Defibrillation efficacy with endocardial electrodes is influenced by reductions in cardiac preload

The optimal phasic relationship between synchronized shock and mechanical chest compressions

OBJECTIVE

Pauses for shock delivery in chest compressions are detrimental to the success of resuscitation and may be eliminated with the use of mechanical chest compressors. However, the optimal phasic relationship between mechanical chest compression and defibrillation is still unknown. We therefore undertook a study to assess the effects of timing of defibrillation in the mechanical chest compression cycle on the defibrillation threshold (DFT) using a porcine model of cardiac arrest.

METHODS

Ventricular fibrillation was electrically induced and untreated for 10s in 8 domestic pigs weighing between 26 and 30 kg. Mechanical chest compression was then continuously performed for 25s, followed by a biphasic electrical shock which was delivered to the animal at 6 randomized coupling phases, including a control phase, with a pre-determined energy setting. The control phase was chosen at a constant 2s following discontinued chest compression. A novel grouped up-and-down DFT testing protocol was used to compare the success rate at different coupling phases. After a recovery interval of 4 min, the testing sequence was repeated, resulting in a total of 60 test shocks delivered to each animal.

RESULTS

No difference between the delivered shock energy, voltage and current were observed among the 6 study phases. The defibrillation success rate, however, was significantly higher when shocks were delivered in the upstroke phase of mechanical chest compression.

CONCLUSION

Defibrillation efficacy is maximal when electrical shock is delivered in the upstroke phase of mechanical chest compression.

Defibrillation delivered during the upstroke phase of manual chest compression improves shock success

OBJECTIVE

The current standard of manual chest compression during cardiopulmonary resuscitation requires pauses for rhythm analysis and shock delivery. However, interruptions of chest compression greatly decrease the likelihood of successful defibrillations, and significantly better outcomes are reported if this interruption is avoided. We therefore undertook a prospective randomized controlled animal study in an electrically induced ventricular fibrillation pig model to assess the effects of timing of defibrillation on the manual chest compression cycle on the defibrillation threshold.

DESIGN

Prospective, randomized, controlled animal study.

SETTING

University-affiliated research laboratory.

SUBJECTS

Yorkshire-X domestic pigs (Sus scrofa).

INTERVENTIONS

In eight domestic male pigs weighing between 24 and 31 kg, ventricular fibrillation was electrically induced and untreated for 10 secs. Manual chest compression was then performed and continued for 25 secs with the protection of an isolation blanket. The depth and frequency of chest compressions were guided by a cardiopulmonary resuscitation prompter. Animals were randomized to receive a biphasic electrical shock in five different compression phases with a predetermined energy setting. A control phase was chosen at a constant 2 secs after discontinued chest compression. A grouped up-down defibrillation threshold testing protocol was used to compare the success rate at different coupling phases. After a recovery interval of 4 mins, the sequence was repeated for a total of 60 test shocks for each animal.

MEASUREMENTS AND MAIN RESULTS

No difference in coronary perfusion pressure before delivering of the shock was observed among the six study phases. The defibrillation success rate, however, was significantly higher when shocks were delivered in the upstroke phase of manual chest compression.

CONCLUSION

Defibrillation efficacy is maximal when electrical shock is delivered during the upstroke phase of manual chest compression.

The role of mechanoelectric feedback in vulnerability to electric shock

Experimental and clinical studies have shown that ventricular dilatation is associated with increased arrhythmogenesis and elevated defibrillation threshold; however, the underlying mechanisms remain poorly understood. The goal of the present study was to test the hypothesis that (1) stretch-activated channel (SAC) recruitment and (2) geometrical deformations in organ shape and fiber architecture lead to increased arrhythmogenesis by electric shocks following acute ventricular dilatation. To elucidate the contribution of these two factors, the study employed, for the first time, a combined electro-mechanical simulation approach. Acute dilatation was simulated in a model of rabbit ventricular mechanics by raising the LV end-diastolic pressure from 0.6 (control) to 4.2 kPa (dilated). The output of the mechanics model was used in the electrophysiological model. Vulnerability to shocks was examined in the control, the dilated ventricles, and in the dilated ventricles that also incorporated currents through SAC as a function of local strain, by constructing vulnerability grids. Results showed that dilatation-induced deformation alone decreased upper limit of vulnerability (ULV) slightly and did not result in increased vulnerability. With SAC recruitment in the dilated ventricles, the number of shock-induced arrhythmia episodes increased by 37% (from 41 to 56) and the lower limit of vulnerability (LLV) decreased from 9 to 7 V/cm, while ULV did not change. The heterogeneous activation of SAC caused by the heterogeneous fiber strain in the ventricular walls was the main reason for increased vulnerability to electric shocks since it caused dispersion of electrophysiological properties in the tissue, resulting in postshock unidirectional block and establishment of reentry.

Cardiac defibrillation and the role of mechanoelectric feedback in postshock arrhythmogenesis

Ventricular dilatation increases the defibrillation threshold (DFT). In order to elucidate the mechanisms responsible for this increase, the present article investigates changes in the postshock behavior of the myocardium upon stretch. A two-dimensional electro-mechanical model of cardiac tissue incorporating heterogeneous fiber orientation was used to explore the effect of sustained stretch on postshock behavior via (a) recruitment of mechanosensitive channels (MSC) and (b) tissue deformation and concomitant changes in tissue conductivities. Recruitment of MSC had no influence on vulnerability to electric shocks as compared to control, but increased the complexity of postshock VF patterns. Stretch-induced deformation and changes in tissue conductivities resulted in a decrease in vulnerability to electric shocks.

