ReviewPhysiology and pathology of TASER® electronic control devices
Introduction
Electrical current is best conceived of as the number of electrons going down a wire per second. Since the number is typically very large, the term “ampere” (abbreviated “A”) is used to represent it and this represents 0.24 × 1018 electrons per second. The term “voltage” merely refers to the electrical pressure pushing the current down its path. One volt (V) is roughly the voltage from a single battery cell. Pathways with high resistance allow less electrical current for the same electrical pressure or voltage. The resistance unit ohm is simply given by the voltage divided by the current.
Continuous currents flowing through the human chest of 0.3 A are typically fatal. Yet a strong static electricity shock can have 100 times the peak current – 30 A – and produce no residual damage.1 The reason that the 30 A peak static shock is not fatal is that the pulse of the static electrical discharge is too short in duration to affect the heart muscle or cause an arrhythmia.2
The term “electrical charge” refers to the area under the electrical current curve. It represents the total number of electrons delivered in a pulse. A charge of 1 C is defined as 6.24 × 1018 electrons. The charge from a TASER X26 ECD (Electronic Control Device) pulse is approximately 100 μC (microcoulombs) or about 6 × 1014 electrons.3
The TASER X26 ECD is a pistol shaped device weighing 205 g. It has a limited power source (a battery of two lithium camera cells – Duracell® CR123 – of 3 V each), and shoots two tethered probes. Together they deliver 19 very short duration pulses per second (19 PPS), with a typical peak voltage of 1900 V (1400–2600 V), over a 5-S pulse voltage is actually only 600 V.4 The device also generates an open-circuit voltage of up to 50,000 V to arc through air or across thick clothing but that voltage is never seen in, or “delivered into” the body.
It is certainly not intuitively obvious how a 50 kV pulse can pass through clothing but then not pass through the body. The technical reason is that the clothing has a very high impedance (nearly infinite) while the body has a much lower impedance (on the order of 600 ohms). In addition, the 50 kV pulse is generated by separate output circuitry from that of the 600 V pulse. The 600-V pulse is generated by a direct connection of charged capacitors which deliver a fixed charge to the body. The 50 kV pulse is generated by a high-impedance transformer output which has low current capacity. An analogy is the van-de-Graff generator which delivers a high voltage – but low current – arc. While impressive, it cannot power a simple lantern while a high current – but low voltage – 1.5 V battery can.
While incapable of stimulating muscles, the “impressive” nature of the arc and the natural fear of electricity adds a significant additional capability to the ECD. Police officers in the United Kingdom have found that the mere display of the arc is enough to gain compliance in the majority of cases.5
The pulse generated is specially designed with a very short duration of 100 μs (microseconds) to efficiently capture alpha motor neurons.2
The TASER ECD should not be confused with generic “stun guns.” Those devices deliver less average current (typical values of 0.3–0.5 mA) and deliver it over a short pathway between two fixed electrodes. The result is pain, but they lack both the electrical charge and the electrode spread to cause motor neuron activation and skeletal muscle capture. With stun gun electrodes only 2–5 cm apart – and the lack of skin penetration – the current flow is primarily through the dermis between the electrodes and there is no significant penetration beyond the fat layer. Thus there is insufficient current in the skeletal muscle layer to capture motor neurons and achieve muscle control.
The average current of the TASER X26 is approximately 1.9 mA (milliamperes) = 19 pulses per second · 100 μC. (This is about 5 times the average current of a typical stun gun.) As seen in Fig. 1, a typical peak current is ∼2.5 A (range of 2–4 A).
The devices use compressed nitrogen to fire 2 small probes at typical distances of 7.7 m.[6], [7] (Other TASER cartridge models can reach a distance of 11 m.) When the trigger is pulled, the high voltage first serves to open the nitrogen cartridges to release the nitrogen to propel the probes towards the target. These probes themselves are designed to pierce or become lodged in most light clothing (which generally offers no protection to the shock delivery due to the 50,000 V arcing capability). The sharp portion of the probe is typically 9 mm long and typically penetrates the epidermis and dermis by 4–5 mm for a good electrical connection.
The probe cartridge can be removed and the device used in a “drive-stun” mode by pushing the front of the weapon into the skin to function as a higher charge stun gun. The drive-stun mode is also available by retaining an expended cartridge on the weapon. Since there is insufficient electrode spread to capture muscles, the drive-stun mode serves only as a compliance technique.
