Letters to the Editor

Predictions from misleading pig model are potentially harmful to humans

This is a study of the influence of transthoracic impedance (TTI) on defibrillation, resuscitation and survival in patients with out-of-hospital cardiac arrest (OHCA), treated with a non-escalating impedance-compensating 150 J biphasic waveform defibrillator.

Cardiac arrest data from two EMS systems were analyzed retrospectively. All witnessed arrests from patients who presented with a shockable rhythm and were treated initially by BLS personnel were included (n = 102). For each defibrillation and resuscitation outcome variable, we tested differences in mean TTI for successful versus unsuccessful outcome. The effect of call-to-shock time on overall outcome was also examined.

Initial shocks defibrillated 90% [83–95%] (95% confidence interval) of patients. Cumulative success with two shocks was 98% [93–100%] and with three shocks was 99% [95–100%]. TTI averaged 90 ± 23 Ω. First-shock success, cumulative success through two shocks and cumulative success through the first-shock series were unrelated to TTI, as were BLS ROSC, pre-hospital ROSC, hospital admission and discharge. In contrast and consistent with previous findings, call-to-shock time was highly predictive of survival.

High impedance patients were defibrillated by the biphasic waveform used in this study at high rates with a fixed energy of 150 J and without energy escalation. Rapid defibrillation rather than differences in patient impedance accounts for resuscitation success.

In-vitro studies indicate that the electric-field intensity, or voltage gradient (VG), generated by a defibrillation shock is a determinant of defibrillation success as well as potential shock-induced cardiac injury. It is not clear how common descriptors of shock dose, e.g., joules (J), relate to VGs.

To assess the relationships between shock energy descriptors and VG.

One monophasic and three biphasic waveforms were compared using transthoracic shocks and standard electrodes in five swine. VG measurements via intracavitary multielectrode-tipped catheters were compared with delivered energy and peak current. Shock variables were recorded at native transthoracic impedance and an adjusted impedance approximating that of typical humans.

For shocks at the same energy setting, peak current and VGs varied widely among the four defibrillators. At simulated human impedance (75 Ω), shocks at each device's maximum energy setting produced similar VGs among the three biphasic defibrillators, despite different delivered energies. For both native impedance (35 Ω) and at 75 Ω, VG correlated with peak current (r = 0.81 and 0.77, respectively) but not energy setting (r = 0.61, 0.52) or delivered energy (r = 0.58, 0.56). At the maximum energy setting of each device, maximum recorded VGs for both monophasic (33 V/cm) and biphasic (24 V/cm) defibrillators were less than those reported to cause myocardial injury (>60 to 80 V/cm).

Energy descriptors correlate poorly to actual shock intensities. When compared with reported VG thresholds of myocardial injury, this study suggests that risk of injury from critically strong VGs is low for all of these defibrillators and equivalent among tested biphasic waveforms.