A taser is a battery-powered, handheld device which delivers a short, low-energy electrical pulse. Two electrode wires are attached to the gun's electrical circuit. Pulling the trigger breaks open a compressed gas cartridge inside the gun and flings the electrodes into contact with a body and a charge flows into the muscles.
The taser delivers 19 short pulses per second over 5 seconds, with an average current of 2 milliamps, according to TASER manufacturer Axon. It creates an electric field, which stimulates nerve cells called alpha motor neurons to send an electrical impulse. The impulse travels to muscles and causes short, sustained muscle contractions.
The taser has two modes: the first, pulse mode, causes neuromuscular incapacitation as the neural signals that control muscles become uncoordinated, and muscles contract at random.
The second mode, drive-stun, uses pain to get compliance. The current -- either direct, DC, or alternating, AC -- is the rate at which electrons going down a wire travel per second. Alternating current is what is typically used in wall sockets and it's more dangerous, according to the Journal of the American Medical Association, causing more extreme muscle contraction.
An ampere, or amp, is the unit used to measure current. A small current -- microamps —- applied directly to the heart can cause a fatal rhythm called ventricular fibrillation.
Humans have protective mechanisms: The skin, which provides high resistance to electricity, and soft tissue, which surrounds muscles and organs like the heart, also reduce the current. The second is by Patrick Tchou, a cardiac electrophysiologist at the Cleveland Clinic, who has tested Tasers experimentally on pigs. You know an engineering problem is difficult when the prevailing technology dates back to the Stone Age.
One reason that finding a good replacement has been such a confounding problem is the nature of the task. Police officers often need to take into custody a violent criminal who has overdosed on a stimulant.
Most people probably would be surprised to learn that, at present, the main methods police use in such situations all rely on inflicting pain. The old standbys are wrist twists and other forms of joint distortion, pepper spray, and clubbing. The problem is complicated by the fact that many illegal drugs are painkillers, and as a result standard subduing techniques are frequently ineffective at bringing troublemaking drug users to heel.
Even worse, many of the dangerously drug-addled perpetrators exhibit superhuman stamina and strength. There are numerous accounts of a person on a drug overdose manhandling half a dozen law-enforcement officers at once. Many officers are injured along with those they are trying to take into custody. The ideal arrest tool, then, must meet a number of requirements.
First, it must be able to temporarily disable even the largest, most determined drug-anesthetized individual. Second, it must do so without causing serious injury to anyone involved. Third, its effectiveness cannot be dependent on causing pain. Fourth, it must work reliably. Some approaches to meeting those criteria have come close, but not close enough.
Under microprocessor control, the device temporarily, and relatively harmlessly, immobilizes a suspect with a carefully engineered electric signal that is specifically designed with human physiology in mind. When you pull the trigger of a Taser gun, a blast of compressed nitrogen launches its two barbed darts at 55 meters per second, less than a fifth the speed of a bullet from a typical pistol.
Each projectile, which weighs 1. Two whisper-thin wires trail behind for up to 9 meters, forming an electrical connection to the gun. Because the barbs get stuck in clothing and fail to reach the skin about 30 percent of the time, the gun is designed to generate a brief arcing pulse, which ionizes the intervening air to establish a conductive path for the electricity.
The X26—the model commonly used by police departments—delivers a peak voltage of V to the body. Once the barbs establish a circuit, the gun generates a series of microsecond pulses at a rate of 19 per second. Each pulse carries microcoulombs of charge, so the average current is 1. To force the muscles to contract without risking electrocution, the signal was designed to exploit the difference between heart muscle and skeletal muscle.
When your brain orders a muscle to flex, an electrical impulse shoots down a motor nerve to its termination at the midpoint of a muscle fiber. There the electrical signal changes into a chemical one, and the nerve ending sprays a molecular transmitter, acetylcholine, onto the muscle. In the milliseconds before enzymes have a chance to chew it up, some of the acetylcholine binds with receptors, called gated-ion channels, on the surface of the muscle cell.
When acetylcholine sticks to them, they open, allowing the sodium ions in the surrounding salty fluid to rush in. As a result, a wave of voltage rolls outward along the fiber toward both ends of the muscle, moving as fast as 5 meters per second.
As the voltage pulse spreads, it kick-starts the molecular machinery that contracts the muscle fiber. The force with which a skeletal muscle contracts depends on the frequency at which its nerve fires. The amount of contraction elicited is proportional to the stimulation rate, up to about 70 pulses per second.
At that point, called tetanus, contractions can be dangerously strong. The same thing happens in the disease tetanus, whose primary symptom, caused by the presence of a neurotoxin, is prolonged contraction of skeletal fibers. The Taser, with its 19 pulses per second, operates far enough from the tetanus region so that the muscles contract continuously but without causing any major damage. Heart muscle has a somewhat different physical and electrical structure. Instead of one long cell forming a fiber that stretches from tendon to tendon, heart muscle is composed of interconnected fibers made up of many cells.
The cell-to-cell connections have a low resistance, so if an electrical impulse causes one heart cell to contract, its neighbors will quickly follow suit. With the help of some specialized conduction tissue, this arrangement makes the four chambers of the heart beat in harmony and pump blood efficiently.
A big jolt of current at the right frequency can turn the coordinated pump into a quivering mass of muscle. The Taser takes advantage of two natural protections against electrocution that arise from the difference between skeletal and cardiac muscle. The first—anatomy—is so obvious that it is typically overlooked.
