Finding The Edge of Heart Safety

Illustration: Bryan Christie Design

HEART ELECTRONICS: An electric impulse starts in the SA node and travels to the two atria, which contract, pushing blood into the lower chambers. Current then passes through the AV node to cause the ventricles to contract.

By Patrick Tchou

With the use of taser Electronic Control Devices by law-enforcement officers on the rise, it's no wonder that questions about the guns' safety come up again and again. As Mark Kroll describes [see “Crafting the Perfect Shock”], Tasers produce uncontrollable muscular contractions, which temporarily immobilize a subject. 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.

As a physician, I contribute to the former perspective by investigating whether Taser shocks can cause serious damage to a heart's normal function.

Let's begin with some basics about how the heart works. Each heartbeat is activated by an electrical impulse that propagates through the four chambers of the heart [see illustration, “Heart Electronics”]. 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. What brings on death is the uncoordinated electrical activation of the heart's main pumping chambers. 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. Taser International covered the costs of the testing equipment and the costs of laboratory use, but none of Taser's funding covered my time or that of any other physicians involved in the studies.

The pigs were under general anesthesia when we performed the experiments. We selected five points on each animal's torso corresponding to sites where Taser darts commonly make contact with human subjects. 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.

We calculated that quantity, cast in terms of multiples of the capacitances, for each of the body sites we'd chosen to test. 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 250 beats per minute, which is about twice the normal rate for pigs. These experiments also showed us that the onset of ventricular fibrillation is related to how fast the heart is driven by the impulses—which scales with the amount of current used.

Because the standard Taser output proved on average to be one-fourth what was needed to cause fibrillation, one is tempted to conclude that the device is fundamentally safe. But there's another factor to keep in mind: a large portion of the violent individuals with whom the police have to deal are under the influence of cocaine, methamphetamine, or other stimulants. So the Taser has to be safe even for those whose physiology is distorted by the presence of such powerful drugs. Cocaine in particular is a concern with respect to cardiac complications because it raises heart rate and blood pressure and significantly increases the risk of a heart attack even without any kind of shock.

My colleagues and I supposed that the presence of such drugs would increase the potential for cardiac arrhythmias, and we later tested this hypothesis in a separate study, published in the Journal of the American College of Cardiology. To our surprise, the amount of current needed to bring on ventricular fibrillation didn't go down; indeed, it increased significantly when the pigs were administered cocaine. After some thought, we realized that our initially puzzling findings were not entirely out of line, because cocaine has certain anesthetic properties that can affect the electrical behavior of the heart in ways that protect it against shocks and decrease its vulnerability to fibrillation. Applying enough voltage to a heart cell will open its sodium-ion channels and start the contraction machinery, but cocaine stops up the voltage-activated sodium channels, making it more difficult for electricity to trigger a muscle contraction.

Another study carried out at our clinic more recently showed that implantable defibrillators and pacemakers function normally after a typical 5-second electric shock from a Taser. It remains to be seen, however, how well such medical devices stand up to repeated or longer shocks.

It is a challenge to relate experiments conducted under controlled laboratory conditions to the vagaries of real life. For one thing, we obtained our results from anaesthetized pigs with ostensibly normal hearts. It's possible that an abnormal or diseased heart—or even a heart under stress or one affected by amphetamines—might be more vulnerable. No one has yet studied the effects of Taser shocks on such hearts, information that is sorely needed to understand what might prove to be the greatest danger from Tasers.

Even so, we were comforted to learn that stun guns do not normally pose any cardiac risk. The full length of the Taser dart tip would have to embed itself into the skin and chest-wall muscle of a relatively small, thin person to get within the range of distances where we found the heart to be most vulnerable. Furthermore, the most sensitive region for the induction of fibrillation covers just a small area. And it is unlikely that two darts would land there.

Much remains unknown about the physiological effects of a Taser shot, but the absence of conclusive medical knowledge doesn't necessarily mean that the devices shouldn't be used—as long as evidence continues to support their safety. Rarely is any biological phenomenon or medical device fully understood and tested, and the Taser is no exception. As more information becomes available, law-enforcement agencies and their officers will better understand the consequences of each pull of the trigger.