Even as a strong static shock will temporarily incapacitate someone, a series of 19 very short duration shocks per second causes temporary incapacitation. The ultra-short electrical pulses applied by TASER ECDs are intended to stimulate Type A-α motor neurons, which are the nerves that control skeletal muscle contraction, but without stimulating cardiac muscle. There are many established reasons why the device should have no effect on the heart; these are enumerated in Table 1.
Current from ECDs is intended primarily to disable the target by preventing voluntary movement. The largest diameter myelinated A-α motor neuron axons (which innervate skeletal muscle fibers) tend to have relatively low electrical thresholds and are fairly easy to stimulate. This is because the stimulation threshold correlates inversely with cell diameter (so larger diameter cells are easiest to stimulate).[8], [9]
Perceptions of discomfort, sensory overload, and pain are carried by myelinated axons (type III A-δ fibers responsible for the sensation of sharp pain) but also by small, nonmyelinated axons (type IV C responsible for dull, aching diffuse pain). The C fibers have stimulation thresholds about 20 times higher than those of sensory A fibers and thus the typical subject describes the effects as sensory overload and sharp pain. However, the primary physiological effect is the local paralysis while the discomfort is actually only a side effect.
By comparison to motor or sensory myelinated nerves, the electrical excitability of the heart is relatively low. This is because the cardiac strength-duration time constant (chronaxie) is about 3 ms (i.e. at least 10–20 times higher than the A-α motor neuron fibers which control skeletal muscle contraction).10 This means that pulses much shorter than 3 ms must have much higher currents to stimulate the heart cells.
The heart is also located deep within the torso as opposed to the skeletal muscle which comprises much of the superficial layers. Fortuitously, electrical current prefers to follow the grain (fiber) of the muscles.11 Thus the ECD current will tend to follow the grain of the skeletal muscle around – rather than into – the thoracic cage. Hence, relatively little, if any, current will pass through the heart.[12], [13], [14] This effect of relatively low penetration into the heart – given surface or near-surface stimulation of tissues – is well known and studied both in the electrical safety literature as well as the medical literature of transthoracic pacing and defibrillation.15
Both in terms of efficacy (efficiently activating motor neurons to affect skeletal muscle between and near the probes) and risk (producing current sufficient to cause ventricular fibrillation), the TASER X26 stimulus pulse appears to be ideal, since the former is maximal and latter minimal.
Section snippets
Electrocution
The term “electrocution” was first coined to describe the judicial execution of a criminal by use of electricity – a contraction of “electrical execution”. Today the term is more broadly used to describe the induction of ventricular fibrillation (VF) by the application of, or exposure to, electrical shock; death occurs virtually immediately. It is now generally accepted that an ECD cannot cause “electrocution” in an adult human.[16], [17], [18], [19], [20], [21]
Primarily from the study of the
Acidosis and rhabdomyolysis
Many arrest-related-deaths present with acidosis and this is to be expected from the effects of a prolonged struggle, hyperactivity, or drug intoxication.[55], [56], [57], [58], [59], [60] Since the ECD causes moderate muscle contractions it is natural to consider the possibility that an ECD application might exacerbate this acidosis. Animal studies with unventilated anesthetized swine do report pH changes.[36], [61], [62] However, in ventilated swine and in human volunteers pH changes are not
Probe application
Central puncture wound. Depending on the postmortem interval a flat erythematous circular lesion (2–4 mm diameter) may be seen. Prolonged application times may result in the proximal surrounding tissue having a cauterized appearance. If the probes were not immediately pulled by police officers, there might be a small bleb or blister in the surrounding skin.
Drive stun application
Central abrasion with underlying tissue having a cauterized appearance. May have scattered abrasions as the electrode tip moves across the
Conclusion
There is essentially zero risk of the induction of VF by shock delivered from TASER ECDs, and it matters not whether single or multiple shock are delivered. All of the available scientific evidence suggests that the output of the ECD is incapable of inducing VF in adult human beings. Human studies find no evidence of acidosis or rhabdomyolysis. ECDs have contributed to fatal fall injuries.
Conflict of interest statement
The author is a member of both the TASER International corporate and the scientific and medical advisory board.
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