The skeletal muscles are on the outer shell of the body; the heart is nestled farther inside. In your upper body, the skeletal muscles are arranged in bands surrounding your rib cage. To lock up skeletal muscle without causing ventricular fibrillation, an electronic waveform has to have a specific configuration of pulse length and current.
The key metric that electrophysiologists use to describe the relationship between the effect of pulse length and current is chronaxie, a concept similar to what we engineers call the system time constant. In successive tests, the pulse is shortened.
A briefer pulse of the same current is less likely to trigger the nerve, so to get the attached muscle to contract, you have to up the amperage.
The chronaxie is defined as the minimum stimulus length to trigger a cell at twice the current determined from that first very long pulse.
Shorten the pulse below the chronaxie and it will take more current to have any effect. So the Taser should be designed to deliver pulses of a length just short of the chronaxie of skeletal muscle nerves but far shorter than the chronaxie of heart muscle nerves. Basically, there are two ways: by using a relatively high average current, or by zapping it with a small number of extremely high-current pulses.
In terms of average current, the 1. As far as single-pulse current goes, the Taser is again in the clear. The single-pulse current required to electrocute someone by directly pulsing the most sensitive part of the heartbeat using 3-ms pulses is about 3 A.
When you factor in that the Taser barbs are likely to land in current-shunting skeletal muscle not near the heart, you wind up with a pretty large margin of safety. For barbs deeply inserted directly over the heart, the margin is slimmer, though, and the key question is whether that margin is adequate.
In the United States, about people die each year under police restraint, according to the U. These incidents include arrests and attempts to control an uncooperative person who needs medical assistance, as well as suicides after arrest.
Studies have shown that stun guns were used during about 30 percent of in-custody deaths in the United States. Although Tasers were involved in a sizable fraction of these deaths, one should not leap to the conclusion that Tasers caused them. Medical examiners have cited Tasers as the primary cause of death in only four cases to date, and three of those were later thrown out of court.
There will always be some degree of violence in many police arrests, and a reliance on handguns and hand-to-hand combat can lead to terrible use-of-force dilemmas for police officers. For example, when a suspect brandishing a knife is within striking distance, law-enforcement officers in the United States are trained to shoot that person. Those questions of safety can be answered in two ways: from a medical standpoint—that is, in terms of the bodily harm that can result from a Taser shock—and from the point of view of someone working in law enforcement.
The second perspective is much broader. How would one minimize injury to both the police officer and the person being taken into custody, not to mention bystanders, while restraining a violent and uncooperative subject?
To probe further, one must ask how alternative means of restraint compare with the use of a Taser. A number of troubles can throw off the internal rhythm of the impulse as it travels along, and the most dangerous kind of these arrhythmias is ventricular fibrillation, which is typically the cause of death in someone who is electrocuted. The heart tissue still carries electrical impulses, but they propagate at chaotic and rapid rates, and the heart ceases to function as a pump, so blood pressure quickly plummets.
It takes 10 to 20 seconds for a person to lose consciousness, less if he or she is standing. So the most important question regarding the safety of Tasers is how likely it is that the use of one will induce ventricular fibrillation. Statistics alone suggest that, so far, the incidence of Taser-induced ventricular fibrillation is low. To investigate this question further in a more rigorous experimental setting, my Cleveland Clinic colleagues and I designed experiments to assess the threshold for bringing about ventricular fibrillation using pigs, taking into account the distance between the heart and the Taser darts at the body surface.
The pigs were under general anesthesia when we performed the experiments. We used a custom-built circuit that matched the waveform and typical 5-second shock duration of an X26 Taser gun, but our device could deliver a much larger shock.
To boost the output current, we increased the capacitor sizes in the device. After inducing ventricular fibrillation, we immediately rescued the animal using an ordinary defibrillator. We then stepped down the current to determine the highest amount that could be delivered without inducing ventricular fibrillation. Of the various positions we exami ned, some were a mere centimeter or two away from the heart, which sits just under the chest wall, touching it on the inside. Not surprisingly, we found that darts near the heart had the lowest thresholds for inducing ventricular fibrillation.
At the closest spots—with one dart hitting at the lower end of the chest wall, and the other at the top of the breastbone—such a cardiac crisis would ensue with about four times the standard Taser capacitance. Our experiments were the first to document that Taser-like impulses, albeit more energetic ones, applied close to the heart on the chest wall in pigs could have serious cardiac consequences.
Even at the standard output of a Taser, we found that current applied to the most vulnerable part of the chest was able to drive the heart to beat up to beats per minute, which is about twice the normal rate for pigs.
Tasers are designed to produce a low-level shock that temporarily immobilizes someone, making it easier to apply handcuffs. Each trigger pull results in a 5-second burst of electricity. Holding down the trigger results in a continuous flow of electricity. The devices also can be placed flush against the skin in what is known as a "drive stun.
The company that produces Tasers, now called Axon Enterprise, told Reuters last year that only 24 people had died from its devices. But Reuters uncovered "1, incidents in the United States in which people died after police stunned them with Tasers. Reuters obtained autopsy findings for of the 1, deaths it documented.
In of the cases, "Taser was cited as a cause or contributing factor in the death, typically as one of several elements triggering the fatality," according to the news organization. Guidelines by the Police Executive Research Forum state that "exposure longer than 15 seconds whether continuous or cumulative may increase the risk of serious injury or death and should be avoided.
Taser use had been declining among Milwaukee police officers, but rose sharply in It declined slightly in
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