable clinical problem after implantation of an implantable cardioverter defibrillator (ICD), little is known about the cause and management of a high DFT in patients with hypertrophic cardiomyopathy (HCM). The purpose of this study was to assess the predictors of a high DFT in patients with HCM.

METHODS AND RESULTS

Twenty-three patients with non-dilated HCM who underwent ICD implantation were included. The DFT at the time of the device implantation was measured in all patients. The patients were divided into 2 groups, a high DFT group (DFT >or=15J, n=13) and a low DFT group (DFT <15J, n=10); and their baseline characteristics were compared. The QRS duration was longer in the high than in the low DFT group (128 +/-31 vs 103 +/-12 ms, respectively; P=0.02). QRS duration, left ventricular (LV) end-systolic diameter, and LV ejection fraction were significant predictors of DFT in univariate analysis. However, in multivariate analysis, the only factor significantly associated with DFT was QRS duration (P=0.002).

CONCLUSIONS

QRS duration is the most consistent predictor of a high DFT in HCM patients undergoing ICD implantation.

Benefit of millisecond waveform durations for patients with high defibrillation thresholds

BACKGROUND

Patients with a high defibrillation threshold (DFT) present an atypical but vexing problem with regard to implantable cardioverter-defibrillator (ICD) therapy. Their implant procedures are lengthy and involve more risk of complications. These patients often sustain a reduced safety margin that may compromise their survival.

OBJECTIVES

The purpose of this study was to evaluate the use of fixed millisecond duration model-optimized biphasic waveforms compared with conventional tilt-based waveforms in patients having a high DFT.

METHODS

We compared a 65%/65% tilt biphasic waveform to a millisecond duration biphasic waveform based on the biphasic burping theory using a 90-microF shock capacitor.

RESULTS

Fifty-four patients were evaluated. Mean DFT with tilt was reduced from 11.0 +/- 5.5 J to 8.8 +/- 4.1 J, for a mean reduction of 20% (P < .0001). For the 13 patients with tilt-based DFTs > or = 15 J, DFT was reduced from 18.7 +/- 4.1 J to 13.4 +/- 3.5 J, for a mean DFT reduction of 28% (P = .009). The population peak DFT was reduced from 29.0 J to 17.5 J, for a 41% reduction (P = .03).

CONCLUSION

Use of simple millisecond biphasic waveforms instead of conventional tilt-based waveforms can lead to substantial reductions in DFT, especially in patients with high DFT.

Defibrillation with a minimally invasive direct cardiac massage device

OBJECTIVE

This study examined (1) the defibrillation efficacy of using a minimally invasive direct cardiac massage (MID-CM) device as one electrode of the defibrillation electrical circuit and (2) the effect on external defibrillation of defibrillating when the MID-CM device is in place and a pneumothorax is present.

METHODS

Part 1: in seven pigs, defibrillation thresholds (DFTs) were determined with a truncated exponential biphasic waveform. DFTs were determined for five electrode configurations: standard transthoracic defibrillation with electrodes on the left and right chest walls (1), with the MID-CM as one of the defibrillation electrodes pressed gently (2) or firmly (3) against the heart with the right chest wall patch as the second electrode, the same as (1) with the MID-CM device in place and the lungs at end-inspiration (4) or at end-expiration (5). Part 2: in six pigs, DFTs were determined with a monophasic damped sinusoidal waveform with external defibrillation electrodes (1) and with the device as one defibrillation electrode and the other electrode on either the anterior (2), lateral (3), or posterior right chest wall (4).

RESULTS

Part 1: the DFTs for (2) or (3) were not different (18.7+/-12.4 vs. 17.0+/-8.3 J), but both DFTs were lower than that for (1) (155+/-45 J). The DFT was elevated for (4) (205+/-69 J) compared with (1). For (5) only one animal could be defibrillated with shocks up to 360 J. Part 2: the DFTs for (2), (3) or (4) were not different (19.5+/-11.0, 25.4+/-9.4, 27.4+/-9.0 J), but all three were lower than the DFT for (1) (198+/-70 J).

CONCLUSIONS

Using the MID-CM device as one electrode of the defibrillation circuit markedly lowers the DFT compared with that for standard transthoracic defibrillation for both a monophasic and biphasic waveform. Defibrillation with the device in place and the chest opened elevates the DFT for external defibrillation much more during end-expiration than during end-inspiration.