Effect of stretch-activated channels on defibrillation efficacy

OBJECTIVES

This study aims to explore whether defibrillation threshold elevation could be caused by sustained recruitment of stretch-activated channels (SACs) and, if so, what are the underlying mechanisms.

BACKGROUND

Clinical studies have demonstrated that patients with dilated and overloaded ventricles have elevated defibrillation threshold. Prolonged ventricular stretch has been suggested as a possible factor in defibrillation threshold elevation; however, its role remains unclear.

METHODS

A two-dimensional finite-element bidomain model of ventricular defibrillation was used in the study. Retaining the geometrical parameters in the model, defibrillation dose-response curves were constructed with and without SACs to isolate the effect of stretch on shock outcome.

RESULTS

Simulations demonstrate that SAC activation leads to flattening of dose-response curve and increases in defibrillation threshold and effective dose for defibrillation by 31.4% and 18.8%, respectively. Examination of the electrophysiologic properties associated with sustained SAC recruitment pinpointed the main mechanisms responsible for the decrease in defibrillation efficacy. The lower conduction velocity of the shock-induced break excitations and the more positive transmembrane potential at the end of the effective refractory period in the tissue with SACs are proposed as main reasons for defibrillation threshold elevation.

CONCLUSIONS

Demonstrating the contribution of SACs to defibrillation threshold elevation identifies SACs as an attractive pharmaceutical target to reduce defibrillation threshold in patients with dilated cardiomyopathy.

Biphasic and monophasic transthoracic defibrillation in pigs with acute left ventricular dysfunction

OBJECTIVE

Our purpose was to compare biphasic versus monophasic shock success for VF termination in a porcine model of acute left ventricular (LV) dysfunction.

BACKGROUND

For the termination of ventricular fibrillation (VF), transthoracic biphasic waveform shocks achieve higher success rates than monophasic shocks. However, the effectiveness of biphasic versus monophasic defibrillation in a setting of left ventricular dysfunction has not been reported.

METHODS

In 23 open-chest adult swine (15-25 kg), LV dysfunction [> or =25% decline in cardiac output (CO)] was induced by continuous inhalation of halothane (1-1.75%). Each pig randomly received transthoracic biphasic and monophasic shocks at three energy levels (30, 50 and 100 J) in two conditions: baseline and LV dysfunction. Halothane effect on left ventricular size and contraction was measured by echocardiography in three additional swine.

RESULTS

With halothane, pigs demonstrated a decline in CO (baseline 4.16+/-0.19, halothane 2.72+/-0.19 l/min, P<0.01), mean arterial pressure (baseline 107.2+/-3.5, halothane 80.1+/-3.4 mmHg, P<0.01) and increased left ventricular end-diastolic pressure (baseline 6.4+/-0.9, halothane 12.7+/-0.8 mmHg, P<0.01). LV diameters increased and fractional shortening fell. During baseline, biphasic shocks achieved significantly greater success (termination of VF) compared to monophasic waveforms (100 J: biphasic 83.3+/-9.5 versus monophasic 38.9+/-9.5%, P<0.01; 50 J: biphasic 67.1+/-8.8 versus monophasic 11.8+/-5.7%, P<0.01; 30 J: biphasic: 31.9+/-6.4 versus monophasic 0+/-0%, P<0.01). The superiority of the biphasic waveform to terminate VF was retained during LV dysfunction at all energy levels (100 J: biphasic 78.3+/-7.3 versus monophasic 37.5+/-8.1%, P<0.01; 50 J: biphasic 65.5+/-11.5 versus monophasic 11.7+/-5.9%, P<0.01; 30 J: biphasic: 40.6+/-8.0 versus monophasic 3.1+/-3.1%, P<0.01). Within both waveforms, there were no significant differences in percent shock success at any energy level comparing baseline with LV dysfunction.

CONCLUSION

In this porcine model of acute LV dysfunction, biphasic waveform shocks were not only superior to monophasic waveform shocks for termination of VF during baseline, but retained superiority to monophasic waveform shocks when LV dysfunction was present.

Inspiration induced by phrenic nerve stimulation increases internal defibrillation energy requirements

INTRODUCTION

The purpose of this study was to systematically evaluate the effects of active inspiration induced by phrenic nerve stimulation on the energy required for 50% successful defibrillation (E50).

METHODS AND RESULTS

Shocks (95-microF biphasic waveform) were delivered after 10 seconds of ventricular fibrillation between a right ventricular coil and left pectoral test can in ten anesthetized pigs (25 to 37 kg). Using a 1-J step size, the E50 was determined with an up/down, three-reversal method. Positive-pressure ventilation was halted just before fibrillation, and shocks were delivered during expiration or at the end of 2 seconds of bilateral phrenic stimulation (50 Hz, 0.3 msec, 5 to 6 V). Phrenic stimulation produced inspiratory volumes that were 15.3 +/- 1.7 mL/kg (mean +/- SD). The E50 was 9.8 +/- 1.9 J during expiration and increased to 13.0 +/- 1.7 during inspiration (P = 0.001). The leading-edge voltage at the E50 was 451 +/- 46 V during expiration and 519 +/- 33 V during inspiration (P = 0.001). The leading-edge current at the E50 was 9.7 +/- 1.0 A during expiration and increased to 11.3 +/- 1.4 A during inspiration (P = 0.002). The average impedance was 47.8 +/- 2.7 omega during expiration and 47.3 +/- 3.3 omega during inspiration (P = 0.12).

CONCLUSION

Inspiration induced by phrenic stimulation results in a 31% increase in the E50 compared with expiration. The decrease in shock efficacy occurs in the absence of a change in impedance. Active inspiration may alter the distribution of the electrical field leading to a decrease in shock efficacy.

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.