Captopril lowers coronary venous free radical concentration after direct current cardiac shocks.

STUDY OBJECTIVES: Direct current (DC) shocks to the heart cause morphologic and functional myocardial damage. Previous studies have suggested that acute DC shock injury is free radical mediated and that the administration of antioxidant enzymes superoxide dismutase and catalase can reduce the level of DC shock-induced free radicals. Angiotensin-converting enzyme (ACE) inhibitors are clinically used drugs that may scavenge free radicals or reduce free radical generation. The objective of our study was to determine whether the ACE inhibitor captopril lowers free radicals after DC shocks. DESIGN: In six open-chest dogs, we administered 100-J DC shocks to the epicardium, before and after administration of captopril, 3 mg/kg. We used electron paramagnetic resonance measurements of arterial and coronary venous ascorbate free radical (AFR) as a real-time marker of free radical generation (total oxidative flux). MEASUREMENTS AND RESULTS: Captopril resulted in a significant lowering of coronary venous AFR concentration: the peak rise in AFR after 100-J shocks was 17.3+/-3.4% (mean +/- SEM before captopril vs. 3.2+/-4.0% after captopril; p<0.05). CONCLUSIONS: Captopril lowers coronary venous AFR concentration after high-energy epicardial shocks.

Encircling overlapping multipulse shock waveforms for transthoracic defibrillation.

OBJECTIVES: This study was performed to determine the efficacy of new encircling overlapping multipulse, multipathway waveforms for transthoracic defibrillation. BACKGROUND: Alternative waveforms for transthoracic defibrillation may improve shock success. METHODS: First, we determined the shock success achieved by three different waveforms at varying energies (18-150 J) in 21 mongrel dogs after short-duration ventricular fibrillation. The waveforms tested included the traditional damped sinusoidal waveform, a single pathway biphasic waveform, and a new encircling overlapping multipulse waveform delivered from six electrode pads oriented circumferentially. Second, in 11 swine we compared the efficacy of encircling overlapping multipulse shocks given from six electrode pads and three capacitors versus encircling overlapping shocks given from a device utilizing three electrodes and one capacitor. RESULTS: In the first experiment, the encircling overlapping waveform performed significantly better than biphasic and damped sinusoidal waveforms at lower energies. The shock success rate of the overlapping waveform (six pads) ranged from 67+/-4% (at 18-49 J energy) to 99+/-3% at > or = 150 J; at comparable energies biphasic waveform shock success ranged from 26+/-5% (p < 0.01 vs. encircling overlapping waveforms) to 99+/-5% (p = NS). Damped sinusoidal waveform shock success ranged from 4+/-1% (p < 0.01 vs. encircling overlapping waveform) to 73+/-9% (p = NS). In the second experiment the three electrode pads, one capacitor encircling waveform achieved shock success rates comparable with the six-pad, three-capacitor waveform; at 18-49 J, success rates were 45+/-15% versus 57+/-12%, respectively (p = NS). At 100 J, success rates for both were 100%. CONCLUSIONS: We conclude that encircling overlapping multipulse multipathway waveforms facilitate transthoracic defibrillation at low energies. These waveforms can be generated from a device that requires only three electrodes and one capacitor.

High perimeter impedance defibrillation electrodes reduce skin burns in transthoracic cardioversion.

During cardioversion, skin burns result from current preferentially flowing at the electrode edge. We tested new electrodes with high perimeter impedance to yield more uniform current distribution; these electrodes reduced histopathologic skin injury.

Nature and determinants of skin "burns" after transthoracic cardioversion.

Skin biopsies obtained 24 hours after elective cardioversion of 30 patients showed variable epidermal necrosis and upper dermal perivascular inflammation, most noticeably in patients receiving high individual peak (> or = 300 J) and cumulative (> or = 350 J) shock energies. Thus, damped sine wave shocks cause skin injury--first degree burns--the severity of which is a function of peak and cumulative shock energy.

Is there an optimal electrode pad size to maximize intracardiac current in transthoracic defibrillation?

Achieving defibrillation depends on adequate intracardiac current. The purpose of this study was to determine, in advance of administering shocks which parameters of body habitus can be used to select the electrode size that maximizes intracardiac current in transthoracic defibrillation. We administered direct current shocks to 18 mongrel dogs over a wide range of weight and size (weight 10-30 kg with chest circumferences 44-77 cm) using a damped sine wave defibrillator and self-adhesive electrode pairs of various diameters (4 cm, 5.8 cm, 8 cm and 10 cm), placed on the right and left lateral chest walls. The energy levels used were 50, 100, and 150 J. Intracardiac voltage gradient, a parameter of intracardiac current, was determined in three orthogonal planes using an intramyocardial electrode array placed in the interventricular septum. The relation between intracardiac voltage gradient magnitude magnitude of VG and various parameters (body weight, chest, circumference, chest volume, chest radius, and heart weight divided by chest radius) was determined. The correlation (r) with the smallest P value was between magnitude of VG and the heart weight divided by chest radius (HW/R) (r = 0.71). Intracardiac current was highest at intermediate pad sizes. The electrode pads that maximized magnitude of VG tended to be large for the larger HW/R dogs, and smaller HW/R dogs. In none of the HW/R groups did the largest electrode pads yield the highest intracardiac voltage gradient. We conclude that there is no simple way to determine in advance an electrode pad size that maximizes intracardiac current. The HW/R ratio influences but does not determine intracardiac intracardiac current.