A novel ultrasound technique to estimate right ventricular geometry during fibrillation

Finite element modelling of the heart for the purpose of studying the electric fields of defibrillation shocks requires knowledge of the geometry of the heart during fibrillation. However, the standard method of measuring this geometry, MRI. cannot be used during fibrillation because the heart geometry changes rapidly and perhaps unpredictably. We present a new ultrasound approach to measuring the right ventricular geometry during fibrillation and preliminary data using this technique. In six anaesthetized pigs, we find that a short axis cross-sectional area of the right ventricle increases by 38% during a 30 s episode of ventricular fibrillation. A long axis cross-sectional area increases by 19% during this same time. By fitting parameters of a simple geometric model to the experimental data, we estimate that the volume of blood in the right ventricular cavity increases by approximately 30% during the episode of ventricular fibrillation. We present the first study of the RV area during-fibrillation with the estimated volume. Our data suggest changes in defibrillation threshold may be linked to current shunting through the increased blood volume.

Pacing following shocks stronger than the defibrillation threshold: impact on defibrillation outcome

INTRODUCTION

A recent study of shocks near defibrillation threshold (DFT) strength demonstrated that at least three rapid cycles always occur after failed shocks but not after successful shocks, suggesting that the number and rapidity of postshock cycles are important in determining defibrillation success. To test this hypothesis, rapid pacing was performed following a shock stronger than the DFT that by itself did not induce rapid cycles and ventricular fibrillation (VF).

METHODS AND RESULTS

Epicardial activation was mapped in six pigs using a 504-electrode sock. The DFT was determined by an up/down protocol with S1 shocks (right ventricle-superior vena cava, biphasic). Ten shocks that were 100 to 200 V above the DFT (aDFT) were delivered after 10 seconds of VF to confirm they always defibrillated. Then, S2, S3, etc., pacing at 5 to 10 times diastolic threshold was performed from the left ventricular apex after aDFT shocks during VF. First, the postshock interval after aDFT shocks was scanned with an S2 stimulus to find the shortest S1-S2 coupling interval (CI) that captured. This was repeated for S3, S4, etc., until VF was induced. To induce VF after aDFT shocks, three pacing stimuli (S2, S3, S4) with progressively shorter CIs were always required; S2 or S2,S3 never induced VF. For the S2-S4 cycles, the intercycle interval was shorter (P < 0.01), and the wavefront conduction time was longer (P < 0.01) for episodes in which VF was induced (n = 57) than for episodes in which it was not (n = 60). Following the S4 cycle that induced VF, two types of spontaneous activation patterns appeared: focal (88%) and reentrant (12%).

CONCLUSION

VF induction after aDFT shocks always required at least three rapid successive paced-induced cycles. Thus, the number and rapidity of the first several postshock cycles rather than just the first postshock cycle may be determining factors for defibrillation outcome.

Prediction of defibrillation outcome by epicardial activation patterns following shocks near the defibrillation threshold

INTRODUCTION

Ventricular defibrillation is probabilistic and shock strength dependent. We investigated the relationship between defibrillation outcome and postshock activation patterns for shocks of the same strength (approximately 50% probability of success for defibrillation [ED50] to yield an equal number of successful and failed shocks).

METHODS AND RESULTS

In five pigs, 10 shocks of approximately ED50 strength (right ventricle-superior vena cava, biphasic, 6/4 msec) were delivered after 10 seconds of ventricular fibrillation (VF). Epicardial activation sequences following shocks were mapped with a 504-electrode shock and analyzed by animating dV/dt of the electrograms. Intercycle interval (ICI, time between the onset of successive postshock cycles), wavefront conduction time (WCT, time between the earliest and latest activation of a cycle), and overlapping index (WCT of cycle[n]/ICI of cycle[n+1]) were determined for the first five postshock cycles. An overlapping index >1 indicates overlap between successive cycles. Of 50 defibrillation attempts, 25 were successes. There was no difference between successful and failed episodes for both ICI (68 +/- 9 msec vs 62 +/- 10 msec) and WCT (97 +/- 24 msec vs 100 +/- 14 msec) of cycle 1. However, starting at cycle 2, the ICI was longer, and the WCT was shorter for successful than failed episodes (P < 0.01). Overlapping cycles (index > 1) were found during the transition from cycles 2 through 5 in all failed (index >1) and in no successful episodes.

CONCLUSIONS

(1) Defibrillation outcome cannot be determined during the first postshock cycle. (2) At least three rapid successive cycles with overlap of cycles 2 and 3 are present in all failed and in no successful episodes. (3) The overlapping index is a marker to predict defibrillation outcome.

Influence of postshock epicardial activation patterns on initiation of ventricular fibrillation by upper limit of vulnerability shocks

BACKGROUND

Shocks of identical strength and timing sometimes induce ventricular fibrillation (VFI) and other times do not (NoVFI). To investigate this probabilistic behavior, a shock strength near the upper limit of vulnerability, ULV(50), was delivered to yield equal numbers of VFI and NoVFI episodes.

