Intrathoracic current flow during transthoracic defibrillation in dogs. Transcardiac current fraction.

Abstract

To achieve transcardiac threshold current during transthoracic defibrillation, a considerably larger current must be delivered to the thorax to compensate for the shunting effect of the lungs, the thoracic cage, and other elements of the torso. This shunting effect is thus an important determinant of transthoracic defibrillation threshold and can be quantified by the transcardiac current fraction (FC, the ratio of transcardiac to transthoracic threshold currents). Previous estimates of FC have ranged from as low as 3% to as high as 45%. The purpose of of this study was to quantify both FC and the major intrathoracic current pathways. Transthoracic and intrathoracic voltages and currents were simultaneously measured during high-voltage transthoracic shocks in 20 dogs. With correction factors determined from another set of 12 dogs, these raw data were corrected to compensate for field distortion caused by the presence of the intrathoracic electrodes, and the adjusted data were fit to a resistive network model. The results showed that 82% of the transthoracic current was shunted by the thoracic cage, while 14% was shunted by the lungs. The remaining 4% (FC) is the portion that passed through the heart. There was good agreement between the two independent methods used to calculate FC. Analysis based on the model indicated that FC was 3.7%, whereas FC determined by direct measurement with calibrated electrodes was 4.2%. Therefore, the results of this study, in contrast to earlier estimates of FC, show that defibrillation in dogs is achieved by only 4% of the total transthoracic current.

Theoretical possibility of ventricular fibrillation during use of TASER neuromuscular incapacitation devices

“…Similarly, Deale and Lerman reported that, in dogs, the thoracic cage shunted 82% of the input current and that the lungs shunted 14%. The remaining 4% passed through the heart [14]. Consistent with research cited above, our FE analyses, conducted for worst-case scenarios, showed that the skin, fat and anisotropic skeletal muscle layers attenuated and diverted a large portion of TASER currents in longitudinal directions, allowing just a fractional amount to penetrate transversally into deeper layers of tissue.…”

Rise in defibrillation threshold after postoperative cardiac remodeling in a patient with severe Ebstein’s anomaly

“…This likely led to shunting of the current across the lung, leading to a drop in the impedance and increased DFT. Prior experiments have shown that 14% of the transthoracic current is shunted by the lung during transthoracic defibrillation 6 . Newman and colleagues 7 assessed the time-dependent effects with a transvenous defibrillation system and found that the impedance rises during follow-up.…”

Principles of External Defibrillators

“…In the absence of biphasic defibrillators, monophasic ones are acceptable. A recommendation for higher initial energy when using a monophasic waveform was weighed by expert consensus taking in consideration the potential negative effects of a high-energy first shock versus the negative effects of prolonged VF [42]. The consensus recommends that rescuers using monophasic AED should give an initial shock of 360 J.…”