Direct current shocks to the heart generate free radicals: an electron paramagnetic resonance study.

OBJECTIVES: We sought to demonstrate that direct current (DC) shocks to the heart generate free radicals. BACKGROUND: Although it is a lifesaving maneuver, defibrillation is known to have myocardial toxicity. The mechanism of this toxicity is unknown. If DC shocks generate free radicals, free radicals could be a mechanism of myocardial injury. METHODS: In a canine model, DC shocks of 10 to 100 J were delivered to the epicardium of both beating and fibrillating hearts, and 200-J transthoracic shocks were administered in dogs with beating hearts. Ascorbate free radical (AFR) concentration was measured in arterial blood and blood continuously withdrawn from the coronary sinus. In some dogs, the antioxidant enzymes superoxide dismutase (15,000 U/kg) and catalase (55,000 U/kg) (SOD/Cat) were administered before shocks. RESULTS: Ascorbate free radicals were generated by DC shocks. A peak AFR increase of 14 +/- 2% (mean +/- SEM) was seen 5 to 6 min after 100-J epicardial shocks. A peak AFR increase of 7 +/- 5% occurred after transthoracic shocks. There was a significant linear relation between the shock energy and peak percent AFR increase: %AFR increase = 0.18 (Shock energy) + 2.9 (r = 0.73, p < 0.0001). Shocks delivered to hearts in ventricular fibrillation (30 s) resulted in generation of AFR equal to but not greater than that observed during similar shocks delivered to beating hearts in sinus rhythm. Multiple successive shocks (100 J delivered twice or five times) did not result in a greater AFR increase than single 100-J shocks, indicating that peak, not cumulative, energy is the principal determinant of AFR increase. Animals receiving SOD/Cat before shock administration showed significant attenuation of the AFR increase. CONCLUSIONS: Direct current epicardial and transthoracic shocks generate free radicals; antioxidant enzymes reduce the free radical generation by shocks.

Transthoracic defibrillation: importance of avoiding electrode placement directly on the female breast.

OBJECTIVES: This study sought to determine the effect on transthoracic impedance of placement of defibrillation electrodes on the female breast versus adjacent to or under the breast. BACKGROUND: Transthoracic impedance is a major determinant of transthoracic current flow in defibrillation. For a given energy setting, a high transthoracic impedance reduces current flow and may adversely affect the ability of electric shocks to accomplish defibrillation. We hypothesized that the increased interelectrode tissue associated with placement of the apex defibrillation electrode on the female breast would result in increased transthoracic impedance compared with electrode placement lateral to or under the breast. METHOD: Transthoracic impedance was measured noninvasively by passing a 5-V, 31.25-kHz square wave current through the chest and comparing the low level current flow to known references. We measured transthoracic impedance associated with three different apex defibrillation electrode positions--on the breast, under the breast and lateral to the breast--in 25 women (brassiere size 34A to 48C, 25 to 75 years old, body weight 128 to 328 lb [58 to 148 kg] and 2 men. The measurements were taken with a modified defibrillator that accurately predicts transthoracic impedance without delivering an actual shock. The measurement sequence was random. RESULTS: The average measured transthoracic impedance with placement of the apex defibrillation electrode on the breast was 95 +/- 25 ohms (mean +/- SD), under the breast 84 +/- 17* ohms and lateral to the breast 83 +/- 20* ohms (asterisk indicates p < 0.01 vs. on the breast by analysis of variance). The study cohort was also classified into two groups: large breasted (brassiere size > or = 40) and small breasted (brassiere size < or = 39). The measured transthoracic impedances for the large-breasted group were 112 +/- 20 ohms for on the breast, 94 +/- 13* ohms for under the breast and 98 +/- 19* ohms for lateral to the breast. For the small breasted group, the similar transthoracic impedance measurements were 81 +/- 21, 77 +/- 16 and 71 +/- 13* ohms, respectively. CONCLUSIONS: In women, placement of the apex defibrillation electrode on the breast results in higher transthoracic impedance, which will reduce current flow. We recommend placing the apex electrode lateral to or underneath the breast.