METHODS AND RESULTS

In 6 pigs, a 504-electrode sock was pulled over the ventricles. ULV(50) was determined by scanning the T wave. S(1) pacing was from the right ventricular apex. Ten S(2) shocks of approximate ULV(50) strength were delivered at the same S(1)-S(2) coupling interval. Intercycle interval (ICI) and wave front conduction time (WCT) were determined for the first 5 postshock cycles. ICI and the WCT of cycle 1 were not different for VFI versus NoVFI episodes (P=0.3). Beginning at cycle 2, ICI was shorter and WCT was longer for VFI than NoVFI episodes (P<0.05).

CONCLUSIONS

The first cycle after shocks of the same strength (ULV(50)) delivered at the same time has the same activation pattern regardless of shock outcome. During successive cycles, however, a progressive decrease in ICI and increase in WCT occur during VFI but not NoVFI episodes. These findings suggest shock outcome is (1) deterministic but exquisitely sensitive to differences in electrophysiological state at the time of the shock that are too small to detect or (2) probabilistic and not determined until after the first postshock cycle.

Can early timed internal atrial defibrillation shocks reduce the atrial defibrillation threshold?

The defibrillation threshold is markedly reduced very early following the initiation of ventricular fibrillation. The purpose of this study was to determine if the same finding holds true for atrial defibrillation. Sustained, reproducible AF was induced with programmed atrial pacing using acetyl-beta-methylcholine chloride (40-640 microL/min) in six adult sheep (heart weight 245-300 g). Seven timing intervals (125 ms, 200 ms, 1 s, 3 s, 10 s, 30 s, and 5 min after AF induction) and two lead configurations: (1) RA as cathode and CS as anode; and (2) RA as cathode and RV apex as anode were tested. Single capacitor biphasic waveforms (3/1 ms) were delivered and atrial defibrillation thresholds (ADFTs) were determined in random order. No significant differences in leading edge voltage and total energy were detected for the RA-CS configuration for the seven timing intervals. For the RA-RV configuration, a significant difference was detected comparing the voltage for 125 ms to the 5-minute timing interval. For all times except 125 ms, the RA-RV threshold was significantly higher than the RA-CS level. In contrast to ventricular defibrillation, the ADFT does not change significantly within the first 5 minutes after the initiation of AF for the RA-CS configuration. However, if the shock is given very early (125 ms after AF induction) with the RA-RV configuration, the ADFT is lowered almost to the RA-CS level.

Energy levels for defibrillation: what is of real clinical importance?

Today, transthoracic and intracardiac defibrillation offer a well-accepted and widely used form of therapy for patients with life-threatening ventricular arrhythmias. Despite the wide clinical use of defibrillators, the mechanisms by which an electrical shock halts fibrillation are still not completely understood. During a shock, different amounts of current flow through the different parts of the heart and the current distribution is highly uneven. This current distribution is affected by changes in the shock potential gradient through the heart, changes in fiber orientation, and changes in myocardial conductivity caused by connective tissue barriers. It would be ideal if the potential gradient distribution throughout the ventricles could be measured directly for each individual patient during defibrillator implantation and follow-up and the shock strength could be programmed based on this measurement, but so far this is not possible. A more feasible approach is to determine, by trial and error, the magnitude of the shock strength delivered through the defibrillation electrodes for successful defibrillation. There is no distinct threshold value above which all shocks succeed and below which all shocks fail to defibrillate. Rather, increasing shock strength increases the likelihood the shock will succeed. Therefore, instead of a distinct defibrillation threshold, a probability of success curve exists. However, increasing the shock strength above an optimal range can actually decrease the success rate for defibrillation. One possible explanation is that the high voltage gradients caused by such large shocks damage cells and result in postshock arrhythmias that may reinitiate fibrillation. Another problem that can affect the probability of defibrillation success for a particular programmed energy setting is that the shock strength required for defibrillation may increase over time due to (1) the growth of fibrotic tissue around the defibrillation electrode; (2) migration of the lead; (3) acute ischemia; or (4) other changes in the underlying cardiac disease (e.g., worsening of heart failure). Such possible increases in the defibrillation shock strength requirement should be compensated for before they occur by adding a margin of safety to the shock strength needed for effective defibrillation. When programming an implantable defibrillator, it is important to keep in mind that the defibrillation shock should be (1) strong enough to defibrillate at least 98% of the time with the first shock; (2) weak enough not to cause severe post-shock arrhythmias or reinitiation of fibrillation; but (3) strong enough to compensate for changes of defibrillation energy requirements over time. This usually can be accomplished by setting the defibrillator 7-10 J higher than the defibrillation threshold determined by a standard step-down protocol.