In response to this concern, a multidisciplinary research collaboration led by the UK Defence Science and Technology Laboratory (Dstl) has examined the possible arrhythmogenic effects of a strike from two commercial TASERs: the M26 and X26. To do this, the researchers applied the TASER waveforms to a digitized human-body representation and modelled the current flow within its heart. These simulated currents were then applied to a guinea-pig heart in vitro (Phys. Med. Biol. 52 7193).

"Given the controversy surrounding the use of the TASER, there has been astonishingly little research into its safety," commented Kenneth Foster, professor of bioengineering and electrical engineering at the University of Pennsylvania (Philadelphia, PA). "We could have guessed that the risks of cardiac events are rather low, since TASER International markets the weapon by sending them to police conventions and by now, hundreds of police officers have tased themselves without mishap. It is nevertheless reassuring see good studies being done that support this conclusion."

Shock results
The TASERS under investigation produce damped sinusoidal waves: the M26 generates a 50 kHz waveform with a current of 10-12 A, a peak voltage of nearly 1000 V and a 50 µs pulse duration; the X26 generates a 120 kHz wave with a peak voltage of around 300 V and a pulse duration of 120 µs. The research team applied these waveforms to the anterior chest wall of a numerical phantom (derived from CT and MR imaging data) using a dart separation of 225 mm - the distance that caused the highest current density on the heart.

By employing computational electromagnetic modelling, the team determined that the M26 TASER induced a peak absolute current density of 0.66 mA/mm2, spread over a circular region of approximately 25 mm in diameter on the right ventricle (beneath the upper probe). For the X26 TASER, the highest peak absolute current density was 0.11 mA/mm2, spread across a similar region.

One possible effect of external electrical stimulation on the heart is the generation of ventricular ectopic beats (VEBs). While isolated VEBs are generally of no clinical significance in healthy individuals, if they occur in the presence of heart disease or certain drugs they can degenerate into life-threatening arrhythmias such as ventricular fibrillation (VF). VF is one of the most common causes of cardiac arrest and can also arise as a direct result of electric shock.

To determine whether the TASER pulses would induce VEB or VF in the guinea-pig heart, the researchers applied the derived current data to a perfused, spontaneously beating, guinea-pig heart preparation. The choice of guinea-pig heart was partly based on the similarity of its electrocardiographic-wave configurations to those generated by a human heart.

The M26 and X26 waveforms were applied to the ventricular epicardial surface of the guinea-pig heart using an 18 mm2 electrode. At the maximum predicted current densities (0.66 and 0.11 mA/mm2), the pulses did not induce VEBs. VEBs could be generated at higher current densities (40.1 mA/mm2 for the M26 waveform and -7.3 mA/mm2 for the X26), but for both TASERs, the current density required to induce this effect was more than 60 times that predicted to arise at the human heart.

To investigate possible VF generation, the researchers applied 5 s pulse trains with progressively increasing intensities to the guinea-pig heart. Neither waveform induced VF when applied at the stimulation system's maximum current densities (51.8 mA/mm2 and -27.1 mA/mm2 for the M26 and X26, respectively). These values are more than 70 (M26) and 240 (X26) times larger than the predicted current densities.

Based on the above results, the researchers concluded that there's a wide safety margin for the generation of VEBs or VF from a TASER strike. Assuming that the guinea-pig findings scale-up to the human heart, they consider it unlikely that the discharge from M26 and X26 TASER devices will influence cardiac rhythmicity via direct electrical action on the heart.

The team does, however, caution that factors such as illicit drug intoxication, alcohol abuse, pre-existing heart disease and cardioactive therapeutic drugs may modify the threshold for generation of cardiac arrhythmias.

"This paper provides an important first step in determining the bioelectric effects of TASERs on the heart," commented Brad Roth, associate professor in the department of physics, Oakland University (Rochester, MI). "I have many concerns about TASERs, but the induction of a cardiac arrhythmia appears to be less of a problem than I would have initially guessed."